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Potrogenesis of calc-alkaline and alkaline magmas from the southern and eastern Aegean Sea, Greece
Wyers, Gerard Paul, Ph.D.
The Ohio State University, 1087
UMI 300 N. Zeeb R4 Ann Arbor, MI 48106
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PETROGENESIS OF CALC-ALKALINE AND ALKALINE MAGMAS FROM THE
SOUTHERN AND EASTERN AEGEAN SEA, GREECE
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of the Ohio State University
By
Gerard Paul Wyers, B.S., M.S.
The Ohio State University
1987
Dissertation Committee: Approved by
M. Barton
K.A. Foland Adviser D.H. Elliot Department of Geology and Mineralogy G. Faure ACKNOWLEDGEMENTS
I would like to sincerely thank Mike Barton for his friendship and superb guidance throughout my stay at Ohio State. I am grateful to Ken Foland for his advice on several aspects of my research, and for his help with the acquisition of the isotopic data. Joep Huijsmans (State University of Utrecht) allowed me to use all of his samples, data and calculations, and spent a great deal of time discussing the petrology and geochemistry of Santorini with me. R.D. Schuiling (State University of Utrecht) permitted me to ship the entire sample collection of the Hellenic arc to Ohio State, and Manfred van Bergen let me use the microprobe at Utrecht free of charge. Many thanks also to David Elliot, Gunter Faure and Ken Foland for their constructive comments on my dissertation, and to Chen Jiang Feng and Jeff Linder for their patience while teaching me the required lab skills. Karen Tyler skillfully drafted most of the diagrams in this dissertation and Carolyn Gribbin typed the tables. I am thankful to the Graduate School for awarding me a University Fellowship and a Presidential Fellowship. Additional funding from Friends of
ii Orton Hall and the Stichting Molengraaff Fonds is also gratefully acknowledged.
Ill VITA
September 15, 1960 ...... Born in Utrecht, the Netherlands
1981 ...... B.S. Cum Laude, Institute for Earth Sciences, Rijksuniversiteit Utrecht 1984 ...... M.S. Cum laude, institute for Earth Sciences, Rijksuniversiteit Utrecht 1984-present ...... Department of Geology and Mineralogy, The Ohio State University
PUBLICATIONS
Henderson, C.M.B., Barton, M. and Wyers, G.P.(1984) The transition from alkaline to sub-alkaline lavas in the low-P environment: the role of volatiles. Transactions of the American Geophysical Union 65, 1153. Wyers, G.P. and Barton, M.(1984) Transitional alkaline— sub-alkaline lavas from Patmos, Greece: an example of a magma series formed by fractional crystallization, assimilation and magma mixing. Transactions of the American Geophysical Union 65, 1123. Wyers, G.P. and Barton, M.(1986) Petrology and evolution of transitional alkaline— sub-alkaline lavas from Patmos, Greece. Evidence for fractional crystallization, magma mixing and assimilation. Contributions to Mineralogy and Petrology 93, 297-311.
Wyers, G.P. and Barton, M. (1986) Calc-alkaline lavas from Nisyros, eastern Aegean Sea, Greece: evidence for assimilation from trace element studies and Sr-isotope
iv results. Transactions of the American Geophysical Union 67, 411. Barton, M., Wyers, G.P., Salters, V.J.M. and Foland, K.A.(1986) Evidence of assimilation during the evolution of two shield volcanoes, Santorini, Aegean Sea, Greece. Transactions of the American Geophysical Union 67, 566. Barton, M. and Wyers, G.P.(1986) The role of subduction in the genesis of primitive alkali basalts from Patmos (Dodecanesos, Greece). Transactions of the American Geophysical Union 67, 1281.
Wyers, G.P. and Barton, H.(1986) Geochemistry of a transitional ne-trachybasalt— Q-trachyte lava series from Patmos, Greece: further evidence for fractionation, mixing and assimilation. Contributions to Mineralogy and Petrology 97, 279-291.
Gulen, L., Hart, S.R., Salters, V.J.M., Wyers, G.P. and Barton, M. (1987) Sr, Nd, Pb isotopic constraints on the petrogenesis of the Aegean arc volcanics. Terra Cognita 7, 170-171.
Wyers, G.P., Barton, M. and Foland, K.A.(1987) Sr- and Nd- isotopic evidence for magma mixing and assimilation in two shield volcanoes on Santorini, Aegean Sea, Greece. Geological Society of America, Abstracts with Programs 19, 900.
Wyers, G.P. and Barton, M.(1986) Polybaric evolution of calc-alkaline lavas from Nisyros, southeastern Aegean Sea, Greece. Journal of Petrology, accepted for publication. Barton, M. and Wyers, G.P.(1988) Conditions of crystallization of Tertiary volcanic rocks from Patmos (Dodecanesos, Greece) and implications for magmatic evolution. Journal of Volcanology and Geothermal Research, accepted for publication.
FIELDS OF STUDY
Igneous Petrology and Isotope Geochemistry
v TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...... ii
VITA ...... iv LIST OF TABLES ...... X
LIST OF FIGURES ...... xiv CHAPTER PAGE I. INTRODUCTION ...... 1
Calc-alkaline volcanism ...... 1 The Aegean Sea ...... 2 Summary of approach ...... 4 Organization of the dissertation ...... 5 II. POLYBARIC EVOLUTION OF CALC-ALKALINE MAGMAS FROM NISYROS, SOUTHEASTERN HELLENIC ARC, GREECE .... 9
Introduction ...... 9 Outline of geology...... 12 Petrography and mineral chemistry ...... 14 Whole-rock major- and trace-element geochemistry ...... 23 Discussion...... 29 Fractional crystallization ...... 29 Major oxide modeling...... 29 Trace element modeling ...... 33 The role of processes other than fractional crystallization ...... 38 Magma mixing ...... 39 Assimilation...... 44 Relationships between the basaltic andesite to andesite series and the dacite to rhyodacite series ...... 46 Estimation of the conditions of crystallization ...... 51 Petrogenetic model ...... 56 Conclusions ...... 62
vi III. THE ROLE OF ASSIMILATION - IN THE EVOLUTION OF CALC-ALKALINE MAGMAS FROM SANTORINI AND NISYROS 87 Introduction ...... 87 Summary of previous w o r k ...... 90 Analytical methods ...... 92 Results ...... 93 Mikro Prof it is Ilias ...... 94 Skaros ...... 95 Nisyros ...... 96 Discussion...... 97 Santorini ...... 99 Intra-cycle variations on Mikro Profitis Ilias ...... loo Intra-cycle variations on Skaros ..... 104 The role of assimilation on Santorini . 106 Nisyros ...... ill Relation between mafic magmas from Skaros, MPI and Nisyros ...... 113 Summary and conclusions ...... 115
IV. THE NATURE OF THE SOURCE REGION OF ISLAND ARC MAGMAS FROM SANTORINI AND NISYROS ...... 142 Introduction ...... 142 Trace element characteristics of the primitive magmas ...... 144 Isotopic characteristics of the primitive magmas ...... 149 Discussion...... 151 Assimilation at the base of the crust .... 152 The role of subducted lithosphere...... 153 The composition of the sub-arc upper m a n t l e ...... 157 Conclusions ...... 164
V. PETROLOGY AND EVOLUTION OF TRANSITIONAL ALKALINE- SUB ALKALINE LAVAS FROM PATMOS, DODECANESOS, GREECE: EVIDENCE FOR FRACTIONAL CRYSTALLIZATION, MAGMA MIXING AND ASSIMILATION...... 170
Introduction ...... 170 Outline of geology...... 170 Petrography...... 171 Analytical techniques ...... 174 Mineral chemistry ...... 174 • Major oxide whole-rock analyses ...... 176 Analyses of groundmasses/mesostasis ...... 178 Discussion...... 178
vii Fractional crystallisation ...... 178 Evidence for magma mixing ...... 180 Evidence for assimilation ...... 181 Conclusions ...... 182 VI. GEOCHEMISTRY OF A TRANSITIONAL NE-TRACHYBASALT - Q-TRACHYTE LAVA SERIES FROM PATMOS (DODECANESOS), GREECE: FURTHER EVIDENCE FOR FRACTIONATION, MIXING AND ASSIMILATION ...... 185 Introduction ...... 185 Summary of geology...... 186 Analytical techniques ...... 186 Results ...... 186 Discussion...... 190 Trace element systematics ...... 190 Comparison of major oxide and trace element d a t a ...... 191 Evidence for magma mixing ...... 193 Evidence for assimilation ...... 193 Conclusions ...... 195 VII. CONDITIONS OF CRYSTALLIZATION OF TERTIARY VOLCANIC ROCKS FROM PATMOS (DODECANESOS, GREECE) AND IMPLICATIONS FOR MAGMATIC EVOLUTION...... 198 Introduction ...... 198 Summary of approach...... 200 A brief description of the lavas of Patmos .. 202 Equilibrium between phases ...... 204 Olivine ...... ;.... 205 Clinopyroxene ...... 205 M i c a ...... 207 Plagioclase...... 208 Summary...... 208 Conditions of crystallization ...... 209 Temperature...... 210 Pressure...... 212 Hater contents ...... 220 Oxygen fugacities ...... 223 Discussion...... 228 Comparison with other lavas ...... 228 Evolution of the MVS lavas ...... 230 Implications for the YVS l a v a s ...... 232 Implications for the intra-crustal evolutionary history of primitive, alkali- basaltic magmas ...... 235 Conclusions ...... 244
viii VIII. PETROLOGY, ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY OF THE VOLCANIC COMPLEX OF 7ATM0S, DODECANESOS, GREECE...... 265 Introduction...... 265 Geology...... 267 Analytical methods ...... 269 Age relationships of the Patmos volcanics ... 271 Petrography and mineral chemistry ...... 272 Main Volcanic Series ...... 272 YVS ne-trachybasalts...... 273 Phonolites ...... 274 Trachytes ...... 275 Rhyolites ...... 275 Whole-rock geochemistry ...... 276 Isotope geochemistry ...... 280 Petrogenesis ...... 282 Main Volcanic Series...... 282 Young Volcanic Series ...... 284 Phonolites and trachytes ...... 287 Rhyolites ...... 289 Conclusions ...... 291 APPENDICES A. Whole-rock major oxide and trace element analyses of lavas from Nisyros ...... 320 B. Whole-rock major oxide and trace element analyses of lavas from Patmos ...... 323 C. Representative mineral analyses for the MVS lavas of Patmos ...... 329 D. The equilibrium composition of phlogopite (addendum to Chapter VII) ...... 335 E. Activities of enstatite and CaTs in clinopyroxene (addendum to Chapter VII) ...... 338 LIST OF REFERENCES ...... 341
ix LIST OF TABLES
TABLE PAGE 2.1. Summary of petrographio characteristics of the Nisyros lavas ...... 64
2.2. Representative analyses of plagiodases...... 65 2.3. Representative analyses of clinopyroxenes ..... 66 2.4. Representative analyses of olivines...... 67 2.5. Representative analyses of orthopyroxenes ..... 68 2.6. Representative analyses of Fe-Ti oxides ...... 69 2.7. Major- and trace element analyses and element ratios of selected Nisyros lavas ...... 70 2.8. Results of least-squares mass balance calculations ...... 71
2.9. Height fractions of mineral phases in the fractionating solid assemblage ...... 72 2.10. Mineral-liquid partition coefficients...... 73 2.11. Comparison of observed and predicted trace element concentrations ...... 74
2.12. Calculated conditions of crystallization of the NisyroB magmas ...... 75
3.1. SiOn, Sr and Nd concentrations and Sr and Nd isotope ratios in lavas from Mikro Profitis Ilias ...... 118 3.2. SiOor Sr and Nd concentrations and Sr and Nd isotope ratios in lavas from Skaros ...... 119
3.3. Si02 r Sr and Nd concentrations and Sr, Nd and 0 isotope ratios in lavas from Nisyros .... 120
x 3.4. Results of simple mixing calculations for MPI .. 121 3.5. Results of simple mixing calculations for Skaros ...... 122 3.6. Proportions of phenocryst phases in fractionating solid assemblage...... < 123
3.7. Results of AFC calculations for HPI ...... 124 3.8. Results of AFC calculations for Skaros ...... 125 3.9. Results of AFC calculations for Nisyros...... 126 4.1. Sr and Nd isotope ratios and selected major - oxide and trace element concentrations in primitive lavas from Santorini and Nisyros ..... 166 5.1. Summary of petrographic characteristics of MVS lavas ...... 172 5.2. Whole-rock chemical analyses and CIPW norms of selected Patmos lavas ...... 177
5.3. Broad-beam electron microprobe analyses of groundmasses ...... 178 5.4. Results of least-squares mass balance calculations which test the feasibility that the magmas of the MVS are related by fractional crystallization ...... 179 6.1. Trace element concentrations in selected Patmos MVS l a v a s ...... 187 6.2. Important elemental and oxide ratios in selected Patmos lavas ...... 188 6.3. Sr- and 0-isotopic data for selected Patmos MVS lavas ...... 189
6.4. Distribution coefficients required to explain trace element variations assuming that D^h1"0 ... 190 6.5. Distribution coefficients required to explain trace element variations using values of F taken from the results of major oxide mass balance models ...... 191
xi 6.6. Estimated proportions of mafic magma which mix with evolved magma to produce the hy-trachybasalts ...... 193 6.7. Results of AFC calculations for Pat-150 ...... 195
7.1. Whole-rock analysis and CIPW norm of a primitive YVS ne-trachybasalt...... 248 7.2. Representative analyses of phenocrysts and xenocrysts in YVS ne-trachybasalt Pat-167 ..... 249 7.3. Comparison of predicted and observed phenocryst/ microphenocryst compositions ...... 250
7.4. Summary of calculated conditions of crystallization of the Patmos magmas ...... 251 7.5. Pre-eruptive temperatures calculated from olivine-liquid equilibria for the ne-trachybasalts ...... 252
7.6. Estimates of pressure and &si02 for the ne“ trachybasalts based upon simultaneous solution of the equations defining equilibria involving olivine-clinopyroxene and anorthite-CaTs ...... 253
7.7. Pressure estimates based upon comparison of temperatures calculated at various pressures using the model of Ghiorso & Carmichael (1980) and Ghiorso et al. (1983) with temperatures calculated from olivine-liquid equilibria .... 254
7.8. Temperatures and pressures estimated for evolved MVS lavas from plagioclase-liquid equilibria, two-feldspar equilibria and Ti-Fe ratios of biotites ...... 255
7.9. Estimates of pre-eruptive water content and Pj{20...... 256 7.10. Estimates of pre-eruptive f02 ...... 257
8.1. K/Ar ages of whole-rock samples from Patmos .... 293 8.2. 40Ar/39Ar analytical data and ages for whole-rock samples from Patmos ...... 294
8.3. Whole-rock chemical analyses and CIPW norms of
xii selected Patmos lavas ...... 295 8.4. Trace element concentrations of selected Patmos lavas ...... 296 8.5. Sr and Nd isotope ratios in lavas from Patmos .. 297 8.6. Results of least-squares mass balance calculations ...... 298 8.7. Results of AFC calculations for the YVS ...... 299
A.I. Whole-rock major oxide and trace element analyses for lavas from Nisyros ...... 321
B.l. Whole-rock major oxide and trace element analyses for lavas from Patm os...... 324 C.l. Representative analyses of olivines and spinel in lavas from Patmos...... 330 C.2. Representative analyses of plagioclases in lavas from Patmos ...... 331 C.3. Representative analyses of clinopyroxenes in lavas from Patmos ...... 332
C.4. Representative analyses of micas and Fe-Ti oxides in lavas from Patmos ...... 333
C.5. Representative analyses of K-feldspar in lavas from Patmos ...... 334
xiii LIST OF FIGURES
FIGURE PAGE
1.1. Major geologic units of the Aegean region ...... 7
1.2. Map of the Aegean Sea, showing the location of major outcrops of volcanic rocks ...... 8 2.1. Geologic map of Nisyros, simplified from Di Faola (1974) ...... 76
2.2. Feldspar compositions plotted in terms of the components Ab-An-Or ...... 77 2.3. Compositions of clino- and orthopyroxenes plotted in the conventional Ca-Mg-Fe quadrilateral .... 78
2.4. Major oxides plotted against Si02 ...... 79 2.5. Variation diagrams of selected trace elements with Rb as a differentiation index ...... 80
2.6. Schematic liquidus surface for natural magmas and surface of equilibration temperatures for natural melts and phenocryst assemblages ...... 81
2.7. 87Sr/B6Sr plotted against Rb contents ...... 82 2.8. Possible relationships between the basaltic andesite to andesite series and the dacite to rhyodacite series ...... 83
2.9. Schematic diagram to explain the observed zonation patterns in phenocrysts from the Nisyros lavas ...... 84
2.10. Log &Sjn2 calculated from Ol-Opx-liquid and plagioclase-liquid equilibria plotted as a function of temperature ...... 85
2.11. Schematic diagram showing the possible evolution
xiv of the Nisyros magmas ...... 86 3.1. The system Fo-Di-Si02 ...... 127 3.2. 143Nd/144Nd versus 87Sr/86Sr for lavas from Santorini and Nisyros ...... 128 3.3. 87sr/86Sr and sio2 as a function of stratigraphic height (relative age) for lavas from MPI ...... 129 3.4. 87sr/86sr and Si02 as a function of stratigraphic height (relative age) for lavas from Skaros ...... 130 3.5. Variations in 87Sr/86Sr and 18O/160 as a function of Si02 for lavas from Nisyros.... 131 3.6. Comparison of Sr-Nd isotopic composition of lavas from Santorini and Nisyros with isotopic compositions of lavas from other island arcs ... 132 3.7. Comparison of Sr-Nd isotopic composition of lavas from Santorini and Nisyros with isotopic compositions of lavas from active continental margins ...... 133 3.8. Simple mixing trends (87Sr/86Sr versus Si02) for lavas from MPX ...... 134 3.9. Simple mixing trends (143Nd/144Nd versus 87Sr/86Sr) for lavas from MPI ...... 135 3.10. 87Sr/88Sr versus Si02 for lavas from Skaros .... 136 3.11. 143Nd/144Nd versus 87Sr/86Sr for lavas from Skaros...... 137
3.12. AFC trends (143Nd/144Nd versus 87Sr/86Sr) for Santorini calculated with three different contaminants ...... 138
3.13. AFC trends (Rb versus 87Sr/86Sr) for Santorini . 139
3.14. AFC trends (143Nd/144Nd versus 87Sr/86Sr) for Nisyros ...... 140
3.15. AFC trends (Rb versus 87Sr/86sr) for Nisyros ... 141
xv 4.1. MORB-normalized trace element patterns for primitive lava compositions from Santorini and Nisyros ...... 167
4.2. 143Nd/144Nd versus 87Sr/86Sr for lavas from Santorini and Nisyros ...... 168 4.3. Range in Pb-isotopic compositions of the Hellenic arc lavas from Gulen et al.(1987) .... 169
5.1. Nap of the Aegean Sea, shoving the location of major outcrops of volcanic rocks ...... 171 5.2. Classification scheme of Patmos MVS lavas in terms of CIPW normative Ne or Q and Differentiation Index ...... 171 5.3. Geologic map of Patmos ...... 172
5.4. Microphotographs of resorbed crystals of olivine and plagioclase and of zoned clinopyroxene .... 173 5.5. Feldspar compositions plotted in terms of the components An-Ab-Or ...... 175
5.6. Compositions of clinopyroxenes plotted in the conventional Ca-Mg-Fe quadrilateral ...... 176 5.7. Major oxides plotted against Si02 ...... 177 5.8. Plot of MgO versus Si02 for the MVS lavas ...... 178 5.9. Interpretation of compositional characteristics of plagioclase and pyroxene phenocrysts in the hy-trachybasalts in terms of magma mixing ..... 181 5.10. Plot of K20 versus Si02 for the MVS lavas ...... 181
5.11. Schematic diagram showing the probable sequence of emplacement and eruption of the MVS magmas .. 183
6.1. Location of Patmos in relation to the Hellenic trench and Tertiary-Recent volcanic arc ...... 186
6.2. Variation diagrams for selected trace elements with Th as a differentiation i n d e x ...... 188
6.3. Plots of Rb, Zr and Nb versus T h ...... 189
xvi 6.4. Chondrite-normalized plotes of REE concentrations in representative samples of the MVS ...... 189 6.5. 87Sr/86Sr plotted against 180/160 for representative samples of the MVS ...... 189 6.6. 87Sr/86Sr plotted against sio2 contents ...... 190 6.7. Observed versus predicted changes in trace element concentration as a function of F ...... 192 6.8. Log(Th/Zr) and Log(Th/Rb) versus Log(Th) ...... 194 7.1. Map of the Aegean Sea, showing the location of volcanic rocks ...... 258
7.2. Geologic map of Patmos...... 259
7.3. Curves showing log &ej[02 versus T calculated from the equilibria Fo * Si02 - En and CaTs + sio2 > An at various pressures for the MVS ne-trachybasalts ...... 260
7.4. Diagram similar to Fig. 7.3, but for a YVS ne- trachybasalt ...... 261 7.5. Plagioclase-liquid temperatures calculated by the method of Ghiorso and Carmichael (1980) plotted as a function of pressure ...... 262 7.6. Pressure-temperature estimates for the evolved lavas ...... 263
7.7. Estimates of f02 plotted against temperature ... 264 8.1. Compositions of clinopyroxenes plotted in the conventional Ca-Mg-Fe quadrilateral ...... 300 8.2. Feldspar compositions plotted in terms of the components An-Ab-Or ...... 302
8.3. Classification of Patmos lavas in terms of CIPW normative Ne or Q and Thomton-Tuttle differentiation index ...... 305 8.4. Major oxides plotted against Si02 ...... 306 8.5. Variation diagrams for selected trace elements with Th as a differentiation index ...... 310
xv ii 8.6. Chondrite-normalized plots of REE concentrations in representative samples ...... 314 8.7. 143Nd/144Nd plotted against 87Sr/86Sr ...... 316 8.8. 87Sr/86Sr plotted against Si02 contents ...... 317 8.9. 143Nd/144Nd plotted against Sio2 contents ..... 318
8.10. The CIPW normative composition of the phonolites plotted in the system NaAlSi04-KAlSi04- Si02-H20 ...... 319
xviii CHAPTER I INTRODUCTION
CALC-ALKALINE VOLCANISM In the late sixties, as the concept of plate tectonics became widely accepted, it was realized that calc-alkaline volcanism is intimately associated with convergent plate boundaries. The origin of theBe magmas has been attributed to partial melting in the subducted oceanic lithosphere or in the upper mantle wedge above the Benioff zone. Calc- alkaline volcanism has been studied extensively because it may reveal information about several fundamental processes that are still poorly understood, such as the recycling of crustal material into the mantle, the formation and growth of continental crust and the origin of certain types of mineral deposits. The following aspects of calc-alkaline volcanism are of special interest from a petrological or geochemical point of view: - How and to what extent doeB subduction of oceanic
lithosphere change the composition of the upper mantle? - How are calc-alkaline magmas generated, and what are the relative contributions of subducted oceanic lithosphere,
subducted sediments and the mantle wedge to the
formation of these magmas? - What is the relationship between calc-alkaline and
alkaline volcanics, which are often found in close association near convergent plate margins? - How and to what extent have the compositions of the magmas been modified en route to the surface?
This last point is important because most erupted magmas do not represent pristine upper mantle melts, but are modified in intracrustal magma chambers by processes such as fractional crystallization, magma mixing and assimilation of crustal material. Therefore the intracrustal evolution of the magmas must be known in detail, and the composition of the unmodified, primary magma must be estimated before more fundamental processes, which occur in the upper mantle, can
be studied.
THE AEGEAN SEA The Aegean region is considered as a separate microplate that is situated in between the larger Eurasian,
African and Anatolian plates (Fig. 1.1, see McKenzie,
1978). The entire area is characterized by strong seismic
activity and high heat flow (Bath, 1983; Jongsma, 1974).
The crust of the Aegean Sea is thin continental crust of highly variable thickness (22-32 km, see Makris, 1978).
Several geotectonic units can be distinguished in the
Aegean region (Fig. 1.1). These units constitute an orogenic belt of alpidic origin that has been wrapped 3 around the Internal Crystalline Massif during the closure of the Tethys ocean (Horvath and Berckhemer, 1982). The islands of the Cyclades are part of the Median Crystalline Massif that extends eastward into the Menderes Massif and westward into the Pelagonian zone (Fig. 1.1). Recent calc-alkaline volcanism (4 Ma - present) has occurred on several islands of the Cyclades and Dodecanesos (Aegina, MiloB, Santorini, Hisyros, Yali, Kos) and on the mainland of the Peloponnesos (Crommyonia, Methana, Poros) (Fig. 1.2). Volcanism along the Hellenic arc is related to subduction of the African plate beneath the Aegean microplate, which started 13 m.y. ago (Le Pichon and
Angelier, 1979, 1981). The Benioff zone can be traced to a depth of 180 km (Papazachos and Comninakis, 1978) and has a dip of approximately 30° North. However, seismic tomography has shown the presence of an inclined slab of high-velocity material down to a depth of at least 600 km (Spakman, 1986). The direction of subduction is approximately perpendicular to the axiB of the Hellenic trench (Fig. 1.1) and parallel to those of the Pliny and Strabo trenches (Le
Pichon and Angelier, 1979), and the rate of convergence has been estimated at 5-7 cm/yr (Angelier et al, 1982). Makris (1978) has argued that the crust south of the trench system is continental crust, which implies that continental collision is already in progress in this area. This makes the Aegean a geologically unique area, since it is the only 4
locality in the world where continental collision is associated with abundant volcanic activity. The presence of a large amount of continent-derived sediment (6-10 km, Biju-Duval et al., 1978) and the continental nature of both plates suggest the possibility of subduction of continental material. Therefore the Hellenic arc is well-suited to study the effects of subduction on the composition of calc- alkaline volcanics and the upper mantle. The occurrence of alkaline volcanics is restricted to the islands of the Dodecanesos (Fig. 1.2) in the eastern Aegean (Kos, Patmos, Samos, Lesbos) and the Turkish peninsula Bodrum. This volcanism is generally older than the calc-alkaline volcanism of the Hellenic arc, and ages range from 4.5 m.y. on Patmos to 22 m.y. on Lesbos. These islands are located near the inferred plate boundary between the Aegean and Anatolian plates, but the relation between this alkaline volcanism and the regional tectonics
is still obscure.
SUMMARY OF APPROACH The scope of this project is limited to the lavas of
three islands in the southern and eastern Aegean Sea. Two islands, Santorini and Nisyros, belong to the Hellenic arc, whereas Patmos is situated approximately 100 km to the
north of the arc and is characterized by alkaline volcanics. The objectives of this study are: 1) to decipher 5
and to quantitatively model (where possible) the intracrustal evolution of the calc-alkaline and alkaline magmas from these islands; and 2) to characterize the nature of the source region and to determine the effects of subduction on the composition of the upper mantle. Mineral chemical, whole-rock chemical and isotopic
data are used to evaluate the roles of fractional
crystallization, magma mixing and assimilation in the evolution of the magmas. In addition, the conditions of crystallization are estimated using mineral-mineral and mineral-liquid equilibria. The most primitive lava compositions, selected on the basis of whole-rock chemistry and isotopic composition, are used to study the upper mantle source region.
ORGANIZATION OF THE DISSERTATION Some of the chapters in this dissertation are manuscripts that have been published or accepted for publication, whereas other chapters will soon be submitted for publication. I want to apologize to the reader for the repetition this has caused, especially in the introductions to the different chapters. Chapter II concerns the petrology of lavas from Nisyros. The isotopic variations on
Nisyros and Santorini are discussed in Chapter III. The
studies of the lavas of Santorini, described in this chapter, are a continuation of the Ph.D.-research of J.P.P. Huijsmans (State University of Utrecht). In Chapter IV the most primitive lavas from Santorini and Hisyros are used to identify processes in the upper mantle source region beneath these islands. Chapters V and VI discuss the evolution of the Main Volcanic Series from Patmos. In
Chapter VII the physico-chemical conditions of crystallization of these magmas are estimated, and the other rock types of Patmos are described in Chapter VIII. The evolution of the upper mantle source region beneath
Patmos is not discussed in this dissertation, but is the subject of a paper in preparation by Barton and Wyers. 7
>,vcm;
uiv!| 11111 (■wl& .'^i^*jV V ^ Mte^*|l,» *Jflgufi Vy*
^k> ** ij?T,M* V#, ■ iC'l-« .<.i *>lfi>fu2»
l l l i i i i i l l
k HM" '.-iAvwjwbhm
^ S S R •s k i
Fig.1.1. Major geologic units of the Aegean region (from Horvath and Berckhemer, 1982). Keys: 1 ■ Internal crystalline massif; 2a - Vardar zone; 2b « Izmir-Ankara zone; 3 » Median crystalline massif; 4 » Felagonian nappe zone; 5 — Pindos zone; 6 - Gavrovo-Tripolitza zone; 7 - Ionian zone; 8 - Lycian zone; 9 - carbonate platform. Inset: plate boundaries and relative motions in the Aegean area after McKenzie (1972). 8
BULGARIA w Rhodopos W estern SALONIKI Thrace o 0 Sam otharki
S ? o Limnos "*** > Haglos o Eustration
Lesbos
Lokris .0 (T>j Chios )»( in ^ATHENE ** Sam os Corinth w lnM ‘ Methana' ^ w * * Patmosfc
\ rpterimos Antimilos^Antiparos
M ilo s'^ Santorini
Christiana
J ' D C T C
50 100km
Trench
Fig.1.2. Hap of the Aegean Sea, showing the location of major outcrops of volcanic rocks (in black). The dashed lines mark the location of the Hellenic trench system and the Hellenic arc. CHAPTER II POLYBARIC EVOLUTION OF CALC-ALKALINE MAGMAS FROM NISYROS, SOUTHEASTERN HELLENIC ARC, GREECE.
INTRODUCTION Volcanism along the Hellenic arc (Fig. 1.2) is related
to an episode of subduction of the African plate beneath the Aegean microplate which began in the Middle Tertiary (see Huijsmans, 1985, for a summary of available geophysical data). No oceanic crust has been identified in
the eastern Mediterranean so that collision of two continental plates is currently in progress. Active subduction may have ceased, and Lister et al.(1984) suggest that the Aegean microplate is being obducted onto the African plate. Volcanics erupted along the Hellenic arc are exclusively calc-alkaline in character (Barton et al., 1983) and the oldest rocks (4.4 Ma - Pe-Piper et al., 1983)
occur on Aegina. Recent eruptions have occurred on Santorini (1950 A.D.) and Nisyros (1422 A.D.) and these
centers are still active. Geophysical data have been interpreted to indicate that crustal thickness increases from 22 km beneath Santorini (located approximately at the center of the arc) to 32 km beneath Aegina and Methana which are in the 10 northwestern part of the arc (Makris, 1978). The Sr-isotope
ratios of lavas erupted on Santorini, Milos, Aegina and » * Methana correlate roughly with crustal thickness (Barton et al., 1983) suggesting that the amount of assimilation of crustal material by arc magmas is directly related to the thickness of continental crust traversed during ascent from the upper mantle (see also Leeman, 1983). Huijsmans and Barton (1988) and Barton et al. (1986) have documented the1 complex evolutionary history of magmas erupted on Santorini and have demonstrated that fractional crystallization, magma mixing and assimilation were important processes in the petrogenesis of these eruptives (see also Chapter III). In addition, evolution was polybaric and the compositions of eruptives reflect mixing of magmas which evolved via AFC processes in separate, but interconnected, chambers sited at different depths within the crust. Sr- and Nd isotope studies (Chapter III) confirm these conclusions. Studies of lavas from Milos, though not yet complete, suggest a similarly complex, polybaric, evolutionary history involving fractionation, mixing and assimilation (Bol and Hasselman, 1985).
He undertook a detailed study of the lavas of Nisyros because 1) reconnaissance work suggested that mixing had played no, or only a minor, role in the evolution of magmas beneath this volcanic complex; and 2) Pe and Gledhill (1975) reported a Sr-isotope ratio of 0.7037 for a basaltic andesite front Nisyros and this value was (at that time) the lowest reported for any lava erupted along the Hellenic arc. It seemed likely on the basis of these observations that evolution of the Nisyros magmas was not as complex as on other islands in the southern Aegean Sea, and that fractional crystallization was the main process that caused the range in composition displayed by the Nisyros lavas. If magma evolution is relatively simple (i.e. involves only fractional crystallization), then it should be possible to use the geochemical characteristics of the most primitive magmas to gain some insight into the processes which occur in the upper mantle source regions, and specifically to evaluate the nature of the component added to the upper mantle source region from the subducted slab. However, it is first necessary to demonstrate that the most primitive magmas represent primary magmas or that the characteristics of the primary-magmas can be inferred with some degree of certainty.
In this paper the petrology and major and trace element geochemistry of the Nisyros lavas are described and are used to quantitatively evaluate the nature of the processes operative during the intracrustal evolution of the magmas. The composition of the upper mantle source regions will be considered in Chapter IV. 12 OUTLINE OF GEOLOGY Nisyros is an entirely volcanic island, roughly circular in shape with a diameter of approximately 7 km (Fig. 2.1). It has a near-perfect circular caldera about 4 km in diameter and about 300 m deep on average. The geology and volcanology have been described in detail by Di Faola (1974) who recognized the following major volcanic rock types: basaltic andesiteB, andesites, pre-caldera dacites, post-caldera dacites, rhyodacites and pyroclastics.
According to the classification scheme of Peccerillo & Taylor (1976), the basaltic andesites and some andesites belong to the medium-K calc-alkaline series, whereas other andesites, the dacites and the rhyodacites belong to the high-K calc-alkaline series. Rhyolites are not present on Nisyros itself, but outcrop on Yali, a small island 3 miles to the north of Nisyros (Di Paola, 1974). According to Di Paola1s (op.cit.) and our own observations, volcanic activity started with submarine extrusions of basaltic andeBite which occur as pillow lavas
(Fig. 2.1). Subsequent volcanism was subaerial, and a series of lavas ranging in composition from basaltic andesite to rhyodacite was produced prior to caldera formation. This activity was accompanied by frequent explosive activity. Pre-caldera lava flows and pyroclastics were erupted from several centers, but according to Di
Paola (1974) only two vents are preserved, one at Ht. Ag. 13 Xoannis and one near Emborio. The entire pre-caldera sequence is young, since one of the oldest lavas on
Nisyros, a dacite, yields a K/Ar-age of 0.2 Ha (Di Paola, 1974). Caldera formation was accompanied by intense pyroclastic eruptions which produced mainly air-fall deposits with some ash flows and surges (J.C. Varekamp, personal communication, 1986). Preliminary work by J.C. Varekamp suggests that the pyroclastics consist of a mixture of mafic and silicic glasses and contain a complex mineral assemblage indicating an origin by magma mixing. The bulk compositions of pumices indicate a high-K rhyodacitic to rhyolitic magma type. Volcanism was renewed with the eruption of domes of dacitic composition which fill the western half of the caldera. These are large, and some spill over the caldera rim towards the west. The most recent activity is phreatic, as evidenced by the occurrence of several explosion craters on the caldera floor. The explosion pits do not contain juvenile magmatic materials and appear to represent the escape of heated groundwater in the poorly consolidated caldera infill material to the surface. In addition, there is intense fumarolic activity inside the caldera, with sulfur deposits around the fumarolic vents. The geothermal potential of
Nisyros is currently being exploited by the Public Power
Co. of Greece. 14 PETROGRAPHY AND MINERAL CHEMISTRY Mineral analyses were performed at the State University of Utrecht, using a TPD microprobe fitted with a Tracor Northern Energy Dispersive System. Operating conditions were: 15 kV accelerating voltage, 3-4 nA sample current and 30-60 s counting time. All analyses are fully corrected for deadtime, background, atomic number, absorption and fluorescence. The petrographic characteristics of the Nisyros lavas are summarized in Table 2.1. The basaltic andesites and andesites are sparsely phyric to porphyritic lavas that are often highly vesicular. The dacites and the rhyodacites are all strongly porphyritic lavas. On the basis of petrographic studies we infer the following crystallization sequence: plag-cpx-ol, plag-cpx-ol-Fe-Ti oxide-(opx), plag- cpx-Fe-Ti oxide-opx-ap-zr-? amph, plag-cpx-Fe-Ti oxide-opx- ap-zr-amph. Hence, all magmas were multiply saturated with crystalline phases prior to eruption.
Plaoioclase is the most abundant phenocryst phase in all lavas and forms euhedral crystals up to 5 mm in size. Representative compositions are reported in Table 2.2 and all analyses are plotted in terms of An, Ab and Or in Fig. 2.2. The average composition of the plagioclase ranges from bytownite in the basaltic andesites to oligoclase in the rhyodacites. Most phenocrysts in the basaltic andesites 15 and andesites show normal with superimposed oscillatory zoning. The cores of phenocrysts in these lavas range in composition from An88_55, whereas the rims are slightly more sodic with compositions from An82_35. The maximum variation observed in a single phenocryst is 30 mol.% An. The cores of phenocrysts in the pre-caldera dacites and rhyodacites range from An66 to An33 and An36 to An27 respectively. However, many of these crystals have rounded cores which are surrounded by optically distinct mantles and rims, whereas some crystals show continuous normal zoning with superimposed oscillatory zoning. The rounded cores of the phenocrysts are relatively sodic (An40 to An33 in the pre-caldera dacites and An30 to An27 in the rhyodacites) compared with the cores of normally zoned crystals (An66 to An40 in the dacites and An56 to An32 in the rhyodacites). The compositions of the rims are intermediate to the two core populations viz. An44 to An39 in the dacites and An32 to An29 in the rhyodacites. The presence of phenocryst cores which define two compositionally distinct populations, together with rims of intermediate composition, is suggestive of magma mixing
(cf. Luhr and Carmichael, 1980; Tsuchiyama and Takahashi,
1983). The plagioclase phenocrysts in the post-caldera dacites show complex zoning and resorption patterns. Some phenocrysts consist of a sieve-textured core surrounded by 16 a mantle of clear feldspar whereas others consist of an inclusion-free core that is surrounded by a sieve-textured • * mantle and a thin, outer rim of clear feldspar* These textures are similar to those reported for plagioclase phenocrysts in many calc-alkaline lavas (cf. Luhr & Carmichael, 1980; Tsuchiyama and Takahashi, 1983) and are thought to indicate magma mixing. We have not analysed the phases in the post-caldera dacites in detail as these are
being studied by J.C. Varekamp (personal communication, 1986), so that we are unable to confirm this conclusion.
Clinopvroxene occurs in all lavas as phenocrysts and microphenocrysts up to 2 mm in size. In the basaltic andesites and andesites the clinopyroxene phenocrysts are euhedral to subhedral, but in the dacites and rhyodacites most crystals are anhedral and are rimmed by fine-grained opaque material. We believe that this is not caused by alteration, but is a result of reaction between clinopyroxene and melt. Representative analyses are reported in Table 2.3 and all data are plotted in the
conventional pyroxene quadrilateral in Fig. 2.3. The range
in Mg/(Mg+ Fe2+) of the clinopyroxene phenocrysts is relatively restricted and the lack of iron-enrichment
shown by these phenocrysts is characteristic of lavas erupted in an arc environment. 17 Host clinopyroxenes in the basaltic andesltea show reversed zoning, in terns of Mg/(Mg+ Fe2-4*) the cores range in composition from 0.814-0.690, whereas the rims have slightly higher ratios of 0.836-0.700. The clinopyroxenes
in the andesites are generally normally zoned, i.e. Mg/(Mg+ Fe2+) is higher in the cores (0.810-0.702) and decreases atj the rims (0.798-0.694). Approximately 50% of the analysed | clinopyroxene crystals in the pre-caldera dacites and i rhyodacites show reversed zoning. The range in Mg/(Mg+ Fe2+) is 0.719-0.665 for cores in reversely zoned crystals,;
0.780-0.691 for cores in crystals that display normal zoning, and 0.734-0.676 for rims of phenocrysts and micropheriocrysts.
In the basaltic-andesite pillow lavas the Al-contents
of the clinopyroxenes are highest at the rims, which probably reflects rapid growth of the crystals during extrusion since experimental studies (Walker et al., 1976; Gamble and Taylor, 1980) have shown that the Al contents of 1
calcic pyroxenes correlate directly with degree of undercooling and with growth rate. In the other rock types i there is no systematic difference in Al- or Ti-contents between cores and rims. The clinopyroxenes as a whole show ! large variations in A1-, Ti- and Na-contents, but the | average concentrations of these elements are lower in the
clinopyroxenes of the more evolved lavas. Thus, the average Al- and Ti-contents of clinopyroxene range from 0.119 atoms j per formula unit (afu) and 0.020 afu respectively in the basaltic andesites to 0.043 afu and 0.006 afu In the rhyodacites. The lower Al and Tl contents of clinopyroxenes In the evolved magmas could reflect directly changing magma chemistry during evolution and particularly an increase in aSi02 , since low values of agj^fmelt) favors the incorporation of Al and Ti in pyroxenes via substitutions such as AlM1+AlT-MgM 1+SiT and TiM1+2AlT-MgM1+2SiT (cf. Barton et al., 1982). It is also possible that the growth rate of clinopyroxenes in the least-evolved lavas was, on average, higher than that in the evolved lavas, or that the residence time of early formed, rapidly grown phenocrysts in the dacitic and rhyodacitic magmas was sufficiently long to allow re-equilibration, via diffusive exchange, of phenocrystB and melts. We have no way of discriminating between these possibilities.
Olivine phenocrysts and microphenocrysts occur in the basaltic andesites and andesites as relatively small euhedral-anhedral crystals (generally < 0.7 mm). In the basaltic andesites olivine is relatively rare and far less abundant than clinopyroxene, but in the andesites olivine is the second most abundant phenocryst phase after plagioclase. Thus the proportion of olivine crystallizing from the melts apparently increased during the early stages of magma evolution, but olivine ceased crystallizing from 19 magmas more evolved than andeslte. Some olivine crystals
are partially serpentinized. The olivine phenocrysts and microphenocrysts are normally zoned. The compositions of phenocryst cores range from FOqq to Fo67 whereas the compositions of the rims range from Fo78 to Fo60 (Table 2.4). Nio contents correlate with Fo contents, as would be expected, and reach 0.3 wt.%, on average, in the cores of phenocrysts.
Orthopvroxene occurs as euhedral phenocrysts up to 2 mm in size in the siliceous _andesiteg. dacites and rhyodacites. It also occurs sporadically as resorbed
xenocrysts in the basaltic andesites and baBic andesites. These xenocrysts mostly are rimmed by clinopyroxene. Like clinopyroxene, orthopyroxene shows a restricted
range of composition in terms of Mg/(Mg+ Fe2+) which varies from 0.674 to 0.580 (Table 2.5, Fig. 2.3). There is little
difference in composition between the phenocrysts in the evolved lavas and the xenocrysts in the most primitive
lavas. More than 75% of the analysed hypersthene
phenocrysts in the dacites and rhyodacites are reversely zoned, although the extent of zoning is very limited.
Mg/(Mg+ Fe2+) ranges from 0.674 to 0.580 in the cores and
from 0.640 to 0.590 at the rims. All of the orthopyroxenes in these lavas are low in Al (< 0.060 afu). EteXi-PXiflgP are extremely rare in the basaltic andesites, and in the andesites they occur only sporadically as inclusions in plagioclase, clinopyroxene and olivine or, sometimes, as microphenocrysts. In the dacites and rhyodacites Fe-Ti-oxides are present as euhedral to anhedral phenocrysts (< 0.3 mm in size) and as inclusions in other phenocrysts. Magnetite is the only Fe- Ti oxide in the basaltic andesites and andesites whereas the evolved lavas also contain ilmenite. Individual phenocrysts are often inhomogeneous and consist of both magnetite and ilmenite and this is taken to represent
unmixing of an initially homogenous phase. Representative analyses (performed with a defocussed beam when appropriate) are reported in Table 2.6. Fe203 contents were calculated assuming that the sum of the cations was 2 or 3 on a 3 or 4 oxygen basis. Mole fractions of ulvospinel or ilmenite and R203 were calculated using Stormer's (1983) method. Ti-magnetite varies in composition from Usp63 in the andesites to Usp23 in the rhyodacites.
A1203 is highest in magnetites in the more mafic lavas and ranges from 1.50-2.50 wt.%; Mgo varies from 0.97-1.94 wt.%.
Ilmenite shows an extremely small range of composition, from 11039 to Ilm82*
Amphlbole is present in small amounts in the pre- caldera dacites and the rhyodacites but is a relatively common mineral in the post-caldera dacites. It is a major constituent of some xenoliths (see below), The amphibole phenocrysts in the dacites are tschermakitic hornblendes according to the classification scheme of Leake (1978) and have K20 contents between 0.34 and 0.70 wt.% and Ti02 contents between 3.20 and 4.19 wt% (Table 2.6). The amphibole phenocrysts in the rhyodacites are magnesio- hastingsites (cf. Leake, 1978) with K20 contents similar to the amphiboles in the dacites but with lower Ti02 contents (2.48-3.21 Wt.%).
Two accessory phases, apatite and zircon, also occur but have not been analysed. Apatite is found as inclusions in clinopyroxene in most lavas. In the dacites and rhyodacites it also occurs as microphenocrysts and is often present in glomeroporphyritic aggregates together with orthopyroxene, clinopyroxene, plagioclase and Fe-Ti- oxides. Zircon is a rare phase that is sometimes present as minute inclusions in Fe-Ti-oxide crystals. The dacites and rhyodacites occasionally contain zircon microphenocrysts.
The main consituents of the qroundmasses in the basaltic andesites and andesites are plagioclase and Fe-Ti- oxides, with minor clinopyroxene. The groundmasses of the dacites consist of plagioclase, Fe-Ti-oxides and interstitial glass. The rhyodacites have a glassy matrix, 22 which is often partly devitrified. In general, the compositions of the crystalline phases in the groundmasses are similar to the compositions of the rims on phenocrysts in the corresponding lavas and for this reason analyses of groundmass phases are not reported.
Xenollths occur in all types of lava erupted on Nisyros and can be divided into three groups: l) cognate xenoliths; 2) amphibole-rich xenoliths; and 3) accidental xenoliths. The cognate xenoliths contain abundant plagioclase and clinopyroxene together with subordinate amounts of olivine and Fe-Ti oxides. This type is common in most lava types, and the minerals in the xenoliths have compositions identical to the phenocryst phases in the lavas. We therefore consider these xenoliths to represent aggregates of the minerals crystallizing from the magmas and hence use the term cognate. The opaques in some xenoliths display skeletal growth forms and clinopyroxene forms radial aggregates of crystals. These textures suggest rapid crystallization, but the contact between the inclusions and the host lava is invariably sharp indicating that the former were solid at the time they were incorporated into the magma. We do not, therefore, view these xenoliths simply as clots of phenocrysts, but as products of earlier stages of crystallization. We interpret the quench textures in some of these xenoliths to reflect rapid cooling in parts of the magma chamber beneath Nisyros. The amphibole-rich xenoliths occur mainly in the rhyodacites, the post-caldera dacites, and in the pyroclastics. They consist mainly of plagioclase, amphibole and Fe-Ti oxides, and contain minor amounts of ortho- and clinopyroxene and abundant interstitial glass. The compositions of the phases in these xenoliths are also similar to those in the host lavas, suggesting that the xenoliths are cognate. The similarity in the composition of amphibole in the xenoliths and rhyodacites is striking and suggests that the amphibole in the rhyodacites is not a phenocrystal phase, but results from disaggregation of the xenoliths. There is some petrographic support for this proposition since interaction between magma and inclusion has resulted in the liberation of crystals from the inclusion and incorporation into the magma. Finally, xenoliths of metamorphic rock are abundant in the pyroclastic formations (see also Di Paola, 1974) but no samples were available for this study.
WHOLE-ROCK MAJOR- AND TRACE-ELEMENT CHEMISTRY
Whole rocks were analysed for major elements and some trace elements (i.e. Rb, Sr, Zr, Y, Nb, Ba and Ni) by XRF.
Other trace element analyses were done by instrumental neutron activation at IRI, Delft. A detailed description of the analytical techniques is given by Barton & Huijsmans 24 (1986) and Huijsmans et al. (1988) and will not be repeated here. Groundmasses of lavas were analysed with the electron microprobe using a defocussed beam (diameter > 50 urn). At least ten spots were analysed per thin section and the results were averaged. New analyses of representative samples are listed in
Table 2.7. Only total iron was analysed (reported as FeO), because Barton and Huijsmans (1986) have shown that the analysed Fe203/Fe0 ratios of the post-caldera lavas from
Santorini are substantially higher than values calculated from oxygen fugacity and temperature and have suggested that these lavas experienced post-eruption oxidation, similarly, Huijsmans (1985) has shown that Fe203/Fe0 ratios of pre-caldera lavas from Santorini are highly variable and it is apparent that the observed variation does not correlate with other compositional parameters (e.g. Si02, Na20+K20). We conclude that analysed values of Fe203 for lavas erupted along the Hellenic arc partly reflect post eruptive oxidation and thuB do not reflect values in the pre-eruptive magma. For this reason, we prefer to report values of FeO (total) and note that a knowledge of the present-day Fe203 /FeO ratio for the Nisyros lavas has no direct relevance to the discussion in this paper. Since we have not analysed Fe203, we do not report values for LOI
(mostly H20+) in Table 2.7. Measured values for the Nisyros lavas range from 1.46% to 0.13% and most values are less 25 than It. The whole-rock major oxide data for lavas and groundmasses are plotted in conventional Harker variation diagrams in Fig. 2.4. Whole-rock analyses published previously by Di Paola (1974) and Davis (1967) are also included in the diagrams. The major element trends are similar to those for most island arc calc-alkaline series and can be qualitatively explained by fractional crystallization involving removal of the observed phenocryst assemblages. The trends for the basaltic andesites and andesites are consistent with removal of olivine, clinopyroxene, plagioclase and minor amounts of
Fe-Ti-oxide. Those for the dacites and rhyodacites are consistent with removal of plagioclase, clinopyroxene, hypersthene, Ti-magnetite and ilmenite together with minor apatite. Groundmass compositions fall within or extend the range defined by the whole-rock compositions, which suggests that phenocryst accumulation did not alter the whole-rock compositions significantly. From Table 2.7 and Fig. 2.4 it can be seen that there is a distinct difference in composition between the pre- and post-caldera dacites. The post-caldera dacites have higher CaO-contents and lower concentrations of Ti02, FeO, Na2o, k2o and P2O5 . The post- caldera dacites of Santorini are also depleted in K20 relative to the pre-caldera lavas (Barton & Huijsmans, 1986). 26 The range in major element compositions is in general the same as that reported for lavas from Santorini (Huijsmans et al., 1988). The Nisyros lavas differ from the Santorini lavas in two important respects: l) Lavas of basaltic composition are absent on Nisyros; Basaltic lavas are likewise rare or absent on other volcanic centers of the Hellenic arc, i.e. on Milos (Bol & Hasselman, 1985), Aegina and Methana (Pe, 1973, 1974). 2) There is a lack of lavas with compositions between 60% and 66% Si02 on Nisyros, and we stress that this compositional gap is not the result of insufficient sampling. The K/6r and K/Cs ratios of the Nisyros basaltic andesites are similar to the average ratios for island arc volcanics as compiled by Morris & Hart (1983). Many of the data utilized by Morris and Hart (1983) were obtained for lavas of basaltic andesitic composition so that comparison with the most primitive Nisyros lavas is warranted. K/Rb is slightly lower than the island arc average, and the Nisyros basaltic andesites are relatively depleted in Ba, as is manifested by the anomalously low Ba/K and Ba/La ratios. Lavas from Santorini show similarily low Ba/K and Ba/La ratios (Barton et al., 1983), and Huijsmans et al. (1988) suggest that these characteristics reflect melting of an anomalous mantle source. We will discuss this possibility in more detail in a forthcoming publication. Selected trace elements are plotted on variation diagrams in Fig. 2.5. Rb was chosen as a differentiation index since it shows maximum enrichment throughout this series (enrichment factor -3.2). The behavior of most trace elements is qualitatively consistent with the hypothesis that these lavas are related via fractionation. Specifically, the observed increases in the concentration of highly and moderately incompatible elements (Zr, Ba, La, Th) and decreases in the concentrations of compatible elements (Sc, V, Sr, Ni) with degree of evolution are predicted to occur during low-pressure fractional crystallization. The observed variations thus broadly confirm conclusions based upon analysis of major oxide variations. Sc and V define convex arrays when plotted against Rb which suggests that these elements were relatively less compatible during the earlier stages of magma evolution compared with the later stages. This could reflect the change in the solid assemblage removed during fractionation from olivine, clinopyroxene, plagioclase and
Ti-magnetite during the early stages to clinopyroxene, plagioclaBe, hypersthene, Ti-magnetite, ilmenite and minor apatite during the later stages.
It is apparent from the data presented in Table 2.7 that a number of elements (i.e. Zr, Hf, Nb, Y and the REE) show unusual behavior in the Kisyros lavas inasmuch as they are not as enriched in the evolved lavas (dacites and rhyodacites) as would be predicted for elements normally considered to be highly incompatible. This is most easily shown by calculating enrichment factors (concentration in most evolved lava/concentration in least evolved lava) from the data in Table 2.7. The enrichment factor for Rb is 2.7, whereas enrichment factors for other elements listed above ares Zr-1.2, Hf-1.3, Nb-1.1, Y-l.l, REE-2.0 to 0.5, i.e. these elements behaved compatibly to moderately
incompatibly during magma evolution. The behavior of most of these elements (Zr, Hf, Y, REE) almost certainly reflects crystallization of small amounts of apatite and zircon, for which there is petrographic evidence. The behavior of Nb probably reflects an increase in the relative proportion of Fe-Ti oxide crystallizing from the magma as evolution progressed.
Two features of the trace element geochemistry of the evolved lavas, however, are difficult to explain in terms of a simple fractional crystallization model. First, the
pre-caldera dacites have slightly higher concentrations of Y, Zr and the middle and heavy REE than the rhyodacites (Table 2.7), and they also have slightly lower
concentrations of Rb (average values-88 ppm for the dacites
and 93 ppm for the rhyodacites). If these lavas are related by fractional crystallization alone, then incompatible
element concentrations in the rhyodacites should be substantially higher than those in the dacites, unless fractionation involves crystallization and removal of substantial amounts of accessory phases such as apatite and zircon. There is no evidence for this from petrographic studies, and there is certainly no evidence for crystallization of a phase which incorporates sufficient Rb to explain the near-equal concentrations of this element in the two lavas. Second, the post-caldera dacites are depleted in Rb, Zr, Nb, Ba, HREE, Hf and Th, but enriched in Sr relative to the pre-caldera dacites (see Table 2.7). We noted earlier that the post-caldera dacites are depleted in Ti02, K20 and P2°5' and enriched in CaO, relative to the pre-caldera dacites. These differences suggest that the pre- and post-caldera dacites, and the pre-caldera dacites and rhyodacites are derived from different parental magmas or that processes other than fractional crystallization were involved in the evolution of the Nisyros magmas.
DISCUSSION
FEflgfrilpnBl crystallisation
Major oxide modeling Although mineral chemical and trace element data suggest that more than one process may have been involved in the evolution of the Nisyros magmas, we discuss first the results of quantitative modeling of fractional crystallization in order to determine the extent to which this process can account for observed chemical variations. As an initial approximation, we assumed that all of the erupted magmas except the post-caldera dacites evolved in a single magma chamber and were derived from a single parental magma. Geochemical differences between the pre- and post-caldera dacites preclude the possibility that they are related to each other by fractionation and suggest that the post-caldera dacites are not directly related to other Nisyros lavas.
Major oxide variations were modeled using the unweighted least-squares mixing technique described by Bryan et al. (1969). Input data included parent and daughter lava compositions and the compositions of phenocryst phases in the parent lava. The compositions of plagioclase, clinopyroxene, olivine and orthopyroxene used in the calculations were close to those appropriate for equilibrium with the host lava based upon criteria described by Roeder and Emslie (1970); Gerlach and Grove
(19S2) and Shibata et al. (1979). The most Ti-rich magnetites, and most R203-rich ilmenites in each sample were also selected for the modeling. The calculations were performed in a stepwise fashion, the daughter magma of one step being used as a parental magma for the subsequent step which involved a more evolved composition. He regard solutions to the mixing calculations to be satisfactory only If the concentration of each oxide in the parent magma, calculated by combination of the compositions of the daughter magma and phenocryst phases, is within analytical uncertainty of measured concentrations (see Chapter V). If necessary, we adjusted the compositions of the phenocryst phases input into the calculations to achieve close agreement between observed and predicted parental magma compositions but we rejected models which required unrealistic phenocryst compositions as mixing components.
The most satisfactory solutions for four models involving the basaltic andesites, andesites, dacites and rhyodacites are reported in Table 2.8. The differences between the calculated and the predicted Ti02 contents exceed analytical uncertainty, but we do not consider this discrepancy to be that significant because relatively few analyses of Fe-Ti oxides have been performed, and it is our experience that successful prediction of Ti02 contents in mixing calculations is strongly dependent upon input Fe-Ti oxide composition* Overall, the solutions for the steps basaltic andesite to andesite, basaltic andesite to dacite and andesite to rhyodacite are excellent and indicate that fractional crystallization was a process of major importance in the evolution of the Nisyros magmas. However, it did not prove possible to obtain a satisfactory solution for a model relating the rhyodacites to the dacites. Predicted K20 contents in the dacites are much lover than the observed concentrations, and this remains true even if the most orthoclase-rich plagioclase composition is used in the calculations. This suggests that processes other than fractional crystallization were also involved in evolution of the Nisyros magmas, or that fractional crystallization occurred under different conditions (e.g. at different pressures). Clearly, if the Nisyros magmas evolved via fractional crystallization from a single parental magma in a single magma chamber (and hence at near constant pressure), then it must be possible to model the dacite- rhyodacite stage of evolution in terms of removal of the observed phenocryst phases. We also note that our model for the step andesite to rhyodacite, though numerically satisfactory, is geologically unrealistic inasmuch as the evolved magmas (dacites and rhyodacites) contain ilmenite and orthopyroxene as phenocrysts and microphenocrysts, and these phases should have been removed during fractionation. Yet satisfactory models relating rhyodacite to andesite do not require involvement of orthopyroxene or ilmenite. This is additional evidence that processes other than fractional crystallization occurred or that fractional crystallization did not involve removal of the observed phenocryst phases. We address the question of polybaric evolution in a later section of the paper.
The calculated proportion of olivine which must be removed from the melts during the early stages of fractional crystallization is much lower than that of clinopyroxene (see Table 2*8). This contrasts with the • relative nodal abundances of these phases in the andesites. The rate at which crystals are removed from a magma are dependent upon a number of factors but, in the case of crystal settling, the size and density of the crystals are important. The clinopyroxene phenocrysts in the andesites are much larger than the olivine phenocrysts and could therefore have been removed more efficiently during fractionation. Nevertheless, the higher abundance of olivine in the andesites compared with the basaltic andesites is unusual. Examination of the phase relationships in the pseudo-ternary system olivine- clinopyroxene-silica (Grove et al., 1983) reveals that the proportion of olivine relative to clinopyroxene crystallizing from melts moving down the olivine-augite- plagioclase cotectic will increase. However, this effect is strongest for primitive (basaltic) magmas and is only minor for intermediate lavas such as basaltic andesite or andesite. We suggest therefore that modal abundances of olivine and clinopyroxene in the basaltic andesites and andesites are inconsistent with a simple, isobaric fractionation model for the Nisyros magmas. In summary, fractional crystallization probably played an important role in the basaltic andesite - andesite - dacite stages of evolution. However, it appears that the 34 Nisyros magmas did not evolve simply via isobaric fractional crystallization from a single parental magma.
Trace element modeling Trace element variations were quantitatively modeled using the Rayleigh distillation equation and the results of the major oxide modeling described above in order to determine the extent to which the behavior of these elements can be explained in terms of fractional crystallization. Since there is petrographic evidence for the crystallization of minor amounts of apatite and zircon we include these phases in the calculations. The low P2°5“ and Zr- concentrations in the most mafic lavas suggest that the magmas were initially undersaturated with respect to apatite and zircon (Watson, 1979; Green and Watson, 1982; Watson and Harrison, 1984), but these phases probably crystallized as a result of local increases in the concentrations of P2o5 and zr around growing phenocrysts
(cf. Chapter .V). The amounts of apatite and zircon that were removed were calculated from P205- and Zr- abundances in the lavas and are listed in Table 2.9. It was assumed that these phases are pure. For each trace element, we computed a possible range of bulk solid-liquid distribution coefficients (D^) from the weight fractions of the minerals in the fractionating solid assemblage (Table 2.9) and from published mineral- liquid partition coefficients. We list a compilation of the latter in Table 2.10 together with references. We used these values of D^, together with the fraction of melt remaining (F) from Table 2.8 and the concentrations of trace elements in the parent magmas, to predict, as closely as possible, trace element concentrations in the daughter
magmas. Calculations were done in a step-wise fashion for the same compositional steps as the major oxide least- squares mass balancing calculations. Observed and predicted trace element concentrations in the daughter magma for each step are listed in Table 2.11. It should be emphasized that
since reported mineral-liquid distribution coefficients for many elements show a wide range of values, so that computed D^'s also show a range of values, we have used values for which lie within the permitted range and which yield
closest agreement between predicted and observed concentrations in the daughter magmas (see Table 2.11). We
consider a model to be successful if the predicted and observed concentrations of a trace element in the daughter magma agree to within analytical uncertainty. Values for analytical uncertainty for each element are taken from
Barton and Huijsmans (1986) and Huijsmans (1985).
The concentrations of most compatible (Sc, V, Hi, Zn,
Sr) and many incompatible (Zr, Ce, Yb, Hf) elements can be predicted to within analytical error thus providing strong support for the hypothesis that fractional crystallization 36 was important during evolution of the Nisyros magmas. Indeed, the concentrations of most trace elements (except
Kb, Ba and Th) in the andesites can be predicted within uncertainty by the fractional crystallization model.
However, predicted and observed concentrations of several incompatible elements in models involving daughter magmas more evolved than andesite differ significantly. For the model involving basaltic andesite and dacite the predicted concentrations of Rb, Ba and Th are lower than measured concentrations in the dacite. on the other hand, concentrations of Rb, Zr, Cs, Ba, La, Ce, Sm, Yb, Hf and Th in the rhyodacite are lower than would be expected if these lavas are related to the dacites via fractional crystallization involving removal of the observed phenocryst phases. Finally, we note that the concentrations of Rb, Cs and the REE in the rhyodacite are also higher than those expected if these lavas are related to andesites via fractional crystallization. Assuming that the crystal-liquid partition coefficients are correct and appropriate for these magma compositions, the results of trace element modeling can be interpreted to indicate that processes other than fractional crystallization involving removal of the observed phenocryst phases occurred during evolution of the intermediate and evolved magmas (andesites, dacites and rhyodacites). Nevertheless, differences between observed
i 37 and predicted trace element concentrations in models involving andesites, dacites and rhyodacites are
sufficiently small (albeit significant) to lead us to conclude that fractional crystallization was of major
importance during evolution of the Nisyros magmas.
Finally, we empasize that our modeling only addresses the possibility that the lavas are related by removal of the observed phenocryst phases, so that the results cannot be used to discount the possibility that fractional crystallization involved phases of different composition, or even different phases. In other words, fractional crystallization may have occurred at pressures greater than
those at which the phenocryst phases grew. If this is the case, then values of F (the fraction of melt remaining) deduced from the major element models and used in the trace
element modeling would be incorrect. Thus, for the step basaltic andesite to dacite a decrease in the value of F from 0.378 (based on major element modeling) could possibly explain the discrepancies between observed and predicted concentrations of incompatible elements in the dacite.
Values of F calculated from incompatible element concentrations and appropriate values for bulk solid-liquid
distribution coefficients range from 0.31 (Ba, Th) to 0.36
(Cs). Since it is extremely unlikely that values of distribution coefficients for highly incompatible elements vary significantly as a function of pressure or, in these lavas, as a function of crystallizing assemblage, we believe that the range of values of F required to explain incompatible element data is further evidence that these lavas are not related by fractional crystallization alone. In the case of the step dacite to rhyodacite it would be necessary to increase the value of F to explain the observed behavior of some incompatible elements (from 0.79 to 0.97 in the case of Rb), but this would not explain the behavior of Ba or Th. Ba contents in the rhyodacites and dacites are approximately equal, whereas the dacites contain slightly more Th than the rhyodacites. *
Unrealistically high values of DBa ( 1.0) and (>1) are therefore required to explain the behavior of these elements.
We conclude that processes other than fractional crystallization occurred during evolution of the evolved Nisyros magmas.
The role of processes other than fractional crystallization
Three processes are traditionally invoked to explain the failure of fractional crystallization models to fully describe chemical variations in a particular series of lavas. These are: magma mixing, assimilation and source region (i.e. upper mantle) heterogeneity, in general, it is necessary to quantitatively evaluate the importance of magma mixing and assimilation before the question of source 39 region heterogeneity can be adequately addressed (e.g. O'Hara and Matthews, 1981) and therefore in this section we discuss the role of nixing and assimilation in the evolution of the Nisyros magmas.
Magma mixing Magma mixing is known to be important in the evolution of many calc-alkaline magmas (Anderson, 1976; Luhr and
Carmichael, 1980), including those from Santorini in the Hellenic arc (Huijsmans and Barton, 1988). Petrographic and mineralogical evidence for mixing has been described by Anderson (1976), Barton et al. (1982) and Tsuchiyama and Takahashi (1983) (see also Chapter V), whereas the effects of mixing on the major oxide and trace element chemistry of magmas have been discussed in detail by O'Hara and Matthews (1981) (see also Chapter VI). We begin with a discussion of petrographic and mineralogical data. The occurrence of abundant phenocrysts of plagioclase and clinopyroxene which show reversed zoning in the dacites and rhyodacites is persuasive evidence for magma mixing, because these lavas also contain plagioclase and pyroxene phenocrysts which show normal zoning. Phenocryst cores therefore define two compositionally distinct populations whereas phenocryst rims and microphenocrysts are of intermediate composition. Barton et al. (1982) and Wyers and Barton (1986a) used mineral chemical evidence of this 40 type for lavas from Vulsini and Patinos (respectively) to argue that magma mixing had occurred. However, phenocrysts in the lavas of Vulsini and Patmos show evidence of strong resorbtion, which is not the case with phenocrysts in the lavas of Hisyros, and therefore some discussion of the relationship between resorbtion and magma mixing is warranted. Fig. 2.6 shows the relationship between initial magma compositions, thermal curvature of liguidus surfaces in multi-component systems and the predicted effects of resorbtion during mixing. In constructing Fig. 2.6 we have assumed that magmas contain phenocrysts and hence that temperatures are lower than liquidus temperatures. We illustrate three different types of mixing. One type (denoted by a in Fig. 2.6) involves mixing of two magmas of very different bulk composition which contain different phenocryst assemblages. The discontinuity in the curvature of the liguidus surface represents the temperature at which the phenocryst assemblage changes (Thompson, 1972). In this case, mixing results in resorbtion of phenocryst phases in both of the original magmas. If one of the original magmas lies close to the natural equivalent of a cotectic, then mixing may result in resorbtion only of phenocrysts in this magma as shown by (b) in Fig. 2.6. Phenocrysts in the other magma involved in mixing (the less-evolved end member in the example shown in Fig. 2.6) will show normal zoning since the hybrid is supercooled relative to the liguidus surface. A third case (c in Fig. 2.6) involves mixing of magmas of similar composition and which contain the same phenocryst assemblage, though the phenocrysts in both magmas are of different composition. Mixing yields a hybrid magma which is supercooled with respect to both of the original magmas and rapid growth should occur on pre existing crystals. Mixing of this type should result in both normally and reversely zoned phenocrysts whereas microphenocrysts precipitated from the hybrid magma should have compositions similar to phenocryst rims and therefore compositions intermediate to the phenocryst cores. Mixing model (a) in Fig. 2.6 explains the resorbtion features described by Barton et al. (1982), Wyers and Barton (1986a) and by numerous other workers. Mixing models
(b) and (c) in Fig. 2.6 can, in principle, explain features observed in phenocryst phases in the Nisyros lavas. For example, the occurrence of both normally and reversely zoned phenocrysts in the basaltic andesites and andesites could be explained by mixing of magmas of similar composition and with identical phenocryst assemblages (i.e. case c in Fig. 2.6), whereas the rounded cores of plagioclase phenocrysts in the dacites and rhyodacites could reflect the mixing of two magmas, one of which lies close to the natural equivalent of a cotectic in composition (i.e. case b in Fig. 2.6). However, more detailed analysis suggests that nixing cannot adequately explain all of the petrographic and nineral chemical data for the Nisyros lavas, or that nixing was of ninor
importance during the evolution of these lavas. For example, it is not clear why the phases which show reversed zoning in the dacites and rhyodacites are exactly the same
phases which show normal zoning. As described above, textureB in these magmas require that the evolved magma
involved in mixing must lie close to the natural equivalent of a cotectic and inspection of Fig. 2.6 shows that this
magma must have contained a different phenocryst assemblage from the more primitive magma involved in mixing. There is,
however, no evidence for this in the dacites or in the rhyodacites. On the basis of plagioclase compositions, the evolved end member involved in genesis of the dacites could have been a rhyodacite. The fact that the dacites and rhyodacites have identical phenocryst assemblages argues strongly, therefore, that the dacites are not hybrid lavas. Similarly, one of the end members involved in the genesis
of the rhyodacites would have to be rhyolitic in composition (nb. similarity in composition of reversed zoned plagioclase phenocrysts and phenocryst rims which are
set in a groundmass of rhyolitic composition) and probably would, therefore, have contained a different phenocryst assemblage than the primitive (? daoitic) end member. Again, no evidence is preserved in the rhyodacites and we suggest that petrographic and mineralogical data for both the dacites and rhyodacites provide no support for the idea that these represent hybrid magmas. Identical arguments apply to the basaltic andesites and andesites, but we cannot completely rule out the possibility that these magmas are hybrids formed by mixing of end members of very
similar composition (see below). Trace element data support conclusions based upon petrographic and mineralogical data. Magma mixing results
in magmas with higher concentrations of both highly incompatible and highly compatible elements than can be produced during fractional crystallization (O'Hara and Matthews, 1981). Due to the large range in reported values of mineral-liquid distribution coefficients for most compatible elements, these elements are less sensitive
indicators of mixing than are the highly incompatible elements (see Chapter VI). From Table 2.11 it is apparent
that the concentrations of incompatible elements (Rb, Zr, Cs, Ba, La, Ce, Sm, Yb, Hf and Th) in the rhyodacites are lower than concentrations predicted if these magmas are related to the dacites by fractional crystallization. The relationship between observed and predicted trace element concentrations in the rhyodacites is thus opposite to that expected if these magmas have evolved by fractional crystallization and mixing. Predicted concentrations of Rb,
Ba and Th in the dacites are lower than observed concentrations indicating that these magmas are not related to the basaltic andesites by fractional crystallization. Trace element modeling is consistent with combined mixing and fractionation but we reject the possibility that mixing has occurred for reasons given in the preceding paragraph. Finally, models which relate the andesites to the basaltic andesites by fractional crystallization are satisfactory for most trace elements, but do not explain Rb, Ba and Th concentrations, which are higher than predicted values in the andesites. This can be interpreted to indicate that some mixing has occurred during evolution of the primitive and intermediate magmas, but that mixing involved magmas of very similar composition. Also, since fractional crystallization can account for the concentrations of many trace elements in the andesites, we conclude that mixing, if it occurred, was of relatively minor importance during the evolution of the primitive and intermediate magmas erupted on Nisyros.
Assimilation Wyers and Barton (1986b) have presented preliminary Sr- isotopic data for the lavas of Nisyros, and in Chapter III the significance of these data is discussed in detail.
In the present paper we use Sr-isotope data to assess the role of assimilation in the evolution of the Nisyros magmas. Sr-isotopes show a substantial range of variation, as shown in Fig. 2.7 in which we have plotted the 87Sr/86Sr ratios of the pre-caldera lavas against Rb which is used as a differentiation index. The lowest value of 87s r /86sr (0.70353) was obtained for a basaltic andesite. Higher values (up to 0.70462) were determined for more evolved lavas. The andesites show very little variation in their
Sr-isotopic composition, but these lavas are characterized by, on average, higher 87Sr/86Sr ratios than the dacites and rhyodacites. The data presented in Fig. 2.7 preclude a simple relationship between the dacites and rhyodacites and the more primitive magmas (basaltic andesites and andesites). The Sr-isotopic ratios of the basaltic andesites and andesites are positively correlated with Rb contents (and with all other differentiation indices). 87Sr/88Sr ratios also correlate positively with d^80 (Hooft
van Huysduynen, unpublished data; Chapter III) and it thus appears that derivation of the andesites from the basaltic
andesites involved assimilation in addition to fractional crystallization (with or without mixing). On the basis of
the results of trace element models reported in Table 2.11,
which shows that trace element abundances in the andesites are well predicted by fractional crystallization, we
suggest that assimilation was of (relatively) minor importance during evolution of the basic and intermediate magmas erupted on Nisyros. 46 The Sr-isotope ratios of the dacites and rhyodacites are lower than those of the andesites so that the former magma types cannot be derived from the latter magmas by
fractionation combined with assimilation. Indeed, the data appear to define two trends, one from basaltic andesite to
andesite, the other from dacite to rhyodacite. Modeling of the isotopic and trace element variations within each trend (i.e. basaltic andesite to andesite and dacite to rhyodacite) is discussed in Chapter III. Calculations involving both average crust and upper crust reproduce the isotopic and trace element composition of the andesites reasonably well (Chapter III). However, none of the assimilants used in modeling the relationship between the dacites and the rhyodacites is entirely successful but AFC models involving lower crust yield better solutions for the evolution of the rhyodacites than do models involving average crust or upper cruBt (Chapter III).
Assimilation combined with fractional crystallization can account for many of the trace element characteristics of the Nisyros magmas, and we emphasize that assimilation is demanded by Sr-isotopic data. The andesites can be derived from the basaltic andesites by AFC processes involving assimilation of average crust or upper crustal material. The rhyodacites can be derived from the dacites by AFC processes involving LIL-element depleted material - 47 probably the lower crust.
Relationships between the basaltic_^ndesite to andesite series and the dacite to rhyodacite series In the preceding section we noted that the Nisyros lavas appear to define two series -one including the basaltic andesites and andesites, the other including the dacites and rhyodacites. These two series are defined in terms of B7Sr/B6Sr ratios and Rb concentrations in Fig.
2 .8 , in which we also show possible relationships between members of the two series. The dotted lines labeled (a) illustrate how the dacites and rhyodacites could in principle be derived from members of the basaltic andesite to andesite series via fractional crystallization. This model is essentially the same as the one proposed by Barton et al. (1983) to explain isotopio variations shown by lavas erupted from ali islands along the Hellenic arc, but fails to account for zoning patterns shown by phenocrysts in the dacites and rhyodacites. Moreover, we have shown that the rhyodacites cannot be derived from the andesites on the basis of petrographic, major oxide and trace element data. We conclude that model (a) is extremely unlikely. The second model (b) involves the genesis of the dacites from the basaltic andesites via combined fractionation and assimilation. The rhyodacites are derived from the dacites also by AFC processes, since we have already shown that AFC 48 processes relating the dacites and rhyodacites require involvement of LIL-element depleted crustal material we
infer that the dacites and rhyodacites were generated by processes occurring at high pressures - within the lower crust - or, because the actual formation of the evolved Nisyros magmas may have involved a combination of models (a) and (b) in Fig. 2.8, that lower crustal material played some role in the genesis of the dacites and rhyodacites. Hence we re-examine petrographic and mineral chemical data
for evidence for a stage of high-presBure evolution. High-pressure crystallization provides an attractive
alternative to magma mixing to explain petrographic and mineral chemical features of the dacites and rhyodacites, and especially the occurrence of reversed zoning in
phenocrysts of plagioclase and pyroxene. Experimental studies (summarized by Green, 19.82) have shown that the composition of plagioclase formed near the liquidus of calc-alkaline magmas becomes more albitic as pressure increases so that the low-An cores of plagioclase phenocrysts in the dacites and rhyodacites can be interpreted to represent phenocrysts formed at relatively high pressures whereas the mantles and rims around these phenocryst cores as well as the normally zoned phenocrysts can be interpreted as the products of crystallization at lower pressures. The occurrence of reversely zoned ortho- and clinopyroxenes in the dacites and rhyodacites could also reflect a stage of high-pressure crystallization. Longhi et al. (1978) found that there is a small but significant increase with pressure in the Mg-Fe2+ exchange distribution coefficient between olivine and liquid at pressures above 5 kilobars which implies that olivine coexisting with a given liquid is more iron-rich at higher pressures, if values of kd for clino- and orthopyroxene respond in an analogous way to changes in pressure (as might be expected), then the relatively iron-rich cores of these phases in the dacites and rhyodacites may represent an episode of high-pressure crystallization. The precise effects of pressure on the distribution of Hg and Fe2+ between ferromagnesian minerals and melts are, however, poorly understood (see Hatton, 1984, for example) and it is thus not clear that an increase in pressure can alone explain reversed zoning of the clino- and orthopyroxenes in the dacites and rhyodacites. Another factor which may be important includes variation in the Fe0/Fe203 ratios of magmas as a function of pressure and the temperature of phenocryst formation. Ho et al. (1982) have shown that the Fe0/Fe203 ratios of magmas increase with increasing pressure at fixed f02 so that high pressures should favor incorporation of Fe2+ in ferromagnesian phases.
Furthermore, as shown in Fig. 2.9, the compositions of phases which form below the liquidus at high pressures of water-undersaturated magmas could become reversely zoned as 50 these magmas approach the surface and lose water whereas phenocrysts grown at lower pressures should show normal zoning. This model is similar to that proposed by Gibb and
Henderson (1978) to explain the occurrence of iron-rich cores in Ti-augite phenocrysts in the Dippen Sill, but also accounts for the occurrence of Ha-rich cores in plagioclase phenocrysts in the lavas of Nisyros. He conclude that polybaric evolution can explain the petrographic and mineral chemical data for the dacites and rhyodacites of Nisyros, and suggest also that the occurrence of both normal and reversed zoned phenocrysts in the basaltic andesites and andesites could also reflect periods of crystallization at different pressures. However, as noted in a preceding section, these magmas may have experienced minor amounts of mixing and this could also account for the presence of normal and reversed zoned phenocrysts in these lavas. Nevertheless, we prefer the model involving polybaric evolution because this also explains the higher proportion of olivine (relative to clinopyroxene) in the andesites compared with the basaltic andesites. Experimental studies have shown that the primary phase volume of olivine in basaltic and peridotitic systems decreases with increasing pressure (Nicholls and Ringwood, 1973). Magmas which evolve at high pressures would thus be expected to contain a smaller proportion of olivine phenocrysts than magmas which evolve at low pressures. He propose that the evolution of the andesites from basaltic andesite parental magmas began at high pressures and continued during ascent, and that the final phase of phenocryst growth occurred in relatively shallow magma chambers. During ascent through the crust the proportion of olivine crystallising from the magmas increased due to expansion of the primary phase volume of olivine. The basaltic andesites which were erupted on Nisyros presumably ascended sufficiently rapidly that abundant olivine did not have time to nucleate and grow. It must be noted that mixing at low pressures cannot explain the relatively high proportion of olivine phenocrysts in the andesites. Phase relationships in the pseudo-ternary system ol-cpx-qtz (Grove et al., 1983) indicate that mixing of primitive and evolved magmas to produce andesite will not necessarily yield hybrids which contain abundant olivine "phenocrysts".
Mixing will more probably yield magmas characterized by the presence of abundant clinopyroxene "phenocrysts".
Estimation of the conditions of crystallization He have estimated the conditions of crystallization of the Nisyros magmas using methods similar to those described in Chapter VII. Our primary objective was to determine quantitatively whether or not the cores of reversely zoned phenocrysts in the dacites and rhyodacites formed at substantially higher pressures than the cores of normally 52 zoned phenocrysts in the basaltic andesites and andesites. However, as described in Chapter VII, estimates of the pressure of crystallization depend upon estimates of values of other intensive parameters (especially temperature and water content) so we report here estimated values of pre emptive temperature, pressure, water content and oxygen fugacity. The results are summarized in Table 2.12 and the methods are described in some detail in the following paragraphs. For all calculations we have adopted the activity-composition relationships for solid phases that were selected in Chapter VII in the study of the conditions of crystallization of the lavas of Patmos. Temperatures were calculated from olivine-liquid (Roeder and Emslie, 1970; Roeder, 1974; Leeman, 1983), two- pyroxene (Wood and Banno, 1973; Lindsley, 1983) and two- oxide (Spencer and Lindsley, 1981; Stormer, 1983) thermometry. As input data we used both phenocryst core and phenocryst rim compositions as well as whole-rock chemical analyses (used with phenocryst cores) and analyses of groundmasses (used with phenocryst rims). Estimates of pre emptive temperature range from 1092-1058°C for the basaltic andesites to 903-830°C for the rhyodacites and, as would be expected, pre-emptive temperatures show an inverse correlation with the degree of magma evolution. In all cases temperatures estimated using different thermometers for each lava showed excellent agreement. 53 Temperatures recorded by phenocryst core - whole-rock and phenocryst rim - groundmass assemblages (olivine-liquid thermometer) and by phenocryst core and phenocryst rim compositions (two-pyroxene thermometer) are more or less identical for the basaltic andesites and andesites. Pressures were estimated by two different methods (see Chapter VII). In most samples pressures were determined by comparison of temperatures determined from plagioclase- liquid equilibrium (Ghiorso and Carmichael, 1980; Ghiorso et al., 1983), which is sensitive to total pressure, with those determined using relatively pressure-insensitive thermometers (i.e. olivine-liquid, two-pyroxene and two- oxide) . Since the temperatures at which plagioclase and melt equilibrate depend not only upon pressure, but also upon the water content of the melt, this technique can also be used to estimate pre-eruptive water contents (i.e. an iterative procedure is adopted to find the pressure at which plagioclase-liquid temperatures agree with temperatures estimated by other methods). Calculated pressures for the basaltic andesites and andesites range from 3.5 to 4.0 kb, whereas the pressure indicated for equilibration of the low-An cores of feldspar phenocrysts in the dacites and rhyodacites is 8 kb. These results suggest that the dacites and rhyodacites began to crystallize at considerably higher pressures than the basaltic andesites and andesites, and since Ghiorso and 54 Carmichael (1980) and Ghiorso et al. (1983) used data for evolved magmas when calibrating their version of the plagioclase-liquid thermometer we suggest that use of this technique for the dacites and rhyodacites of Nisyros is permissible. However, we emphasize that calculated values of pressure are relatively imprecise and can be assigned uncertainties of +/“ 2 Kb (see Chapter VII for a discussion). Therefore we interpret these results to indicate that phenocryst cores in the dacites and rhyodacites record a stage of high-pressure crystallization compared with phenocryst cores in the basaltic andesites and andesites. Estimates of pre-eruptive water contents range from 1.5 wt.% in the basaltic andesites to 6.5 wt.% in the rhyodacites and correlate positively with the degree of magma evolution (see Table 2.12). Comparison with data for the solubility of water in magmas given by Hamilton et al. (1964) indicates that none of the magmas was water-
saturated prior to eruption. The andesites contain phenocrysts of olivine and
orthopyroxene along with plagioclase which allows pressure to be determined by a slightly different method. For these
lavas we have calculated asio2 afl a function of P and T from the equilibrium:
Mg2Si04 + Si02 - Kg2Si206 Olivine + Melt ■ Pyroxene 55 using thermodynamic data taken from Bacon and Carmichael (1973). We then compared as^02 calculated in thie way with that calculated using the thermodynamic model of Ghiorso and Carmichael (1980) and Ghiorso et al. (1983) for equilibrium between plagiodase and liquid (see Fig. 2.10). The pressure at which a6 ^02 defined by both methods is equal is 3.5 to 4.0 kb which is in excellent agreement with the range of values derived using the method described
above. Values of pre-eruptive f02 were calculated from coexisting Fe-Ti oxides (Spencer and Lindsley, 1981; Stormer, 1983) in the dacites and rhyodacites and from the coexistence of olivine, magnetite and liquid in the andesites. The occurrence of olivine, magnetite and melt allows calculation of pre-eruptive f02 from the
relationship
3Fe2Si04 + 02 * 2Fe304 + 3Si02 Olivine + Gas- Oxide + Kelt
since agj.021 p an The results of the calculations presented in this section thus confirm that the dacites and rhyodacites experienced a prolonged period of crystallization at high pressures and thus support the model derived from consideration of major oxide, trace element, Sr-isotopic, petrographic and mineral chemical data. Although these magmas began to evolve in a deep magma chamber, they continued to evolve during ascent - hence the reversed zoning of the phenocrysts indicates a stage of evolution at relatively low pressures. All magmas were undersaturated with water prior to eruption and f02 decreased with temperature parallel to the Ni-NiO buffer. PETROGENETIC MODEL On the basis of the data presented in the proceeding sections, we propose that a fairly complex plumbing system, such as that shown in Fig. 2.11, existed beneath NiByros during evolution and eruption of the pre-caldera lavas. In constructing Fig. 2.11, we have assumed that two discrete magma chambers, one located at about 27 km depth and the other located at 12-14 km depth, are connected by a single conduit. These depths correspond to the pressures calculated for the equilibration of phenocryst cores and host magmas for the dacite-rhyodacite sub-series and basaltic andesite-andesite sub-series, respectively. The 57 Hoho beneath Nisyros is located at a depth of approximately 27 km (Makris and Stobbe, 1984) and we suggest that the * deeper chamber is located at the crust-mantle boundary. It seems likely that the shallower chamber is also located at a major seismic/petrological discontinuity, but there are insufficient geophysical data to confirm this. The basaltic andesites erupted on Nisyros do not have petrological and geochemical characteristics appropriate for primary, mantle-derived magmas (i.e. Mg/(Mg+ Fe) ratios, Ni and Cr contents are too low) and, by analogy with calc-alkaline volcanics erupted on Santorini (Huijsmans et al., 1987) and elsewhere (see Gill, 1981, pp. 298-309), we conclude that these magmas were derived from parental high-alumina basalts via fractional crystallization. This may have occurred within the upper mantle or within the crust. Ascending batches of basaltic andesite (or basalt) ponded in the deep (lower crustal) magma chamber and evolved via fractional crystallization and assimilation of LXL-element depleted lower crustal material to produce the dacite-rhyodacite series. During ascent through the crust, or possibly in the shallower magma chamber, fractional crystallization and, perhaps, some assimilation modified the composition of the dacites and rhyodacites. Evolution of the basaltic andesite- andesite series also began at high pressures, but was predominantly the result of low-pressure fractional. 58 crystallization combined with assimilation of LlL-element enriched (upper crustal) material* Hence we conclude that the magmas erupted on Nisyros experienced polybaric evolution and that the chemistry of the eruptive products represents a random sampling of magmas produced by fractionation and assimilation at both high and low pressures. The occurrence of two types of cognate xenoliths (i.e. amphibole-bearing and amphibole-free) in the Nisyros lavas is consistent with, but is not proof of, the model outlined above. Amphibole-bearing xenoliths, which occur exclusively in lavas of the dacite-rhyodacite series, must have formed at higher Pjj20 than the pyroxene-bearing xenoliths and this can be interpreted to indicate crystallization at higher total pressures (see Green, 1982, for a discussion of the relevant phase relationships) since magmas are likely to lose water during ascent (see Barton and Huijsmans, 1986;). The occurrence of amphibole-bearing xenoliths in the evolved lavas is thus consistent with crystallization at high pressure and with relatively high water contents. •The textures of the cognate xenoliths indicate fairly rapid crystallization (see above) and we suggest that the xenoliths formed by crystallization in thermal boundary layers at the walls, floor and roof of the magma chambers. This is consistent with work by others (McBimey and Noyes, 59 1979; McBirney, 1980, 1985; Barton and Huijsmans, 1986; Huijsmans and Barton, 1988) which indicates that fractional crystallization does not necessarily result from crystal settling, but largely reflects preferential crystallization and growth in cool boundary layers at the margins of magma bodies. The model proposed here can account for the compositional gap (i.e. absence of lavas with silica contents between 60 and 66 wt. %) observed in the chemistry of the lavas of Nisyros. The most recently advocated model for the occurrence of bimodal calc-alkaline volcanism is that of Grove and Donnely-Nolan (1986). These authors adopted a proposal by Wyllie (1963), and explained the rarity or absence of intermediate magmas in terms of variations in the slopes of liquidus surfaces in temperature-composition space. If the liquidus surface is relatively flat in temperature-composition space, then a small decrease in temperature will result in a large shift in composition. Compositions lying along a near-horizontal liquidus surface will thus be subject to a lower sampling probability with falling temperature than will compositions lying along steeper (in terms of dT/dX) liquidus surfaces. Grove and Donnely-Nolan (1986) showed that the liquidus surface for intermediate calc-alkaline magmas at the Medicine Lake volcano is much shallower than that for primitive or evolved magmas which probably explains the occurrence of bimodal volcanism at this volcanic center. We suggest an alternative model for the lavas of Nisyros, which was first proposed by Thompson (1972) to account for chemical discontinuities observed in many anorogenic lava series. According to this model, the dacites and rhyodacites represent late-stage residual liquids that separate from a body of crystallizing mafic magma at depth (i.e. at the crust-mantle boundary) by filter pressing,’ whereas the basaltic andesites and andesites erupted from a magma body located at shallower depths within the crust. We prefer this model because it explains the compositional gap in terms of the two-chamber model developed above. However, we do not wish to imply criticism of the model of Grove and Donnely-Nolan (1986) and we believe that both models can account for compositional discontinuities in the chemistry of erupted lavas. It should be noted that the occurrence of chemically continuous, coherent, series of calc-alkaline lavas at many localities (e.g. Santorini, Aegean Sea - Huijsmans et al., 1988) is not evidence against the Grove and Donnely-Nolan (1986) hypothesis since it is being increasingly recognized that magma mixing is important in the evolution of magmas erupted at many calc- alkaline complexes and mixing of magmas of differing composition produces magmas of intermediate composition. On the other hand, the model presented in this paper should also be considered for magma series with compositional gaps 61 which record evidence of polybaric evolution. He emphasize that the model described here is over simplified in as much as we cannot establish with certainty that the Nisyros magmas evolved in two discrete magma chambers and we do not account for the origin of the post- caldera dacites. He acknowledge that our evidence for two discrete chambers is tenuous, but stress that the model emphasizes polybaric evolution within the crust, and not a specific number of magma chambers. A more complex model, involving a larger number of magma chambers or a process of continuous magma evolution within the crust could also explain the available data. Zt should be stressed, however, that the chemistry of erupted calc-alkaline lavas reflects the interaction of processes occurring at different depths within the crust, as discussed by Huijsmans and Barton (1988) for the lavas of the Santorini volcanic complex. This possibility must be taken into account in future studies of even a relatively simple series of lavas, Buch as those of Nisyros. Nevertheless, it is not clear why, in the case of Nisyros, relatively silicic magmas form largely in a chamber sited at the base of the crust whereas primitive magmas evolve in shallower chambers. Our conclusions contradict current dogma, but should be considered by those working on other calc- alkaline volcanic complexes. He have not studied the post- caldera dacites in detail, but the fact that these lavas 62 represent hybrid magmas (based upon petrographic and mineralogical evidence), yet are depleted in LIL and associated elements suggests that these magmas are derived from a parental magma which is depleted in LIL etc, elements. It therefore appears that the post-caldera dacites are derived from parental magmas which are themselves derived from a depleted upper mantle. By analogy with the lavas of Santorini (Huijsmans et al., 1988; Huijsmans and Barton, 1988) we tentatively suggest that the upper mantle source region beneath Nisyros became depleted in LIL elements with time through extraction of magma. Further studies are obviously necessary to elucidate the origin of the post-caldera lavas. CONCLUSIONS The most important conclusions of this study are: 1. The evolution of the Nisyros magmqs was simple relative to the evolution of magmas erupted elsewhere along the Hellenic arc. 2. The pre-caldera lavas are the products of fractional crystallization and assimilation, with fractional crystallization being the dominant process. 3. The lavas can be divided into two series on the basis of chemistry and mineralogy: one series includes basaltic andesites and andesites, whereas the other series includes dacites and rhyodacites. There is a chemical discontinuity between these series, i.e. lavas with Si02 contents between 60 and 66 wt. % are absent. 4. Basaltic andesites and andesites have largely evolved in a shallow magma chamber (12-14 km depth), whereas the dacites and rhyodacites largely evolved in a deep magma chamber (-27 km depth) sited at the base of the crust. 5. Eruptive products represent the sampling of magmas evolving at different depths within the crust. Polybaric evolution may be more important during the evolution of calc-alkaline magmas than is currently realized. 6 . Polybaric evolution can account for the chemical discontinuity between the least-evolved and most-evolved magmas erupted on Nisyros. 64 Tabic 2.1. Simary of petrographic characteristic* of the Hiayros lava*. Lava type Flag Cpx Ops Fe-Ti 01 Ap Zr Aaph Osidc Baaaltic Andcaite P,H P,K - - P.H I - - Andcaite P,H P.H (P,M) K P.H 1 (1) to Pre-Caldera Dactte P,M P.H P.H P.H - K Rhyodacit* P,M P.H P.H P.H - H (H.l) (X7) Post-Caldara Dacitc P.H P.H P.H P.H XM Notes: V * phenocryst; H " nlcrophenocryst; X * xenocryat; 1 " inclualoo in other phenocryst phaaea; ( ) ■ rarp oecurrente. Table 2.2. Representative analyse* of plagioclaaes. Baa.And. Baa.And. And. And. Dae. Dac. Dac. Rhyod. Rhyod. Rhyod. core H a core ria core care ria core core ria Si02 A9.09 53.51 51. BA 56.19 57.63 55.26 57.AO 60.54 57.10 60.22 A12° j 31.82 29.16 29.16 25.77 25.6A 27.7A 27.20 23.98 26.A9 24.29 f *2o 3* 0.78 0.76 0.78 0.81 0.A6 0.5A 0.32 0.28 0.24 0.40 CsO 15.72 12.61 12.92 9.56 8.16 9.97 8.72 5-76 8.57 6.13 NajO 2.AA 3.98 3.95 5.57 7.02 5.AS 6.22 7.80 6.31 7.75 K 20 0.07 0.18 0.11 0.96 0.A0 0.29 0.57 0.71 0.36 0.76 Total 99.92 100.20 99.21 98.86 99.31 99.25 100.A3 99.07 99.07 99.55 Atoolc proportions on the baala of 8 oxygens Si 2.2A5 2.A19 2.37A 2.567 2.605 2.507 2.565 2.721 2.583 2.700 Al 1.716 1.55A 1.599 1.388 1.367 1.A8A 1.433 1.271 1.413 1.284 Te 0.027 0.026 0.027 0.028 0.016 0.019 0.011 0.010 0.008 0.014 Ca 0.771 0.611 0.63A 0.A68 0.395 0.A85 0.418 0.277 0.416 0.294 Ka 0.217 0.3A9 0.351 0.A9A 0.616 0.A79 0.539 0.680 0.554 0.674 K O.OOA 0.011 0.007 0.056 0.023 0.017 0.033 0.041 0.021 0.044 t cations A.980 A. 970 A. 992 5.001 5.022 A.991 A.999 5.000 4.995 5.010 An 0.777 0.629 0.639 0.A60 0.382 0.A9A 0.A22 0.278 0.A20 0.291 Ab 0.219 0.359 0.35A 0.A85 0.596 0.A88 0.544 0.681 0.559 0.666 Or O.OOA 0.012 0.007 0.055 0.022 0.017 0.033 0.041 0.021 0.043 Ct Ut ♦Total Fe as F«20j Table 2.3. Representative analyses of clinopyroxenes. Baa.And. Bas.And. And. And. Dac. Dac. Dac. Rhyod. Rhyod. Rhyod. core ria core ria core core ria core core ria SlOj 53.01 52.54 50.30 51.46 52.79 51.87 52.72 53.10 52.66 52.62 TiOj 0.50 0.41 1.00 0.72 0.36 0.40 0.40 0.30 0.24 0.23 A120 j 1.81 2.37 3.32 2.06 1.33 1.77 1.15 0.95 1.43 0.84 Cr2°3 - 0.12 ------0.51 - FeO 7.62 5.68 9.72 10.17 11.06 9.25 10.32 10.25 7.67 9.54 HnO 0.16 0.14 0.24 0.41 0.60 0.33 0.42 0.64 0.52 0.55 HgO 15.92 16.26 15.06 14.65 14.02 14.74 14.36 14.28 15.28 14.23 CaO 20.59 22.09 19.64 19.21 20.30 20.32 20.77 20.92 21.86 21.38 Hs20 0.62 0.43 0.42 0.43 0.26 0.64 0.38 0.35 - 0.52 Total 100.23 99.61 99.70 99.11 100.72 99.32 100.52 100.79 100.17 99.91 Atonic proportions on the basis of 6 oxygens si 1.952 1.931 1.883 1.937 1.965 1.946 1.963 1.972 1.952 1.971 Ti 0.014 0.012 0.028 0.021 0.010 0.011 0.011 0.009 0.007 0.007 Al 0.079 0.103 0.147 0.091 0.059 0.079 0.051 0.042 0.063 0.037 Cr - 0.004 --- -- 0.015 - Fe 0.235 0.175 0.305 0.320 0.345 0.290 0.322 0.319 0.238 0.299 Mn 0.005 0.004 0.008 0.013 0.019 0.011 0.013 0.020 0.016 0.018 0.874 0.891 0.841 0.822 0.778 0.825 0.797 0.791 0.844 0.795 Ca 0.812 0.870 0-78B 0.775 0.810 0.817 0.829 0.833 0.868 O.B58 Ha 0.045 0.031 0.031 0.031 0.019 0.047 0.028 0.026 - 0.038 £ cations 4.016 4.021 ■4.031 4.010 4.005 4.026 4.014 4.012 4.003 4.023 Hg/(Mg*£Fe2*) 0.768 0.836 0.734 0.720 0.693 0.740 0.712 0.712 0.780 0.726 O •Total Fe as FeO Baa And. Baa.And . And. ‘ And. And. And. COl■e ria core ria core core 5iOj 39 22 38.74 37.63 37.84 37.53 36.27 AljO, m 0.21 0.13 0.28 0.35 FeO* 18, 99 20.41 23.53 24.08 26.83 31.37 HnO 0.36 0.37 0.50 0.43 0.64 0.54 h bo 41.91 40.11 37.07 33.88 34.53 30.22 NlO - - 0.33 0.30 - CaO 0.29 0.18 0.28 0.27 0.27 0.26 .. Total 100.97 99.81 99.24 98.96 100.38 99.01 Atopic proportions on the bsits of 4 oxygens si 0.996 1.001 0.993 1.006 0.997 0.998 Al - 0.007 0.004 0.009 0.012 Fs 0.603 0.441 0.321 0.335 0.596 0.722 Hn 0.012 0,008 0.011 0.010 0.014 0.013 H» 1.386 1.544 1.460 1.422 1.368 1.239 Ni -- 0.007 0.006 - Ca 0.D081 0.003 0.008 0.008 0.008 0.008 I cations 3. >03 2.999 3.002 2.992 2.998 2.992 Fo 0. ^97 0.778 0.737 0.726 0.697 0.632 I | •Total Fe as Fail Table 2.5. Representative analyses of orthopyroxenes And. And. Dac. Dac. Rhyod. Rhyod. core ria core ria core ria SiO, 52.58 52.63 53.44 52.62 53.07 53.05 TiOj 0.37 0.24 0.10 - 0.14 0.16 AljO, 1.53 1.34 0.65 0.67 1.36 0.55 FeO* 18.70 19.75 23.19 21.74 22.83 23.20 MnO 0.88 0.62 0.86 0.88 1.22 1.26 HgO 24.3B 23.23 21.20 21.75 19.98 20,58 CaO 1.63 1.63 1.16 1.27 1.14 1.12 HijO 0.28 - 0.49 - 0.39 0.38 Total 100.IS 99.44 101.09 98.98 100.13 100.30 Atoaic proportions on the basts of 6 oxygens St 1.935 1.956 1.984 1.983 1.986 1.989 Tl 0.010 0.007 0.003 - ■ 0.004 0.005 Al 0.067 0.059 0.029 0.030 0.060 0.025 Fe 0.576 0.614 0.720 0.687 0.715 0.727 Mn 0.021 0.020 0.027 0.028 0.039 0.040 M* 1.337 1.287 1.173 1.222 1.115 1.150 Ca 0.064 0.065 0.046 0.051 0.046 0.045 Na 0.020 - 0.036 — 0.028 0.028 I cations 4.030 4.008 4.018 4.001 3.993 4.009 Hg/iMg+EFe2*) 0.699 0.677 0.620 0.640 0.609 0.612 ♦Total Fe as FeO Table 2.6. Repreaentatlve analyaea of Fe-Tl oxldea and anphibolea 69 And. Dac. Dac. Rhyod. Rhyod. Dac. Rhyod. Xenolith Tl-nag T£*»ag ila Ti-nag 11a anpb anph anph SiO, _ _ 43.01 41.31 41.82 TIO, 19.68 13.06 46.39 6.75 44.72 3.20 2.79 2.48 AtjOj 2.50 2.16 0.45 1.52 0.39 10.88 12.72 13.06 CrjOj 0.26 0.15 *■ 0.30 0.15 -- - FtO* 70.52 76.50 48.80 85.21 51.67 13.62 14.45 13.60 NnO 0.26 0.43 0.86 0.53 0.38 0.17 0.00 0.19 HgO 1.94 1.71 1.88 1.16 2.39 12.87 12.48 12.93 KiO 0.34 0.51 0.21 4 * ---- CaO 0.12 0.06 0.08 - 0.14 11.31 11.05 11.70 ZnO 0.39 0.35 0.24 ---- Ha,0 - -- - - 2.11 2.47 2.62 KaO - tm - — - 0.52 0.34 0.34 Total 96.21 94.93 98.91 97.47 99.84 97.69 97.81 98.74 Atonic proportiona on the baate of 12 or 23 oxygena Si - • -- 6.369 6.164 6.143 Ti 1.841 1.291 3.629 0.881 3.497 0.357 0.313 0.275 Al 0.364 0.336 0.056 0.240 0.048 1.900 2.227 2.262 Cr 0.026 0.016 - 0.032 0.013 - - - Fe 7.262 8.407 4.245 9.535 4.494 1.687 1.795 1.671 tfn 0.028 0.049 0.076 0.060 0.033 0.021 - 0.024 Kg 0.356 0.337 0.293 0.233 0.371 2.843 2.764 2.630 Mi 0.035 0.055 0.018 ----- Ca 0.016 0.008 0.009 - 0.016 1.794 1.758 1.842 Zn 0.036 0.035 0.019 *• -- 4 * - Na a # - - -- 0.607 0.712 0.746 K -- • * - 0.098 0.066 0.066 E cationa 9,964 10.534 8.345 10.951 6.472 15.676 15.799 15.659 OS? or 1LM 0.558 0.370 0.872 0.241 0.823 - - - ♦Total Fa aa FeO Table 2.7. Major- end trace eleaent analyaea and elenent ratloa of aelected HI ayroe lavat. H-24 R-6 M-10 H-13 H-22 Baa. And. And. Dac. Bhyodac. Toat-Cald. Dac. sio. 55.39 59.65 66.37 70.30 67.51 TiO, 0.96 1.18 0.63 0.35 0.41 Al jOj 18.A7 17.11 16.16 15.52 16.07 FeO* 6.18 6.81 3.88 2.42 2.98 MnO 0.13 0.15 0.09 0.09 0.07 MgO 4.00 2.59 1.28 0.78 1.73 CaO 9.67 5.66 3.46 2.50 4.14 Ha,0 3.70 4.48 4.70 4.80 4.29 KjS 1.28 2.11 3.21 3.16 2.71 P205 0.21 0.26 0.15 0.08 0.09 Sc 23 19 7 4 7 V 211 198 61 26 64 Hi 17 6 4 3 5 2n 73 75 50 38 41 Rb 30 60 89 92 80 Sr 561 426 337 289 480 T 18 24 24 22 20 Zr 148 206 243 228 184 Nb 11 15 18 16 12 Ca 1.19 1.94 3.31 3.34 3.24 Ba 226 492 715 715 607 U 16.64 29.87 36.07 32.93 33.02 Ce 32.96 54.92 55.86 53.21 47.73 Sa 3.81 4.73 3.75 2.68 3.28 Zu 1.12 1.54 1.00 0.77 0.77 Tb 1.12 1.39 0.45 0.34 0.53 Tb 2.32 2.51 2.23 1.91 1.77 Lu 0.43 0.52 0.32 0.38 0.28 Hf 3.10 4.79 4.98 5.08 4.13 Tb 3.20 6.77 10.14 9.95 9.60 K/Rb 355 291 299 285 281 (501 t 61 K/Ca B93B 9029 8051 7854 6943 (11744 ± 2700) K/Sr 19.0 41.1 79.1 90.8 46.9 (19.5 i 3.7) K/Ba 47.1 35.6 37.3 36.7 37.1 (33.1 t 6.4) Be/La 13.6 16.5 19.8 21.7 18.4 (27.3 i 6.6) ♦Total Fe at FeO. Analyaea noraallted to 100* on a volatile-free batia. Between parentheaea; laland arc average ratloa from Morria aod Hart (1983). Table 2.8. Result* of the leaat-aquare* easa balance calculation*. N-24— H-3 N-24— N-10 N-10— N-13 N-3— N-13 Baa.And And. Baa.And Dac. Dac. Hhyodac * And. Rhyodac. N-24 N-24 N-10 N-3 Oba. Calc. Dlff. Oba. Calc. DIET. Oba. Calc. Diff. Oba. Calc. sin. 55.40 55.40 0.0064 55.40 55.41 -0.0090 66.43 66.48 0.0476 59.95 59.96 0.0181 TiOj 0.94 0.87 -0.0865 0.96 1.08 -0.1193 0.63 0.63 0.0019 1.17 1.35 0.1821 Al,03 IB.47 18.46 -0.0100 18.47 18.49 -0.0246 16.17 16.02 -0.1475 17.07 17.09 0.0225 FeO 6.IB 6.20 -0.0149 6.18 6.15 0.0345 3.88 3.88 0.0040 6.74 6.68 -0.0561 MnO 0.13 0.12 -0.0141 0.13 0.09 0.0422 0.09 0.10 0.0109 0.14 0.10 -0.0368 HgO 4.00 3.99 -0.0131 4.00 4.01 -0.0080 1.28 1.16 -0.1168 2.49 2.52 0.0276 CaO 9.67 9.66 -0.0076 9.67 9.65 0.0215 3.46 3.44 -0.0245 5.51 5.46 -0.0492 Na}0 3.70 3.72 0.0161 3.70 3.48 0.2171 4.70 4.89 0.1874 4.58 4.40 -0.1798 k 2o 1.2B 1.19 -0.0902 1.28 1.33 -0.0536 3.21 2.64 -0.5740 2.09 1.90 -0.1868 Mix Propor Mix propor Mix propor Mix propor- vari tions vari tions vari tions vari tiona able able able able 11-3 0.5248 N-10 0.3781 N-13 0.7920 N-13 0.5766 01 0.0132 01 0.0259 CPX 0.0246 01 0.0347 CPX • 0.1278 CPX 0.1438 0PX 0.0077 CPX 0.0545 Flag 0.3177 Flag 0.4130 PI*R 0.1498 Flag 0.2799 Tl-aaj 0.0139 Ti-*ag 0.0367 ri-Mg 0.0167 Ti-»ag 0.0494 Iln 0.0024 SSR - 0.0166 SSR • 0.0684 SSR - 0.4030 SSR - 0.1089 Note*: All Te calculated aa PeO. Analysed phenocryst phases in each lava used as input data. 72 Table 2.9. Weight fractioni of aineral phaiea in the fractionating eolid aaarablege. 01 Cpx Opx Flag Mag 11a Ap 2r H-24 ♦ N-3 0.0270 0.2697 - 0.6705 0.0293 - 0.0024 0.00012 H-24 + N-10 0.0414 0.2302 - 0.6612 0.0588 - 0.0082 0.00025 N-3 N-13 0.0821 0.1290 - 0.6626 0.1169 - 0.0090 0.00025 H-10 H-13 — 0.1219 0.0382 0.7422 0.0827 0.0119 0.0030 0.00012 Table 2.10. Mineral-liquid partition coefficients OLCPX OPX PLAC MAC 1LM AP ZR Sc •02-.37 1.6-17 .53-7.5 •01--15 -.8-3.3 1.8 .22 V .04-.09 .9-18 .05-7.2 .01-.07 8.7-63 12 .01 - Cr .3-34 1.9-245 2-143 .01-.04 1-620 6 .2 - mi 4-38 26-9 1.1-24 •01-.26 1.4-77 10 .2 - Zn 1.2-1.5 .31-12 2.6-4.4 .04-.25 3.1-13 -- 8.5 Rb .008-.011 .001-.04 •01-.03 .01—.2 .01 .01 .01 - Sr .003-.02 .06-.21 .01-.1 1.3-3.2 .01-.1 .01 2 - Zr .01 • 12-.34 •02-.22 •01-.02 .14-1.7 .28 .01-.636 - Ba .005-.02 .001-.30 .01-.23 .01-.59 .12-.40 .01 .01 — ta .003-.01 .08-.23 .02-.3 •12-.21 •05-.45 .05 14-30 3.11-5.25 Ce .003-.009 .04-.51 .003-.33 .023-.3 .05-.82 .05 16.6-53 2.29-5.14 Md .003-.01 .065-.70 .006-.43 .023-.2 •05-.55 .05 27-81 1.97-4.77 Sa .003-.011 .09-1.4 •014-.43 .024-.2 .05-1.4 .05 20.7-90 2.58-5.16 Eu •005-.01 .09-1.2 .043-.42 .055-1.3 .05-.66 .05 14.5-50 1.07-5.22 Tb .009-.023 .09-1.5 .U-.67 .006-.3 .05-1 .05 9.4-37 128-254' Hf .01 •18-.48 .001—.22 .01-.02 .14-1.7 .25 .73 958-997 Th .01 .001-.03 .001-.22 .004-.01 .05-1.2 .01 1.3 21-94 References: Arth (1976); Soatal et al. (1983); Fujiuki: (1986); Gill (1981); Irving (1978); LeRoex and Erlank (1982); bidden (1978); Inhr and Carmichael (I960); Woodruff et al. (1979); Evart (19B2)(xircon-liquid partition coefficients only). Table 2.11. Comparison of obterved and predicted trace element concentrations 74 H-24 ♦ N-3 H-24 ♦ H-10 Baa.And. And. Baa.And. Dac. «-3ob. «-3pr.d D D-range N-10ob. H-10Pr«d D D-range SC 19 19 1.30 0.46-4.79 7 7 2.22 0.42-4.22 V 180 180 1.25 0.51-6.75 61 61 2.28 0.73-7.90 Ni 7 7 2.38 0.23-6.47 4 4 2.49 0.32-9.17 Zn 82 82 0.82 0.24-3.83 so 50 1.39 0.33-3.76 Rb 61 57 0.01 0.01-0.15 B9 79 0.01 0.01-0.14 Sr 421 421 1.45 0.89-2.21 337 337 1.52 0.89-2.19 Zr 209 209 0.46 0.18-0.62 243 243 0.49 0.34-1.16 Cs 2.55 2.27 0 T 3.31 3.15 0 ? Ba 515 428 0.01 0.01-0.49 715 592 0.01 0.01-0.48 La 32 30 0.14 0.14-0.29 36 36 0.23 0.22-0.47 Ce 57 57 0.15 0.07-0.49 56 56 0.46 0.14-0.80 Sn 5.13 5.13 0.54 0.09-0.77 3.75 3.75 1.02 0.21-1.28 Eu 1.42 1.42 0.63 0.10-1.34 1.00 1.00 1.12 0.18-1.59 Yb 2.57 2.73 0.75 0.05-0.75 2.23 2.39 0.97 0.11-0.97 Hf 4.89 4.89 0.29 0.18-0.31 4.98 5.14 0.4B 0.30-0.48 Th 7.15 6.06 0.01 0.01-0.06 10.14 8.30 0.02 0.02-0.12 Table 2.11, cont. H-10 ♦ N-13 N-3 ♦ N-13 Dac. Rhyod. And, Rhyod. H-l3ob. «-13pred D D-range N’13ob. **“13pred D D-range Sc 4 4 2.77 0.31-2.77 4 7 2.71 0.31-2.71 V 26 26 4.66 0.98-7.87 26 26 4.51 1.14-9.74 Hi 3 3 2.23 0.32-8.69 3 3 2.54 0.53-15 Zn 38 38 2.18 0.42-2.89 38 38 2.40 0.53-3.36 Rb 92 108 0.16 0.01-0.16 92 105 0.01 0.01-0.14 Sr 289 289 1.66 0.98-2.42 289 289 1.68 0.89-2.18 Zr 228 262 0.67 0.18-0.67 228 228 0.84 0.33-1.22 Cs 3.34 4.18 0 T 3.34 4.42 0 T Ba 715 uuo 0.52 0.02-0.52 715 745 0.40 0.02-0.48 La 33 42 0.32 0.15-0.32 33 42 0.49 0.22-0.49 Ce 53 62 0.53 0.08-0.53 53 62 0.84 0.18-0.84 Sa 2.68 4.00 0.72 0.10-0.72 2.68 4.37 1.29 0.22-1.29 Eu 0.77 0.93 1.33 0.10-1.33 0.77 1.05 1.55 0.19-1.55 Yb 1.91 2.41 0.66 0,05-0.66 1.91 2.70 0.91 0.11-0.91 Hf 5.08 5.80 0.35 0.16-0.35 5.08 6.33 0.53 0,29-0.53 Th 9.95 12.42 0.13 0.01-0.13 9.95 11.17 0.19 0.03-0.19 Table 2.12. Calculated condition* of cryatal I ization of the Hiayroa Lavs type Method T(*C) P(kbar) fOj (bar) Baa. And. 01-L1 1058-1092 Plag-L2 1073 3.5 1.5 And. 01-L1 994-1026 Opx-Cpx3 1000-1020 01-0px-L and PlagrL*1 1000-1025 3.5-4.0 2.25-2.5 01-Mag-L* 10-9.11 Dac. Opx-Cpx3 900-918 Plag-L2 911 8 2.75 Hag-11a* 8B0 10-11.9 Rhyodac. Opx-Cpx3 830-903 Plag-L2 872 8 6.5 Kag-IIa® 860 I0-II.5 1. Olivine-liquid theraoaeter* froa Roeder and Easlie (1970), Roeder (1974} and Leeaan(1978) 2. Plagioclaae-liquid theraoaeter froa Chiorao and Caraichael (1980) 3. Two-pyroaene theraoaetera froa Wood and Banno (1970) and Lindsley (1983) 4. See teat 5. Oxygen fugacity froa QFM-equilibrtua 6. TWo-oxlde theraoaeter froa Spencer and T.indatey (1981). Hole fractions ofulvospinel and ilaenlte calculated aa suggested by Stoner (1983). ISLE OF NISYROS (simplified from Di Paola 1974) ■ Basaltic Andesites 0 1 2km Evl A ndesites E3 Pre-Caldera Dacites E=3 Post-Caldera Dacites MANDRAKI Rhyodacites EMBOR0 1 Pyroclastics I I Detrital Deposits I :• Ag. loannis-#; m N1KIA Fig.2.1. Geologic nap of Nisyros, simplified from Di Paola (1974). 7 1 ------7f------K ------X ------A ------B ------K ------7C------IT Or Fig.2.2. Feldspar compositions plotted in terms of the ^ components Ab-An-Or. -j Hti ••• En Dac. Rhyodac. En Fig.2.3. Compositions of clino- and orthopyroxenes plotted in the conventional Ca-Mg-Fe quadrilateral. n| o 79 •N’** 70 74 * * Filled circles 0 > 66 si02 . 62 •V 64 1 ■ I 4 2 0 4 3 2 0 10 0.6 S S it 71 74 70 ago. * r 66 6 >02 >02 6 Si02 62 Majorplotted oxides against ••• I 2 6 2 s 4 4 ■ 3 2 0 a 0 0 6 4 2.4. 10'i OS m o »j o z *n crosses crosses « groundnass analyses. Fig. = pre-caldera lavas; = pre-caldera lavas; open » post-calderasquares dacites; 80 •oo 340 TOO 330 A300 HO 400 300 140 300 BO 13 40 30 JS 340 34 300 30 „ 130 40 30 BtO • 30 4B0 13> A 440 z 400 330 30 40 00 100 30 40 •0 too Fig.2.5. Variation diagrams of selected trace elements with Rb as a differentiation index. ei T Degree of evolution ► Fig.2.6. Schematic liquidus surface for natural magmas (solid line) and surface of equilibration temperatures for natural melts and phenocryst assemblages (dotted line). Mixing of magmas very different in composition (a) results in resorbtion of phenocryst phases in both of the original magmas. Mixing of one magma close in composition to a "natural" cotectic with another magma (b) will result in resorbtion of one set of phenocryst phases. Mixing of magmas of similar composition (c) promotes development of normal and reversed zoning, but does not result in resorbtion of phenocrysts. The kink in the liquidus surface represents a change in the phenocryst assemblage. 0.7046 - □ □ J” 0 .7042 - oo CO 5 0 .7038 - 0.7034 + 0 20 40 60 80 100 Fig.2.7. 87Sr/86Sr plotted against Rb contents. Crosses ~ basaltic andesites; filled circles = andesites; diamonds = dacites; squares = rhyodacites. Analytical uncertainty in 87Sr/86Sr is 2-3 in the decimal place (1 standard deviation of the mean). toCD I 0.7046 - | to ^ 0.7042 - to 03 0.7038 - 20 40 60 80 100 Rb Fig.2.8 . Possible relationships between the basaltic andesite to andesite series (1-2) and the dacite to rhyodacite series (3-4). The latter may be derived from the former by fractional crystallization as shown by the dotted lines labeled (a) or by fractional crystallization and assimilation as shown by the dash-dot line labeled (b). 84 • A I P Fig.2.9. Schematic diagram to explain the observed zonation patterns in phenocrysts from the Nisyros lavas. Near-adiabatic uprise of magmas from A to B results in reversed zoning of phenocrysts. Near-isobaric evolution of magmas from B to c results in normal zoning of phenocrysts grown at low pressures (i.e. B) and in normal zoning of rims of phenocryst cores grown from A to B. 85 0 3.5 4.0 1100 3.0 ? 1000 900 -0.200 -0.300 log a y SiO, Fig.2.10. Log &5jn2 caicuiateo.calculated fromiron 01uj -0px-liquid and plagioclase-liquid equilibria plotted as a function of temperature. Parallel curves « 01-0px-liquid equilibrium; filled circles = plagioclase-liquid equilibrium. The numbers on each curve refer to pressure in kbar. 86 AFC (basaltic andesite, andesite) (—13km) AFCMacite, rhyodacite) Moho (—27km) t Ascent of primitive basalts Fig.2.11. Schematic diagram showing the possible evolution of the Nisyros magmas. CHAPTER III THE ROLE OF ASSIMILATION IN THE EVOLUTION OF CALC-ALKALINE MAGMAS FROM SANTORINI AND NISYROS INTRODUCTION Calc-alkaline lavas from intra-oceanic island arcs and active continental margins commonly have variable radiogenic isotope ratios. Intra-oceanic island arc lavas probably have inherited these isptopic variations from the upper mantle source region. This indicates that long-lived heterogeneities exist in the upper mantle, or that the magnitude of the contribution from the subducted oceanic slab to the overlying mantle wedge is highly variable in space and/or time. Recent studies of 10Be in lavas from several island arcs have shown that subducted sediments have been involved in the generation of some calc-alkaline magmas (Tera et al., 1986), and obviously the nature and the amount of subducted sediments also influence the isotopic composition of the magmas. In calc-alkaline magmas from active continental margins the isotopic characteristics acquired in the source region may change during ascent by interaction of the magmas with continental crust. The effects of crustal assimilation on the magma composition must be taken into account before the isotope 87 88 ratios of island arc volcanics can be used to evaluate the nature of the source regions of calc**alkaline magmas. Different models have been proposed to describe the process of assimilation/ the most simple of which is binary mixing (Faure et al., 1974; Langmuir et al.r 1978; Vollmer, 1976). However/ since magmas are generally not superheated, assimilation of the wall rock through binary mixing is unrealistic (cf. Bowen, 1928), unless the necessary heat is provided from an external source (e.g. underplating of the magma body with hot, mafic magma; cf. Grove and Kinzler, 1986}. A more realistic model involves coupled wall rock assimilation and fractional crystallization (AFC), as originally proposed by Bowen (op.cit.), which is based on the principle that the heat required for assimilation is balanced by the latent heat of crystallization. The trace element and isotopic compositions of the contaminated magma can be modeled with the equations derived by DePaolo (1981), and by comparing these to the observed lava compositions the ratio Ma/Hc (rate of assimilation/rate of crystallization) can be estimated. Contamination will have little direct effect on the major element composition of the liquid, because the magma must remain in equilibrium with its phenocryst phases (Bowen, 1928; O'Hara, 1980). Contamination will principally lead to changes in the mineral compositions and in the relative proportions of the crystallizing phases. This is so because the magma will move off natural equivalents of cotectlcs into the primary phase volume of one of the crystallizing phases, but subsequent crystallization will move the melt back towards and onto a cotectic (see Fig.3.1)* Therefore major elements should not be used to quantitatively estimate the amount of contamination. Trace elements, especially the highly incompatible elements, are not very sensitive to small changes in mineral compositions or to the proportions of phases crystallizing, and can therefore be used together with isotope ratios to solve (numerically) the AFC- equations for Ma/Mc and F (remaining fraction of magma). Assimilation and fractionation may also be accompanied by magma mixing. Underplating with hot magma and subsequent mixing may cause vigorous convection which will facilitate the disintegration of the wall rock or xenoliths (cf. O'Hara and Matthews, 1981; van Bergen and Barton, 1984). Furthermore, concurrent AFC and mixing may be necessary to obtain large amounts of assimilation relative to fractionation. Taylor (1980) estimated the upper limit of Ma/Mc to be about 0.3, assuming assimilation of cold (150°C) wall rock into a magma at ll50°c. Mixing of an evolved magma with a primitive magma at high temperature could provide the extra heat necessary for Ma/Mc ratios to exceed 0.3. The equations for simultaneous assimilation, fractional crystallization and mixing have been derived by O'Hara and Matthews (1981). In the present paper we discuss the pre-caldera lavas from the volcanic complex of Nisyros, and lavas from two shield volcanoes on Santorini. Nisyros and Santorini are two islands of the Hellenic arc (Fig.1.2). The crust on both sides of the trench system has a continental character. Crustal thicknesses range from 27 km below Nisyros to 22 km below Santorini (Makris and Stobbe, 1984) and therefore contamination of the magmas during ascent could have occurred. In this paper we quantitatively evaluate the intra-crustal processes that caused the variations in isotopic composition in the lavas from Nisyros and Santorini. SUMMARY OF PREVIOUS WORK The plate tectonic and geophysical data of the Aegean area are summarized in Huijsmans and Barton (1988). The shield volcanoes of Skaros and Mikro Profitis Illias (MPI) are situated in the northern part of the main island of Santorini. A caldera that formed as a result of the Minoan eruption in 1390 B.C. dissected these volcanoes and all lava flows above sea level are now accessible for sampling. Huijsmans and Barton {1988) report mineralogical and geochemical data for each flow from these volcanoes, and the following is a brief summary of their work. Whole-rock major and trace element concentrations for the lava flows of Skaros and MPI define cyclic compositional variations with stratigraphic height (Huijsmans and Barton, 1988). These compositional cycles are separated from each other by pyroclastic deposits, which have indurated upper surfaces and sometimes show evidence of soil development. The cycles typically start with more evolved lavas (basaltic andesites or andesites) whereas younger flows are more mafic in composition (basalts or basaltic andesites). Several eruptive products of the Skaros volcano are more siliceous in composition. The two lowermost flows of the Skaros sequence are dacites, but they are separated from the overlying andesites by an erosional surface and probably belong to an older cycle, or even to another (older) volcanic sequence. The upper cycle of Skaros shows a reversed trend: from basaltic andesite through rhyodacite to rhyolite (Upper Pumice series, see Fig. 3.4). Petrographic and mineral chemical data for both volcanic sequences indicate that most lavas are hybrids that formed by mixing of a basaltic and a low-silica andesitic magma. However, intra- and inter-cycle variations in trace element concentrations cannot be explained by the mixing of magmas which are related by fractional crystallization and indicate that magma evolution was more complex and must have involved assimilation of crustal material (see Huijsmans and Barton, 1988). The volcanic complex of Nisyros has been previously described by Di Paola (1974) and in Chapter II. In that chapter it was shown that the pre-caldera lavas on Nisyros constitute two sub-series that evolved at different pressures: a low-pressure (3.5 kbar) basaltic andesite- andesite series and an intermediate pressure (8 kbar) dacite-rhyodacite series. Trace element variations and preliminary Sr-isotope results show that fractional crystallization combined with assimilation (both at deep and shallow levels within the crust) can account for the chemical and isotopic variations in both series. ANALYTICAL METHODS The samples for the isotopic measurements were selected after careful petrographic studies to avoid any effects arising from secondary alteration. Powdered samples were dissolved in a 1:10 mixture of HC104 and HF, and the alkalis were removed by precipitation with HClo4 . Sr and the REE were separated on ion exchange columns containing AG50H-X8 resin by elution with HC1. Nd was separated from the REE fraction by ion exchange on columns with Teflon powder coated with HDEHP following the procedure of Richard et al.(1976). Sr was loaded in nitric acid on single oxidized Ta filaments and the 87Sr/86Sr ratios were measured on a Nuclide 12" single collector maBS spectrometer at The Ohio 93 State University. The Isotope ratios were normalised to 86Sr/88Sr » 0.11940 and are reported relative to 87Sr/86Sr - 0.71022 for the NBS-987 standard. Nd was loaded in HC1 on a previously dried droplet of H3PO4 onto the side filament of a double Re filament assembly. The Nd isotope ratios were measured as Nd+ with Finnigan HAT 261 multicollector mass spectrometers either at the Finnigan factory, Bremen, or at The Ohio state University. The data were normalised to 146Nd/144Nd ■ 0.72190. At Bremen, the Nd measurements were made using static (no peak jumping) multicollection; the measured value of 143Nd/144Nd for the La Jolla Nd (standard distributed by G. Lugmair) was 0.511841 +/~ 0.000010 (one standard deviation of 21 runs). At Ohio State, the Nd measurements were made by using dynamic (four collectors and a single peak jump) multicollection; the measured 143Nd/144Nd for the La Jolla Nd was 0.511852 +/- 0.000005 (one standard deviation, 43 runs). No adjustments to the measured and normalised Nd isotope ratios were made. RESULTS The results of the isotopic measurements for MPI, Skaros and Nisyros are listed in Tables 3.1, 3.2 and 3.3 respectively. Whole-rock major and trace element analyses for the Santorini samples are reported by Huijsmans (1985) and Huijsmans and Barton (1988); those for Nisyros are 94 given in Chapter II. All isotopic data are compiled in Fig. 3.2 where 143Nd/144Nd is plotted against 87Sr/88Sr. The isotopic data for Santorini and Nisyros define two distinct, parallel trends, with the Nisyros isotope ratios shifted towards relatively lower 87Sr/86Sr. For Nisyros and MPI there is a well defined negative correlation between Sr- and Nd-isotope ratios. The range in 87Sr/86Sr and 143Nd/144Nd for the more mafic lavas from Skaros is relatively restricted, and there is no clear correlation between Sr- and Nd- isotope ratios for these lavas. However, all Skaros lavas define a negative correlation between Sr- and Nd-isotope ratios. Mikro Profitis Illias The variations in 87Sr/86Sr and whole-rock Si02 with stratigraphic height are illustrated in Fig. 3.3. The first cycle of MPI (with exception of the lowermost flow) shows excellent correlations between 87sr/86Sr and 143Nd/144Nd and differentiation indices such as Si02. The first and second flow have identical Sr and Nd isotope ratios, but show a large difference in bulk composition. Correlations between isotope ratios and differentiation indices are less regular in the second cycle, and cycle III is anomalous in that the four analysed samples have virtually identical major element compositions, whereas the Sr- and Nd-isotope ratios vary considerably. Only a few samples from Skaros and MPI have been analysed for oxygen isotopes (Hoefs, 1978; de Jong, 1985, pers.comm.). Unfortunately these analyses were not done on the samples studied here so that it is not possible to relate directly the oxygen data to the Sr- and Nd-isotopic data. Plagioclase separates from four MPI samples which range in composition from basalt to andesite yield d180 values of +6.1 - +6.4 °/oo (A.J. de i Jong, pers. comm. 1985), and these variations are within analytical error (+/- 0.2 °/oo). Skaros The variations in 87Sr/86Sr and whole-rock Si02 are plotted as a function of stratigraphic height in Fig. 3.4. The Skaros lavas are characterized by lower Sr- and higher Nd-isotope ratios relative to lavas of comparable bulk composition from MPI (Tables 3.1 and 3.2). Furthermore, for a given range in bulk composition the range in isotopic compositions is considerably smaller for the Skaros lavas. In the lower two cycles of Skaros (with exception of the lowest flow, see above) there is a poor negative correlation between 87Sr/86Sr and Si02 (Fig. 3.4). In cycles III and IV this correlation is reversed, and the uppermost rhyodacitic and rhyolitic units of cycle IV are characterized by substantially elevated 87Sr/86Sr. Nd- and Sr- isotope ratios for lavas from Skaros have been reported previously by Briqueu et al. (1986), and generally the two 96 data sets agree fairly well. Their analyses show that isotope ratios as primitive as 87Sr/86Sr-o.70381 and 143Nd/144Nd*0.512873 occur in at least one basalt from the Skaros volcano. Apparently we did not analyze this flow for Sr- and Nd- isotope ratios. Plagioclase phenocrysts in three basaltic andesites from Skaros yield d180 values of +5.7 to +5 .8°/oo (A.J. de Jong, pers. comm. 1985), slightly lower than the oxygen isotope ratios for HPI. Hoefs (1978) also reported oxygen isotope ratios for some lavas from Skaros, but most analyses were performed on whole-rock samples, and the results were interpreted to reflect minor low-temperature alteration (Hoefs, 1978). In the Upper Pumice series plagioclase phenocrysts yield a d180 value of 8.0 °/oo (Hoefs, op.cit). Therefore the total range in d180 for Skaros exceeds the maximum enrichment in 180 expected as a result of fractional crystallization in calc-alkaline magmas (from 5.8 to 6.8 °/oo, Hatsuhisa, 1979). Nlsvros The results for Nisyros are listed in Table 3.3, together with d180-values for plagioclase separates reported by Hooft van Huysduynen (1985). The variations in 87Sr/86Sr and d180 with whole-rock Si02 are illustrated in Fig. 3.5. In the basaltic andesites and andesites there is a poorly defined positive correlation between Sr- and o- 97 isotope ratios and Si02. One sample (N-24) has an anomalously high d180-value of 8.2 °/oo. since this lava flow occurs at sea level, the high O-isotope ratio probably reflects low-temperature alteration by sea water (e.g. Faure, 1986). Two samples, an amphibole-rich inclusion (N- 14) and a cumulate rock of basaltic andesite composition (N-21), are characterized by very low Sr- and high Nd- isotope ratios. The dacite-rhyodacite subseries has relatively low d180 and 87Sr/86Sr and high 143Nd/144Nd, if compared to the andesites, which is in accordance with the observation made in Chapter II that this sub-series is not directly related to the more mafic lavas on Nisyros by assimilation and/or fractional crystallization. DISCUSSION The isotope ratios of the lavas from Santorini and Nisyros are compared to the isotopic compositions of volcanics from other island arcs and from active continental margins in Figs. 3.6 and 3.7 respectively. Also shown in these diagrams are the fields for Mid-Ocean Kidge Basalts and Ocean Island Basalts. The lavas from Santorini and Nisyros define two distinct, near-linear trends on this diagram, which are parallel to the main axis of the field for OIB and MORB. All Santorini lavas plot within the field for oceanic basalts, whereas lavas from Nisyros plot just below this field. New Britain and the Marianas (De Paolo and Johnson, 1979; De Paolo and Wasserburg, 1977, Stern, 1981) are Intra-oceanic island arcs for which there is no evidence that subducted sediments have affected the isotopic composition of the source region (Tera et al., 1986). Furthermore, it seems unlikely that the isotopic composition of calc-alkaline magmas can be substantially changed by assimilation of oceanic crust. The lavas from Santorini and Nisyros are characterized by lower 143Nd/144Nd and higher B7Sr/B6Sr relative to the lavas from New Britain and the Harianas, but they are comparable in isotopic composition to calc-alkaline volcanics from the Sunda arc (Whitford et al., 1981), Patagonia (Hawkesworth et al., 1979), Mexico (Verma, 1983) and S. Chile (Frey et al., 1984). It has been suggested by Whitford et al. (1981) that the Sunda lavas acquired their relatively high B7Sr/B6Sr and low 143Nd/144Nd by contamination of the source region with subducted (old) sialic sediments. The calc-alkaline magmas from Central and South America have ascended through continental crust of substantial thickness, and assimilation of continental material has been invoked to explain the high Sr- and low Nd-isotope ratios from these localities (e.g. James, 1982). The Hellenic arc is underlain by continental crust, and crustal contamination may have played an important role in the evolution of the magmas. In the following sections we examine the isotopic variations on Santorini and Nisyros in 99 terms of crustal assimilation. Santorini Huijsmans and Barton (1988) have shown that the cyclic compositional variations on Skaros and MPI reflect eruptions from chemically and thermally zoned magma chambers, but that the zonation in the chambers resulted from processes other than crystal settling. On the basis of mineral chemical and whole-rock chemical data, Huijsmans and Barton concluded that the chemical variations on Skaros can be explained by mixing of batches of magma that were mutually related by fractional crystallization. Magma evolution on MPI must have been more complex, because trace element concentrations are inconsistent with fractional crystallization followed by magma mixing. Barton et al. (1986) used preliminary Sr-isotope results to show that magma mixing between batches of magma which had previously been contaminated to varying degrees can qualitatively explain the observed chemical and isotopic variations on Skaros and MPI. The order in which the magmatic processes (fractionation, assimilation, mixing) occurred has to be known, or appropriate assumptions have to be made, before the chemical and isotopic variations can be quantitatively modeled. He will make the assumption that magma mixing was the final process that has affected the magma compositions on Skaros and MPI, i.e. nixing was not accompanied by, or followed by, fractional crystallization or assimilation. This assumption seems justified because: 1) The hybrid lavas contain mineralogical evidence of magma mixing. Apparently there was insufficient time for these disequilibrium phenocrysts to settle out or to be resorbed: i.e.eruption followed mixing in a fairly short time (cf. Chapter V); 2) Intra-cycle variations of incompatible and moderately compatible trace elements are nearly linear when plotted against conventional differentiation indices (data from Huijsmans, 1985); 3) Thermal gradients in the magma chambers underlying the two volcanoes were very high, about 300°c and resulted from underplating and mixing of more evolved magmas with hot, primitive magma (Huijsmans and i Barton, 1988). Such high thermal gradients cannot be maintained over long periods of time, and eruption must have occurred shortly after the mixing event. (i) Intra-cycle variations on Mikro Profitis Illias The variations of 87Sr/86Sr with Si02 are illustrated in Fig.3.8. This diagram clearly shows that each compositional cycle defines a distinct trend. The first cycle displays the largest range in bulk composition and provides a good example of an eruption sequence from a zoned magma chamber. We tested the hypothesis that the basaltic andesites in this cycle formed by magma mixing of a high 87Sr/86Sr, low 143Nd/144Nd andesitic magma (SI-160) with a low 87Sr/86Sr, high 143Nd/144Nd basaltic magma (SI- 140) against all available chemical ai\d isotopic data. The end member magmas are the first and last lavas (respectively ) of cycle I. The proportions of these end members in the hybrid magmas were determined on the basis of major element mass-balance calculations using the unconstrained least-squares mixing program of Bryan et al. (1969). The trace element concentrations and isotope ratios were calculated with the equations for simple mixing provided by Langmuir et al. (1978) The observed and calculated major oxide-, trace element- and isotopic compositions for two basaltic andesites of cycle I (SI-137 and Si-135) are listed in Table 3.4, and the calculated mixing curves are shown in Figs. 3.8 and 3,9. The calculated compositions for these two samples generally agree very well with the actual compositions. Small discrepancies between observed and predicted compatible element concentrations can be explained by either minor crystallization or phenocryst accumulation in one of the end member magmas or in the hybrid magma after the mixing event. The lower observed concentrations of V, Cr and Ni, for example, can be accounted for by removal of only 1.5-6 wt.% of crystals from the hybrid magma. The match between calculated and measured isotope ratios is near-perfect. These results suggest that magma mixing between a 1 0 2 contaminated andesite and an uncontaminated, or leee contaminated high-Al203 basalt could have caused the chemical and isotopic variations in the first cycle of HPI, and confirm that magma mixing (rather than fractional crystallization) caused the magma chamber to become zoned. The first and second lava flows of cycle I (M-7 and SI-160, see Fig. 3.8) are very different in whole rock composition, but have identical Sr- and Nd-isotope ratios, and in this case differentiation involved only fractional crystallization. During formation of this siliceous cap assimilation may have been prevented by a layer of cumulates along the roof and walls of the chamber, that separated the magma from the country rock. Analogous calculations were carried out on compositions from the second cycle of HPI, to test whether a similar model explains compositional variation in these lavas. It was assumed that contaminated andesitic magma (SI-142, lowermost flow of cycle II) mixed with basaltic magma (SI-140, uppermost flow of cycle I) to produce cycle II magmas of intermediate composition (SI-145 and SI-147). The calculated and observed values for major- and trace element concentrations and isotope ratios are listed in Table 3.4. Again the predicted concentrations agree well with the measured concentrations, considering analytical uncertainty. In both lavas FeO, Ti02 and V are significantly higher than the calculated values, which 103 could bo caused by loss of snail anounts of Fe-Ti-oxides fron one of the end nembers after the nixing event. Magna nixing nay lead to supersaturation of the nagna with respect to certain phases, which will cause crystallization in thernal boundary layers (see Huijsnans and Barton, 1988). Calculated values for Cr are higher than the actual concentrations and nay reflect post-nixing crystallization of e.g. dinopyroxene. The variations in chenical and isotopic conposition in the upper cycle of MPX are highly unusual. The lavas of the third cycle all have sinilar najor elenent compositions, but differ considerably in isotopic conposition (see Fig.3.3). The upper two flows (SI-121 and SI-118) are strongly enriched in both highly conpatible and highly/noderately inconpatible elenents (Huijsnans, 1985; Huijsnans and Barton, 1988) relative to all other lavas of MPI. Huijsnans and Barton (1988) concluded that these are hybrid lavas, and that the anonalous trace elenent conpositions of these lavas reflect nixing between a nafic nagna (nore prinitive than SI-140) and a highly evolved nagna (of rhyodacitic or rhyolitic conposition). The lowernost flow of this cycle (SI-126) is isotopically and chemically identical to lavas SI-130 and SI-135 of the first cycle of MPI (see Figs. 3.8 and 3.9) and probably also represents a hybrid magma. The unusual trend in the third cycle nust have been caused by nagna mixing, since no 104 other magmatic process can cause Isotope ratios to shift, while the major element composition remains essentially constant* We tested whether the composition of flow SI-122 is consistent with an origin through mixing of magmas with compositions similar to 81-126 and SI-121. The mixing proportions could not be estimated with least-sguares mass balancing methods, since there is a lack of variation in major element compositions. Instead the mixing proportions were determined which yielded the best agreement for trace elements and isotope ratios. The results are listed in Table 3.4 and show that the calculated composition closely matches the analysed trace element and isotopic composition of SI-122. (ii) Intra-cycle variations on Skaros The variations in Sr-isotopic composition as a function of differentiation are illustrated in Fig. 3.10, and in Fig. 3.11 87Sr/86Sr is plotted against 143Nd/144Nd. These diagrams show that for the lavas of cycles I, II and III there is a negative correlation between 87Sr/86Sr and whole-rock SiC>2/ and no clear correlation between Sr-and Nd-isotope ratios. This peculiar behaviour can only be explained by mixing of a mafic magma characterized by higher 87Sr/86Sr and 143Nd/144Nd with a more evolved magma with less radiogenic Sr- and Nd-compositions. These magmas cannot be mutually related by assimilation as upper crustal 105 contamination should nearly always result in increasing Sr- and decreasing Nd-isotope ratios with increasing differentiation. Evidence for magma mixing is present in all lavas (except for the rhyodacite) and also in the Upper Pumice Series (Bond and Sparks, 1976)• However, Briqueu et al.(1986) did not recognize the importance of magma mixing in the evolution of the Skaros magmas and concluded that Nd- and Sr-isotope variations are consistent with fractional crystallization combined with minor assimilation (Ma/Mc-0.1-0.2) of metavolcanics or schists. To test whether magma mixing can generate variations such as illustrated in Figs. 3.10 and 3.11, a similar procedure as described above for lavas from HPI was followed. He assumed that the end members were SI-181 (basalt, Skaros-I) and SI-173 (basaltic andesite, Skaros- I). The calculated compositions for two basaltic andesites (SI-106 and SI-182) are given in Table 3.5. Overall there is good agreement between predicted and observed compositions. However, for some other lavas it was impossible to obtain reasonable fits for compatible element concentrations. Thus far it has been assumed that the hybrid magmas resulted from simple mixing between two end member magmas. However, the scatter of the data points for the mafic lavas in Figs. 3.10 and 3.11 indicates that mixing between two magmas cannot completely account for the observed chemical and isotopic variations and suggests that 106 more than two end members were involved in the mixing process. Another complicating factor is the presence of highly forsteritic olivine xenocrysts with Cr-spinel inclusions in at least one of the flows of the first cycle (Huijsmans and Barton, 1988). The data for the mafic lavas on Skaros seem to indicate magma mixing between several batches of magma that had slightly different isotope ratios. The magmas inherited these differences in isotopic composition from a heterogeneous source region, or acquired these characteristics by varying degrees of contamination before they mixed in the plumbing system beneath the Skaros volcano. The top cycle of Skaros is very different from the other cycles in that there is a large range in bulk compositions. The basaltic andesites of thiB cycle are hybrids that are abnormally enriched in incompatible and compatible elements. Mineral chemical data and high compatible element concentrations suggest that the mafic end member involved in the mixing process must have been more mafic than any of the Skaros basalts (Huijsmans and Barton, 1988). The origin of these lavas is therefore similar to that proposed for the upper flows of MPI. However, there are distinct differences in trace element- and isotopic composition between the basaltic andesites from Skaros-lV and MPI-III. 107 (lii) The role of assimilation on Santorini In the previous section it was concluded that mixing of magmas with different Sr- and Nd-isotopic compositions may have caused the intra-cycle isotopic variations on HPI and in the lower three cycles of Skaros. In this section we will attempt to determine whether the andesitic mixing end members of HPI-I and II could have been derived from the basaltic end member by assimilation. The primitive and evolved end member magmas of Skaros are not mutually related by assimilation (see above). In addition, we will test whether or not the variations in Skaros-IV are a result of crustal contamination. The main problem with modeling assimilation on Santorini and Nisyros is that metamorphic xenoliths are extremely rare in the lavas of these islands. Only one flow of HPI contains metamorphic xenoliths and one of these (SI-122X) has been analysed for trace elements and radiogenic isotope ratios (Table 3.1). The nature and the age of the crust in the Aegean are largely unknown. Ages for basement rocks of the islandB Naxos, los and Sikinos range from about 25 Ha to 515 Ha (Andriessen et al., 1979; Hensjes-Kunst and Kreuzer, 1982; Andriessen et al., 1987) and suggest that the upper crust is fairly young. As crustal end members in the calculations we used the xenolith from HPI and compositions of average crust and upper crust taken from Taylor and HcLennan (1981). All three crustal end members were assumed to have 108 isotopic compositions identical to that of the xenolith (SI-122X), namely 87Sr/86Sr - 0.7081 and 143Hd/144Nd - * 0.51242. In the following section we determine whether the andesitic, high 87Sr/86Sr, low 143Nd/144Nd end members of the first and second cycle of HPI (SI-160 and SI-142) are related to the most mafic lava on HPI (SI-140) by coupled assimilation and fractional crystallization (AFC). Assimilation by simple mixing was tested only with the xenolith as the crustal end member, but a very poor match was obtained between calculated and analysed lava composition and the results are not given here. Other crustal compositions (average crust and upper crust) have higher Sr-concentrations than most lavas on Santorini. Contamination of the magmas with these crustal endmembers by simple mixing would cause an increase in Sr- concentrations of the magmas which is not in agreement with data reported for the Santorini lavas (Huijsmans and i Barton, 1988). AFC-calculations ware done for HPI using these three crustal end members. Initially we used only the observed or inferred 87Sr/86Sr, 143Nd/x44Nd, Sr- and Nd- concentrations. The bulk solid-liquid distribution coefficient for Sr (DSr), along with Ha/Hc and F were allowed to vary between realistic limits (i.e. Dgr-0-2, Ha/Hc*0-1, F-0-1). On the basis of these results several sets of Dgr, Ma/Mc and F were selected that yielded good matches with the measured Sr-and Hd-concentrations and isotope ratios, and these sets of parameters were used to calculate the trace element compositions of the contaminated magmas. The bulk solid-liquid distribution coefficients for all trace elements were estimated from mineral-liquid partition coefficients compiled from the literature (Table 2.10), and from the relative proportions of the phenocrysts in the solid. The latter were taken from major element least-squares mass balance calculations (Huijsmans, unpublished work; Huijsmans and Barton, 1988) and are listed in Table 3.6. The AFC models that yielded the most satisfactory results for Sr- and Nd-isotope ratios and fourteen trace elements are listed in Table 3.7. The AFC trends are illustrated in terms of 143Nd/144Nd vs. 87Sr/86Sr and Rb vs. 87Sr/86Sr in Figs. 3.12 and 3.13. A comparison of the calculated and measured lava compositions (Table 3.7) shows that: 1) Calculated ranges of compatible element concentrations are quite large, which reflects the large range of reported mineral-liquid partition coefficients (Table 2.10). Clearly, as might be predicted, compatible elements are not very sensitive indicators of contamination. 2) AFC calculations with the xenolith as crustal end member yield poor fits between predicted and observed concentrations of incompatible elements (i.e. Rb, zr, Ba). Relatively high values of Ma/Mc (0.3-0.7) are needed because the xenolith contains little Sr (65 ppm). This xenolith is depleted in incompatible elements, and probably represents a restite that has already lost a partial melt. 3) Calculations with average crust and upper crust yield better results for incompatible trace elements at much lower values of Ma/Mc (0.05-0.3). However, the calculated concentrations of highly incompatible elements (Rb, Th, Ba) for SI-142 (MPI-II) are systematically too low, suggesting that the contaminant must have been enriched in these elements relative to upper crust. 4) Sr- isotope ratios are predicted within analytical uncertainty, but calculated Nd-isotope ratios are lower than the measured ratios if the xenolith is used in the calculations, or higher if total crust or average crust are used. Considering all available data we conclude that the calculations with upper crust yield the best overall match between predicted and analysed trace element concentrations and isotope ratios. Analogous AFC calculations were performed for the fourth cycle of Skaros, to determine whether the rhyodacite (SH-153) and the Upper Pumice (SH-100) could have evolved by AFC from a basaltic andesite magma (SI-22). The AFC models that yield the most satisfactory results for each of Ill the three crustal compositions are listed in Table 3.8. The AFC curves are illustrated for 143Nd/144Nd versus 87Sr/86Sr and for Rb versus 87Sr/86Sr in Figs. 3.12 and 3.13 respectively. The four points made above for MPI also apply to the AFC calculations for Skaros. However, in the case of Skaros the AFC model involving average crust yields the best match between observed and predicted trace element concentrations. Only minor assimilation is necessary as indicated by the low Ma/Mc value of 0.1, and this value is similar to that of Briqueu et al* (1986) who estimated that the isotopic compositions of lavas from Skaros and Nea Kameni are consistent with AFC at Ma/Mc-0.1-0.2. Nisvros The same crustal end members were used to calculate AFC-trends for the two sub-series on Misyros. The proportions of the crystallizing phases needed to calculate bulk solid-liquid distribution coefficients are listed in Table 3.6. The results of the calculations are reported in Table 3.9, and the AFC-trends are illustrated in terms of Sr- and Nd-isotope ratios and Rb in Figs. 3.14 and 3.15. Calculations with the xenolith yielded extremely poor results which have therefore been omitted from Table 3.9. Basaltic andesite N-24 was taken as the parent magma for the basaltic andesite/ andesite sub-series. Two samples, an amphibole-rich inclusion (N-14) and a cumulate rock of basaltic andesite conposition (N-21) have lower Sr- and higher Nd-isotope ratios than N-24, but unfortunately the compositions of these samples do not represent true nelt compositions. The compositions of the contaminated andesitic magma (N-3) calculated with average crust and upper crust closely match the analysed composition at Ma/Mc*0.15-0.3 (Table 3.9). Assimilation of average crust or upper crust by a dacitic magma (N-10) yields daughter magmas that are more enriched in incompatible elements than rhyodacite N-12. Therefore we performed additional calculations with average lower crust (Taylor and McLennan, 1981) which is depleted in LIL-elements relative to average crust and upper crust. The results (Table 3.9, Fig. 3.15) show that incompatible element concentrations agree much better with the concentrations measured in N-12, but are still slightly overpredicted, and calculated Nd-isotope ratios are again higher than the measured values. Xn the calculations wa assumed that the isotopic composition of the lower crust is identical to that of the xenolith (Si- 12 2X). This is almost certainly an inappropriate assumption, as the lower crust is believed to have relatively low sr-isotope ratios (e.g. Mcculloch et al., 1987). However, the isotopic composition of the lower crust is poorly known, and because there are very few isotopic data available on the metamorphic basement of the Cyclades it is impossible to obtain a more reasonable estimate of 113 the isotope ratios of the lower crust. In Chapter II it was suggested on the basis of trace element calculations and mineral chemical data that the dacites and rhyodacites on Nisyros evolved through coupled fractional crystallization and assimilation of lower crustal material at a pressure of approximately 8 kbar, and the present calculations are consistent with that hypothesis. Relation between mafic magmas from Skaros. MPI and Nlsvros The mafic lavas from Skaros plot on the low- 87Sr/86Sr, high-143Nd/144Nd extension of the trend defined by the lavas from HPI (Fig.3.2). On the basis of isotope ratios alone, it seems reasonable to suggest that the mafic lavas from HPI are related to those of Skaros by assimilation. The most primitive basalt on Skaros (SI-181) has considerably less radiogenic Sr- and more radiogenic Nd-isotopic compositions than the most primitive basalt on HPI (SI-140). The HPI basalt is characterized by higher incompatible element concentrations and lower Ni- and V-, but higher Cr-contents. However, the major element compositions of these two basalts are very similar, which implies that only minor crystallization could have occurred. Least-squares mass balance calculations for the step SI-181 — SI-140 indicate that only 6 wt.% of crystals were removed from Si-181, assuming that the two magmas are related by fractional crystallization only. AFC calculations for trace elements and isotope ratios indicate that much higher degrees of differentiation are needed: Fa p C-0.6-0'.8 for upper crust with Ha/Hc**0.1-0.2. This is the exact opposite of what one would expect; the F-values for AFC should always be higher than the F-values calculated for fractional crystallization, because material is added to the melt during assimilation. The similarity in major element compositions of SI-181 and SI-140 also precludes the possibility of an origin of SI-140 by magma mixing between SI-181 and one of the high 87Sr/86Sr, low 143Nd/144Nd magmas of Skaros or HPI. The isotopic and trace element differences between the mafic magmas of Skaros and HPI may be explained by one of the following models (Huijsmans and Barton, 1988): 1) the two basalts are ultimately derived from the same primitive magma by different degrees of contamination with a common crustal source; 2) they are derived from a common primitive magma by assimilation of different crustal compositions; 3) they inherited the different isotopic and trace element characteristics from a source region that was heterogeneous in space and/or time. It seems plausible that the extent and nature of the modification of the upper mantle by slab- derived fluids change with distance and time. Huijsmans et al.(1988) note that incompatible element concentrations in three eruptive centers on Santorini (HPI, Skaros, Nea Kameni) decrease with the age of the eruptive center. They 115 suggested that this could have been caused by continuous depletion of the upper mantle as a result of melt extraction. The Nisyros lavas are characterized by low-radiogenic Nd- and Sr-compositions relative to lavas of comparable bulk composition from Santorini, and most of the Nisyros lavas even plot below the field for oceanic basalts (Fig. 3.6). This implies that the source region below Nisyros is different from that beneath Santorini. The characteristics of the upper mantle source regions of Nisyros and Santorini will be discussed in detail in Chapter IV. SUMMARY AND CONCLUSIONS The new isotopic data for Santorini and Nisyros, together with previously reported whole-rock chemical and mineral chemical data show that magma evolution was complex and involved fractional crystallization, assimilation and magma mixing. To a large extent, the following model follows that proposed by Huijsmans et al. (1988) and Huijsmans and Barton (1988). The chemical and isotopic variations for MPI can be explained as-follows: An andesitic magma forms from a high- alumina basaltic magma by AFC involving upper crustal material. Injection of new basaltic magma and subsequent mixing cause the magma chamber to become thermally and chemically zoned. Shortly after the mixing event the 116 chamber Is tapped from the top and a series of lavas Is erupted. This period of volcanic activity is followed by a relatively quiet period, as indicated by the existence of a pyroclastic interval (erupted from a different, nearby center, see Huijsmans and Barton, 1988) with evidence of soil development on its upper surface. During this period another contaminated andesitic magma evolves from a basaltic magma by AFC, and the sequence of events described for the first cycle (underplating, mixing, eruption) is repeated* Magma mixing is also responsible for the unusual compositional trends in MPI-IIX, but in this case the mixing end members are hybrids themselves. The mafic lavas from the lower three compositional cycles of Skaros formed from hybrid magmas, but, in contrast with MPX, magma mixing on Skaros involved several end member compositions with slightly different chemical and isotopic characteristics. These batches of magma were not mutually related by AFC, but may have evolved from a common, more primitive magma by different degrees of crustal contamination, or, alternatively, they may have formed in a heterogeneous source region. The rhyodacite and Upper Pumice of the upper cycle of Skaros could have evolved by AFC from a basaltic andesite parent. The contaminant was similar in composition to average crust. The geochemical characteristics of the basaltic andesites and andesites on Nisyros are consistent with 117 differentiation by AFC involving average crust or upper crust. The dacites and rhyodacites are also related by AFC, but the crustal endmember was depleted in LIL-elements, and could have been of lower crustal origin. The AFC calculations for the lavas from Santorini and Nisyros clearly show that assimilation can generate trends within and parallel to the field for MORB and OXB. The fact that volcanics plot within this field cannot be used to conclude that assimilation was of minor importance. Assimilation played a major role in the evolution of the magmas from the southeastern Hellenic arc, but the most mafic lavas from Skaros, HPI and Nisyros are not mutually related by AFC processes. The different geochemical characteristics of these magmas seem to indicate that the upper mantle source region is heterogeneous in space or time, although it cannot be completely ruled out that these magmas are ultimately derived from a common, primitive magma which was contaminated to varying degrees during ascent through the crust. Table 3*1- SiO*, Sr and Nd concentration* and Sr and Nd isotope ratios in lavas 1 1 8 fro* Kikto Profitls Xlias (Santorini) Ssaple flow # SiO1 87Sr/86Sr t 2 “ Sr1 miMd/l^iM £ 2 o Hd* 5 SI-118 28 54.8 .70576 £ 6 228 m _ 23 SI-121 26 54.9 .70580 £ 6 229 .612542 t 9 24 St-122 25 55.1 .70561 ± 6 236 .512563 1 9 23 SI-126 24 54.6 .70509 1 6 262 .512663 ± 9 17 81-147 20 57.4 .70483 £ 6 237 .512693 ± 9 30 SI-145 18 58.3 .70479 £ 8 225 .512707 1 9 31 SI-142 IS 60.4 .70498 £ 5 210 .512704 ± 8 33 SI-140 14 51.2 .70457 £ 6 243 .512745 1 8 13 SI-137 12 52.9 .70489 1 5 256 .512696 ± 9 18 SI-135 10 54.6 .70502 £ 6 262 .512648 ± 9 19 SI-130 8 54.6 .70503 £ 5 265 .512659 t 9 19 M-10 5 56.2 .70506 £ 6 253 .512656 £ 9 19 SI-160 2 56.0 .70520 £ 6 248 .512626 ± 10 19 H-7 1 60.9 .70519 £ 4 238 .512624 £ 9 28 SI-122X1* Xenolith 67.1 .70810 ± 6 65 .512420 ± 9 ISlOj and Sr concantrations fro* Huijsnans (1985). JNd concantrations deteraincd by linear interpolation of the chondrite— nornalized concentrations for Ce and Sa. *Hd concentration assumed to be identical to that for SAM129 (netavolcanlc xenolith) from Briqueu et al. (1986). ''Partial analysis SI-122X: Zn - 10 ppm; Rb - 28 ppn; Zr - 148 ppm; Be - 47 ppm. Table 3.2. Sl02 , Sr and Hd concentration! and Sr and Nd isotope ratios in lavas 119 from Skaros (Santorini) Sanple flow # SiO1 •7Sr/B6Sr t 2 a Sr1 ^Nd/l^Nd £ 2 a Nd3 2 SK-100 30 70.6 .70513 £ 7 95 .512691 ± 9 28 SH-153 29 67.1 .70498 £ 6 136 .512722 t 8 37 SI-22 28 55.5 .70429 ± 6 186 .512816 i 9 18 SI-19 26 54.5 .70419 ± 5 190 .512808 £ 9 17 SI-106 22 52.1 .70419 £ 6 228 .512809 £ 9 12 SI-103 19 53.0 .70427 £ 4 227 .512804 £ 7 15 81-98 15 51.8 .70424 £ 5 238 .512813 £ 9 10 St-183 12 52.0 .70421 t 9 257 .512836 £ 9 8 St-182 11 52.8 .70414 t 6 212 .512805 £ 9 12 SI-181 10 51.2 .70424 £ 6 221 .512846 £ 8 7 SI-178 7 54.4 .70409 £ 5 221 .512792 £ 9 15 SI-173 4 54.7 .70405 £ 6 209 .512833 £ 8 16 SI-171 2 63.4 .70434 1 5 188 .512747 £ 9 28 *,3See footnotes Table I. Table 3.3. SiO*, Sr end Nd concentration! end Sr, Nd and 0 Isotope 120 ratios in lavas from Rlijrroi, Staple SIO1 B7Sr/8fiSr ± 2 o Sr2 l^tid/i^Nd t 2 o Hd2 J1B0(o/oo)7 9 R-21 56.1 .70353 ± 5 1194 .512790 i 9 17 5.7 N-14 55.7 .70357 ± 4 670 .512827 t 10 21 5.8 N-24 55.6 .70418 ± 6 561 .512705 t ’9 18 8.2 N-9 58.5 .70458 t 5 410 .512673 ± 9 24 6.3 N-8 58.7 .70460 ± 4 417 .512693 t 13 25 - N-ll 59.1 .70461 * 5 413 .512657 ± 9 22 7.1 N-19 59.2 .70423 t 3 438 .512726 t 9 27 6.9 N-6 59.6 .70457 ± 6 426 .512664 t 9 27 - H-3 59.9 .70461 t 6 421 .512645 1 10 29 7.3 N-10 66.3 .70397 t 6 337 .512706 t 6 26 6.6 N-20 66.8 .70414 t 4 337 .512707 ± 14 31 6.9 N-12 70.6 .70451 i 4 288 .512624 t 9 25 7.4 lSl02 and Sr concentrations from Uyera and Barton (1987) 2See footnote Table 1 *fron Hooft van Nuyaduynen (1985) Table 3.4. Reaulta of •inpie nixing calculation* for HPI HPI—I HPI-II MPI-ltl end aeibtri: Sl-140 and Sl-160 end member* : 81-140 and SI-142 end member*: SI-126 and SI-121 SI-137 SI-137 SI-135 SI-135 SI-147 SI-147 51-145 St-145 SI-122 St-122 calc oba calc oba calc oba calc oba calc oba SiO, 52.8 52.9 54.9 54.6 57.5 57.4 58.4 58.3 TiOj 0.89 1.0 0.93 0.92 1.1 1.4 1.2 1.3 At,Oj 18.9 18.0 19.0 20.2 16.7 16.3 16.S 16.2 FeO 7.9 9.3 8.0 7.5 7.9 9.4 8.0 9.0 MnO 0.16 0.19 0.16 0.15 0.16 0.19 0.16 0.20 H/A H/A H*0 4.4 4.8 3.8 2.6 2.7 2.7 2.4 2.5 (aee teat) CaO 9.6 9.5 8.9 9.1 6.4 6.5 6.0 6.0 ita2n 3.2 3.2 3.6 3.5 4.1 4.1 4.2 4.1 k 2o 0.97 0.89 1.2 1.1 1.9 1.6 2.1 2.1 P2°S 0.13 0.13 0.14 0.15 0.23 0.26 0.24 0.27 Sc 29 33 26 21 23 25 22 23 27 28 V 272 290 294 209 135 167 126 160 193 191 Cr 54 18 33 9 28 7 21 6 130 157 Hi 16 10 9 2 ---- 42 49 Zn 75 81 78 72 92 101 94 104 72 78 Rb 33 30 39 32 68 51 72 71 52 58 Sr 245 256 247 262 218 237 216 225 237 236 Zr 124 120 137 134 230 213 243 248 155 161 Ha 163 164 197 197 305 351 326 357 249 282 l,a - - -- 27 27 29 31 24 26 Ce -- — - 53 56 56 58 42 40 Nd 16 18 17 19 28 30 30 31 22 23 Hf 3.0 3.0 3.4 3.5 6.1 6.2 6.4 6.1 4.2 4.3 Th 4.4 4.4 5.2 5.6 11 10 12 12 9.2 10.3 87Sr/M Sr 0.70486 0.70489 0,70502 0.70503 0.70486 0.70483 0.70490 0.70479 0.70560 0.70561 l'3Nd/l" N d 0.512680 0.512696 0.512652 0.512 659 0.512709 0.512693 0.512707 0.512707 0.512566 0.512563 Z mafic . end member S4Z 28Z 24Z 16X 2SZ 121 Table 3.5* Keiulct of staple alxing celculetions for Skero* 1 2 2 end aeabers: SI-181 end S1-173 SI-106 SI-106 SI-182 SI-182 calc ob» calc obe SLOj 52.1 52.1 52.8 52.8 Ttoa 0.80 0.99 0.82 0.82 A120j 18.2 18.3 18.2 18.0 FeO 8.2 8.6 8.2 8.2 HnO 0.17 0.17 0.17 0.18 MgO 6.3 5.5 6.3 5.9 CeO 10.6 10.7 10.5 10.8 H*20 2.5 2.9 2.6 2.6 K20 0.56 0.56 0.63 0.66 P2°5 0.10 0.11 0.11 0.10 Sc 36 37 35 37 V 261 298 255 265 Cr 115 95 130 79 Ni 46 27 50 31 Zn 70 70 70 68 Rb 18 16 20 22 Sr 217 228 216 212 Zr 90 82 96 93 Be 91 105 100 108 U 8 8 9 8 Ce 16 19 17 20 Hd 10 12 11 12 Rf 2.5 2.3 2.6 2.3 lb 2.8 1.8 3.3 3.4 B7gr/B6Sr 0.70416 0.70417 0.70415 0.70412 i»*Nd/ m t ,M 0.512839 0.512609 0.512838 0.612805 X aaftc and oaaber 682 6o: Table 3.6. Proportion* of phenocrjrat phaaea in fractionating aolld aaaemblage OL Cpx Opt Flag Hag 11m Ap Zr SI-160 9.3 24.5 62.3 3.7 1.2 0.19 . Sl-142 9.6 21.5 - 64.8 3.8 - 0.24 - SH-153 - 29.6 12.6 50.3 7.1 - 0.50 SH-100 - 25.5 14.4 51.3 7.8 - 1.0 - N-3 2.8 27.0 - 67.1 2.9 - 0.24 0.012 N-12 tm 12.2 3.8 74.2 8.3 1.2 .30 0.012 Table 3.7. Reaulte of AFC calculation* for Nlkro Profit!* Itiaa. SI-HO •» SI-160 (HPI-1) SI-140 S1-1A2 (MPI-H) SI-160ob. S I - 1 6 0 „ ,C S I - 1 6 0 „ ,C SI-160« l c SI_1*2oba a2-*«c.l« S1-,42c*le SI-,42c.le Contoeinant: SI-122X average upper SI-122X average upper cruet cruat cruat cruat Sc 23 . 4—46 6-38 19 1-63 1-61 V 318 - 3-298 3-255 102 Cr - 1-379 1-371 11 1-76 1-77 7 1-81 Hi 2 - 1-81 1-21 2-22 1-19 1-18 Zn 81 5-75 - 24-88 Rb 99 4-111 - 6-143 46 46-50 40-43 43-45 83 55-62 Sr 58-66 64-72 248 220 262 255 210 213 Zr 204 181 151 220-239 167-178 159-165 273 257-287 251-283 Da 235 263-296 118-172 178-229 204-237 373 154-251 203-325 228-359 Ca - - 18-20 17-18 33 Ce - 25-29 26-30 - 33-41 33-38 63 41-62 Nd 19 43-64 32 23 20 33 36 34 35 Hf - 3.8 4 .0 -4 .3 3 .7 -3 .9 7.3 5.9-6.8 6.1-7.0 Th 6.0 5 .8 -5 .9 5 .5 -5 .6 14 8.7-9.3 9.3-9.8 87Srm Sr 0.70520 0.70518 0.70522 143M m i i M 0.70513 0.70498 0.70490 0.70490 0.70408 0.512626 0.512577 0.512710 0.512688 0.512704 0.512646 0.512733 0.512725 AFC paroeetera Ka/Mc AFC paroeetera 0.70 0.20 0.30 0.30 0.30 0.05 0.05 F 0.75 0.60 0.75 0.45 0.45 0.35 0.33 Sr 0.8 1.0 1.0 0.9 0.9 1.2 1.3 DSK 0.11 0.11 0.11 0.13 M 0.13 0.13 0.13 124 Table 3.8. Results of AFC calculations for Skaros. Sl-22 •» SH-153 (SKA-1 V) 31-220 ♦ sn-100 (SKA-1V) s»-1530b. SH-153„lc SH-153c.Ic Sl-153„ic sn-iooob, sn-iooc-lc sh-ioom 1 c SH-iooc a U I —— Contaainant SI-122X average upper — SI-122X average upper crust cruet cruat crust Sc 11 1-39 1-38 9 1-41 1-39 V- - 1-248 1-220 15 - 1-230 1-220 Cr 5 - 1-66 1-78 2 - 1-99 1-97 Hi 1 - 1-16 1-14 1 - 1-16 1-16 Zn 71 1-86 - 2-112 46 2-73 - 4-102 Rb 93 90-101 92-102 112-126 95 86-97 92-102 100-111 Sr 136 n o 137 140 95 95 100 97 Zr 290 299-362 281-335 327-397 273 293-358 281-338 280-356 Ba 453 227-350 288-409 356-522 490 211-326 2B8-409 326-457 La 35 - 26-31 29-36 28 - 22-29 24-31 Ce 69 - 43-69 48-82 50 - 34-65 37-68 Nd 37 37 31 36 2B 33 28 30 Hf B.3 - 8-10 9.5-12 6.0 - 8.4-10 8.8-11 Th 16 — 17-19 19-22 15 - 16-18 17-19 87Sr/BSSr 0.70498 0.70489 0.70506 0.70505 0.70513 0.70513 0.70519 0.70510 ^Nd/^Hd 0.512722 0.512697 0.512791 0.512774 0.512691 0.512664 0.512790 0.512776 AFC paraaeters: AFC paraatera: Ha/Hc 0.35 0.10 0.10 0.45 0.10 0.1 F 0.50 0.45 0.40 0.55 0.45 0.45 d |/Srl 1.3 1.5 1.4 1.4 1.9 l.»' Ds/i* 0.21 0.21 0.21 0.33 0.33 0.33 Hd * Table 3.9. Resulta of AFC calculation* for Nlsyroa H-24 ♦ lt-3 H-10 ♦ H-12 "-3ob. W « l c M c.le " - ^ b a *-l*c.lc **"^7cale «*-12calc ContuIninC: — average upper - lover average upper cruat cruat cruat cruat crust Sc 19 2-36 4-28 4 8-11 7-11 V 5-8 180 6-303 13-242 29 28-78 22-73 Iti 16-61 7 1-33 2-25 3 3-8 2-7 2-6 Zn 82 — 19-101 38 — Rb 35-56 61 60-67 59-63 92 95-97 57-95 108-111 Sr 421 398 438 288 311 293 289 Zr 209 192-272 192-235 219 236-259 245-271 259-286 Ca 2.6 2.7 2.4 3.3 3.6* 3.9 4.1 11a 515 353-510 364-449 711 723-793 749-830 La 786-869 32 30-34 27-29 38 38-39 39-41 41-42 Ce 57 50-69 47-57 55 58-63 60-65 62-68 Nd 29 35 29 23 28 30 31 Sa 5.1 4.5-7.7 4.4-6.0 24 3.9-4.4 3.9-4.5 4.0—6.6 Ru 1.4 0.9-2. 3 0.9-1.7 0.7 0.9-1.2 0.9-1.2 0.9-1.2 Yb 2.6 2.8-4.8 2.6-3.6 1.7 2.4-2.6 2.4-2.7 2.4-2.7 Ilf 4.9 5.3-5.8 4.8-5.1 4.9 5.2-5.4 5.4-5.7 5.8-6.0 Th 7.2 6.9-7.2 6.3-6.5 10 10-11 11-12 12 87Sr/B6Sr 0.70461 0.70457 0.70458 0.70451 0.70449 0.70449 0.70441 0.512645 0.512683 0.512659 0.512624 0.512694 0.512689 0.512680 AFC parameters AFC paraaeters Ha/Me 0.15 0.30 0.55 0.50 0.30 r 0.50 0.70 0.90 e It 0.90 0.90 “Sro5'“ ' 1.4 1.4 1.8 1.8 1.8 tjS/L 126 nMd 0.14 0.14 0.14 0.14 0.14 127 Di Fo+L Fo SiO Fig.3.1. The system Fo-Di-SiC>2. If a magma at b has Si02 added to it, it will leave the field boundary, precipitate Di, and move back towards the field boundary. The actual path followed by the magma will lie somewhere between b-c and b-d-e. 128 .6129 ■ MPI + typical T 0° ° ▼ Nisyros uncertainty O Skaros .5128 & 8 T ■o z o V ▼ If o "V .5127 ▼ u ▼ Z CO rf * ▼ ▼ .5126 ■ ■ .5125 .703 .704 .705 .706 87Sr/86Sr Fig.3.2. 143Nd/144Nd versus 87Sr/86Sr for lavas from Santorini and Nisyros. flow# 30 35 20 25 10 15 74 .08 75 .06 76 5 5 5 5 5 60 62 0 6 58 56 54 52 50 .7060 .7056 .7052 .7048 .7044 0 5 - ■ typical— height (relative age) for lavas from MPI. Horizontal lines from Horizontal (relative forlavasMPI. age) height represent pyroclastic intervals between the cycles. the intervals between pyroclastic represent Fig.3.3. 87Sr/86Sr and SiO, as a function of stratigraphic of function asa and SiO, 87Sr/86Sr Fig.3.3. ■ II ■ ■ ■ ■ 7 8S Si 2 i0 S /86Sr r 87S ■ uncertainty ■ ■ ■ ■ ■ - . yia uncertainty typical . - hi i i _i i i i i ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ----- ■ ■ SKAROS 35 ■ * typical uncertainty typical uncertainty ■ ■ 3 0 ■ ■ ■ ■ ■ ■ 25 ■ ■ 20 • ■ ■ 15 ■ -■ ■ ■ . - ■ - B ------■ 10 1 ■ ■ 5 ■ ■ ■ ■ 0 ------1. .— .. ...i------1------i------i ------1------1---- — .7040 .7043 .7046 .7049 .7052 51 55 59 63 67 71 87Sr/86Sr Si02 Fig.3.4. 87Sr/86Sr and SiO, as a function of stratigraphic height (relative age) for lavas from Skaros. Horizontal lines represent pyroclastic intervals between the cycles. The lowermost flow (#2) does not belong to cycle I (see text). 131 NISYROS .7048 typical uncertainty .7046 . 4 - rfV V .7044 Wkm ■ £ -7042 - ■ oo T of0 .7040 00 T .7038 ■ Bas. And,-And. .7036 ▼ Dac.-Rhyodac. ■ ■ .7034 — 1— _1------1 — i----- 1------1------1------1--- 8.5 typical uncertainty —j— 8.0 7.5 ■ O ■ ® 7.0 to 6.5 6.0 5.5 54 56 58 60 62 64 66 68 70 72 Si02 Fig.3.5. Variations in 87Sr/86Sr and 180/160 as a function of Si02 for lavas from Nisyros. 132 .5136 MORB rv Marianas S. Sandwich Isl. L. Antilles .5130 New Britain Japan OIB •o z Banda co Sunda .5124 ■ MPI T Nisyros O Skaros .6118 .701 .703 .705 .707 .709 Fig.3.6. Comparison of Sr-Nd isotopic composition of lavas from Santorini and .Nisyros with isotopic compositions of lavas from other island arcs. Fields for MORB and OIB from Zindler et al. (1984). Isotopic data for island arc volcanics from DePaolo and Johnson (1979), DePaolo and Wasserburg (1977), Stern (1981), Nohda and Hasserburg (1981), Hawkesworth et al.(1977, 1979), Cohen and O'Nions (1982), Whitford et al.(1981), Von Drach et al.(1986), Morris and Hart (1983), Mcculloch and Perfit (1981), White and Patchett (1984), Dixon and Stern (1983) and Davidson (1985). 1 3 3 .5136 MORB v\ \ _N_ \ f \ ^>Ecuador .5130 - Patagonla"*7^ * ^ * T3 Z V OIB S. Chile rV. •o Z N. Chile P) Mexico .5124 - 'Peru ■ MPI ▼ Nisyros O Skaros .5118 .701 .703 .705 .707 .709 .711 87S r/8 6 Sr Fig.3.7. comparison of Sr-Nd isotopic composition of lavas fron Santorini and Nisyros with isotopic compositions of lavas from active continental margins. Isotopic data from Francis et al.(1977), Frey et al.(l984), Hawkesworth et al.(1979), James (1982) and Verma (1983). lavas from MPI. Sample numbers are referred to in text.totne arereferred numbers from Sample MPI.lavas (87Sr/86Sr for SiO,) versus trends Simple mixing rig.3.8. oo Sr/ Sr 74 * .7048 74 L .7044 75 - . - .7052 .7060 75 - .7056 50 140 2 4 6 8 0 62 60 58 56 54 52 uncertainty typical 126 121 MPI 160 Cce tl Cycle ▼ yl I Cycle ■ yl 111 Cycle I 142 134 MPI .51278 140 typical uncertainty— ^ .51271 142 XI 126 Z ^ .51264 160 "O z n ■ Cycle I ▼ Cycle II 121 Cycle III .51250 .7045 .7048 .7051 .7054 .7057 .7060 87S r/8 6 Sr Fig.3.9. Simple mixing trends (143Nd/144Nd versus 87Sr/86Sr) for lavas from MPI. Sample numbers are referred 135 to in the text. Sr/ Sr .7042 .7044 00 1 * 1 1 1 7040 .7046 .7052 .7048 .7050 Fig.3.10. 87Sr/86Sr versus Si02 for lavas froralavas for Si02 87Sr/86Sr versus Fig.3.10. Skaros. 1 5 9 3 7 71 67 63 59 55 51 • . V - - 8 A t f A 181 T - uncertainty j- | typical typical | "■1173 22 i ------Skaros Cce II Cycle ▼ yl IV Cycle • yl 1 Cycle ■ sio2 yl III Cycle ------t ----- ■ 153 • 1 100 # 136 137 Skaros .51288 181 typical uncertainty | 173 ■ . 22 .51282 •o ■ z .51276 ■o z ■ c*> ■ Cycle 1 153 • T Cycle II .51270 ^ Cycle III 100 • 0 Cycle IV .51264 —i i------i------.7040 .7043 .7046 .7049 .7052 87S r /86Sr Fig.3.11. 143Nd/144Nd versus 87Sr/86Sr for lavas from Skaros. 138 .51288 22 typical uncertainty— ^ , .51278 A.C. r*=0.2 z 140 U.C. r=0.1 163 A.C. r=0.05 TJ U.C. r=0.05 z 1 4 2 CO ^ - A.C. r«0.2 r- .51268 X r=0.35 ■ Skaros X r=0.45 T MPI X r=0.7 .51258 .7042 .7046.7050 .7054 87S r /88Sr Fig.3.12. AFC trends (143Nd/144Nd versus 87Sr/86Sr) for Santorini calculated with three different contaminants: average cruBt (A.C.), upper crust (U.C.) and xenolith SI- 12 2X (X). r is the ratio Ma/Mc, V 139 160 U.C. r=0.1 typical uncertainty X r=0.35 120 U.C. r=0.1 ■ Skaros A.C. r=0.1 ▼ MPI „X r=0.45 *~A.C. r=0.1 £ 80 ^ i iU.C. r r=0.05 A.C. r=0.05 X r=0.7 160U.C. r=0.3 40 A.C. r=0.2 .7042 .7046 .7050 .7054 87S r /B6Sr Fig.3.13. AFC trends (Rb versus 87Sr/86Sr) for Santorini. Curve labels and symbols as in Fig.3.12. NISYROS .51274 typical uncertainty "10 24 •5127T r-0 .5 5 r=0.50 r=0.15 .51267 r=0.50 r=0.30 ■ .51264 Bas. And.-And. 12 'Dac.-Rhyodac. T .51260 J. _L .7035 .7041 .7044 .7047 87-S r/ ,8 6 - Sr Fig.3.14. AFC trends (143Nd/144Nd versus 87Sr/86Sr) for Nisyros. L.C. refers to lower crust; other curve labels as in Fig.3.12. 141 NISYROS 120 U.C. r=0.S0 typical uncertainty AX. r=0.50 L.C. r=0.55 90 A.C. r=0.15 60 U.C. r=0.30 24 30 ■ Bas. And,-And. ▼ Dac.-Rhyodac. 0 0.7038 .7041 .7044 .7047 Fig.3.15. AFC trends (Rb versus 87Sr/86Sr) for Nisyros. Curve labels as in Figs.3.12 and 3.14. CHAPTER XV THE NATURE OF THE SOURCE REGION OF ISLAND ARC MAGMAS FROM SANTORINI AND NISYROS INTRODUCTION The composition of the source regions of magmas from island arcs and active continental margins is a matter of controversy. The source materials that have been suggested for island arc magmas include subducted oceanic lithosphere (altered to varying degrees), subducted sediments, the upper mantle wedge, and the lower crust (see Gill, 1981, for a detailed overview of current and older ideas). Most models for the origin of island arc magmatism fall into two groups: those involving melting within the subducted slab and those involving melting within the upper mantle wedge. Some workers have proposed that arc magmas form by partial melting of subducfced oceanic crust with some sediment (e.g. Marsh and Carmichael, 1974; Marsh, 1982; von Drach et al., 1986). Large degrees of melting are necessary to totally consume garnet, since REE patterns in high- alumina basalts do not show any significant enrichment in LREE (Gill, 1974; Johnston, 1986). Brophy and Marsh (1986) proposed that large degrees of melting can be attained during diapiric ascent of a crystal mush from the subducted 142 143 slab. The diapir continues to melt until, at 15-20 km above the slab, the melt separates from the solid and ascends to the surface. During ascent the mush or melt may partially equilibrate with surrounding upper mantle material. Many others favour an origin of island arc magmas by partial melting in the mantle wedge overlying the subducting lithosphere (Nicholls and Ringwood, 1973; McCulloch and Perfit, 1981; Gill, 1984). The mantle wedge is chemically modified (and its solidus lowered), prior to melt generation, by a fluid or a partial melt derived from the subducted slab. Gill (1981) has compiled data for lavas from many island arcs and active continental margins and discusses the advantages and disadvantages of the two models mentioned above. He concludes that formation of magmas by partial melting of slab-modified upper mantle material is most consistent with the available data. However, it is very difficult to argue against a model such as that proposed by Brophy and Marsh (1986). The composition of the mantle wedge prior to modification by a subduction component is also much debated. Pearce (1983) suggests that the mantle beneath island arcs is initially strongly depleted and similar to a MORB-type source. However, Morris and Hart (1983) noted that the trace element ratios of many arc lavas are more similar to ocean island basalts than to MORB, and Gill 144 (1984) suggested that both MORB and OIB sources are Involved In the genesis of island arc magmas. In this chapter, the geochemistry of primitive lavas from Santorini and Nisyros are used to investigate the source regions of magmas from the Hellenic arc. The magmas erupted on Santorini consist of tholeiitic basalts, calc- alkaline andesites, dacites and rhyodacites (Huijsmans et al., 1988). The lava series from Nisyros (basaltic andesite through rhyodacite) are entirely calc-alkaline (Chapter II). It has bean suggested that the crust in the Aegean area is continental on both sides of the trench system (Makris, 1977) which indicates that either the subduction process has ceased since no oceanic lithosphere is present anymore, or that continental crust is being subducted. A large amount of continent-derived sediment is present in the eastern Mediterranean (6-10 km, Biju-Duval et al., 1978), although most of this sediment is accumulating south of the Mediterranean Ridge (see also Le Pichon and Angelier, 1979, 1981). This tectonic environment makes the Hellenic arc an ideal location to study the influence of subduction -of continental material on the composition of island arc magmas. TRACE ELEMENT CHARACTERISTICS OF THE PRIMITIVE MAGMAS Before compositions of lavas can be used to estimate the composition of the source region a correction has to be made for. changes in magma composition after melt segregation as a result of processes such as fractional crystallization. In order to correct for fractionation one has to assume that the primary magma was in equilibrium with peridotite. A magma in equilibrium with upper mantle peridotite should satisfy the following requirements: Mg/Mg+Fe>0.68, Ni>200 ppm and Cr>600 ppm. The fractionation correction is then carried out by adding olivine, or olivine and Cr-spinel, to primitive lava compositions until the magma has the appropriate composition to be in equilibrium with upper mantle peridotite (see Wright et al., 1975; Nicholls and Whitford, 1976; Feigenson et al., 1983). Problems may arise when clinopyroxene has been removed from a magma in addition to olivine and Cr-spinel. It is easily shown that an attempt to calculate the primary magma composition on the basis of the above listed criteria by addition of olivine, clinopyroxene and Cr-spinel to a specific lava composition will result in three equations with four unknowns, and hence the primary magma composition cannot be uniquely estimated. Furthermore, lava compositions that are used to calculate the composition of the primary magma may have been affected by assimilation or magma mixing, and must be corrected for the effects of these processes. The lava compositions that will be used here as probes of the upper mantle source region of the Hellenic arc are two basalts from Santorini (samples SH-32 from C. Balos and SI-181 from Skaros) and two basaltic andesites from Hisyros (samples N-21 and N-24). These four samples were selected because they represent the most primitive lava compositions known from Santorini and Nisyros, and because they contain no petrographic or mineral chemical evidence for magma mixing. The major element compositions of these lavas have been given by Huijsmans et al. (1988) and in Appendix A. The concentrations of incompatible trace elements and the isotopic compositions are listed in Table 4.1. The lavas are characterized by low ®7Sr/®6Sr and high 143Nd/144Nd relative to other lavas from these islands. Oxygen isotope analyses (A.J. de Jong, pers.comm. 1985) for two of these lavas yielded values characteristic of the upper mantle (SI-181"5.6°/oo and N-21«5.7°/oo). Sample N-24 from Nisyros has a higher oxygen isotope ratio, but this is caused by low-temperature sea water alteration (Chapter II). It will be assumed that contamination of the magmas with upper crustal material has not significantly changed the compositions of the primitive magmas. Possible effects of assimilation of lower crustal material on the composition of the magmas are discussed in a later section (see below). None of the primitive lavas of Santorini and Nisyros represent primary magma compositions, and therefore a correction for fractional crystallization is needed. However, prior to eruption four phenocryst phases have been removed from these magmas: olivine, clinopyroxene, Cr- spinel and plagioclase (see below). Nevertheless, Nicholls (1978) reported a primary magma composition for Santorini that was obtained by addition of olivine (6-13 wt*%) to the most primitive lavas, and by adjustment of the Cr content through addition of small amounts of clinopyroxene and Cr-spinel. Nicholls1 procedure is in principle incorrect, because his calculations do not take into account the simultaneous crystallization of the three phases. Furthermore, the Santorini basalts contain small negative Eu-anomalies, which indicate that plagioclase was also removed from the magmas. However, we will use his results to make the assumption that the primitive lavas on Santorini have lost a total of about 10 wt.% of solid phases. This indicates that the concentrations of incompatible elements in the primary magma must have been about 10% lower than the measured concentrations in the Santorini basalts. The correction for fractionation is small and is well within analytical uncertainty, and therefore it can be assumed that the concentrations of incompatible trace elements in the primary magma for Santorini were similar to the concentrations that have been measured in the primitive Santorini basalts. The same four phenocryst phases (i.e. olivine, clinopyroxene, cr-spinel and plagioclase) were removed from the primary magmas beneath Nisyros. However, the most 148 primitive magma compositions on Nisyros are considerably more evolved than on Santorini, and a much larger » correction is needed for fractional crystallization. Application of Nicholls1 procedure to the Nisyros magmas will result in large errors in the estimate of the total amount of solid phases lost from the primary magma, and therefore we cannot estimate the concentrations of incompatible elements in the primary magma with any confidence. The Incompatible trace element concentrations for the primitive lavas of Santorini and Nisyros are normalised to average MORB following Pearce (1983) and the results are plotted in Fig. 4.1. The trace element patterns for Santorini and Nisyros are similar, but the lavas from Nisyros are enriched in incompatible elements relative to the lavas from Santorini due to a larger extent of fractional crystallization in the Nisyros magmas. The lavas are enriched in K, Rb, Ba and Th relative to HORB, but depleted in Ti, Y and Yb, which is typical for lavas erupted in an arc environment. The MORB-normalised trace element patterns for Santorini reveal two interesting features: 1) The concentrations of incompatible trace elements are extremely low when compared to other calc-alkaline lavas from island arcs and continental margins (see Ewart, 1982: Pearce, 1983). The depleted character of the Santorini 149 basalts has bean noted previously by Huijsmans et al. (1988). 2) The patterns are characterized by negative Nb anomalies, but the other High Field Strength Elements (HFSE) do not show significant depletion relative to adjacent elements (Fig. 4.1). However, calc-alkaline lavas from other localities are characterized by pronounced, negative, HFSE anomalies (Pearce, 1983). Both the low concentrations of incompatible elements and the lack of strong HFSE-depletion are characteristics shared by island arc tholeites. Similar incompatible element depletion has been observed in tholeiitic lavas from e.g. the Scotia arc, St.Vincent (L. Antilles), Tonga, the Marianas, the Kermadec arc and New Britain (Pearce, 1983; Meijer, 1982; Ewart et al., 1977; Basaltic Volcanism Study Project, 1981), but the significance of this has not, to date, been adequately explored. ZSOTOPIC CHARACTERISTICS OF THE PRIMITIVE MAGMAS The Nd-Sr isotopic composition of the lavas from Santorini and Nisyros is illustrated in Fig. 4.2. The isotopic data define two sub-parallel arrays (see Chapter III). The lavas from Santorini plot within the field for ocean island basalts (OIB), but near its left boundary, whereas the Nisyros lavas plot below this field. The primitive lavas from Nisyros are characterized by lower 87Sr/86Sr and slightly lower 143Nd/144Nd relative to the lavas from Santorini* Also shown on Pig. 4.2 are the various upper mantle components identified by Zindler and Hart (1986). These are depleted mantle material (DMM A, B), enriched mantle components (EM I,II), bulk silicate earth (BSE), high U/Pb material (HIMU) and prevalent mantle (PREMA). Lower custal material is similar in isotopic composition to EM-I, and continental upper crust and pelagic sediments are similar to EM-II. Zindler and Hart (1986) have suggested that PREMA is a discrete mantle component because of the large number of measured isotope ratios that plot within or near this field. PREMA includes the isotopic signature of lavas from Hawaii and Iceland as well as those of lavas from several intra-oceanic island arcs and continental suites (Zindler and Hart, op.cit.). The Sr- and Nd-isotopic trends for Santorini and Nisyros extend between PREMA and EM-II, which is also the case for lavas from other active continental margins (see Chapter III). Gulen et al. (1987) report Pb-isotope ratios for the Hellenic arc lavas and the total range in 207Pb/204Pb and 206Pb/204Pb for these lavas is shown in Fig. 4.3. The field in Fig. 4.3 includes lavas from Santorini, Nisyros, Aegina, Methana, Milos and Antiparos and falls to the right of the Geochron, above the Northern Hemisphere Reference Line from Hart (1984) and also between PREMA and EM-II (or pelagic sediment). Most island arcs 151 show a relative enrichment in 207Pb (e.g. S.Sandwich islands, Barreiro, 1983; Tonga, Sun, 1980) which is often attributed to subduction of sediments and involvement of these sediments in the generation of island arc magmas. However, some arcs lack this distinct Pb-isotopic signature (e.g the Aleutians, Morris and Hart, 1983; the Marianas, Meijer, 1976), which suggests that the contribution from sediments to these island arc magmas must have been small. DISCUSSION In the previous sections it was shown that the primitive magmas for Santorini and Nisyros are characterized by unusually low incompatible trace element concentrations relative to other calc-alkaline magmas, and by low 87Sr/86Sr and/or low 143Nd/144Nd relative to OIB. The composition of island arc magmas is a function of 1) the composition of the upper mantle wedge; 2) the magnitude and the nature of the contribution from the subducted lithosphere; and 3) the effects of intra-crustal processes such as assimilation, fractional crystallization and magma mixing. It has been shown earlier that the primitive lavas of Santorini and Nisyros do not contain any evidence of magma mixing, and that the composition of the Santorini basalts is only slightly modified by fractional crystallization. In the following sections the unusual trace element and isotopic character of the primitive 152 Hellenic arc magmas will be examined in terms of assimilation of lower crust, subduction of crustal material, and processes in the upper mantle wedge prior to melt generation. Assimilation at the base of the crust It is possible that the depleted character of the magmas of the Hellenic arc is not inherited from the upper mantle source region, but is caused by assimilation of lower crustal material which is depleted in incompatible elements (Taylor and McLennan, 1981). Assimilation of lower crustal material by magmas ponding at the base of the crust has been suggested previously for e.g. the evolved magma series of Nisyros (see Chapters II and III) and for the Cascades (W.P. Leeman, pers.comm. 1987). Involvement of the lower crust in the evolution of the mafic magmas of Santorini and Nisyros could in principle account for the anomalous isotopic signature of these magmas, since granulites are characterized by relatively low 87Sr/86Sr and 143Nd/144Nd (e.g. McCulloch et al., 1987). However, the oxygen isotope ratios in the primitive lavas of Santorini and Nisyros (see above) are similar to upper mantle values, and do not indicate substantial interaction of these magmas with crustal material. The oxygen isotopic compositions reported for crustal rocks are highly variable (e.g.Hoefs, 1980) and it is possible (in principle) that the lower crust beneath the Hellenic arc has values similar to the upper mantle (e.g. 5.5-6 °/oo), but this would be highly coincidental. The primitive Santorini magmas are characterized by very low Sr concentrations (221-236 ppm, see Table 4.1), whereas the lower crust has much higher concentrations of Sr (425 ppm, see Taylor and McLennan, 1981). Hence a significant contribution from the lower crust to the primitive magmas of Santorini is unlikely on the basis of the low Sr concentrations of the magmas. Furthermore, the Hellenic arc lavas have high 207pb/204Pb and 206Pb/204Pb ratios (Gulen et al., 1987), whereas Proterozoic granulites generally have less radiogenic Pb isotope ratioB that fall to the left of the Geochron in Fig. 4.3 (e.g. Zindler and Hart, 1986). We conclude therefore that the low concentrations of incompatible elements in the primitive magmas of the Hellenic arc are not a result of assimilation of lower crust, but that the magmas were depleted in incompatible elements prior to ascent. The_role_of subducted lithoBPhere The enrichment of island arc magmas in Sr, K, Kb, Ba and Th relative to MORB is commonly attributed to the addition of a subduction component (altered oceanic lithosphere or sediments) to the upper mantle source region. Another characteristic of island arc volcanics is the depletion of the magmas in HFSE relative to other incompatible trace elements* It has been suggested that HFSE depletion of island arc magmas results from retention of these elements by a Ti-rich mineral phase such as rutile or perovskite within the slab or in the upper mantle wedge (e.g. Morris and Hart, 1983). However, Watson and Ryerson (1986) have shown that basaltic and andesitic liquids in equilibrium with a Ti-rich phase such as rutile must have very high Ti02-concentrations (5-9 wt.%). Such high Tio2 concentrations exceed concentrations measured in island arc magmas, so HFSE depletion cannot be attributed to melts in equilibrium with residual rutile or perovskite. The occurrence of negative HFSE anomalies in island arc magmas could be due to either 1) retention of HFSE in rutile during dehydration of the subducted slab; or 2) an upper mantle source that was depleted in HFSE prior to partial melting. Salters and Shimizu (1987) analysed the trace element compositions of spinel lherzolites from several localities and found that all samples show relative depletion in Ti and Zr. This suggests that HFSE depletion is not necessarily a result of slab-mantle interaction, but that it is a feature characteristic for parts of the upper mantle (including sub-arc mantle). Melts in equilibrium with perovskite in the lower mantle will be depleted in HFSE. Salters and Shimizu (op.cit) propose that migration of melts from the lower mantle to shallower levels may 155 have caused HFSE depletion in parts of the upper mantle. The concentrations of incompatible trace elements in the primitive lavas from Santorini and Nisyros are very low in comparison with other calc-alkaline lavas, but similar to concentrations measured in island arc tholeiites. Furthermore, MORB-normalised trace element plots do not show pronounced negative HFSE anomalies. Apparently the modification of the upper mantle source region of the Hellenic arc by slab-derived fluids was of a limited extent or of a different nature in comparison with sources for other calc-alkaline magmas. The depleted character of the primitive magmas of the Hellenic arc is unexpected, because as noted previously a large amount of continental material is available for subduction in the eastern Mediterranean. It was shown above that the low concentrations of incompatible trace elements in the primitive magmas of the Hellenic arc are not a result of assimilation of lower crustal material. Alternatively, the depleted character and the anomalous iBotopic composition of the magmas could be caused by subduction of sediments derived from lower crustal material. A nearby source for such sediments is provided by the granulites of the African shield. However, most of the arguments used earlier to reject the possibility of lower crustal assimilation also apply to contamination of the upper mantle source with lower crustal 156 material. Also, the Sr-isotope ratios of granulite-derived sediments would have increased by interaction with sea water during transport from the African shield to the Hellenic trench system, and therefore subduction of granulitic sediments cannot account for the low sr and Nd isotope ratios observed in the primitive magmas of the Hellenic arc. Independent evidence against the subduction of abundant sediments beneath the Hellenic arc is provided by the very low B concentrations of these magmas' (W.P. Leeman, pers. comm. 1987). The concentrations of B are low in fresh MORB ( The elevated and variable 207Pb/204Pb and 206Pb/204Pb of the Hellenic arc lavas (Gulen et al., 1987) still require explanation* High 207Pb/204Pb ratios in island arc magmas have often been attributed to subduction of pelagic sediments (see above), but the low B concentrations indicate that the elevated 207Pb/204Pb ratios in the lavas of the Hellenic arc are not a result of sediment subduction. In Chapter III it was shown that the trends towards increasing 87Sr/86Sr and. decreasing 143Nd/144Nd in the more evolved lavas of Santorini and Nisyros (Fig. 4.2) can be generated by assimilation of continental crust. This suggest that the variations in Pb-isotope ratios (Fig. 4.3) are probably also a result of crustal contamination, since the upper crust is characterized by high 207Pb/204Pb ratios relative to the upper mantle. The composition of the sub-arc upper mantle in terms of 143Nd/144Nd versus 87sr/86sr the primitive lavas of Santorini and Nisyros plot near the left Bide of the field for ocean island basalts (Fig. 4.2), i.e. relatively displaced towards EM-I (or lower continental crust), although 206Pb/204Pb of the lavas (Gulen et al., 19B7) is considerably higher than that of EM-X (Zindler and Hart, 1986). It was shown earlier that this unusual isotopic signature is not caused by assimilation or subduction of lower crustal material. Also, modification of the upper mantle wedge by a dehydration fluid derived from altered oceanic crust will not produce the observed isotopic compositions, because hydrothermal alteration tends to increase 87Sr/86Sr in oceanic crust while leaving 143Nd/144Nd unaffected. Therefore it is concluded that the low 87Sr/86Sr and/or 143Nd/144Nd (relative to OIB) of the primitive magmas of the Hellenic arc are characteristic of the upper mantle in this area. Similar Sr and Nd isotopic compositions have been reported for calc-alkaline lavas from continental margins (e.g Patagonia, Mexico, S. Chile) and from island arcs that are (at least partially) underlain by continental crust (Japan, Sunda) (see chapter XXX and references therein). Furthermore, many lavas from continental anorogenic suites have isotopic compositions intermediate to PREMA and EM-X (see H o m e r et al., 1986). However, intra-oceanic island arcs are characterized by Sr and Nd isotopic compositions similar to PREMA. The difference in isotopic composition between these two types of island arc magmas probably reflects the different nature 159 of sub-oceanic versus sub-continental mantle wedge, rather than crustal contamination, since assimilation of (upper) crust will generally result in isotopic trends toward EM-II (see Chapter III). This is another indication that island arc magmas are generated in the mantle wedge and not in the subducted slab. Menzies and Hass (1983) have pointed out that upper mantle isotopic compositions that fall to the left or right of the oceanic array reflect changes in the Rb/Sr or Sm/Hd of the source region that are caused by mantle metasomatism. Isotopic compositions within the oceanic array indicate coherent fractionation in Rb/Sr and Sm/Nd as a result of partial melting (Menzies and Wass, op. cit.). Isotopic compositions that plot to the left of the field for OIB (i.e. towards EM-I in Fig. 4.2) can be attained if at some stage in the evolution of the upper mantle Sm/Nd is decreased whereas Rb/Sr is only mildly increased, or vice versa. Menzies and Wass (1983) have described pyroxenite xenoliths from Kiama, New South Wales, that are characterized by low ®7sr/86sr and 143Nd/14*Nd. This unusual isotopic signature (i.e. EM-I) was attributed to a decrease in the Sm/Nd ratio of the upper mantle caused by infiltration of a C02-rich, LREE-rich fluid into the source area. Similar data have been reported by Salters and Barton (1985) and Barton (1986) for lavas from the Leucite 160 Hills, Wyoming. However, magmatism in Hew South Wales and the Leucite Hills associated with LREE-enriched upper mantle source regions is strongly alkaline in character. A LREE-enriched upper mantle source region beneath the Hellenic arc is difficult to reconcile with the low concentrations of incompatible elements in the magmas of Santorini and Nisyros. Also, REE patterns for Santorini are only slightly LREE-enriched (LaN/YbN«1.6-2.4, see Huijsmans et al., 1988), and are inconsistent with an EM-I type enriched upper mantle source. Alternatively, isotopic compositions to the left of the field for OIB could result (over a certain time span) ♦ from a selective decrease in the Rb/Sr ratio of the upper mantle. It has been suggested that LIL element depletion observed in granulites is a result of fluxing of the lower crust by C02-rich fluids (Touret, 1971). Granulites have low Sr and Nd isotope ratios, but are often LREE enriched (e.g. McCulloch et al., 1987) which suggests that the Rb/Sr ratio has been lowered, but that the depletion event did not result in a significant increase in the Sm/Nd ratio. The occurrence of similar processes in the upper mantle beneath the Hellenic arc (i.e. LIL element depletion by C02-rich fluids) could explain the isotopic compositions and the low concentrations of incompatible elements observed in the magmas of Santorini and Nisyros. 161 It ig not possible to determine when this depletion event has occurred, because the effects of such a process on Rb/Sr and Sm/Nd are not quantitatively known. However, the Sr-Nd isotopic compositions of the primitive lavas of Santorini and Nisyros still plot within (Santorini) or near (Nisyros) the field for OIB (Fig. 4.2). The Sr and Nd isotope ratios indicate that the source region for the Hellenic arc is depleted relative to bulk earth on a time- integrated basis (i.e. the source region evolved with a lower Rb/Sr ratio and a higher Sm/Nd ratio relative to bulk earth) but less depleted than the source region for intra- oceanic island arcs (PREMA, see Fig.4.2). This observation seems to contradict the conclusions reached on the basis of incompatible element concentrations: the trace elements indicate a source highly depleted and similar to the source region for island arc tholeites (and MORB), whereas the isotope ratios suggest a less depleted source region, similar to that for many calc-alkaline suites from continental margins. This paradox can be explained by assuming that the depletion event, responsible for the LIL element depleted character of the Hellenic arc magmas, occurred relatively recently, so that not enough time has elapsed for a clear imprint of this depletion event on the Sr and Nd isotope ratios in the upper mantle source region. The continental crust in the Aegean is relatively young: ages for the crystalline basement of the Cyclades range 162 from 25 Ha to 515 Ma (Andriessen at al., 1979; Hensjes- Kunst and Kreuzer, 1982; Andriessen et el., 1987). The depletion of the upper mantle may veil be related to recent * crust formation in this area. The following model for the upper mantle source region is compatible with the available data: Originally the upper mantle source region beneath the Hellenic arc consisted of moderately depleted (in between PREMA and BSE in Fig. 4.2) sub-continental upper mantle material, comparable in composition to the source regions for other calc-alkaline suites from active continental margins. At some fairly recent time, the upper mantle source region became depleted in incompatible elements. Depletion may have resulted from fluxing of the upper mantle by C02-rich fluids, which has previously been proposed to explain LIL element depletion and low 87Sr/86Sr and 143Nd/144Nd in granulites (see above). The source region of the Hellenic arc was isotopically heterogeneous, as indicated by differences in* isotopic composition between the primitive magmas of Santorini and Nisyros. Heterogeneity of the upper mantle may have existed prior to the depletion event, or developed as a result of this event. Later, after the onset of subduction, the composition of the upper mantle wedge may have been modified by a fluid derived from the subducted slab. The subducted slab was relatively young, unaltered oceanic lithosphere that was stripped of its sediments (Huijsmans et al., 198B). Fluids released during dehydration served to lower the solidus and may have enriched the upper mantle in highly incompatible elements (notably K, Rb, Ba and Th, see Fig. 4.1). Another possibility is that subduction has stopped prior to formation of the magmas of Santorini and Nisyros, and that there is no further contribution from the subducted slab to the overlying mantle wedge. If there has been any subduction-induced enrichment at all in the mantle wedge, then the pre-subduction upper mantle source region must have been even more depleted in incompatible elements than is apparent from the magma compositions of Santorini and Nisyros. The MORB-normalised trace element composition of primitive upper mantle from Taylor and McLennan (1981) is plotted in Fig. 4.1. Similarities between the trace element plots of the Santorini basalts and primitive upper mantle suggest that such magmas can be produced by melting of relatively primitive mantle. Accordingly, it is possible to obtain a rough estimate of the maximum degree of melting necessary to produce the observed trace element composition of the Santorini basalts, assuming that the source region was not significantly modified by slab-derived fluids. This is 6.2% for sample SH-32 (average value calculated for K, Rb, Ba and Th, assuming equilibrium melting and D-0). This is a maximum value since the Sr and Nd isotopic 164 compositions indicate that the source is depleted in incompatible elements relative to bulk earth. On Santorini the transition from tholeiitic basalts (low-K) to calc-alkaline andesites, dacites and rhyodacites (intermediate-K to high-K) is accompanied by increasing 87Sr/86Sr and decreasing 143Nd/144Nd. This transition from tholeiitic to calc-alkaline compositions is probably caused by assimilation through addition of potassium and other incompatible elements to the magmas. Assimilation could also have resulted in an increase in f02, thus causing magnetite to precipitate and preventing further iron- enrichment of the magmas. CONCLUSIONS The primitive lavas from Santorini and Nisyros are strongly depleted in incompatible elements relative to other calc-alkaline lavas. Similarly low concentrations of incompatible elements have been reported for tholeiitic lavas from intra-oceanic island arcs. The Sr-Nd isotopic composition of primitive magmas from the Hellenic arc and other arcs underlain by continental crust is very different from the isotopic composition of magmas from intra-oceanic island arcs. It is suggested that this is not a result of crustal contamination, but caused by compositional differences between sub-continental and sub-oceanic lithosphere in the mantle wedge. No evidence has been found for contamination of the source region of the Hellenic arc with subducted sediments. Low B-concentrations in the magmas indicate that the subducted oceanic lithosphere was relatively unaltered and therefore young. Alternatively, subduction may have ceased, and there is no further contribution from the subducted slab to the upper mantle wedge. The upper mantle beneath the Hellenic arc is moderately depleted on a time-integrated basis,, but experienced a strong, relatively recent depletion event prior to the onset of subduction. Table A.I. Sr- and Nd- isotope ratios and selected major oxide 166 and trace element concentrations In primitive lavas from Santorini and Nisyros. SH-32 51-181 N-21 H-24 HO. 0.872 0.75Z 0.70X 0.962 K-0 0.48 0.31 1.14 1.28 0.15 0.08 0.19 0.21 ? 2 ° 5 Rb 10 ppm 9 ppm 16 ppm 30 ppm Sr 236 221 1194 561 V 16 15 14 18 Zr 69 65 146 148 Nb 3 3 6 11 Ba 69 60 219 226 Ce 14 12 30 33 So 2.7 2.0 3.9 3.8 Yb 1.9 1.7 1.4 2.3 Hf 1.9 1.8 3.0 3.1 Ta - -- 3.2 Th 1.2 1.1 2.3 3.2 87Sr/86Sr .70408(6) .70422(3) .70351(6) .70416(6) 143Nd/144h’d .512859(8) •512B46(9) .512790(9) .512705(9) Between parentheses: uncertainty In Isotope ratios (i.e. 2 standard deviations of the mean) 167 ao- 10- ROCK MORB — N-31 PUM 0 .1 - 0.01 YBRB Fig.4.1. MORB-normalized trace element patterns for primitive lava compositions from Santorini and Nisyros. PUM is the trace element pattern for Primitive Upper Mantle from Taylor and McLennan (1981). MORB-normalised Ta concentrations for SH-32, SI-181 and N-21 were assumed to be identical to the values for Nb. .709 EM-11 .707 .705 OIB EM-I 87Sr/86Sr SANTORINI PREMA NISYROS .703 OMM H1MU L i .701 et al.(1984). et al.(1984). mantle The components proposedby zindlerand Fig.4.2. Fig.4.2. versus 143Nd/144Nd 87Sr/86Sr for lavas from arediscussed the text. in Santorini and Nisyros. Santorini Nisyros. and HORB Fields for andOIB from Zindler Hart (1986) Hart DMM, HIMU, PREMA, (1986) EM-I, (i.e. EM-II andBSE) .512 .5128- . 5 1 3 6 PNtTl/PNEM 207Pb/204Pb 1 B.l S I eipeeRfrneLn rmHr (1984). labels Other from Hart Line Reference Hemisphere Fig.4.3. Range in Pb-isotopic compositions of the of Hellenic the compositions in Pb-isotopic Range Fig.4.3. r h aea nFg 4.2.in Fig.as same the are is Northern the NHRL et al.(1987). from Gulen lavas arc 1« OMMB s O EM-I T I DMM A DMM BSE. b P 4 0 2 / b P 6 0 2 18 PREMA ELNC ARC HELLENIC i a EM-II MORB so 21 HIMU 22 169 Contributions to Contrfb Mineral Petrol (1916) 93:297-311 Mineralogy and Petrology O Springer-Vetlsg 1916 CHAPTER V PETROLOGY AND EVOLUTION OF TRANSITIONAL ALKALINE - SUB ALKALINE LAVAS FROM PATHOS, DODECANESOS, GREECE: EVIDENCE FOR FRACTIONAL CRYSTALLIZATION, MAGHA MIXING AND ASSIMILATION G. Paul Wyers ind Michael Barton Department of Geology and Mineralogy, The Ohio Slate University, Columbus, OH 43210, USA Abitract. Petrographic, mineral chemical and whole-roek many magmas erupted through the continental crust are major oxide data are pmented Tor the lavas of the Main Influenced by magma mixing and assimilation in addition Volcanic Scries ofPatmot, Dodecaresos, Greece, These la to fractional crystallization. As emphasized by O'Hara vas were erupted about 7 m.y. ago and range In composi (1977) and O’Hara and Matthews (1981), it Is essential that tion from ne-tiachybasalts through hy-trachybasalts and the operation o f such processes are recognized, and the trachyandesites to Q-trachytes. To tome extent, the ne-tra- cITects quantified, before attempts are made to reconstruct chybaults are Intermediate In composition to the alkaline upper mantle source region characteristics. lavas found on oceanic islands and the calc-alkaline lavas The Tertiary to recent lavas from the southern Aegean of destructive plate margins. Major oxide variation is large Sea, Greece, have evolved via fractionation, mixing and ly explicable in terms or fractional crystallization involving assimilation (Barton etal, 1983; Huijsmans etal. 1983, removal of the observed phenocryst and mlcrophenocryst 1986). Most of the volcanic complexes thus far described phases viz. olivine, plagloclasc, dinopyroxene and Ti-mag- in detail occur along the Hellenic arc which is associated nctite in the mafic lavas, plagloclase, dinopyroxene, mica with the overriding or the African Plate by the Aegean and Ti-magnetite in the evolved lavas. Apatite, which oc microplate (Huijsmans et al. 1936). In this paper, we de curs at an indusion in other phenocrysts or as mlcropheno- scribe the petrology ofalkaline-sub-alkaline lavas from Pat- crysts must also have been removed. However, mass bal mos, which is located some 100 km to the north of the ance calculations indicate that the chemistry o f the hy-tra- Hellenic arc lu -s.t). Specifically, we use mineral chemical chybasalts is inconsistent with an origin via fractional crys and whole-rock major oxide dau to evaluate the processes tallization alone and the complex xoning patterns and re- operative during evolution o f the Main Lava Series (tee sorbtion phenomena shown by phenocrysts in these lavas Wyers et al. in prep.). Our major objective Is to demonstrate show that they are hybrids formed by the mixing o f80-77% that significant conclusions may be drawn on the basis of ne-traebybasalt with 20-23% trachyandesitc. It Is estimated major clement data alone. In companion papers (Wyers that the mixing event preceded eruption by a period of and Barton in prep.; Barton and Wyers in prep.) we use 12 b-2 weeks suggesting that mixing triggered eruption. trace element and isotopic dau to further constrain the Combined fractionation and mixing cannot explain the rel ideas developed in the present communication and we esti atively low MgO contents o f the hy-lraehybasalts and it mate the conditions o f crystallization. is concluded that assimilation also occurred. Assimilation, and especially addition of volatile! to the magmas, may be responsible for the evolutionary trend from ne-normative Outline of geology to by-normative magmas and was probably facilitated by Pstmoi fa an almost entirely volcanic Island with an area of approx intensified convection resulting from mixing. A modd Is imately 38 km*. The geology has been described in some detail presented whereby primitive magma undergoes fractiona by Robert (1973) who neognlacd the following major volcanic tion in an intracrustal magma chamber to yield more ualu: bigh-K, Mgb-Al basalts; intermediate poussk lavas (latite, evolved liquids. Influx o f hot primitive magma into the quartz-la tile, potassk trachyte); quartx-potassk alkaline lavas (quaru-trachytes, pbonolltes) and pyrodejtics; and sodie alkaline base o f the dumber facilitates assimilation, but eventually lavas (pbonolltes, trachytes). We have adopted a daasificaiion mixing yields the by-trachy basal la and finally the ne-traehy- scheme based entirely upon whole-rock chemistry. This scheme, basalts are erupted. which was previously used by Johnson et a l (1976) for alkaline vokanks on islands off the coast of Papua (New Guinea) is Qlut- tratcdinnsse and Is based upon the amount of normative ocphe- One or quaru (which Includes SO , horn normative bypemhene) Introduction and the Thomlon-Tuttie (I960) dUTertntlatioa index. We deliber In recent yean there has been increasing recognition of ately choee the scheme to avoid the necessity of using names non oally associated with cak-atkaHns or typically alkaline lava series the complexity o f the processes involved in magma evolu and it has been used successfully for lavas on some oceanic Islands tion, It is now widely realized that the compositions of (c.g* Tristan da Cunha; Baker et aL 1964), Tbs distribution of the otajor volcanic units is shown in , OJJprkt refwstr re; O.P. Wycn which Is modified (torn Robert (1973). The predominant rock-typcs 170 171 ■ U I 0 A A t A * air* flhodopoi Western J f j f " ^ Threee-^*- / a n > Samatharti o ai B? „ Ibnnot 20 > Hegioa tf c* tuitretion rta.s,! Classification of Patmot MVS lavas in terms of CIPW nor mative Ne or Q and Thomton-Tuttle differentiation index. Classifi " ■ h cation scheme after Johnson et al. (1976) ^ * CMi Robm and Cantagrcl 1977) and indicate at least two episodes of volcanic activity, one about 7 m.y. ago (Q-trachyiet) and one about 4 m.y, ago (dikes and ne-trachybasalts from Chiliomodi), On the basis of major oxide, tract dement and Sr-lsotopic dau, Wyers cl al. (in prep.) recognise a main volcanic series (MVS) which consists of the ne-irtehybaultt found as volcanic bombs, Pttmoif the hy-irathybasalii, hy-irachyandetlies and Q-trachytts. Recent detailed field studies suggest that the trachytes were erupted before o O ° *■(pserimoe the basalts and that the volcanic sequence Is essentially inverted Amiparoi , / ? (I.e. more evolved lavas were erupted before the least evolved la - Petrography The petrographic characteristics of the MVS lavas are summarited i in w i-i . Detailed petrographic descriptions will be presented elsewhere (Wyrrt et al. in prep.). All rocks are strongly porphyriiie SO 1 0 0 km with phcnocryst contents up to about 30% (ritual estimates). Fresh oihlnt is round only in the ne-irachybataltt, In which Treneh il occurs at aenocrystt and phenocrysts and at microphenocrysit. The xcnocrytit show embayed margins indicative of reaction with the host magma, and contain Inclusions of brown chrome-sptncl rti.S .I Simplified geologic map for the Aegean showing the location and opaque magnetite. Irregular extinction and strain lamellae are of the Hellenic trench system. the Hellenic arc, and Patmoi common, The phenocrysts lack strain lamellae. Serpentine often occurs along the rims or in cracks which penetrate the olivines. Pteudomorpht after olivine occur In the hy-trachybatalts are hy-trachybatalts1, hy-iruchyandesitei and Q-trachytes (which («*.».*• ). The original olivine has been entirely replaced by deep form an undivided tenet on the map), rhyolitet, trachyttt and ltd-brown colored material which It rimmed by opaques, or by unclassified pyroclasiics. Ne-lraehybaultx are relatively icarce. fine-grained opaque mticrial. These pteudomorphs are Interpreted The largest outcrop occurs on the email itland of Chiliomodi. but to result from reaction between olivine and melt rather than from these rocks alto occur as dikes in the central and southern part sub-tolidut hydrothermal alteration. of the main island and at volcanic bombs found in or near small flafbclair itthe dominant phcnocryst phase in all of the lavas. ciploaion vents to the east of Grikou. Phonoliles arc likewise rare, It shows radial and oscillatory toning, The phenocrysts show com occurring only at two localities in the'northern pan of the {stand. plex morphologies which art rtmlnisctnl or those shown by feld A series of steeply dipping trachytes, rhyolitet, pyrodattict and spars in oceanic basalts (Dungan and Rhodes 1971; Rhodes ct al. mafic dykes occurs In the south, and hat been called the Old Volca- 1979; Kuo and Kirkpatrick 1963) and in calc-alkaline lavas (Luhr nic Series by Robert (197}). The only non-volcanic rocks arc mar and Carmichael I960) inasmuch at sieve leisures and embayed bles, which art only cipoied in the south west, near Cape Ghenou- margins are common. The cores of some phenocrysts in the ne- pis. Field studies indicate that the marbles have been lectonically trachybasalts, hy-irachybasalit and hy-irachysndctites are riddled cmplaccd and that they may not necessarily represent the local with inclusions of groundmass material and are surrounded by basement. rims of clear feldspar («*■•-•* ). On the other hand, tome of the According to Robert (1973), the central part of Patmot lies phenocrysts in the ne-lrachybaisalts and hy-irachyandcslies show in a NW-SE trending graben. This implies that lavas which arc embayed, inclusion-free cores surrounded by a sieve-tenured found exclusively in the north or south of the island, such as the mantle around which there it a rim or dear feldspar («»■»-*•). trachytes and the rhyolitet. constitute the oldest eruptive products. Textures such as these suggest periods of rapid growth or partial A few K-Ar age determinations are available (Fytikas ct al. 1976; reaction with the hotl melt and are absent In the phenocrysts in the most evolved lavas (l.e. the Q-trachytes). The plagiodase phe- 1 For the sake of brevity, we have abbreviated terms such at hyper- noaysts in the hy-lrechybxsalts, irachyandesiies and Q-itachyiet sihenc normative trachybatait to hy-trachybatalt etc. commonly have a very thin outer rim of sanidine - a feature often 172 w ISLE OF PATMOS (SIMPLIFIED FROM ROBERT 1979) 0 1 2 Km CCHtKANOU B H Ns ■ Tttcfcy tuu>l H> • TrKhr bHtlt ■HI H»• Traeftr tfldnitt 0 • TrKhyN B S j TtKDflt RhfCtiii L^j) n Ttkli i.i, Summary of petrographic diaracteriitics of Main Volcanic Series la m Lava type 01 Flag Cp* F e-T i Mica Ktpar Ap Oaide Ne'trachybasalt P,M,X P.M P.M P.M 1 Hy>trachybasa!t S P.MX P.MX P.M MX X M Hy>tracbyandcsite — P,M P.M P.M P.M P.M M Q-trachyte — P.M P.M P.M PJd P.M M P •• pbcnocryit; M " micro phenocrytt; X “ acnocrytl; S “ pseudomorph j I - Inclusion In other phcnocryit phasea considered to be characteristic or thoshonitei. The calcic rddspars phenocrytt phase. The pyroaene phenocrysts in the mafic lavas in w im lavas show varying degrees of alteration to day minerals show ejlremcly complez toning patterns. The phenocrysts in the and seriate. ne-lrachybaults are medium-grttn to colorless and sr character' Clinofyfoitnt, which occun as cuhedrahanhedral phenocrysts lead by radial, oscillatory and sector toning (ru iu ). Some pheno- and as mloophenocryits is generally the second most abundant crysu have abundant indusiont of groundmass material in the 173 300 i Ftg. 4. a Pseudomorph of olivine in hy-traehybaxalt. The olivine I* replaced by deep red-brown colored material, b Plagioclate phcnocryst wiib aim lutured core in hy-lraebybasall. c Plaglodase phenocrytt with dear, embayed core, sieve Matured mantle and dear rim in fay-tnchybasalt. 4 Pyroxene phcnocryst in nc-tradiybasait showing sector and oidllatory toning cores and others have embayed margins suggesting retorbtion. This xenocrytlt. Needle-like inclusions of rutile are common In the bio- it especially true in the cate of the hy-irachybasaltt which contain lites of iht trachyandcsiics. two distinct types of phcnocryst. one with a colorless core, a light- SmlJiiK it found at resorhed xenocrytlt In the hy-irachybatalis green mantle and a darker-green rim, the other with an embayed, and as phenocrysts in the trachyandcsittt and Q-trachytes. The dark-gtccn core, a light-green mantle and a darker-green rim. The sanldines in the trachyandcsiics show tome retorbtion cITettt (sieve mantlet and rims of these two types of phenocrytt are identical lecture, embayed margins), but It is not clear from petrographic in appearanct and the mantles arc identical to the microphcno- observations whether these represent phenocrysts or xenocrytlt (tee cryttt, >be rims identical to the groundnuts pyroxcntt. These py below). roxenes arc almost identical to those described by Barton cl al. Apatite it a ubiquitous accssory phase in the MVS lavas. It (19S2) from the K-ridt Ians of Vulxinl (Italy), The green cores occurs mostly at small inclusions, orientated parallel to the crystal arc characterised by radial coning and occasionally contain inclu faces, In plagioclate, dinopyroxene and blotlte phenocrysts. but sions of opaques and groundmast material. The colorless cores ii alto forms microphennerystt In the hy-iraehybasalis. trachyandc- show radial, osdltatory and sector zoning. In contrast, only radial tiles and Q-traehytes, The mierophcnoerysts in the hy-trachyande- toning it shown by the pyroxenes in the hy-trachyandesi^cs and sites arc anhcdral and art dusted with fine, brown specks and Q-intchyies. The phenocrysts tn these lavas vary in color from are Interprcitd to be xcnocrysis. dark-green (hy-trachyendctltct) to light yellowish-green (Q-tra- All or the minerals described above may occur In errital rloti chytes). It should be apparent that the colorleqs and green cores or f/omnoefgrrgarrs and, in general, the minerals found in the of the pyroxene phenocrysts In the hy-trachybasalit arc similar glomcroaggrcgates are those found as phenocrysts in the host lavas. in appearance and xonlng characteristics to the pyroxenes in the However, tanidinc does not occur in glomcroaggrtgatts in the hy- ne-trachybasalts and hy-trachyandesiies respectively. trachybasalts which supports the contention that the anhcdral tani 4/ofwr/rr occurs as anhcdral phenocrysts and mlcrophcno- dinc crystals in these lavas represent xcnocrysis. crytit in all lavas though it it relatively rare in the ne-irachybasalit, The greutukuauti of the lavas are mostly flne-gratncd, hoto- Sioihe it a phtnocryst and micro phcnocryst phase in the hy- crystalline, and commonly display pflotaxhlc texture. The ground- trachybasslis, trachyandcsiics and Q-trachytct. The phenocrysts mass of the Q-traehytes occasionally indudes minor brown glass. always show a thin rim of opaque material (T magnetite) which The main crystalline phases ore plagioclate, sanidinc, pyroxene, presumably reflects oxidation and retorbtion during eruption. magnetite, apatite and biotite. Carbonate occurs in the groundnuts However, the biotitt “ phcooctytil" in the hy-trschybasxlts show or some lavas and is interpreted to be secondary. In some cases, extensive retorbtion features (complete or nearly complete replace the carbonate clearly replaces dinopyroxene. ment by turbid, opaque material) and most probably represent Some of the MVS lavas contain xtnelitht and M utionj. We 174 301 hart not jret completed deuiltd tiudiet of these, but the octuntnce Plagloelase or a diitinetive type of inclusion in tome of the trachyandesiics is worthy of aoie. The inclusions are sub-spherical with a diameter The plagioclases show a wide range in composition but of 2 cm. Contacts with the host lava ire not sharp, but art finely the overage composition changes progressively from bytow- contorted. The Inclusions are holo-crystatline and. In contrast with niie in the ne-trachybasalts to andcsine in the Q-trachytes. the lava, contain numerous Irregular vesicles. The dominant miner- The phenocrysts in individual lavas show extensive zoning at constituents art plagioclate and sVeletal opaques which often (up to 35% An), as illustrated In ru .s.ii, which is in most form radial aggregates. Clinopyroscne and biotite (rimmed by cues normal inasmuch as Ca decreases and Na and K opaques) occur in subordinate amounts. Inclusions showing tex increase towards the rim. BaO comenu are low tural features similar to those described here have been reported (<0.05 wt.%). from intermediate and acid lavas (e.|. Eichelberier and Oooley 1977; Van Ber|tnetal. 1913). The cores of the majority of the phenocrysts in the ne- trachybasalts are A n ,,-.,, whereas the rims vary from Anti,. The cores of a few phenocrysts are richer in an* Analytical techniques orthite(An„ . „ ) but tf(e rims ofthese crystals are compost- Reconnaissance mineral analyses were performed at the Free Uni tionally identical to those of the other phenocrysts. The versity, Amsterdam, utlnf a Cambridge Mark 9 automated micro cores of the microphenoctyiu have compositions which fall probe and wavclenf th dispersive techniques. Operating conditions within the range shown by the phcnocryst rims. were: accelerating voltage - 30 k V, sample current - 30 n A, count The plagioclases in the hy-trachybasalu exhibit complex ing lime - 30 S for each pair or elements. Detailed mineral chemical compositional variations. The cores of the phenocrysts de studies were carried out at the State University of Utrecht using fine two distinct populations (m .s.st), one with A n „ .,» a TPD microprobc fitted with a Tracer Northern Energy Disper and the other with An««.,e. The mlcrophenocrysts are in sive System. Operating conditions were: 13 kV accelerating volt termediate in composition (-A n ,,). In general, crystals age, M nA sample current and (0-100 s counting time. All analy ses are fully corrected for deadtimc. background, atomic number, with anorthlte-rich cores show normal zoning to -A n ,,, absorption and fluorescence. Agreement between analyses obtained although many crystals have a very thin outer rim which on the two Instruments it excellent. is very poor in An (A n ,-,). Crystals with cores of A n ,,.,# Major oxide whole-rock analyses were done at Utrecht by au commonly show abrupt reversals in zoning, the core giving tomated XRF on fused glass discs. The instrument was regularly way to a mantle o f AnM. „ which is in turn surrounded calibrated using a variety of international and internal standards. by a zone o f intermediate composition (A n,,) and by a Precision or the XRF analyses is the same at that given by Barton thin outer rim of extremely An-poor material. and Huijsmans (1936) (see Hujjsmant etal, 1936, for a detailed The phenocrysu in the trachyandesites and Q-trachytes discussion), vii, SiOj-0.7%, AI|O,-0.5%, TiO,-0.04%. FeO- show normal zoning. The cores of the phenocrysts in the 0.3%. MgO-0.3%, MnO-0.01%. CaO-O.3%. Na,0-0,4%, K-O- 0.01%, PjO|-0.04%. These figures are based upon replicate analy trachyandesites are mostly in the range A n ,,.,,, but a few ses of well-characterized standards and are reported at 3 o devia are more anorthitic (A n,,) and these most probably repre tions in wt.% from the true value. sent xenocrysu. The rims of the phenocrysts and xenocrysts Fc,0, and FcO have not been determined independently be show a restricted range o f composition ( A n ,,.,|) whereas cause there Is evidence (petrographic) or post-eruptive oxidation the microphenocrysu (A n ,,.,,) are o f intermediate com and alteration In many of the lavas. For similar rtatons, all analy position. The cores of the phenocrysu in the Q-trachytes tes have been recalculated to 100% on a volatile-free basis. Mea range.from A n ,, to An**, and the rims (A n „ .,« ) have sured LOI's range from 0.63-3.43 wt.%. similar compositions to the cores of the microphenocrysu (A n „ .,o ). Mineral chemistry Ollcint Clinopyroxene Representative analyses arc available from G.P. Wyers The clinopyroxene phenocrysts show a relatively restricted upon request. The cores o f the xenocrysu are highly forster- range o f composition in terms of Mg/(Mg+Y Fe3 *), from ilic (F o,#.,,) and contain 0.3-0.4 wt.% NiO. They are thus 0.84 to 0.68. The cores o f the phenocrysts in the ne-traehy- comparable in composition to the olivines which occur In basalts tend, on average, to be characterized by higher Mg/ xenoliitu o f upper mantle material (cf. Boyd and Nixon (M g + j F e 1*) than the cores of the phenocrysu in the 1973; Reid etal. 1973). The Fo content decreases to more evotved lavas (0.84 and 0.74-0.78 respectively), but F oio-it M the rim. The phenocrysts show a wide range there is a poorly defined correlation between Mg/(Mg + in composition, mostly from F ou (cores) to F o,T (rims). JTFe1*) and whole-rock chemistry due partly, no doubt, The data indicate, therefore, that the olivine xcnocrysis par to the extensive zoning or the phenocrysts. It is, however, tially re-equilibrated with the host melt. The micropheno- noteworthy that the dinopyroxenes in the trachyandesites crysts are relatively iron-rich (F o ,t . , i ) and are also nor are indistinguishable in composition from those in the Q- mally zoned. trachytcs. In terms o f the conventional pyroxene quadrilat eral (ht.s.*4, the dinopyroxenes range in composition from diopside to salite. Spinel Al, Cr, Ti and Fe1* (calculated assuming that £ ca The small size o f the crystals made analysis difficult. The tions - 4,000 on a 6 oxygen basis) contenU are also variable spinels are rich in A 1,0, (—16 wt.%). SiO„ TiO,, MnO, but are, on average, higher in the dinopyroxenes in the NiO and ZnO are present in low amounts (<0.5 wt.%), nc-trachybasaUs than in the dinopyroxenes in the trachyan- The spinets are similar in composition to those which occur desites and Q-trachytes. The appropriate ranges in compo in Ihereolile xcnoliths entrained in alkali basalts and basan- sition shown by the pyroxenes in the ne-trachybasalts and ites. trachyandesites/Q-trachyies are; Al 0.116-0.393 and 175 302 An An OT Or A* t rii.S.S • Feldspar compositions plotted in terms of the compo nents A n-A b-O r. KeTh ntphclinc tnchybault; HyTb hyper- sthene trachybasall;HyTa hypcrsthene trechyandcsitt;QT quartx-trachyte. b The com of phtnociysts in the hy-traehyba- salts define two distinct populations whereas rims and microphe- nocrystt are of intermediate composition b 0016-0.075 tfu. Tl 0.012-0.066 and 0.001-0.030 afu, Fe>* lions which are identical to those o f the microphenociytti (expressed as F e , 0 F e O T ) 0.53-0.96 and 0.21-0.32 re in these lavas. spectively. Na contents (0.012-0.074 afu) are approximately the same in pyroxenes in all lava types. As would be expected from the petrographic descrip Mica tions, zoning patterns in the pyroxene phenocrysts in the ne-trachybasalts and hy-trachybasalu are complex but they It proved impossible to obtain satisfactory analyses o f the are, in general, similar to those described by Barton et al, strongly resorbed mica phenocrysu in the hy-lrachybasalts (1992) for pyroxene phenocrysu in Vulsini leudte tephrites. so only analyses o f the microphenocrysu are considered. The cores of the clinopyroxene phenocrysts in the hy-tra* The micas in these lavas are phlogopites and arc richer chybasalu define two compositionally distinct populations in Mg than those in the trachyandesites and Q-trachytes. (Fig. 6b). one with M f/(M g+£F e**) 0.820-0.951. the In the latter micas, Al is just about sufficient to balance other with M g/(M g+£Fe**) 0.702-0.748. The mantlet Si-defidendes in the tetrahedral sites whereas in the former, surrounding both types of core have intermediate compori- (A1+Si) <8,000. Tl contenu are uniformly high 176 101 Hd f Hr-Tb - y & * O-T •• i. In Ft PAT 14 WO 101 ru.s.s ■ Composition! of dinopyroxenes ploittd in the conventions! C a-M g-F e quadrilateral. Th<: relatively wide range in Ca-contcm un- doubtedly reflects complex oidllilory and lector xoning. Abbreviation! ai Tor Fig. i. b The com of pyroxene phenocrytu In the hy-traehyba- tain define two diitinct populations. The mantles around the pheno- 20 cryiti and the microphenocrysts occupy an intermediate Held In / b (0.61-0.72 afu) and do not correlate with Mg/(Mg + crysu in the hy-trachybasalu is the tim e as that or pheno* £ Fe1*). Ba contenu are relatively low, <0.09 afu. crysu in the trachyandesites. The analyses are plotted in the A n -A b -O r diagram In ria.s.s*. M a in tiiit Major oxide whole-rock analyses The magnetites contain up to 16.2 wt.% TiO„ 3.3wt.% AltO) and 2.6 wt.% M |0 . TiOt contenu are highest in Representative whole-rock major oxide analyses together the magnetites in the ne-trachybasalu and are lowest in with C1PW norms are given intur, s.i. Since only total the magnetites in the Q-trachytes. Calculations based upon iron was analysed, the norms were calculated assuming stoichiometry (i.e. assuming £ cations - 3,000 on the basis Fe,0,/FeO"O.I5 (Brooks 1976). of 4 oxygens) indicate 30-10 wt.% F e ,0 ,. Some magnetites As shown in rt,.s.i, the ne-trachybasalts are alkaline contain exsolution lamellae o f llmenite, but unfortunately according to the definition o f Shand (1922), whereas the the coarseness of the cxsolution prohibiu successful reinte intermediate and evolved lavas are sub-alkaline. The switch gration of the component phases to yield a reliable estimate from alkaline to sub-alkaline occurs at an early stage in of the composition of the original phase. the evolution of the lava series and in this respect the Pat- mos Main Volcanic Series differs from truly alkaline volca Sanldint nic series as found, for example, on Tristan da Cunha (Baker ct al. 1964), St. Helena (Baker 1969) and Skye The phenocrysu range in composition from Or,,An, to (Thompson et al, 1972), There are other differences between Or?,An, and individual crystals are normally zoned - that the Patmos ne-trachybasalu and the alkali-otivine basalts is, Or contents decrease and An contenu increase towards of oceanic islands. The former have higher SiO, and lower the rims. However, the range o f composition found in each TiO, and total iron, characteristics often associated with type of lava is relatively small, viz. Or„An(,f -O r„A n*,t calc-alkaline basalts. To this extent, the Patmos MVS may in the hy-trachybasalu, 0 r ,» A n ,-0 r „ A n , in the tra be considered to be transitional between the alkaline series chyandesites and O r,,A n ,-O r,,A n , in the Q-trachytes. o f intra-oceanic islands and the calc-alkaline series of de The rims are identical in composition to the micropheno structive plate margins. crysu and to the rims of the plagioclate phenocrysu in Major oxide variations shown by the MVS are illus the same lava. Note that the maximum Or content o f pheno* trated in variation diagrams inrt«.s.r, in which SiO, has T.M. 1.1 Whole-rock chemical inilyies *nd CIPW normi of (elected Pitmot l i m N e-Tb H y-Tb H y-T i Q-T Pit 127 Pit 126 Pit 54 Pit III Pit 74 Pit 75 Pit it Pit 56 SiOi 50.5 50.5 54.5 54.1 61.7 6160 65.9 66.4 TiO, 1.17 1.17 1.15 1.51 0.17 0.11 0.59 0.59 AljOj 11.5 17.7 17.2 11.2 17.0 17.1 16.5 16.0 FeOT* 7.09 7.29 6.72 6.66 4.11 45.59 5.41 5.40 MnO 0.17 0.15 0.14 0.51 0.10 0.16 0.07 0.07 M*0 6.51 7.14 5.65 2.70 1.14 1.12 0.17 0.19 CiO 9.22 10.1 7.15 6.59 4.55 4.59 2.91 2.99 N t|0 5.44 5.05 2.92 5.15 5.15. 5.06 5.46 5.51 KjO 2.95 2.71 5.15 5.55 5.62 5.59 6.19 6.05 P .0 , 0.72 0.70 0.71 0.(1 0.49 0.41 0.26 0.29 Mi . 0.621 0.656 0.492 0.419 0.402 0.414 0.515 0.511 {M |+£Fe**) CIPW normi Q _ ——— 9.5 10.1 15.1 15.1 or 17.7 16.0 50.5 511 55.2 55.0 36.6 35.7 •b 19.5 19.4 24.7 26.6 264 25.9 29.3 21.6 in 25.1 26.7 11.7 19.1 15.9 16.4 10.7 11.1 ne 5.5 5.4 ——— — —— dt 15.5 15.0 12.4 5.7 10 1.9 2.0 1.5 by —— 1.5 5.4 9.5 1.9 5.2 5.4 ol 15.6 15.9 6.9 6.5 ———— ml 1.7 1.1 1.6 1.6 1.2 1.1 0.1 0.1 11 12 12 11 15 1.7 1.7 i.l 1.1 *P 1.7 1.7 1.9 2.1 10 1.1 0.6 0.7 * Totil F c ti FeO. Aml)iei rormiliicd to tOO.Oon * volitile-rrec batii. Normi calculated with FcjC^-OU FcO • •• • u M .m «• li t.l .i. Major oxides ploutd siu n tt SiO; The trends shown by most oiides ire broadly consistent with fractional o crystallisation 178 Tin* i.i Bro«d-beim electron micro probe snslyses or ground- m u m If H»trw>T>lll SiO, 5)32 66.30 a4 TiO, 1.37 0.26 * A t,0, 19.60 17.1) i FeO 6.52 1.44 MnO MgO 2.96 0.14 CaO 7.9) 1.21 ■i «• Na,0 III 3.15 41 K ,0 5.99 s.so P ,0, 0.40 “ rts.s.s A plot or M |0 versus SiO, suggests the existence or two magma series Analytes normalized to 100%. Total Fe rtportrd u FeO been uted it i differentiation index. MgO, FeOT, CaO Care was taken to avoid microphcnocrysts and the larger and AljOi decrease with increasing SiO„ whereasN biO groundnuts crystals to that the analytes most probably remains approximately constant and K ,0 increases. TiO| do not represent the composition oF the liquid quenched and P ,0 , show slight initial enrichment and then decrease. upon eruption. Rather, they represent the Final liquid after These variations are qualitatively explicable in terms of crystallization oF part oF the groundmats. Analytes are re crystal-liquid differentiation processes involving crystalliza ported inTttu l.^Relative to the whole-rock compositions, tion ot the observed phenocryst phases viz., olivine, dlnopy- the groundmasses art enriched in SiO, and K ,0 and are roxenc and plagiodase in the mafic lavas and biotite, dino depleted in MgO, FeOT, CaO and A l,0». They thus con pyroxene and plagiodase in the more evolved lavas; the form to the trends shown by the whole-rock analytes, as change in the rate or enrichment or K ,0 at SiO)« 54 wt.% shown if the groundnuts analytes are compared to whole- marks the orfset of biotite crystallization. The behaviour rock analytes with about the tame SiO, content. Although or N a ,0 indicates that plagiodase was the dominant crys it is Impossible to And a perfect match between the ground- talline phase, in accordance with pctrographic observations. mass analytes and the whole-rock analyses - presumably The decrease in TiO» and P ,0 , in lavas with > 54% SiO, because the Former represent the Anal liquid after partial requires involvement or Ti-magnetlte, biotite, and apatite crystallization of the groundmats - it is noteworthy that in the differentiation or the intermediate and evolved lavas. high K ,0 contents and high K ,0 /N a ,0 ratios can be gen However, extrapolated P ,0 | and TiO, contents at 40 wt.% erated during crystallization. The low NajO content, rela SiO| (the minimum SiO, content or any realistic crystalline tive to the whole-rock analysis, of the mesostasis o f Pat auemblage) are > 0 , indicating that crystallization ofTi- 26 may reAect the crystallization of albltic feldspar as a magnetite, biotite, and apatite must have occurred in the groundmats phase. ne-trachybasalts, for which there is petrographic evidence. Nevertheless, detailed examination or the variation dia Discussion grams reveals that certain oxides exhibit unusual behavior. N atO and A 1,0, contents remain approximately constant Fractional cryitalllzatlon - quantltatkt modtUng during differentiation, which suggests that the processes in volved in the evolution o f the Patmos MVS were not identi Although there is evidence that more than one process may cal to those involved in the evolution of other alkaline lavas. have operated during evolution of the Patmos MVS, we Also, the variations of MgO and to a lesser extent, FeO Artt consider fractional crystallization as this process it depart from a linear or a smooth curvilinear trend which readily amenable to quantitative modeling. Major oxide is normally developed as a result of differentiation. Indeed, variations were modeled using unweighted least-squares MgO contents of the hy-trachyba salts are so low in compar mixing calculations (Bryan et ai. 1969) involving subtrac ison to those of the ne-trachybasalts and trachyandesitea tion o f the observed phenocryst phases From successive pa as to suggest the existence o f two distinct lava series - one rental magmas to generate more evolved compositions. For From ne-trachybasalt to hy-lrachybasalt, the other From ne- each model, a large number of plausible solutions were ob trachybasalt through trachyandesite to Q-trachyte (lu.s.d tained depending upon a) the type and number o f minerals The existence oF two series is precluded upon the basis oF used and b) the compositions o f the minerals, especially field relationships, but it is noteworthy that the hy-trachy- the compositions or the Fe—Ti oxides and plagiodase. Un basalts also appear to be relatively enriched in P ,0 „ TiO, fortunately, there is no sure way to discriminate between and KiO in addition to FeO (secru.s.i). These observations the various solutions (Le Maitre 1982). Some previous strongly suggest that processes other than Fractional crystal workers have arbitrarily accepted models as being satisfac lization have been involved in the evolution or the magmas. tory if the sum o f the squares or residuals (SSR) is less than 1.5 (Luhr and Carmichael 1980), or less than 0.1 (Le Rocx and Eriank 1982). Others have applied a number oF Analyses of groundmssses/racsostasis criteria, induding the absolute and/or relative difference The groundmasses of two lavas were analysed with the elec for cadi oxide between the calculated daughter and the tron microprobe using a wide (20-50 pm) spot diameter. proposed daughter (Fisk et al. 1982). We adopted a two- 179 W6 TtMt ) ‘ Reiulll of least-tquare* m m balance calculation! which 104 the feasibility that the majrnil ©f the MVS *rt related by fractional crystallisation Pat 26-Pat 54 Pit 54-Pat 74 p it 74-Pat St 1 Pat 26 o £ Pat 74 Obi. Calc. Diff. Calc. Diff. Obi. Calc. Diff. SiO, 50.04 50.01 -0.03)4 34.49 54.31 0.0399 61.69 61.69 -0.0026 TiO, 1.17 1.09 -0.0104 1.1) 1.56 0.4)4) 037 0.94 0.0716 A1,0, 17.71 17.10 0.0731 17.31 17.19 -0.0209 17.0* 17.02 -0.0197 FeO* 7.39 7J1 0.0156 6.72 6.65 -0.0740 4.31 4.13 -0.0027 MnO O.IJ 0.16 0.0291 0.14 0.12 -0.0154 0.10 0.07 -0.0295 MgO 7.14 7.16 0.02)6 ).65 3.72 0.0745 1.34 1.94 0.0004 CaO 10.06 10.11 0.0509 7.1) 7.60 —0.2295 '4.)) 4)4 0.003) N«,0 3.0) 2.30 -0.7277 2.92 2.74 -0.11)4 3.1) 3.2 7 0.1)39 K ,0 2.71 3.15 0.4)54 5.13 4.4) -0.6930 5.62 5.61 -0.0054 P ,0, 0.70 0.63 -0.0670 0.71 1.01 0.)022 0,49 0.43 -00070 Mix Propor S.D. Mix Propor S.D. Mix Propor S.D. Variable tions Variable tions Variable tions Pat 54 0.6051 0.0)36 Pat 74 0.6444 0.0)32 Pal 53 0.7355 0.0067 01 0.0917 0.0150 Cpx 0.0730 0.0372 Cpx 0.0)4] 0.00)9 Cpx 0.0740 0.0)52 Plag 0.1544 0.0273 Plag 0.1149 0.0054 Piig 0.3072 0.0254 phiog 0.0723 0.0265 fii 0.0705 0.003) TJ-msg 0.0153 00059 Tt-mag 0.029) 0.0060 Ti-trag 0.0090 0.0013 Ap 0.00)7 0.0092 Ap 0.0173 0.0096 Ap 0.0076 0.0016 SSR-0.7403 SSR-017)2 SSR -0.0151 * All Ft calculated it FeO Pit 2S-ne-trichybmU: Pit 34-hy.trachybesali; Pit 74»h)'-trich)indoitt, Pat JI-Q-lrachjie. Analysed phrnocryit phases In each livi uitd u input diu state approach to the problem. Initially, we accepted all analytical uncertainly. These models involve the ne-trachy- solutions for which the SSR was lest than 1.2 as solutions basalts, hy-trachybaults and trachyandesites and the poor In this rante are within TOTAL analytical uncertainty for solutions presumably reflect the anomalous compositions the daughter magma. We then attempted, via adjustment of the hy*trachybasalts which were documented in the pre of the mineral compositions, to reduce the absolute differ* ceding section. In particular, the (Its for K ,0 , NatO, P ,0 , cnees for each oxide between the calculated daughter and and TiOj are poor and cannot be reconciled with the hy proposed daughter to the values o f analytical uncertainty pothesis that major oxide variations reflect fractional crys for each oxide. Before presenting the results, three points tallization alone. Nevertheless, the results or the calcula should be emphasised; I) The best SSR’s (generally <0.1) tions suggest that fractional crystallization was an impor for each model were obtained using plagiodase composi tant process in the evolution of the MVS and confirm that tions which are far too albitic to represent the osvrogr com olivine, clbopyroxene and plagiodase were the major position of the removed feldspar and solutions involving phases removed from the mafic magmas whereat plagio- unrealistic plagiodase compositions were thus rejected; 3) date, dinopyroxene and phlogoplte/biotite were the major Variations in plagiodase and F e -T i oxide composition phases removed from the intermediate and evolved mag have a'disproportionally large influence upon F, the frac mas. Furthermore, the solutions to the mixing models re tion of liquid remaining; 1) The role of aanidine in htigmi quire that minor F t-T i oxide and apatite played an impor- evolution it ambiguous - in most.cascs, inclusion o f uni* unt role in magma evolution in accordance with qualitative dine (an observed phenocryst phase in the cvolvcddavu) predictions made on the basis of major oxide variations significantly improves the SSR (e.g. the by*trachybasalts and with pctrogrsphic observations. It it noteworthy that to the trachyandesites) but often sanidine is a redundant the low P ,0 , contents or the mafic MVS lavas preclude phase inasmuch as it must be added to parent magmas apatite uturau'on (Green and Watson 1982) in these liquids from which other phases must be removed. Since unidine and that the apatite appears to have crystallized in response is a redundant phase in models involving the by-trachyan- to the local increase in P ,0 , in liquids adjacent to growing deciles and Q*irachytes - in which unidine occurs as a phenocryst phases. phenociysta! phase - we have ignored all solutions for the It is not possible to identify other processes which have hy-trachybasalts which involve unidine. affected the Patmos MVS from the results of the mass- IniMi* s.* the most utisfactory solutions for three mix balance calculations atone. Two additional processes, ing calculations involving a ne*trachybasalt, a hy-trachyba- magma mixing and assimilation, are discussed in the follow salt, a trachyandesite and a Q-trachyte are listed. All meet ing sections. To avoid confusion, it must be emphasized the criterion that SSR < 1.2, but two fail the criterion that that these conclusions do not necessarily conflict with the the misfit for individual oxides should be similar to the data presented for the groundmasses of two o f the lavas. 1 8 0 W The latter demontirate that composition* showing extreme o f equilibrium with the hott magma, which it the case for enrichment in Ka0 and high KjO/NajO ratios can be gen* the Patmos hy-trachybasalts. Indeed, the complex zoning’ erated by crystallization alone, but it must be borne in mind patterns of the plagiodase and pyroxene phenocrysts pro that these liquids do not correspond in detail with any of vide strong evidence for the mixing of two magmas, one the erupted lavas and that the analytes represent the prod* of which was rdativdy primitive and carried phenocrysts uctt of extreme fractionation on the scale of a thin section. or diopsidic pyroxene and anonhitie plagiodase whereas As noted previously, certain characteristics of the analyses the other was more evolved and carried phenocrysts o f sa of the groundmasses (i.e., the low Na*0 content of the litic pyroxene and more albitic plagiodase. During mixing, groundmass of Pat 26) are suggestive of modification of these phenocrysts may be resorbed, depending upon the the liquid composition by quench crystallization during er* proportions in which the magmas mix and upon the thermal uption. It is thus unlikely that such compositions are gener curvature of the liquidus surface (Barton et al. 1982), since ated during equilibrium or fractional crystallization involv they are out o f thermal and chemical equilibrium with the ing the observed phenocryst and microphenocryit phases. host magma. After mixing, pyroxene and plagiodase of intermediate compositiop precipitate from the hybrid magma at microphenocrytts or nucleate and grow at man Evidence fo r magma mixing tles around the xenoerysts (ru.s.d. This indicates that the Peirographic and mineral chemical data provide the most melt was homogenized prior to eruption and it is notewor compelling evidence that magma mixing has occurred. In thy that the mantles do not occur on the fractured surfaces particular, the complex zoning patterns shown by the dino- of crystats which were broken during eruption. The thin, pyroxene and plagiodase phenocrysts in the hy-trachyba extreme rims o f the crystals and the groundmass crystals salts art difficult to recondle with a simple, isobaric, frac nucleated and grew during eruption. tional crystallization model. Similar features have been de The xenoerysts o f biotite and unidine in the hy-trachy- scribed for phenocrysts In MORB's, calc-alkaline lavas and basalts were presumably derived from the relatively evolved alkaline lavas and have been interpreted to indicate poly- magma involved in the mixing event whereat the resorbed baric fractionation (Fisk et al. 1982) or magma mixing olivines were derived from the relatively primitive magma. (Dungan and Rhodes 1978; Rhodes etal. 1979; Barton The latter mineral was undoubtedly resorbed during or et al. 1982; Kuo and Kirkpatrick 1982). As noted by Barton after the mixing event because the hybrid magma did not and Van Bergen (1981) and Barton etal. (1982), complex lie on the olivine uturation surface. The small, dusty-brown zoning patterns could also result from disaggregation ofcrystals of apatite probably also represent xenoerysts de cognate or non-cognate xenoliths with reaction between li rived from the evolved magma since the PjO, contents of berated xenoerysts and the host magma. This possibility the hy-trachybasalts are too tow for these magmas to be it the most difficult to evaluate in the cate o f the Patmos saturated with apatite (cf. Green and Watson 1982). Some lavas, as many of the minerals which are obviously xeno- resorbtion of this phase would be expected to occur, there crystal in the hy-trachybasalts occur as glomeroaggregates fore, during mixing. in other lavas and hence could be derived by disaggregation Peirographic and mineral chemical data allow the end- and partial ingestion of cumulates. The arguments pre members involved in the mixing process to be identified sented by Barton et al. (1982) against this possibility in the and an estimate o f the proportions in which the end- case o f pyroxene xenoerysts in the lavas of Vulsini are alto members mixed to be made. Both the hy-trachyandesites applicable to the hy-trachybasalts. There is no evidence, and the Q-trachytes carry phenocrysts of salitic pyroxene, in the form of partially disaggregated xenoliths, to support plagiodase, unidine, biotite and apatite, but the composi the assimilation hypothesis and, moreover, it is difficult tions of the most sodle plagiodase cores and the unidine to envisage how two compositionally distinct types o f py xenoerysts indicate that hy-trachyandesite it the most likely roxene and plagiodase phenocryst cores originate by disag evolved end-member. The occurrence of olivine and diop- gregation o f xenoliths and why such cores are found only sidic pyroxene suggests that the mafic end-member was sim in the hy-trachybasalts. The occurrence o f diopsidic and ilar in composition to the most primitive analysed ne-tra- salitic cores in a wide variety o f alkaline lavas (Brooks and chybault, but it may have been slightly leu evolved (i.e. Printzlau 1978; Barton and Van Bergen 1981) alto mitigates higher M g/(M g+£ Fe**)) in terms of major oxide chemis against this hypothesis. try since the diopsidic cores o f pyroxene phenocrysts in The occurrence of diopsidic and salitic pyroxene cores the by-tnchybasalts are slightly more magnesian and con is also unlikely to reflect variation in /Hlo and/or/O* during tain more Cr than the phenocrysts in the ne-trachybaultt. crystallization (Frisch and Sehmincke 1969), as discussed Mixing proportions may be estimated from the composi in some detail by Barton et al. (1982). In particular, there tions o f the pyroxene and plagiodase phenocryst cores and is no evidence that crystallization or magnetite brought mantles In the by-trachybaults and arc 80-77% mafic end- about a change in the Fe** and Fe** content or the melt member and 20^-23% evolved end-member. In calculating and thereby influenced the composition o f the pyroxene, these proportions we have assumed that only one cycle of as appears to have occurred in the case of K-ricb lavas mixing occurred, which it reasonable on the bans o f the from Muriah, Java (I.A. Nicholls, per*, comm. 1981). avaitable data, and that no other proceu (e.g. assimilation) The presence, in any lava, or two compositionally dis operated concurrently with mixing (see, however, below). tinct generations of pyroxene and plagiodase one of As emphasized above, and elsewhere (Barton etal. which is in equilibrium with the host magma - might be 1982), sufficient time elapsed between mixing and eruption ascribed to polybaric fractionation (Fisk et al. 1982; Ven to allow the hybrid magma to become completely homoge turis and Barton 1983), but it it extremdy improbable that nized. An approximate estimate of the length o f time can this process can account for the presence of three genera be made becauu olivine phenocrysts became completely tions o f pyroxene and plagiodase, two o f which are outresorbed during residence in the hybrid magma. Thomber 1 8 1 SOS Wo •0 , In / rti.i.s Interpretation of compoiltional characterlnlci or plagioelsse and pytoxtne phtnocrytts In (he hy-trachybxialn In lermi of mjpna mixing. Note especially the similarity to the dlagtimi presented by Barton et al. (191)) whichilluiiraic tlTetti of mixing on pyroxenes In the lavas orVuUini and Huebner (1982) and Donaldson (1964) have published data Tor the rate of dissolution of olivine in basaltic melts. Using these data it may be shown that the olivine phcno- crytts in the hy-traehyandesites would be resorbed over a period of 8-373 h, depending upon the degree o f superheat* ing o f the hybrid magma and upon the extent or chemical disequilibrium between olivine and melt. Using data pre sented by Donaldson (op. tit.) Tor the rate of dissolution o f plagiodase in basalt, we estimate that the resorbtion 01— features observed in the hy-trachybasalt could have devel <1 as oped in 12-14 h. It is thus apparent that eruption occurred M. W1« shortly alter mixing and such data support models in which rti.J.to a plot o fK |0 versus S10a for the MVS lavas illustrating mixing triggers eruption (Sparks etal, 1977; Huppert and (be effects of mixing (hybridism) between nt-uaehybault and tra- Sparks I9S0). chyandcslte. Note that the tlevated Ka0 contents of the hy-tiachy* basalts are inconsistent with mixing alone (see also Fig. 1) Although the most convincing evidence for mixing is found in the hy-lrachybatalu, it seems postible that other evolved rock-types represent hybrid magmat. Definitive and plagiodase phenocrysts provide no evidence for post mineral-chemical evidence is lacking, but the occasional mixing fractionation. Grove etal, (1982) have recently An-rich plagiodase phenocryst cores, di-rich pyroxene phe shown that magmas undergoing reaction with olivine may nocryst cores and the vesicle-rich inclusions in the trachyan become enriched in FeO at approximately constant SiOa, desites are certainly suggestive o f mixing. but this does not explain the enrichment in Ka0 or the depletion in MgO shown by the hy-trachybasalts. It is thus Ecldenctfor assimilation necessary to invoke the operation of another process during evolution o f the intermediate lavas and the most likely pro Magma mixing can account for some or the unusual chemi cess is assimilation or crustal material. Since we haw not cal characteristics of the hy-tnchyandesitea - the high PaOa found, to date, evidence for assimilation in the form of contents for example - but it cannot account for all of xenoerysts or xenoliths and hence cannot deduce the nature them. In terms of magma chemistry, mixing it a linear pro of the crustal end-member involved, it is impossible to pre cess (Brooks and Printzlau 1978) and a hybrid magma dict the effects o f assimilation with any certainty. However, should thus be a linear combination or the end-members. it is known that, in general, assimilation will lead to an This is dearly not so in the cate o f the hy-trachybasalts increase in Ka0 content and because of this the inability (rts.s.u) which are relatively enriched in K |0 and FeO of the ieast-squares mixing calculations to fully describe and relatively depleted in MgO. Such characteristics coutd KjO variation it believed to be highly significant. Further conceivably reflect post-mixing fractional crystallization more, K»0 is likely to be enriched relative to NaaO during but we are reluctant to accept this possibility because analy assimilation (Watson 1982) which may explain the unusual ses of microphenocrysts and mantles around the pyroxene behavior o f these oxides in the Patmos MVS and the poor 182 309 fill for both of these oxides in the least-tquares m ist bal tallization of mineral assemblages involving two silica-free ance calculation!. Assimilation will normally be accompa- phases (Ti-magnetite and apatite) in the ne-trachybasalts nied by fractional crystallization (Bowen 1921) and if the can yield hy-normative residual liquids and it seems signifi assimilated material is poor in MgO (as are most cnmat cant that these two phases have been identified in the lavas rocks) this could lead to relative depletion in MgO as is o f both Skye and Ascension. HjO and/or F, which are observed in the hy-trachybasalts. required to stabilize apatite could be added to the magma Simultaneous operation or assimilation and fractiona during assimilation, and it may be noted that Thompson tion probably explains another unusual feature of the Pat- et al. (1983) have recently argued that the Skye basatts have mos MVS. The transition from ne-normative to hy-norma* been contaminated by continental crust. live trachybasalts occurs at any early stage of magma evolu It seems likely, therefore, that apatite plays a crucial tion, corresponding to a DI or about 53. Many alkali-basalt rote in tl^e derivation of hy-normative liquids from ne-nor - trachyte suites on oceanic islands are entirely ne-norma- mative parental magmas at low pressures and that the vola tive, for example those on Tristan da Cunha (Baker et al. tiles necessary to stabilize magnetite and apatite, are, in 1964) and St. Helena (Baker 1969), whereas in the continen at least some cases, enriched in the magmas by interaction tal alkali-basalt - trachyte suite of Skye (Thompson et al. with the continental crust. 1972) the transition occurs at a DI of about 72. The lavas o f Ascension are similar to those or Patmos inasmuch at Concluitont ne-normative mafic lavas are associated with hy-normative intermediate lavas (Harris 1983). In terms of the conven 1. Fractional ciystallizaiion can account for much of the tional basalt tetrahedron (Yoder and Tilley 1962), the tran compositional variation displayed by the MVS but can sition from ne-normative to hy-normative malic magmat not account for the alkali oxide and MgO contents of is equivalent to crossing the plane ol-di-plag which approxi the hy-trachybasalts. mates a thermal divide at tow pressures and therefore places 2. Peirographic and mineral chemical data provide conclu severe constraints upon the path followed by crystallizing sive evidence that the hy-trachybasalts are hybrid mag magmat. Prcsnall ctat. (1971) have shown that the plane mas formed by mixing between ne-trachybasalts and it, in Tact, a complex curved surface which at I atm. is trachyandesites. The mixing event preceded eruption by located mostly in the tholciile volume of the basalt tetrahe a very short time period - less than 375 h - during which dron but partly (due to solid solution toward monticcllite olivine xenoerysts were resorbed and mantles grew ar in olivine) in the alkali basalt volume. These workers have ound pyroxene and plagiodase xenocryst cores. The hy also shown that the tendency for dinopyroxenc to become brid magmas were completely homogenized during or more aluminous as pressure increases, shifts the whole sur after the mixing event. face into the alkali basalt volume, and that the divide ceases 3. Mixing combined with fractionation cannot account for to operate at a pressure of 3-5 kb. We show elsewhere the low MgO contents o f the hy-trachybasalts which (Wycrs and Barton 1984; Barton and Wyers in prep.) that indicates that assimilation also occurred. Assimilation the Patmos MVS evolved at P ■ 2-3 kb, so that the crystalli also accounts In part for the large increase In KjO/NajO zation trend o f these lavas requires explanation. which accompanies evolution of the MVS. We contend that assimilation combined with fractiona 4. Assimilation, combined with fractionation, caused the tion may alter the course of crystallization sufficiently that transition from ne-normative to hy-normative composi the thermal divide is breached at tow pressures. Interaction tions at any early stage or evolution. In particular, vola with crustal material may not directly induce a trend to tiles (H |0 , F, 0 ), etc.) may be added to the magmas wards silica enrichment if a thermal barrier is operative and induce crystallization or Fe—Ti oxides and apatite between the assimilant and the magma (O'Hara 1910), but (in some cases, perhaps, amphibote) which can signifi assimilation could lead to enrichment of the magma in oxy cantly allect the liquid line o f descent. gen (cf. Osborn 1939) and other volatiles (especially H ,0 ) We envisage the following scenario for emplacement which would promote crystallization of minerals which sig and eruption o f the MVS magmas (seerti,s.tt). Emplace nificantly alter the course of crystallization. A number o f ment o f primitive trachybatalt into shallow magma workers (e.g. Yoder and Tilley 1962; Presnall etal. 1978) chambers (Wyers and Barton 1984) where fractional crys have suggested that crystallization of spinel allows magmas tallization occurs to produce more evolved magmas. During to breach the olivine-gabbro divide and this mechanism fractionation, repealed influx or hot primitive magma leads is o f particular interest as the mafic Patmos lavas contain to vigorous convection which facilitates assimilation (Van phenocrysts and roicrophenocrysts o f Ti-magnetite. Osborn Bergen and Barton 1984) and triggers eruption. Eventually, (1959) showed that early crystallization o f magnetite it fa mixing o f primitive and evolved magma yields the hy-tra- vored by high / O i (cf. Wyers and Barton 1914) and will chybasalts. Following eruption of the latter, the ne-ttachy- lead to residual liquids enriched in silica. It is noteworthy basalts are erupted cither because al evolved magmas have that Thompson et al. (1972) concluded that the transition been expelled from the chambers or because they followed from ne-normative to hy-normative lavas on Skye reflects a different plumbing system and were not intercepted by crystallization o f abundant magnetite in the benmoreitcs. the evolved products in the chambers. Throughout much However, comparison with alkali-basalts from oceanic o f the volcanic history, the shallow chambers thus acted islands suggests that other factors must be involved. For as a density Alter, preventing eruption o f ne-trachybasalt example, early crystallization o f magnetite in the basalts and the magma in these chambers was compositionally o f Tristan da Cuhna (Baker et al. 1964) and St. Helena stratifled. (Baker 1969) has not led to the development o f hy-norma- The Patmos MVS thus experienced a complex evolu tive derivative magmas. The results o f the major oxide mass tionary history involving fractionation, assimilation and balance calculations for the Patmos MVS indicate that crys- mixing and may serve as an example for the evolution of 1 8 3 310 ria.S.ll Schematic diagram showing the probable sequence of emplacement and eruption of the MVS magmas. J Ne trachybasalt it emplaced into a relailvtly shallow magma chamber and difTerentiatet to produce trachyandesite and trachyte, probably in a toned chamber. 3 Underplating by hot, primitive ne- trachybasalt Instigates vigorous convection in the chamber and induces partial digestion of wall-rock material. Some mixing or trachyandesite and Q-traehyte may occur. 3 Mixing occurs between trachyandesite and ne-trachybsislL 4 Eruption of hybrid hy- trachybasalt.3 Eruption of ne-trachybasalt, probably along grabcn faults but possibly also from original vents alkaline and arc magmas in other region* i.e. the alkaline dement evidence for the role or continental crust in calc-alka tenet or Central Italy (Barton et al. 1982). line volcanism, Santorini and Milos, Aegean Sea, Greece. Earth Planet 5d Leu 63:273-291 AeknenMttnmit. The analytical work upon which thit piper It Barton M, Van Bergen MJ (1981) Green dinopyroienet and asso biied was done largely at the Slate Univenity ol Utrecht, The ciated phases in a potastium-rieh lava from the Leueite Hills, Netherlands, and we with to thank R.D, Schuiling In particular Wyoming. Contrib Mineral Petrol 77:101-114 Tor making the facilities available. J. tan der Wat and P. Antcn Barton M, Vartkamp JC, Van Bergen MJ (1982) Complex toning helped with the wholt'toek analytes and it is appropriate to at- of dinopyroxenes in the lavas of Vultlnl, Latium, Italy: Evi knowledge the efforts made by J.P.P. Huijsmans to improve the dence for magma mixing. J Vole Geolhcrm Res 14:361-388 quality of whole-rock analytes at Utrecht. The microprobet in Bowen NL(I928) Evolution of theigcnous rocks. University Press, Utrecht and Amsierdam receive financial and personnel support Princeton from ZWQ-WACOM. We alto with to thank HJS. Pictertcn for Boyd FR, Nixon PH (I97S) Origins of the ultramafle nodules from assistance in the field and in the laboratory, WJ, Lustcnhouwer some kimbcrliies of northern Lesotho and the Monastery mine, and C. Kieft for reconnaissance microprobe analyses, and MJ. South Africa. Phys Chem Earth 9:431-434 van Bergen for maintaining the microprobc in Utrecht. We are Brooks CK (1976) The Fe|0(fFeO ratio of basalt analyses: an especially grateful to IGME. Athens, for permission to do field appeal for a standardised procedure. Bull Geol Soc Denmark work on Patinos and to the State University or Utrecht and the 23:117-120 Stichting Motengraaff Fondt for financial support for field work. Brooks CK, Printslau 1 (1978) Magma mixing in mafic alkaline Finally, M.B. wishes io acknowledge Hattoh'S. Yoder, Jr. and volcanic rocks: the evidence from relict phenocryst phases and Tim L, Grovt whose perceptive comments at or after the 1914 other inclusions. J Vole Geotherm Res 4:313-331 Fall AGU meeting prompted him to critically reexamine the evi- Bryan WB, Finger LW, Chayes F (1969) Estimating proportions dcncc for fractionation, mixing and assimilation. However, we in peirographic mixing calculations by least-squares approxi accept full responsibility for the views expressed in this paper. mation. Science 163:926-927 Donaldson CK (1984) Crystal dissolution rates in a basaltic melt. In: Progress in experimental petrology. NERC, London, pp 173-178 References Dungan MA, Rhodes JM (1978) Residual glasses and mtlt Indu- tionj in basalts from DSDP Legs 43 and 46: Evidence for Baker I (1969) Petrology of the volcanic rocks of Saint Helena magma mixing. Contrib Mineral Petrol 67:4|7-4JI Island, South Atlantic. Geol Soe Am Bull 80:1113-1310 Eidtelbergcr JC, Gooley R (1977) Evolution of silicic magma Baker PE, Gass 10, Harris PG, Le Maltre RW (1964) The volcano- dtxmben and their relationship lo basaltic volcanism. Geophys logical report or the Royal Society expedition to Tristan da Monop Am Geophys Union 20:37-77 Cunha. Phil Trans Roy Soc Lond Ser A 236:439-578 Fisk MR, Bene* AE, Schilling J-G (1982) Major element chemistry Barton M, Huijsmans JPP (1986) Post-caldcra lavas from the San- of Galapagos Rift Zone magmas and Ihdr phenocrysts. Earth - torini volcanic complex, Aegean Sea, Greece: an example or Planet Sd Lett 61:171-189 the eruption of lavas of near constant composition over a Frisch T, Schmincke H-U (1969) Petrology of dinopyroxene-am- 2,200 year period. Contrib Mineral Petrol (in press) phibolc indusiont from the Roque Nublo volcanic*. Gran Can Barton M, Salters VJM, Huijsmans JPP (1983) Sr-Isotopt and trace aria. Canary Islands. Bull Voleanol 33:1073-10S8 184 111 Fjrtiku MD, Giuliani O. Innocent! F, Marinetti C, Mazzuoli R the join diopside-fonterite-anonhiie from I atm to 20kbar: (1976) Gtoehronological data on recent ma|matitm of the Ac- Their bearing on the generation and crystallization or basaltic jean Sea. Tectonophytics 31:29-34 magma. Contrib Mineral Petrol 66:203-220 Green TH, Wation EB (1911) Crystallization of apatite in natural Reid AM, Donaldson CH. Brown RW, Ridley WL. Dawson JB magmat under high pretiutt, hydrous conditioni. with partlcu- (1975) Mineral chemistry of periodotite xenoliths from the La- lar reference to the “orogenIcH rock series. Contrib Mineral traine Volcano, Tanzania. Phys Chem Earth 9:523-543 Petrol 79:96-103 Rhodes JM, Dungan MA, Blanchard DP, Long PE (1979) Magma Grove TL, Gerlach DC, Sando TW (1912) Origin of calc-alkaline mixing al mid-ocean ridges: evidence from basalts drilled near aeriea at Medicine Lake volcano by fractionation, attimilation 22* N on the Mid-Atlantic ridge. Tectonophyaics 55:35-61 and mixing. Contrib Mineral Petrol 90:160-112 Robert U (1973) Let rochet volcanique de File de Patmos (Dode Harrii C (1913) The petroloiy of lavas and associated plutonie canese, Orece). These 3* Cycle, Laboratoire de Petrographie, inclusions of Ascension Island. I Petrol 24:424-470 Universitl parit-6 Huijsmans IPP, Barton M, Sitters VIM (1913) Evidence for multi Robert U, Canlagrel JM (1977) Le volcanisme bataliique dans ple processes in the genesis of calc-alkaline lavas from Santor le ^ud-Est de la Mer Egee. Donncet geochronologiques et rela ini, Greece. EOS Trans Am Oeophys Union 64:45 tions avec la lectoniquc. Sixth Coll on the Geology or the Ae Huijsmans JPP, Barton M, Sailers VJM (1916) Tectonic setting, gean Region, Athens, pp 961-967 volcanic stratigraphy and geochemistry of calc-atkaline voka* Shand SJ (1922) The problem of the alkaline rocks. Proc Geol nism on Santorini, Aegean Sea, Oreecc. J Vole Geotherm Res SocS Africa 25:19 (in press) Sparks RSJ, Pinkerton H, MacDonald R (1977) Transport of xeno- Huppcrt HE. Sparks RSI (1910) The fluid dynamics or a basaltic lilhs in magmat. Earth Planet Sd Lett 35:234-231 magma chamber replenished by inilua of hot, dense ultrabatic Thompson RN, Etson J, Dunham AC (1972) Major element chem magma. Contrib Mineral Petrol 75:279-219 ical variation in the Eocene lavas of the Isle of Skye, Scotland. Johnson RW, Wallace DA, Ellis DJ (1976) Feldspathoid-bearing J Petrol 13:219-233 potassic rocks and associated types from volcanic islands ofT Thompson RN, Morrison MA, Diekin AP, Hendry OL (1913) the coast of New Ireland, Papua. New Guinea: a preliminary Continental flood basalts ...arachnids rule OK7 In: Hawkes- account of geology and petrology. In: Johnson RW (cd) Volca worth CJ. Norry MJ (eds) Continental basalts and mantle xeno- nism in Australia, Elsevier, New York, pp 297-316 liths. Shiva, Nantwich (UK), pp 151-116 Kuo L-C, Kirkpatrick RJ (1912) Pre-eruption history of phytic Thombcr CR, Hucbncr JS (1912) Dissolution m et of olivine in basalts from DSDP tegs 45 and 46: evidence from morphology basaltic liquids. EOS Trans Am Geophys Union 63:452-453 and toning patterns in plagioclasc. Contrib Mineral Petrol Thornton CP, Tuttle OF (I960) Chemistry of igneous rocks. I. 79:13-27 Differentiation Index. Am J Sd 251:664-614 LeMaitre RW (1912) Numerical petrology: Statistical interpreta Van Bergen MJ, Barton M (1914) Complex interaction of alumi tion of geochemical data. Elsevier, Amsterdam nous m eta sedimentary xenoliths and siliceous magma; an ex LcRocx AP, Erlank AJ (1912) Quantitative evaluation or fractional ample from Mu Amiata (Central Italy). Contrib Mineral Petrol crystallitation In Bouvet Island lavas. J Vole Geotherm Ret 16:374-315 13:309-331 Van Bergen MJ, Ghezzo C. Rlcd CA (1913) Minette inclusions Luhr JF, Carmichael ISE (1910) The Colima volcanic complex, In the rhyodadtic lavas of Ml. Amiata (Central ltaly):/nincral- Mexico. I. Pott-caldera andcsitct from Volcan Colima. Contrib ogical and chemical evidence of mixing between Tuscan and Mineral Petrol 71:343-372 Roman type magmas. J Vole Geotherm Res 19:1-35 O'Hara MJ (1977) Geochemical evolution during fractional crys Venhuis G-J, Barton M (1915) Major oxide chemistry or Precam- tallization or a periodically refilled magma chamber. Nature brlan dolrrite dikes of tholeiltic affinity from Rogaland, south 266:503-307 western Norway. Norsk Geol Tidssk (In press) O'Hara MJ (1910) Non-llnear nature of the unavoidable long-lived Watson EB (1912) Basalt contamination by continental crust; some Isotopic. trace and major clement contamination of a develop experiments and models. Contrib Mineral Petrol 10:73-17 ing magma chamber. Phil Trans Roy Soc Lond Scr A Wyers GP, Barton M (1914) Transitional alkaline - subalkaline 297:213-227 lavas from Patmos, Orecce: an example of a magma series O'Hara MJ, Matthews RE (1911) Geochemical evolution in an formed by fractional crystallization, attimilation and magma advancing, periodically replenished, periodically tapped, con mixing. Trans Am Geophys Union 65:1123 tinuously fractionated magma chamber. J Geol Soc London Yoder HS Jr. Tilley CE (1962) Origin of basalt magmas: an experi 131:237-277 mental study of natural and synthetic rock systems. J Petrol Osborn EF (1959) Role of oxygen pressure in the crystallization 3:342-532 and differentiation ofbasaltic tnigma. Am J Sd 257:609-647 Presnal) DC, Dixon SA, Dixon JR. O’Donnell TH, Brenner NL, Schrock RL, Dycus DW (1971) Liquidut phase relations on Received October 13,1915 / Accepted March 14,1916 Contributions to Contrib Mineral Petrol (1987) 97:279-291 Mineralogy and Petrology O Sprfe|tr*VcrU| 1917 CHAPTER VI GEOCHEMISTRY OF A TRANSITIONAL NE-TRACHYBASALT - Q-TRACHYTE LAVA SERIES FROM PATMOS (DODECANESOS), GREECE: FURTHER EVIDENCE FOR FRACTIONATION, MIXING AND ASSIMILATION G. Paul Wyers and Michael Barton Department or Otology and Mineralogy, The Ohio Stale Univenity, Columbut, OH 4)210, USA Abstract Trace-clement and preliminary Sr- and O-iso to pic end-member enriched in LREE relative to the average cruit. data are reported Tor a transitional alkaline-sub-alkallne Zr and Hf data are also not well explained and indicate lava series (MVS) from Patmos, Greece. The lava types that the assimilant was depleted in HFSE relative to avenge belonging to this series are ne-trachybasalt, hy-trachybasalt, crust or that HFSE are held back in relatively insoluble hy-trachyandesite and Q-trachyte. Rb, Sr and Ba contents, phases such as zircon in the rest lie during assimilation. Nev as well as K/Rb ratios, of the ne-trachybasalts differ from ertheless, the results or the modeling demonstrate that Ba those o f alkali basalts o f oceanic islands and those ofK-rich concentrations may decrease during AFC processes and alkaline lavas of continental regions and are consistent with that high Sr contents («• 1500 ppm in the MVS nc-trachyba- the occurrence or these volcanlcs in a destructive plate mar salts) do not render made, parental magmas immune to gin environment. Qualitatively, the variations shown by the effects o f assimilation in terms ofthdr ,TSf/, ‘Sr ratios. many trace elements throughout the MVS are explicable The results o f this study confirm conclusions based upon In terms o f magma evolution via fractional crystallization major-oxide and mineral chemical data for the MVS lavas involving removal of the observed phenocryst phases. but, more importantly, show that careful analysis of trace Cross-cutting REE patterns can be explained by. removal element data allows the various processes involved in o f small amounts o f apatite. However, certain features of magma cvotution to be identified and quantified, even in the data cannot be reconciled with the operation of frac the absence o f major oxide and isotopic data. Finally, it tional crystallization alone. These are: a) the compatible is reiterated that magma mixing and assimilation may be behavior of Ba throughout the MVS; b) the moderately coupled processes in the magma chambers beneath many (as opposed to highly) incompatible behavior o f Zr, Rb volcanic centers, and recognition of this ffcet has profound and Nb relative to Th; and c) the significant decrease of implications for studies of magmat erupted at continental K/Th, Rb/Th, Zr/Th, Zr/Nb, Nb/Th, Yb/Th, Ta/Th, U/Th margins and through continental crust. and Zr/Ta ratios especially (but not exclusively) in the made part o f the series. Quantitative modeling indicates that the hy-trachybasalts are anomalousty enriched in both highly incompatible and highly compatible elements and these la vas are shown to be hybrids formed by mixing o f ne-trachy Introduction basalt and hy-trachyandesite. Mixing proportions o f the Recent work hat documented the complexity o f processes end members calculated from incompatible element abun involved in the genesis and evolution of magmas erupted dances ( —19% ne-trachybasalt) differ from those calcu at convergent plate boundaries. Understandably, much at lated from compatible element abundances («-6274 ne-tra tention has focused upon the role of subduction in arc vol- chybasalt) and are inconsistent with proportions calculated caniun and has led to the development of geodynamic mod from published mineral chemical data. In addition, mixing els for the evolution or the upper mantle and the crust. cannot account for the observed variations in incompatible However, it is important to recognize that few erupted mag element ratios and this is taken as evidence for the simulta mas represent primary melts - that is, pristine upper mantle neous operation or assimilation. Isotopic variations (,TSr/ melts. It has been demonstrated convincingly that several **Sr from 0.7049 to 0.7076 and , , 0 /'* 0 from 4.7 to 8.6%J processes may operate simultaneously during evolution or and the positive correlation o f isotope ratios with SiO, and arc-magmas in imra-crustal storage chambers. The most Th contents provide conclusive proof that assimilation oc commonly identified processes are fractional crystallization curred. Calculations show that the isotopic characteristics and magma mixing (Anderson 1976; Luhr and Carmichael and the concentrations o f many trace dements in the Q- 1980; Huijsmans and Barton 1983, 1984) but the rote of trachytes can be explained by fractional crystallization of assimilation is being increasingly established (Briqueu and ne-trachybasalt combined with assimilation o f ovtrati con Lancelot 1979; Taytor 1980; DePaolo 1981; James 1982). tinental crust (*’Sr/, *Sr—0,710), and that large amounts Since the combined effects o f fractionation, assimilation of assimilation are not necessary (M a/M oO .J5). REE data and mixing may be exceedingly complex (O’Hara and Mat are not well explained by this model and suggest a crustal thews 1981), it is imperative that the possible operation o f these processes is taken into account before more funda Offprint rtquaUIt; G J. Wyers mental processes, which occur in the upper mantle, are tack- 185 186 280 leJ. Failure to Fully evaluate the intravrustal evolutionary and phonolitc. Banon and Wyerx (1986: also in prcparatinr.i -ho* hittory or magma series can lead to serious errors of inter* that all of the ne-trachybasalts erupted on Patmos were Jcriicd pretalion of melting processes, source region characteristics from an upper mantle source which contains a substantial compo etc. (O’Hara 1977). nent from a subducted slab, possibly that which undctlict the Hel Trace element and isotopic studies are especially useful lenic arc. These workers also show that the source region ccnuins an "enriched** component (similar to that in the source regions for constraining and extending models based upon major of oceanic island basalts) and that the nature of the source region oxide, whole-rock chemical and phenocryst data. In the changed with lime. This reflects vertical or, more probably, lateral present paper, we present trace element and preliminary heterogeneity in the mantle wedge beneath Patmos (see Barton Sr* and O-isotopic data for a transitional alkaline - sub- and Wyers, in preparation for a fuller discussion). The only non- alkaline lava series from Patmos, Dodecanesos. Greece. The volcanic rocks are marbles which are exposed in the south western petrology of these lavas was discussed in a previous paper part of the island and which appear to have been cmplaced tcctoni- (Wyers and Barton 1986) in which it was concluded that cally. fractionation, magma mixing and assimilation had played Wyers and Banon (1986) identified a Main Volcanic Series a rote In magma evolution. We use the trace element and (MVS) consisting of ne-trachybasalts, hy-trachybasalts, hy-tra- chyandcsites and Q-trachytes which was probably erupted isotopic data to confirm this hypothesis. 5.6 m.y. B.P. (Barton and Wyers 1986). This series also forms the subject of the present paper. The relationship of other rock-types Summary or geology on Patmos (Including tome ne-trachybasalts) to the MVS is not fully understood and forms the basis for ongoing research. Major Patmos is located in the southeastern Aegean some 100 Itm north oxide whole-rock and phenocryst data for the MVS lavas indicate of the Hellenic arc (Fig. I). Volcanism along the latter it exclusively a complex evolutionary history involving fractional crystallieation. calc-alkaline in character ond began 4.4 m.y. B P, (Pe-Pipcr et al. magma mixing and assimilation of crustal material. 1987) in the north west. The last eruption occurred on Santorini in 1950. This activity is related to tubduction of the African plate beneath the Aegean microplate (Ninkovitch and Hays 1972: Papa- Analytical techniques radios and Comninakit 1971) or, possibly, obductlon of the latter Trace element analyses were done at Utrecht by XRF and at IRI. over the former (Litter et al. 1984). DelA, by INAA. Established techniques were used and a variety The geology of Patmos was described by Wyers and Barton of International and well-charactcrired internal standards were in (1986), and only a brief summary Is given here. Volcanism occurred cluded in each batch for analysis. Precision of the analyses has *■6.0*4. J m.y. B P. (Barton and Wyers 1986) and yielded a wide been determined by J.P.P. Huijsmans and is: XRF - better than variety of lava-lypei. from ne-trachyhault to trachyte, rhyolite 10V. (relative) for Ni. Zr, Rb. Sr and Ba, better than 25% for Nb: INAA - better than 10% Tor Sc. V, Cr, Ce. Sm. Eu. Yb. Hf. Th, and U, and better than 15% for La, Tb, Lu and T«. •’S r/'S r ratios were measured at the ZWO Laboratory for Isotope Geology. Amsterdam, using techniques described by An- drictscn el al. (1979). Analytical uncertainty is better than 0.06'M and all samples were analysed in duplicate. " 0 / " 0 analyses were performed in Utrecht by A. de Jong. Oxygen was liberated by reaction with BiFt following the method of Clayton and Mayrda (196)). The dO " values are reported as permit differences (*„) relative to the SMOW standard and all results ore reported relative to a value of 9.65£» for the NB5*28 standard. Analytical precision is better than 0.I-4.2X,. Because some of the lavas have been af Uwn g Hlfw# fected by alteration (nb. Iron hydroxides, clay minerals and scrictie are observed In thin sections of some samples), most isotope analy ses were done on fresh plagiodase phenocrysts which were sepa rated from the rocks and purified using heavy-liquid and magnetic methods. For one sample, *’Sr/'*Sr ratios were measured for both a plagiodase separate and the whole-rock in order to examine the possible effects of alteration. Results Results are presented in sa t. s.t for those samples for which major oxide whole-rock data have been previously reported (Wyers and Barton 1986) and for samples for which isotopic data have been acquired. Major oxide data for the latter are available from O.P. Wyers upon request. Trace element data for the ne-trachybasalts confirm the alkaline character o f these lavas. The elevated Rb, Sr and Ba contents are especially characteristic o f alkaline magmas. As will be not ed later, there it no evidence that the concentrations of these strongly incompatible elements have been significantly affected by post-eruptive alteration processes (cf.. Hart et al. 1974). The ne-trachybasalts are strongly enriched in Ba and Sr compared with alkali-olivine basalts from most rit-t.i. Location of Patmos in relation to the Hellenic oceanic islands (Le Maitrc 1962; Baker et at. 1964; Baker Utnch and Tcrtiary-Rceenl volcanic arc 1969; Clugue and Frey 1982; Le Roex and Erlank 1982) 187 281 and continental rcgioni (Thompson et at. I9S0; Kyle 1981). S' R2a 5R & most shoshonitet (e.g.. Gill 1984), and even most malic d rJ or -a rj » — members of the alkaline LKS series or Roccamonfina (Ap 3Sf*sjs*8§3Ris2- jsja? pleton 1972). Emici (Civetta etal. 1981) and Murjah (Ni- s - eholls and Whitford 1983). Rb contents are similar to those ; SRSSsSS S of tome oceanic island alkali basalts (e.g., those from Tris j|g,-SI!M g 2 | 5 £ p ;fjV-dride;H-d tan da Cunha - Baker et at. 1964) and the LKS basalts from the localities cited above. K/Rb ratios (196: t*«< *•>) R “ are very low in comparison to many alkaline lavas (cf., R Clague and Frey 1982) but are similar to those or K-rieh 3 SSRSSSPi 3 tavas from Central Italy (121-202). These data reinforce £ 6 3gr'S£,S2g§gR2'r"ei"efB:r,Sf®; the conclusions o f Wyers and Barton (1986) that the ne- trachybasalts are not directly comparable to the alkaline lavas o( oceanic Islands or the typical K*rich alkaline lavas c 5^ 2 »R R 2 R ? of continental regions. Following Barton and Wyers (1986). i| = gs-TR2g2g32"'J ta tu ».!. Important elemental and otlde ratios in selected Patmos lata* ' Lava type Comments Nc-Tb HyTb H>-Ta OT K.TTi 2499.1 1321 1363 1223 RbiTh I I I 7.S 7.33 6.19 K.Rb 195.6 202.1 113.6 197.7 Ave all lavas-195.4±9 Zr/Th 24.4 10.4 9.41 1.6 NbiTh 1.67 0.19 1.03 0.95 Ave evolved lavas *0.96 ±0.07 Ce/Th 11.7 3.96 3.41 167 Eu/Th 0.27 0.012 0.057 0.042 Tb/Th 0.119 0.034 0.027 0.011 Yb/Th 0.2022 0.013 0.075 0.012 Ave evolved lavas-0.081 ±0006 Hr/Th 0.444 0.263 0.252 01305 Taith 0.0123 0.0604 0.0521 0.0564 Ave evolved lavasav0.056 l0.004 U/Tti 0.4466 0.2367 0.220 0.2351 Ave evolved l*v*i«0.237±0.02 R.O./Ce 66.67 70.27 41.37 23.2t Tl.V 34.21 33.21 47.11 57.91 Ti/Zr 31.11 23.36 16.49 9.13 ZrjHf 43.47 39.01 37,43 37.19 Ave all lavas-39.3±4 Zr/Nb 14.67 11.60 9.14 9.00 Zr.Ta 293.3 171.60 110.79 131.9 Nb/Ta 20.0 14.1 19.77 16.17 Ave all lavas-17.9 ±3 Abbreviations 11 In Table I 100 1000 too • s • « a > I | too 100 » 10 100 T* IOOO *1 10,000 4 t* | IOOO I 10 too I 10 100 I 10 100 100 -| to .000 « < ■ rti.a.i. Variation diagrams Tor aelected I trace etcmenli with Th as a diirercntiotion Indea. Note the unusual behavior of Ba. which it normally a strongly incompatible too ion dement. See teat Tor further discussion • to too I 100 100 I til ITK members o f the MVS. The behavior o f Rb may alto be Rb versus Th would result in a discontinuous trend con anomalous, but since phlogopite or biotite is a phenocrystal sisting or two or more segments as shown in sts.s.s*^ The phase in the intermediate lavas (hy-trachybasalt to hy-lra- initial rate or enrichment or Rb is 2.21 whereas the rate chyandesite) it is possible that Rb behaves as a highly in* or enrichment in intermediate and evolved lavas is 1.17. compatible element during the initial stages of evolution Corresponding values for Th are 1.8} and 2.48 respectively and at a moderately incompatible element in the intermedi to that if Rb variations are entirely due to fractional crystal ate to late stages of evolution (nb. the evolved magmat lization then this element was more incompatible than Th also contain unidine phenocrysts). In this case, a plot of during evolution o f the primitive magmas. 189 383 too a lo0 Th 10 ■Tfc 400 700 0 T 10 »Th 1m In Tfe ri|.t.s*-«.ploii or Rb, Zr and Nb vtnus Th. Straifhi lines repre rii.S.s. Chondrite-normalitcd plou of REE concentration! in rtpre- sent the best fit to the data and were fitted visually. In a we show lentatlve samples of the main lava types. Abbreviations: Ne-Tb that Rb variations can alto be described by two sttuliht lines, nepheline normative trachybatalt; Q-T quirtt trachyte. Note that one of which piuea throujh the origin. See tu t for detail* the ne-trachybaialu arc enriched In the intermediate and light REE, but depleted in the heavy REE, relative to more evolved member! of the Main Volcanic Series Probably (he moit unusual behavior of all trace ele T*H» s.s. Sr- and O-itotopic data for selected Patmos MVS lavas ments is that shown by Ba. Concentrations of this normally incompatible element decrease with increasing Th and com Sample Rock type *’Sf/*‘S r= ld 4 "o r„ > parison with studies of other alkaline lava series suggest that this behavior it unlikely to reflect fractional crystalliza PAT-187 Ne-Tb 0.70507 = 9 5.) 070489+16 tion alone. PAT-tB7* Nc-Tb 5.3 PAT-t28 Ne-Tb 0.70492 ±22 4.7 REE data are shown in chondrile-nomtalized plots in PAT-180 Hy-Tb 0.70687 ±29 7.0 ru.s.t. The light rare earth elements (LREE) remain ap PAT-165 Hy-Ta 0.70667±t6 6.4 proximately constant from ne-trachybasalt to Q-trachyte. PAT-105 Hy-Ta 0.70684 = 7 7.6 whereas the HREE increase systematically throughout the PAT-1 SO Q-T 0,70761 ± 8 8.6 series. In contrast, the middle REE (Sm—Tb) decrease from ne-trachybasalt to Q-trachyte. These variations result in All measurements made on plagioclase separates, cxctpt Pat 187* cross-cutting REE patterns. The LREE enrichment (relative which it a whole-rock analysts.Abbreviations as in Table 1 to the HREE) o f the Patmos MVS is, however, a character istic shared by alkaline basalts from both oceanic islands and continental regions (Thompson et al. 1980; Ctaguc and Frey 1982). All lavas are characterized by a small negative Eu anomaly though the ubsolute magnitude or the anomaly ^ rasa cannot be determined In the absence of Gd analytes. ,TSr/**Sr and * '0 /u 0 analytes are listed In m u s.s. ■ rota *’Sr/*‘Sr and "O /^O isotope ratios both show a wide range of values, from 0.7049 lo 0.7076 and from 4.7 to l"o 8.6%. respectively. Both , , Sr/MSr and " 0 /“ 0 increase rit.s.s. ,,Sr/,*Sr plotted against <5l,0 for representative samples from ne-trachybasalt to Q-trachyte and there is a fairly of the MVS good correlation (rw0.960) between u 0 / ‘*0 and ' ’Sr^'Sr (ru.*->). Since most of the isotopic measurements reported in t«si> s.s were made on plagiodase separates (with low Rb/Sr), variations in ' ’Sr/'‘Sr cannot be due lo the decay pie 165 is alto due to alteration (teerti.t.s). ' ’Sr/'^Sr versus or*’Rb since extrusion. In the case of the single whole-rock SiO) systematics (ua.i.t) suggest thut Sr-isolope ratios have measurement (Pat 187), the reported *’Sr/“ Sr ratio has been affected by hydrothermal alteration to a lesser degree been corrected to an age of 6.6 m.y. which Is suggested than oxygen isotope ratios and support Tor this contention by K -A r age data (see earlier discussion). it provided by the plagioclase separate/whole-rock analyses We interpret the low <50*’ value for one of the ne- of one or the ne-trachybasalts (Pat 187): l , Sr/**Sr ratios trachybasalt ( # 128) to reflect hydrothermal alteration, of the plagioclase separate (0.70507 ±9) and whole-rock and it seems probable that the relatively low value for sam- (0,70489 ±16) are identical within analytical error. On the 190 284 tavw s.t- Diitribulion otcfTiucnlx required to explain trace clancm variation! attuning that />?» *0 \s Itlt • .* J* ■ % Sample No. Pal 54 Pat 78 Pal 5X ■ »»•« ■ Value of F 0.12 026 0.21 to 10 10 Sc I.I6H 1.888 1.772 H i.t.t. a,Sr;**Sr plotted against SiO, V 1.0 JH 1.485 1.787 contend Cr I.J9(> ).4 2X5 or Rb (and, alto or K) and the possibility exists that Rb tional crystallization must have operated during evolution behaved at a highly incompatible element during the initial of the MVS. itagei or magma evolution and at a moderately incompati ble element in the intermediate and later ttagei ofevolution Comparison o f major oxideandtract elementdata (rse.s.i). |f during the Initial ttaget or magma evolution then /yjfM'ece may be calculated from ria.s.s The distribution coefficients required to explain trace ele and it -*0.24, an impottibly high value given that apatite ment behavior in the MVS may also be calculated by using (the only significant repotitory Tor Th) it a minor compo the value of f determined in major oxide, lesst-squarcs mass nent or the fractionating solid assemblage. The maximum balance calculations, since D ■ 1 + tog {Cj-/Cf)/1ogF where pottible value or ( ^ “’•“calculated from apatite abun Cf and Cf are the concentrations of any element in the dance it 0.07, We contider it to be unlikely that Rb it magma of interest and in the initial magma, respectively, more incompatible than Th during the initial itagei or and F is the fraction o f remaining melt. The advanuge magma evolution became, at thown in ris.t.s, this would of this approach is that the results o f major element model require that a phate tuch at phlogopile or unidine became ing can be directly tested using truce element data. Bulk itable on the liquidut in the ne-trachybault to hy-trachyba- solid-liquid distribution coefficients for all trace elements tall compotiiional range and there it no peirographic evi are listed in tisi* s.s together with values o f F taken from dence Tor this. Indeed, Wyen and Barton (1986) argue that Wyers and Banon (1986). The proportions o f the fraction phlogopite and unidine were not phenocrytt phases in the ating phases (Wyers and Barton 1986) have been recalcu hy-trachybaulli to that there it no reaton Tor lated to cumulative values taking ne-trachybasalt (Pal. 26) to change dramatically during the initial ttaget or magma lo be the parental magma and were used to calculate bulk evolution. We conclude that apparent diitribulion coefD- solid-liquid distribution coefficients. The relative change in cientt for Rb and K are abnormally high. Since identical concentration, Cf/C? for selected trace elements in respre- argument! apply to the other incompatible trace elements, tentative samples is plotted against Finrn.i.r. we Infer that these data are inconsistent with a simple frac tionation model. It must be emphasized that the relatively high distribu tion coefficients required to explain incompatible element 1>XU S.S .Distribution coefficients required to explain trace element behavior result from the assumption that the behavior of variations using values or F taken from the results or mujor-oxide muss-batancc models Th can be explained in terms fractionation, or _ q and F" Cn,/Cr\ where f i t the Traction of liquid remaining Sample No. Pat 54 Pat 75 Pat 58 at any stage of the fractionation process. C%, is the concen tration of Th in the ne-trachybaults and is the concen Value of F 0.61 039 0.31 tration of Th in any residual magma. If this assumption Sc 1.38 1.80 2.01 it invalid, then the high distribution coefTirients have no V 1.13 1.64 2.02 absolute significance. Thus, ifTh has been enriched by pro Cr 2.32 4.40 3.32 cesses other than fractionation, distribution cocfllcienis for Ni 2.42 4.40 3.73 other elements will apparently be high, i.e., will be an arti Rb (-0.2) 0.18 0.31 fact of the assumptions made in the method o f calculation. Sr 1.91 1.93 1.91 For example, ir the apparent distribution coefficient for Zr 0.45 0.60 0.58 Th is -0 .2 (see below) rather than 0 in the initial stages Nb (-0.02) 0.10 0.17 1.34 of magma evolution (ne-trachybault to hy-trachybasalt), Ba 1.46 1.48 ^ " ^ - 0 . 3 7 and 0 £ i" ,” “ -O .7l. Obviously. Th must La 1.18 1.06 1.05 Ce 0.89 0.89 0.95 have been very strongly enriched by processes other than Sm 006 1.13 0.96 fractional crystallization (hence the use o f a negative value Eu 1.14 1.25 1.2V o f DTlun* )if realistic values of were applicable Tb 1.26 1.17 128 during magma evolution. On the other hand, the more real Yb 0.47 0.65 0.46 istic auumption (on the basis o f peirographic evidence) that Lu 0.61 0.46 0.35 />{»*lt'*Mwal greater than zero compounds the problem Hf (-0.23) 0.20 0.25 of interpretation of the behavior of other incompatible de- Ta (-0.62) 009 0.03 menu. Thus, if 1* taken to be 0.07, then Th (—1.26) (-0.41) (-0.30) U (-0.16) 0.34 0.24 - 0 .5 1 and 0 Z"m‘»’,“ -O.77. K (-0.29) 0.23 0.29 Finally, we note that during at least the initial suges or fractional crystallization, incompatible element ratios Values of F reported in Wyers and Barton (1986) recalculated to should remain approximately constant. It is dear from Ta- cumulative values assuming Pat 26 is the parental magma. Values blet.zthat this is not the case in the MVS. Some ratios of D fL for most elements (i.e., excluding the highly incompatible (K/Th, Rb/Th, Zr/Th and Zr/Nb) decrease throughout the elements In Pat 34, Th and Ba) arc entirely consistent with values series although the greatest decrease occurs between the calculated from the proportions of the mineral phases obtained ne-trachybasalu and the hy-lrachybaulls i.e., during the from the major oxide mass-balancing models and mineral-liquid tarllest stages o f magma evolution. Other ratios (Nb/Th, distribution coefficients from the following sources: Arth (1976). Yb/Th, TafTh, U/Th. Zr/Ta) decrease during the early Baker etal. (1977), Irving (1978), Luhr arid Carmichael (1980), Gill (1981), Kyle (1981), Roden (1981). Ewart (1982), Mahood stage of evolution and thereafter remain approximately and Hildreth (1983) and Salters (unpublished date). Analytes or constant. The departure from predicted behavior, as docu micas and coexisting groundmasses done by VJ.M. Salters were mented by these incompatible element ratios, probably pro used to calculate Kd's for Ta, Hf, Th. Sc and La. We arc grateful vides the strongest evidence that processes other than frac- to Salieri for permission to use these data 192 216 bution coefficients calculated for nearly all other elements are similar lo, or show similar variations as, the ones re ported in T>it* s.t. For most compatible elements (including Ba), required distribution coefficients are slightly higher (nb. Ni, Cr, Sc, V) but for Ni and Cr there is the same A tendency for calculated distribution coefficients to increase i l and then lo decrease. Thii observation is important inas e" much as it demonstrates that this phenomenon is real and is not the result o f the method of calculation. Distribution coefficients required to explain the behavior of the REE are slightly higher (in the case of La, Eu and Tb) or lower or! (in the case or Ce, Yb and Lu) than those reported in Ta bles.* which is consistent with the more or less neutral be havior o f these elements in these lavas (ru.s.t) and with the differences between distribution coefficients calculated for compatible and incompatible elements using the two different approaches described above. The distribution coefficients listed in m il s.s may be compared with permitted values, calculated from mineral- liquid partitioning data and the proportions of the solid phases determined by least-squares mass-balancing tech niques. With the exception of the highly incompatible ele c1 ments during the earliest stages of evolution, and of Th c* and pa, the required distribution coefficients are consistent with published data and with the results of major element 0 1 modeling. Specifically, it should be noted that a) the rela tively high values o f Du required by the data can be ex plained by removal ofTi-magnelite and biotiti; b) the high values o f Du can be reconciled with the removal ofapaiiie, b for which there is peirographic evidence (Wyers and Barton 01 «• Oi 01 01 SI 1986); c) the behavior o f the light and intermediate REE rii.t.r*,i. Oburvcd versus predicted chanttt In tract tltment eon- can likewise be reconciled with the removal o f apatite; d) ctntntion at a function of F, the fraction of liquid remaining al any stage during fractional crystallization. Value* or F were calculated distribution coefficients for highly compatible el taken from the multi of major element, kut-tquiret mau balance ements (Sr, Ni, Cr, Sc, V) are well within the range of calculation* (Wytri and Barton 1916). a (incompatible dementi) values allowed by mineral-liquid data; e) increasing values clrclrl Rb; ijuarti Th; trlanflti Zr; h (compatible demenu) open of D u >nd Dv with evolution are explicable by removal tiftlei Ct;/UlrJ ciixlti Sr; tquarrt Sc; iriwitla Ba of mica from the evolved magmas. We Interpret the results of this analysis to indicate that fractionation hat played an important, though not exclusive role in the evolution The main difference between the retulti presented in of the MVS. We comment further on the behavior orcertain M il a.* and those presented in to il s.* is that calculated elements (e.g., Ba) in the following sections. distribution coefficients for several incompatible elements Comparison of the data reported innst™ s.* «a syn d i (Rb, Nb, Hf, Ta, Th and U) are itfgatln for the early cates that distribution coefficients extracted from log-log stages of evolution in the former table. The distribution plots for compatible dements are also consistent with pub coefficients for Th remain negative throughout the series, lished data and it is thus appropriate to comment upon whereas those for the Other incompatible elements are posi the relative merits of the different approaches adopted in tive for steps involving the intermediate and evolved lavas. this study. When using trace clement systcmatics the as Negative distribution coefficients are cltarly impossible, sumption must be made that the element showing the great and indicate the operation o f processes other than fraction est degree of enrichment behaves as if and, ation. The negative values are evidence that during the early as noted previously, this assumption is invalid if processes stages of evolution incompatible elements are enriched to other than fractionation have occurred. On the other hand, a greater degree than can be achieved through fractional coupling o f trace element and major oxide models requires crystallization atone (see rts.s.r). The behavior of Th sug that the latter are reliable, but there is no sure way to gests that the processes which lead to this incompatible evaluate the accuracy, or validity, o f major oxide models element excess operated (to a lesser extent) alto during the (cf., Wyers and Barton 1986). Indeed, we have shown that hy-trachybasalt-trachyandesite-Q-trachyte stages of evolu major oxide variations in the MVS are not entirely consis tion. tent with fractional crystallization so that there is some Disregarding the negative values noted above, it is note doubt about the validity or values or F determined in mass- worthy that the calculated distribution coefficients for Rb, balance, major oxide calculations. It is for these reasons Nb, H f and Ta in the more evolved lavas are substantially that we have used two different approaches to trace element lower than those calculated In the previous section and are modeling and it Is encouraging that both lead to identical in belter accord with values expected for such incompatible conclusions. Nevertheless, we feel that coupling o f trace elements. Nevertheless, distribution coefficients required to and major element data yields the most satisfactory method explain the behavior of U and Zr are relatively high. Distri o f evaluating the processes involved in magma evolution 193 since in most cam the result! or major element modeling t.tl. s.s. Estimated proportions of mafic mapru which mix with provide the best fit to the majority of the data. evolved magma to produce the hy-irachyhaulti Incompatible elements Compatible elements Ecideme for magma mixing Th 24% Ni 46% Wyers and Barton (1986) have presented convincing miner Hr 25V. Sc 66% al chemical evidence that the hy-trachybasalts are hybrid u 9V* V 16*4 magmas formed by mixing o f ne-trachybasalt and trachyan- Ta **/. Cr 49% dcsite. O'Hara (1977) and O'Hara and Matthews (1981) Rb 29V. have shown that mixing results in anomalous enrichment Average 19V. 62V. o f highly incompatible elements in the hybrid magmas and hence the results presented above are entirely consistent with simultaneous operation of mixing and fractionation. The highly incompatible elements are particularly sensitive to make such variations noticeable and we have noted else indicators of proceues other than fractionation since miner where fWyera and Barton 1986) that mineralogic and peiro al-liquid distribution coefficients for these elements are, by graphic data provide evidence for only one mixing cycle. definition, dose to zero and there it thus no uncertainty Additional evidence that simple back mixing of the ne-tra- In the selection of values for the purpose of modeling. There chybasalts and the trachyandesites cannot yield the hy-tra- is tome peirographic and mineralogic evidence that mixing chybasalts is obtained by calculation of the proportions may have played a role in the evolution of the most evolved of mafic and evolved end-member involved in the mixing lavas on Patmos (hy-trachyandesitet and Q-trachytes). process from incompatible and compatible element data. (Wyers and Barton 1986) and the behavior of Th (t *s l s.s) Results are listed fnt*»t* s.s, and show that a wide variation in these lavas is consistent with this. The behavior of other in the proportion of the mafic end-member is required to elements (Rb. Nb. Ta. etc.) provides no evidence for mixing explain the trace element data. Moreover, it is apparent or these lavas, however, and even though these elements that In general a higher proportion of ne-trachybasalt (aver- are not truly incompatible at the more advanced stages age-62% ) is necessary to explain compatible clement o f evolution and hence would not show the same amount abundances in the hy-trachybasalts than is necessary to ex o f enrichment as Th. these data strongly suggest that mixing plain incompatible element abundances (average propor was of limited importance in the genesis or the trachyande- tion 19V* mafic end-member). Neither value corresponds sites and Q-trachytes. to that deduced from mineral chemical data for pyroxenes Although both mineralogic and incompatible trace ele and plagioclases (80-77% mafic end-member; Wyers and ment data provide extremely strong support for the magma Barton 1986) so that either multiple mixing events occurred, mixing origin of the hy-trachybasalts. It must be emphasized and are not recorded in the phenocryst assemblages due that other processes, e.g.. assimilation, can also result in to homogenization of the hybrids, or other processes oper anomalous, high incompatible clement concentrations (e.g.. ated concurrently with fractionation and mixing. DePaolo 1981). It is thus not possible to discriminate be tween mixing and assimilation using incompatible elements Etidence/or assimilation alone. Compatible trace element data are, however, useful in this regard since mixing, unlike assimilation, leads lo The Sr- and O-isotopic data for the MVS lavas (r*su s.s; anomalous enrichment of strongly compatible elements in ri'.t.i) indicate that assimilation played an Important role the hybrid magmat (O'Hara 1977; O'Hara and Matthews in their evolution. The observed, positive correlation be 1981). The initial increase and subsequent decrease in calcu tween ,1Sr/, *Sr and " 0 /'* 0 is precisely that expected to lated values for the bulk solid-liquid distribution coeffi result from assimilation (Taylor 1980; James 1982). More cients for Ni and Cr f»n*> t.t «4«. vreflects relative enrich over, since both isotopic ratios correlate positively with con ment of these elements in the hy-trachybasalts (secrt*.*.)) ventional differentiation indices such as SiO) (rn.s.s) and and the tendency for calculated distribution coefficients for Th, it is dear that fractionation accompanied assimilation. Sc and V to increase during magma evolution can also However, inspection o f ru-.t.s **e s.s indicates that the be interpreted to reflect magma mixing (seet*»i* s.s). We amount of assimilation did not increase uniformly during conclude, therefore, that trace element data provide in differentation. "Sr/'^Sr ratios for the ne-trachybasalts are disputable evidence that mixing accompanied fractionation substantially lower than those for the hy-trachybasalts, tra- and that mixing was especially important in the genesis chyandcsites and Q-trachytes and it appears that the evoked of the hy-trachybasalts. magmas have experienced a simitar degree of contamina It is necessary to determine whether combined mixing tion. This is consistent with the model presented by Wyers and fractionation can account for all o f the trace element and Barton (I9B6) for the evolution o f the Patmos MVS. characteristics o f the MVS lavas and specifically for the in which ne-trachybasalt is cmplaccd in intracrustal magma observed variau'ons in incompatible clement ratios. It is chambers, differentiates to produce Q-trachytcs and tra- well known that hybridism is fur more effective than frac chyandcsites, with later mixing of trachyundcsite and a new tional crystallization (at least over much or the range of batch o f ne-trachybasalt to yield hy-trachybasatt. It is ap crystallization) to generate variations in the ratios o f very parent that significant contamination or the Q-trachytes highly incompatible and moderately incompatible elements and trachyandesites occurred during their residence in the (O'Hara and Matthews 1981; Eq.42) and the decreases chamber. However, rti.s.s demonstrates that the hy-trachy- in certain elemental ratios reported im*»u s.s for the MVS batalis cannot simply be mixing products of the evolved may thus be attributable lo the efTects of mixing. However, lavas and a new batch o f ne-trachybasalt as ,TSr/**Sr ratios a targe number o f mixing cycles (-1 0 ) are required in order for these lavas do not lie on a mixing line betwen the primi 194 311 tive and evolved magmat. Thii indicate* that attimilation accompanied mining and fractionation and that mining probably hat operated concurrently with attimilation in the moit evolved magmat. Thii it an important conclusion at previoui worker* (O'Hara and Matthews 1911; Van Bergen and Barton 1984) have noted that mining, or underplating of evolved magma by hot primitive magma may supply I the additional heat required for attimilation to be effective and alto may trigger vigorous convection which facilitate* disruption of the wall-rock and any entrained xenolitht. Indeed, mining or underplating may be necessary if relative* ly cool, evolved magmat (e.g.. trachyandesites, trachytes) are to assimilate substantial amounts o f crustal material a (cf., Van Bergen and Barton 1984). The very fact that mining and attimilation operated con* currently makes it extremely difficult to separate the effects of these processes on trace element concentrations. This Is especially true if the crustal end-member involved in the process is unknown, as is the cate for the Patmos MVS. The isotopic data indicate that the contaminant was charac terized by ’’Sr/’^ r i 0.7076 and d‘'0!58.6% , but these c-j characteristics apply to a wide range of cruttal rock*types. I However, at shown below, trace element data for the MVS .ii lavas also provide clear evidence that assimilation occurred. At noted previously, there is a discrepancy between the mixing proportions calculated from major element (mineral chemical) data, incompatible trace element data and com b patible trace clement data. Such discrepancies are entirely u«.l.t».v. Log (Th.Zf) and log (Thi'Rb) versus log (Th). Solid consistent with concurrent operation of fractionation, mix tints show trends expected for fractional crystallization using the ing and assimilation. Specifically, incompatible elements are distribution coefficients given on the diagram. The uppermost line more enriched than can be explained by mixing alone on each diagram represents the maximum rate of change in (Th/Zr) whereas compatible elements are more depleted than can and (Th/Rb) attainable via fractional crystallization because maxi be explained by mixing alone. Since the crust is generally mum values of Du and £>«, (strictly applicable to only the hy- enriched in incompatible elements, and depleted in compati trachyandesites and Q-lrachylcs) were used for construction. ble elements, relative to magmas derived from the upper DashedUnts show the trends expected for fractional crystallization mantle, we believe that these discrepancies support the frac* If £>rs>0. See text for details tlonation-mixing-atsimilaiion model. In this context, It it probably significant that the K-contents of the hy-trachyba- salts are inconsistent with an origin by simple mixing of It Is dear that trace element characteristics of the hy-trachy- the ne-trachybasalt and hy-trachyandesite (Wyers and Bar basalts and Q-trachytes reflect the operation or assimilation ton 1986). Specifically, the proportion of mafic end-member in addition to fractionation. For these lavas, Th hat been required by K-concentrations, 16%. is similar to the aver enriched relative to Zr and Rb (and other incompatible age amount required by other incompatible element concen elements) via assimilation. trations but is far lower than that required by mineral chem The behavior o f Zr and Ba is espedally suggestive of ical data. Wyers and Banon (1986) argued that the behavior assimilation. Both elements normally behave as strongly of K in the Patmos MVS is indicative o f assimilation and incompatible elements. In the Patmos lavas, Zr is more mixing. compatible than Rb, K, Nb, Th, U. etc., which may indicate Concurrent operation of fractionation, mixing and as that Zr was held back in relatively insoluble phases such similation a n alto explain the observed variations in in at zircon in the restite during assimilation (cf., Harris and compatible trace clement ratios. Specifically, variations in Bell 1982). The decrease in Ba during magma evolution Zr/Th, Ce/Th, Tb/Th, Yb/Th. Hf/Th and Ta/Th, (m u t.r) is inconsistent with fractional crystallization unless suggest that Th is enriched via assimilation relative to other was abnormally high for the MVS. Reported values elements (REE and HFS elements). It also appears that o f DE,**'""* are mostly £0.6 (cf., references in explanation Th is enriched relative to K and Rb. These data thus suggest o f t*xu s.s), but Sun and Hanson (1973) reported a value that Th is preferentially incorporated Into' the magmas o f 1.47 based on analysis of plagioclase phenocrysts and through interaction with the crust. This it illustrated in groundmats in a Ross Island trachybasalt. Using their data, rts.t.s in which log (Th/Zr) and log (Th/Rb) are plotted however, we obtain a value for o f 0.72 and we against log (Th). The dutn points for a series of lavas gener strongly doubt that values greater than 1.0 are realistic for ated by fractional crystallization should define arrays with basaltic magmas. It follows that Ba should behave as an zero or small positive slopes. Data for the MVS define incompatible element during at least the initial stages or an array with substantial positive slope, and the trends of evolution o f the MVS. and that its failure to do so is an the data lie well above those expected to result from frac important indication o f assimilation. To support this con tional crystallization. Since mixing is inferred to have been clusion, the results o f assimilation-fractional crystallization important mainly in the evolution o f the hy-trachybasalts. (AFC) modeling using the equations given by DePaolo 195 2X9 t*»t> s.r. Results of AFC-caleulationi Tor Pat-150 C.,w ■'Sr.*‘Sr.,k ,1Sr;**Sr-> Sc a- 67 9 0.70771 0.70763 V II- 233 16 Ni t- 47 4 Rb 179- 259 229 Parent mattna -. Pat-1S7 Sr 531 539 Daughter magma : Pat-150 Zr 397—1008 338 Contaminant : Average cnm. ” Sr/'‘S r - 0.710 fix I393-J7I0 1400 Hr 12- 17 9 Ma.Mc-0.55 Th 10- 39 40 F-0,2* Cni. it the range of tract element concentration! calculated Tor Pat-150 with mineral-liquid partition coefficient! from the literature (ice Table 5) ,,Sr/, *Sr and the Sr-concentration were calculated aitumini 0J"4 l4**i<«t.l Ma/Me li the ratio mat! aliimilated'matt cryitalliied F ii the remaining fraction of melt (1981) are given in m u ».». Input data for the calculation! Conclusions were: trace element and Sr-itotopic data for a ne-trachyba u lt and a Q-trachyte; trace element data for average crust The trace element and preliminary isotopic data presented (Taylor and McClennan 1981) and an assumed Sr/*‘Sr in this paper confirm the conclusions of Wyers and Barton ratio of 0.710 for the cruital end-member. The latter value (1986) and allow quantitative evaluation of the processes it appropriate for average emit and Ii dote to the value important in the evolution o f the Main Volcanic Series on (0.7081) measured for crustal xenoliths in calc-alltaline la Patmos. The major conclusions are: vas from Santorini in the Hellenic arc (C.P. Wyers. unpub 1. The Patmos MVS evolved largely via fractional crystalli lished data). Since we report the results for illustrative pur zation. poses only, our assumptions about the composition of the 2. Inconsistencies between incompatible element and major crustal end-member are adaquate. From m u s.r, it is clear oxide data, and between incompatible element and compati that a) AFC processes can closely reproduce the Sr-isotope ble element data indicate the operation of other processes. ratios, and reproduce compatible element concentrations 3. Trace element data can be lariely explained by Invoking and many incompatible element concentrations in the Q* concurrent operation o f magma mixing and fractionation. trachytes to within analytical error and b) Zr and H f con 4. Isotopic data (although limited) together with the ob centrations are inconsistent with the specific model tested, served behavior of certain trace elements (especially the de probably for the reason given above or because the actual crease or Ba contents throughout the MVS) provide conclu crustal end-member involved in AFC processes was de sive evidence for the operation o f assimilation. pleted in HFS elements relative to average crust; also LREE 5. The results of AFC modeling indicate that the isotopic concentrations (not reported in m u s.t) are not well pre data and many of the trace element data can be explained dicted by this model, and require a crustal end-member by assimilation of average crust. High values of Ma/Mc more enriched in LREE than average crust. are not necessary to explain the variation ofBa in the MVS The main point, however. Is that the behavior of Ba but the actual material assimilated must have been richer in the MVS can be explained by AFC processes and does in LREE than average crust. The assimilant moy also have not require assimilation of an unusual crustal end-membcr. been relatively depleted in HFS elements, or those elements Furthermore, the observed decrease in Ba concentration were held back in refractory phases such as zircon in the (and concomitant increase in Rb, K, Th etc.) does not re restite during assimilation. quire high values ofMa/Mc (mass assimilated/mass crystal 6. Primitive magmas with high initial Sr contents lized). The calculated value is 0.55, indicating that assimila {-1500 ppm) may experience substantial contamination as tion was relatively leu important than crystallization during they traverse continental crust, i.e., high Sr contents do evolution o f the MVS. We conclude that careful analysis not buffer the ’ ’Sr/'^Sr ratios o f magmas efficiently. of trace clement data allows identification o f the operation 7. Mixing and assimilation were coupled processes during o f assimilation in the evolution or magma series even if evolution o f the MVS. We suggest that this possibility be isotopic data are not available. explored in future studies or magmas erupted through con Two additional comments are appropriate. First, the tinental crust. Many recent studies emphasize the impor results of the modeling described above together with "O / tance of assimilation and fractional crystallization, but our u O data for the ne-trachybasalt and Q-trachyte allows the work demonstrates that mixing of magmat related via AFC " 0 / “ 0 ratio of the assimilant to be predicted as approxi processes and mixing accompanied by assimilation are also mately 9.SX.. This value is within the range or values re important. Failure to recognize the importance of mixing ported for a wide variety o f sedimentary rocks (Hawkes- will lead to erroneous conclusions based upon AFC model worth 1983), Second, the primitive Patmos basalts contain ing. In particular, simple AFC modeling will fail to account ~ 1500 ppm Sr, whereat the Q-trachytes contain - 500 ppm for variations in trace element concentrations since the lat Sr, Gearty, even magmas relatively rich in Sr ore not im ter are sensitive indicators o f mixing. It is thus essential mune to crustal contamination as has been previously In that future studies incorporate both isotopic and trace ele ferred (e.g., Hawkcsworth and Vollmcr 1979). ment data and that attempts are made, as and when appro- 196 290 priatc, to model a) simultaneous mixing and assimilation Clayton RN, Mayeda TK (19*3) The use of bromine peniaDuoride and b) mixing of lavas related by AFC procases and hence in the extraction of oxygen from oxides and silicates Tor isotope to document the complexity of the evolutionary procases analysis. Geochim Cotmothim Acta 27:43-52 occurring in sub-volcanic plumbing systems. Only then can DePaoto DJ (1911) Trace element and isotopic effects of combined attempts be made to reconstruct the upper mantle processes wattruck assimilation and fractional crystallisation. Earth Planet Sd Lett 33:189-202 which ate responsible for magma genesis. Ewart A (1982) Peirogenetis of the Tertiary anorogenie series of The lavas of Patmos are an excellent example o f mag Southern Queensland. Australia, in the light of trace element mas which experienced a complex evolutionary history in geochemistry and O. Sr and Pb isotopes. J Petrol 23:344-3l2 a fractionating, advancing, periodically replenished, period Gill J (1981) Orogenic andcsiies and plate tectonics. Springer, Ber ically tapped magma chamber. The origin of there magmas lin Heidelberg New York will be discussed in a forthcoming paper. Gill JB (1984) S r-P b —Nd isotopic evidence that both MORB and OIB tourtes contribute to oceanic island arc magmat in Aiknm/tdtrmmi. Analytical data were acquired with Invaluable Fiji. Earth Planet Sd Lett 68:443-458 help from J. van dcr Wat, H.M.V C. Covers, WJ.H. Eutren, H.S. Harris C, Bell JD (1982) Natural partial melting or syenite blocks Pietenen and A. de Jong at the Slate University of Utrecht, and from Ascension Island. Contrib Mineral Petrol 79:107-11) H.N.A. Prirm and co-worken at the ZWO Laboratory for Isotope Hart SR, Davis KE (1978) Nickel partitioning between olivine Research at the Free University of Amsterdam. Field work in the and silicate melt. Earth Planet Sd Lett 40:203-219 Aegean was funded by the Slate University of Utrecht and H.S. Hart SR. Erlank AJ, Kable JD (1974) Sea floor basalt alteration: Pietersen it thanked for hit hetp and companionship in the field. tome chemical and Sr isotopic effects. Contrib Mineral Petrol Many or our ideas concerning the origin and evolution or the 44:219-230 Patmos lavas were critically evaluated by participants in the PIO Hatties worth CH (1982) Isotope characteristics of magmat erup seminar teria at Utrecht and we are grateful to those who con ted along destructive plate margins. In: Thorpe RS (ed) Ande- strained our occasional flights or fantasy. Jan Primus tncouraged sites: orogenic andesites and related rocks. John Wiley and discussion, and R.D. Schuiling encouraged freedom or thought Sons, New York, pp 549-571 and research. The comments of an anonymous reviewer led to Hawkctwonh CJ, Vollmer R (1979) Crustal contamination versus substantial improvement of the manuscript. enriched mantle: ‘‘'N d/'^N d and ’’Sr/^Sr evidence from the Italian volcanics, Contrib Mineral Petrol 69:151-165 Huijsmans JPP, Barton M (1983) Fractional crystallisation and Referenca magma mixing In toned magma chambers, Santorini, Greece. 1UOO XVIII General Assembly (IAVCEI), Hamburg 67 Anderson AT (IVJi) Magma mixing: petrological process and vol- Huijsmans JPP. Barton M (1984) Geochemical evolution of two canotogical tool. J Vole Geotherm Ret 1:1-31 shield volcanoes on Santorini, Aegean Sea, Greece. Trans Am Andriesten PAM, Boclrijk NAIM. Hebeda EH, Priem HNA, Ver- Ocophyt Union 65:1123 durmen EATh, Vcrxchure RH (1979) Dating the events or Irving AJ (1978) A review of experimental studies of crystal-liquid metamorphlsm and granitle magmatism in the Alpine orogen trace element partitioning. Geochim Cosmochim Acta of Naxos (Cyclades, Greece). Contrib Mineral Petrol 42:743-770 69:315-223 James DE (1982) A combined O. Sr, Nd and Pb Isotopic and Appleton JD (1972) Peirogenetis of potassium-rich lavas from the trace element study of crustal contamination in the Central Roccamonflnu Volcano, Roman region, Italy, J Petrol Andean lavas. I. Local geochemical variations. Earth Planet 13:425-456 Sd Lett 57:47-62 Anh JG (1976) Behavior of trace elements during magmatic pro Kyle PR (1981) Mineralogy and geochemistry of a basanite'to cesses. A summary of theoretical models and their applications. phonolile sequence at Hut Point Peninsula, Antarctica, bated J Ret US GeolSurv4;|;4t-47 on cores from Dry Valley Drilling Projccl drillholes 1, 2 and Baker BH, Goles GO, Leeman WP, Lindstrom MM (1977) Geo 3. J Petrol 22:451-500 chemistry and pctrogencsit of a basali-bcnmorriie-irachyte Le Maltre RW (1962) Petrology or volcanic rocks, Gough Island, suite from the southern part of the Gregory Rift, Kenya. Con- South Atlantic. Bull Geol Soc Am 73:1309-1340 trib Mineral Petrol 64:303-132 Le Roex AP, Erlank AJ (1982) Quantitative evaluation or frac Bak,er I (1969) Petrology of the volcanic rocks of Saint Helena tional crystallization in Bouvet island lavas. J Vole Geotherm Island. South Atlantic. Oeol Soc Am Bull $0:1213-1310 Res 13:309-338 Baker PE. Gass IG, Harris PG, Le Maine RW (1964) The volcano- Utter GS, Bangs O, Fccnstra A (1984) Metamorphic core com logical report or the Royal Society expedition to Tristan da plexes of Cordilltran type in the Cyclades, Aegean Sea, Greece. Cunha. Philos Trans R Soc London Ser A 256:4)9-57$ Geology 12:221-225 Barton M, Huijsmans JPP (I9$6) Post-caldera lavas from the San Luhr JF, Carmichad ISE (1980) The Colima volcanic complex, torini volcanic complex, Aegean Sea, Oreccc: an example of Mexico. I. Post-caldera andesites from Volcun Colima. Contrib the eruption or lavas of near constant composition over a Mineral Petrol 71:343-372 2,200 year period. Contrib Mineral Petrol 94:472-495 Mahood G, Hildreth W (1983) Large partition coclfidcnts for tract Barton M, Wyers GP (1986) The role of subduction In the genesis dements in hlgh-silica rhyolitcs. Ocochim Cosmochim Acta of primitive atkali-basaltt from Patmos (Dodecancsos, Greece). 47:11-30 Trans Am Geophys Union 67:1281 Nicholli 1A, Whitford DJ (1983) Potassium-rich volcanic rocks Bol LCGM, Hasseiman JF (1985) The petrology and geochemistry of the Muriah complex, Java, Indonesia: produets of multiple of volcanic rocks from Milos (Aegean Sea, Greece). Unpub magma sources? J Vole Oeoihcrm Ret 18:3)7-339 lished MSc thesis, Utrecht, p 80 Ninkovilch D, Hays JD (1972) Mediterranean island arcs and the Briqueu L, Lancelot JR (1979) R b-Sr systematica and crustal origin of high potash volcanoes. Earth Planet Sd Lett contamination models for calc-atkatine igneous rocks. Earth 16:3)1-345 Planet Set Lett 43:385-396 O'Hara MJ (1977) Geochemical evolution during fractional crys Ciuctta L. Innocent! F, Manetti P, Peccerillo A, Poll G (1981) tallisation of a periodically refilled magma chamber. Nature Geochemical characteristics of potastie volcanica from Ml. Er- 226:503-507 nid (Southern Latium, Italy). Contrib Mineral Petrol 78:37-47 O'Hara MJ, Matthews RE (1981) Otochcmical evolution in an Claguc DA, Frey FA (1982) Petrology and trace dement geochem advanting, periodically replenished, periodically Upped, con istry of the Honolulu vokanlcs, Oahu: implications for the tinuously fractionated magma chamber. J Geol Soc London oceanic mantle bdow Hawaii. J Petrol 23:447-504 1)8:237-277 197 291 Paparachot BC, Comminakit PE (1978) Gcoledonic ilgnificaiKe Thompion RN, Gibion IL. Marrincr GF. Matlcy DP, Morrivon of the deep icitmk rone* in the Aegean area, tn: Doumat MA (1980) Tiace-elcmeni evidence of mullittag* mantle fution C (cd) Thera and the Aegean World, vol 1 London, pp 121-1)0 and polybaric fraaional cryttaDiaalion in the Palacoccne tavai Pe-Pipcr GG. Piper DJW, Reynoldi Pll (191)) Paleomagnetle lira* or Skye, N.W. Scotland. I Petrol 21:265-29) tigraphy and radiometric dating of the pliocene volcanic rocki Van Bergen Mi. Barton M (1984) Com pic* Interaction of alumi- or Aegina, Greece. Bull Volcanol 46:1-7 nou* mctaiedimentary aenotilh* and tiliceoui mifma; and et- Roden M (1911) Origin or coetittlng minute and ultramafic brec ample from Ml. Amiata (Central Italy). Contrib Mineral Petrol cia, Navajo Volcanic Field. Contrib Mineral Peirol 77:195-206 86:)?4-)85 Sun SS, Hanton GN (1975) Rare-earth element tvidcnce for dilTer- Wycn GP, Barton M (1986) Petrology and evolution ortranitlion- entiation or McMurdo Volcanic*. Roil Hland, Antarctica. *1 alkaline-tub-alkaline lavai from Patmot. Greece: evidence Contrib Mineral Petrol 54:1)9-155 for fractional cryttalliMtion, magma miaing and attimilation. Taylor HP (1980) The elTecti or attimilation or country rock* by Contrib Mineral Pcirol 9): 297—)l 1 magma* on '*0;u 0 and ' ’Sr/^Sr lyitematic* in igneou* rockt. Earth Planet Sd Lett 47:24)-254 Taylor SR, McOennan SM (1981) The compoiition and evolution o( the continental cruit: rare-earth element evidence from tedi* mentary rock*. PhiloiTram R Soc London A)0! :)ll-)99 Received May » , 1986/Accepted May 18.1987 CHAPTER VII CONDITIONS OF CRYSTALLIZATION OF TERTIARY VOLCANIC ROCKS FROM PATMOS (DODECANESOS, GREECE) AND IMPLICATIONS FOR MAGMATIC EVOLUTION INTRODUCTION In recent years there has been significant progress toward quantification of the processes involved in the origin and evolution of magmas. It is now possible to model in detail the changes in magma chemistry which accompany melting, fractionation, mixing and assimilation. However, there have been relatively few attempts to estimate the conditions under which magmas form or evolve. Such estimates are essential if petrogenetic models are to be adequately constrained. Estimates of the temperatures of crystallization of a suite of rocks can potentially be used to constrain models involving magma mixing and assimilation. A knowledge of 02 f , fH2 0, fC02 , fS2 , etc., is necessary for a full understanding of the cause of explosive volcanism and of the effects of volatile species on magma evolutionary trends (e.g. Osborn, 1959). At least a rough estimate must be made of the pressure of crystallization and, hence, of depth to the magma chamber, 198 199 since Che evolutionary paths of magmas are subject to severe constraints imposed by the existence of thermal divides, but the latter may operate over relatively restricted pressure intervals (e.g., O'Hara, 1968). Estimation of pre-eruptive intensive variables is also important because certain physical properties of magmas (e.g., density and viscosity) can be calculated using established algorithms (e.g., Bottlnga and Weill, 1970, 1972; Shaw, 1972) and can be used to evaluate the physical aspects of magma emplacement and evolution, such as ascent rate and the role of crystal settling (Barton and Huijsmans, 1986). This approach is required if observations on erupted lavas are to be extended to predict the processes which occur in intracrustal magma chambers and have obvious applications to research into geothermal energy resources. In this chapter estimates are made of the conditions of crystallization of alkaline and sub-alkaline lavas from Patmos, Aegean Sea, Greece. The petrology and geochemistry of many of these lavas have been described in Chapters V, VI and VIM. We use these estimates to place constraints upon models for the intracrustal evolutionary history of the magmas and to assess the significance of the occurrence of upper mantle xenoerysts in non-primary magmas. 2 0 0 SUMMARY OF APPROACH Many previous workers have attempted to estimate the conditions of crystallization of igneous rocks via comparison of phenocryst assemblages with the results of phase equilibrium studies of natural lavas. With few exceptions (see Eggler, 1972; Eggler and Ritchey, 1978), this approach yields poor and inaccurate results slncet a) phenocryst assemblages often do not change dramatically over relatively large pressure Intervals (Green, 1982); b) it is difficult to firmly establish the order of crystallization from petrographic studies; c) the order of crystallization is strongly dependent upon volatile contents (e.g., H2 0 ,0 2) and there have been too few experimental studies in which the concentrations of volatile species have been adequately controlled; d) it is extremely difficult to correct for differences in the composition of the magma of Interest and that used in experiments; e) many experimental studies of natural samples have been beset by problems involving Fe-loss to the container, nucleation failure, and failure to demonstrate equilibrium. It is thus currently impossible to erect an accurate and internally consistent petrogenetlc grid for partially crystalline magmas of even a restricted compositional range. 2 0 1 I.S.E. Carmichael and co-workers (Carmichael et a l ., 1974, 1977; Ghiorso and Carmichael, 1980; Ghiorso et a l., 1983) have developed an alternative approach which utilizes the principles of classical thermodynamics. We have adopted this approach (with some modifications) in our study of the lavas of Patmos, and of lavas from the Hellenic arc, Greece (Barton and Huijsmans, 1986). The basic relationship is ii* - ;i° + RTlnaf (1) where i refers to a component in phase A, \l is the chemical o A potential, n^ is the standard state chemical potential, a^ is the activity of component i in phase A, and R is the gas constant. For two or more phases, the following relationship holds -AG£ " RTlnK (2) where the left hand term represents the Gibbs Free Energy change for the reaction between pure phases under standard state conditions (see Nordstrom and Munoz, 1985) and K is the equilibrium constant. Standard state thermo chemical data are readily retrieved from tables of the thermodynamic properties of geological materials (e.g ., Helgeson et a l., 1978) or from experimentally determined melting curves of minerals (e.g., Bottlnga and Richet, 1978). The major uncertainties in application of this approach thus lie in 2 0 2 demonsCration of equilibrium and inadequate knowledge of activity-composition relations. As more calorimetrlc and experimental data accumulate, more sophisticated models of activity-composition relationships for geologic materials are being developed. We have elected to use relationships reported in the literature and, where necessary, have tested different models against available experimental data and have used the results to select the most appropriate model. No doubt, future work will invalidate some of the adopted models so that our conclusions will be subject to refinement. Nevertheless, we believe that the thermodynamic approach offers the most satisfactory method for estimation of the conditions of crystallization and that while the absolute values of the pre-eruptive intensive parameters reported in this paper cannot be assumed to be 1 0 0 % accurate, they constitute an internally consistent set of data, which allow considerable constraints to be placed upon petrogenetic models. We address the problem of equilibrium in a following section of the paper. A BRIEF DESCRIPTION OF THE LAVAS OF PATMOS The geology of Patmos (Figs. 7.1 and 7.2) was described most recently by Wyers and Barton (1986, see Chapter V). Several phases of volcanic activity can be recognized, the most Important of which are a) eruption of ne-trachybasalts, hy-trachybasalts, hy-trachyandesites and Q-trachytes 5.5-5.7 m.y BP and b) eruption of ne- trachybasalts 4.5-4.6 m.y. BP. The ages reported in this paper are different than those proposed by previous workers (Fytlkas et a l., 1976; Robert and Cantagrel, 1977) and are based upon new K-Ar determinations (see Chapter VIII). Volcanism appears to be related to graben formation in the eastern Aegean which began in the late Tertiary (Segnor and Canltez, 1982). Trace element data (Barton and Wyers, 1986) suggest that the ne-trachybasalts were derived from a mantle source modified by interaction with a subducted slab. Recent work (see Huijsmans, 1985, for a summary) suggests that a slab dips at 30* beneath the Aegean and that the site of subduction is located to the south of Crete. The older (5.5-5.7 m.y. BP) lavas noted above constitute the Main Volcanic Series (MVS) and are unusual inasmuch as the transition from ne-normative to hy- normative compositions occurs at an early stage of evolution. There are significant geochemical differences between the parental magmas of the MVS and the younger ne- trachybasalts (YVS), and these will be discussed in detail elsewhere (Barton and Wyers, in prep.). For the purposes of the present paper it is sufficient to note that these lavas represent primitive, but not primary magmas. Mg/(Mg 204 + EFe) ratios are ~ 0.62 - 0.64 for the MVS ne- trachybasalts and 0.52 - 0.62 for the YVS ne-trachybasalts. These values are too low for the magmas to represent partial melts of olivine-rlch mantle material, yet many of these lavas contain xenocrysts of olivine (F 0 9 0 ) and Cr- splnel of upper mantle origin as well as phenocrysts of olivine, clinopyroxene and plagloclase and Ti-magnetite (see Chapter V). EQUILIBRIUM BETWEEN PHASES Representative whole-rock analyses for the MVS lavas have been reported in Chapter V. A whole-rock analyses of a primitive YVS ne-trachybasalt is given in Table 7.1 and representative microprobe analyses are reported in Table 7.2. We use published data to predict equilibrium olivine, clinopyroxene, mica and plagloclase compositions assuming that these minerals crystallized from a melt equivalent to the whole-rock analysis. We compare predicted compositions with analyses of phenocrysts in the lavas to Identify equilibrium compostions. Since most of the published data on ollvine-and cllnopyroxene-liquid equilibria are strictly applicable only to basaltic compositions, we identify equilibrium olivine and clinopyroxene compositions in only the least-evolved lavas (ne- and hy-trachybasalts). Results are summarized in Table 7.3. 205 Olivine Equilibrium olivine compositions may be predicted from Che relationship ♦ [ xfj*’ + Kd( l-x'j*'’) ] where Kd is the Mg-Fe exchange distribution coefficient with a value between 0.27 and 0.33 (Roeder and Emslie, 1970; Roeder, 1974; Bender et a l., 1978; Leeman, 1978). The data of Roeder (op. c i t .) , Thompson (1974a) and Edgar et a l., (1976) indicate that these values are applicable to alkaline magmas, whereas work by Hatton (1984) suggests that the dependence of Kd upon pressure is small, at least at low pressures (< 5 kb). Predicted olivine compositions for the MVS netrachybasalts are Fo8 7 -8 i,t in agreement with the observed range (Fo87_81) for the cores of phenocrysts in these lavas. Predicted olivine compositions for the YVS ne-trachybasalt (Foe3 -7 9 ) also agree well with the observed range of Fogi,-71* for the cores of phenocrysts in these lavas. Clinopyroxene Equilibrium high Ca-pyroxene compositions (Mg/(Mg+ZFe2+)) can be predicted using a relationship analogous to that given above but with Kd ■ 0.23 (Gerlach and Grove, 1982). These are 0.88 and 0.81 for the MVS ne- trachybasalts and hy-trachybasalts respectively, and 0.83 for the YVS ne-trachybasalts. The clinopyroxene 206 phenocrysts In the ne-trachybasalts are more iron-rich than the predicted compositions « 0.85 in the MVS ne- trachybasalts and 0.80 - 0.68 in the YVS ne-trachybasalts), and this remains true if higher values of Kd, up to 0.30, are used. Higher values are suggested by the experimental results of Thompson (1974b), by experiments on K-rlch lavas (Barton and Hamilton, 1979, 1982), and by data for naturally occurring K-rich lavas (Barton, 1979; Barton and Van Bergen, 1981) C1). The pyroxenes in the ne-trachybasalts contain substantial amounts of ferric iron (6.1 - 1.9 wt.%, calculated assuming that £ cations ■ 4.000 on a 6 oxygen basis) and taking this into account yields better agreement between measured and predicted compositions (see Table 7.3). As described in Chapter V, pyroxene phenocryst cores in the hy-trachybasalts of the MVS define two distinct compositional populations, neither of which is in equilibrium with the host magma. These lavas are hybrids, formed by mixing of ne-trachybasalt and (i)Data for K-rich lavas were examined to evaluate the effects of alkalinity, and particularly K2 0 content, on distribution coefficients defined in experiments on basaltic magmas. Appropriate values of Kd for K-rlch alkaline lavas are 0.25 - 0.26. 207 hi-trachyandeslte, and the pyroxene phenocryst cores represent xenocrysts inherited from the pre-mixing end members. Clinopyroxene microphenocrysts have compositions appropriate for equilibrium with the host rock. It proved unnecessary to correct the pyroxene microphenocrysts for the presence of ferric iron, and calculations based upon stoichiometry indeed suggest that these pyroxenes contain less Fe3+ than pyroxenes in the ne-trachybasalts. Mica Mica occurs as microphenocrysts or phenocrysts in the hy-trachybasalts, hy-trachyandesites and Q-trachytes and ranges in composition from phlogopite to' b lotlte. A survey of published data for K-rlch alkaline lavas (see Appendix D for details) indicates that the compositions of coexisting phlogopite or biotite and clinopyroxene are related via the relationship xJ|jgCa - xMg*/K» where K - 0.994 ± 0.027, i.e. coexisting mica and clinopyroxene have essentially identical Mg/(Mg + SFe2+) ratios. The composition of mica In equilibrium with liquid can therefore be predicted as described above for olivine and * pyroxene but with Kd suitably adjusted. Predicted and observed compositions for the hy-trachybasalts and Q- trachytes show excellent agreement (Table 7.3), but the micas in the hy-trachyandesites are more Fe-rich than 208 predicted. There are two possible reasons for thlsi a) ferric iron is present In these biotites; b) the blotites are xenocrysts and the hy-trachyandesites are also hybrid magmas. We favor the first alternative though the latter possibility cannot be entirely ruled out (see Chapter V). Plagloclase Equilibrium plagloclase compositions were calculated from the relationship - xca^ * t ^Ca** + (* " X ^q)Kd*1] where Kd - 0.9 - 1.2 (Shlbata et a l., 1979; Gerlach and Grove, 1982). Agreement between the calculated compositions and the average measured phenocryst core compositions (nb. the phenocrysts are strongly zoned - see Chapter V) is excellent for the ne-trachybasalts, hytrachyandesites and Q-trachytes. In the case of the hybrid hy-trachybasalts, predicted compositions closely correspond to the compositions of microphenocrysts, and are intermediate to the two phenocryst core populations described in Chapter V. Summary The data presented above indicate that in most cases the phenocryst cores have compositions appropriate for equilibrium with the whole-rocks and suggest that the latter can be taken to represent liquids. In the following calculations we use phenocryst compositions which are Identical to, or very close to, the predicted values for each magma type. The microphenocrysts in the hy- trachybasalts of the MVS have compositions appropriate for equilibrium with the whole-rock, but this result is surprising since these are hybrid magmas formed by mixing of ne-trachybasalt and hy-trachyandesite. In general, there Is no reason to expect that the compositions of minerals precipitated from hybrid liquids should correspond to those predicted to be in equilibrium with a hybrid magma. The bulk compositions of the hy-trachybasalts clearly do not represent liquids, but rather mixtures of liquid, inherited phenocrysts (- xenocrysts) and microphenocrysts. We are thus uncertain how to interpret the data for the hy-trachybasalts and exclude these rocks from the following discussion. CONDITIONS OF CRYSTALLIZATION Various equilibria have been used to obtain estimates of pre-eruptive temperature, pressure, water content and water pressure, and oxygen fugacity. The results are reported in Table 7.4 and details of the calculations are described below. 2 1 0 Temperature In general, it is necessary to fix one of the intensive parameters listed above before the others can be determined. In the case of the Patmos lavas, temperatures can be calculated from olivine-liquid equilibrium and from the Ti and Fe2 + contents of biotites. Several workers have studied the distribution of Mg and Fe2 + between olivine and mafic liquid experimentally, and have used the results to construct olivine-liquid geothermometers (Roeder and Emslie, 1970; Roeder, 1974; Bender et a l., 1978; Leeman, 1978). As noted previously, this thermometer is probably relatively insensitive to pressure, at least at pressures below 5 kb. For the ne- trachybasalts, temperatures were calculated using equilibrium phenocryst core compositions and the whole-rock analyses. Temperatures were also calculated for the MVS ne-trachybasalt from the compositions of microphenocrysts and coexisting groundmass (see Chapter V). Results are listed in Table 7.5, from which it is evident that: 1) temperatures for the MVS ne-trachybasalts are significantly higher than those for the YVS ne-trachybasalts and, as would be expected, there is a negative correlation between equilibration temperature and Mg/(Mg+EFe2+) of the host lava; 2 ) the results for each lava obtained using four different calibrations of the thermometer differ by only 60aC which is within experimental and analytical 2 1 1 uncertainty; 3) there is a substantial (150*C) difference between temperatures calculated for phenocryst cores and whole rock analyses and for microphenocrysts and groundmass analyses for the MVS ne-trachybasalts. As there is no satisfactory way of discriminating between the various versions of the olivine-liquid geothermometer, we also report average temperatures in Table 7.5. The maximum deviations from the mean temperature are +34 and -33’C, and we believe that these average temperatures are close to the true, pre-eruptive temperatures. Luhr et a l . , (1984) have used a geothermometer formulated by I.S.E. Carmichael which is based upon the Tl/Fe2 + ratios of biotites. They noted that temperatures estimated using this.geothermometer agree well with temperatures estimated from isotopic thermometers for the same lavas (Rye et al., 1984). It is likely that temperatures calculated using this geothermometer are also relatively Insensitive to pressure, at least over a restricted range of pressure. The temperature calculated for a Q-trachyte Of the MVS using a biotlte with a Ti/Fe2+ ratio of 0.3398 (measured) is 935*C. Several other geothermometers (e.g., plagioclase- liquld) are also available but are sensitive to other parameters (e.g ., P, q) and are utilized in the following sections. 2 1 2 Pressure Estimates of have been made using three Independent methods. 1. Given suitable expressions for the activities of components in the solid phases and standard-state thermodynamic data, expressions equivalent to ( 2 ) for the reactions Mg2 Sia, + Si02 ♦ Mg2 Si2 Q6 (3) Olivine Melt «• Pyroxene and CaAl(AlSi)06 + Si02 + CaAl2 Si2 0a (A) Pyroxene Melt «■ Feldspar may be solved simultaneously to yield a value of ^TOTAL ^ assuming that pre-eruptive temperatures are equivalent to the temperatures estimated from olivine-liquid equilibria. For (3) we adopted the sub-regular solution model of Wood and Kleppa (1981) to describe actlvity- composition relations in olivine: Olivine fvoct v°ctM 2 aMg2 SlOi, ■ ^ Mg ' YMg ) <5> where - exp [2000(1- X°aC)2]/RT (6 ) For the forsterltic olivines described here, this model yields values of ^g^SiC^ wh*0*1 are nearly identical to those calculated by assuming ideality. 213 'Low Ca-pyroxene does not occur in the Patmos lavas and It was necessary to select a model for the activity of Mg2 Si2 0g in calcic pyroxene. We tested several possible models against experimental data (see Appendix E for details) and on the basis of the results we adopted two models, one derived by DePaolo (1979): _Cpx yM1vM2yM2m yM2\ "l /7x Mg2 Si2 Ofc Mg Mg Mg' " Na^ where v M 2 - 1 /u r 9 vM2r ymV + YMg p RT WMg-Ca[(xCa)2 * *22)*] ‘ and the other proposed by Holland et a l., (1979): Cpx Cpx Cpx ... Mg2 Si2 06 Mg2 Si2 06 *Mg2 Si2 06 where yCpx yMl„M2 (10) Mg2 Si2 0fe Mg Mg and YMg2 Si2 0te “ ®XP tWCa-MgXCa( l "XMg) /RT^ (ll) Values for the Interaction parameters were taken from the papers by DePaolo (1979) and Holland et al., (1979). For reaction (4) we assumed that plagloclase behaves as an ideal solution (Kerrlck and Darken, 214 1975; Ghiocso et a l., 1983) and adopted a procedure analogous to that described above to evaluate models for the activity of CaAl(AlSi)0^ in clinopyroxene (see Appendix E ). We selected the Ideal two-site mixing model of Herzberg (1978), viz. acaAl(AlSl)0 . „M2yM1 6 Ca Al* We used the reaction constants given by DePaolo (1979) for reaction (3), whereas for reaction (4) we modified the reaction constants given by Gasparlk and Llndsley (1980) to take into account the change in the standard state from quartz to liquid S1 0 2 . Silica activities calculated from (3) and (4) for a MVS ne-trachybasalt at various pressures are plotted as a function of temperature in Figs. 7.3a and b. In each diagram, intersection of the two sets of curves yields a single curve in P-T- logagj^^space along which the equilibria are simultaneously satisfied . Taking the average temperature calculated from olivine-liquid thermometry (1192*C), we estimate a pressure of 2.9 kb if the model of DePaolo (1979) is used to calculate acaMgSi2 0 6 * and a Pressuce of 2 , 0 ^ the model of Holland et a l., (1979) is used instead (Figs. 7.3a and b ). As noted in Appendix E , we believe that the model of DePaolo (op. c it.) is 215 superior at temperatures > 1150*C, and we therefore prefer the higher pressure estimate. As shown below, this estimate is consistent with estimates based upon feldspar barometry for the evolved MVS lavas. We have also estimated the pressure of equilibration of the microphenocrysts and obtain a value of 3.0 kb, in good agreement with the preferred value obtained for the phenocrysts. Note that the higher value of calculated for the mlcrophenocryst assemblage compared with that calculated for the phenocryst assemblage (Table 7.6) Is consistent with the inferred evolution of the MVS magmas from ne-normatlve to hy-normative compositions (Chapter V) Pressures have also been estimated for crystallization of the YVS ne-trachybasalt using the solution model of Holland et al. , (1979) for the component Mg2 Si2 06 in clinopyroxene (nb, the olivine-liquid temperature is substantially lower for the YVS ne- trachybasalt) and is ~ 7 kb (Fig. 7.4). The results therefore suggest that the younger ne-trachybasalts crystallized at substan tia lly higher pressures than the MVS ne- trachybasalts. These results are reported in Table 7.6. 216 Ghiorso and Carmichael (1980) and Ghiorso et a l ., (1983) have calibrated a plagloclase-liquid thermometer on the basis of available experimental data and an assumed regular solution model for silica te melts. The temperatures calculated from this thermometer are strongly dependent upon assumed values of PfOTAL• ^ pre-eruptive pressures of 2.9 kb for the MVS and 7 kb for the YVS are assumed (see above), then the temperature calculated for the MVS ne-trachybasalt is ~ 1160*C, whereas that calculated temperature for the YVS ne-trachybasalt is 1118. These temperatures are substantially lower than those estimated from the olivine-liquid thermometry, but are within the range permissible by the latter technique and suggest that the pressures estimated above are not grossly in error. Alternatively, if pre-eruptive temperatures are assumed to be equivalent to the average temperatures calculated from olivine-liquid thermometry (i.e . the reasonable assumption is made that olivine and plagloclase crystallized simultaneously), then equilibration pressures can be determined as shown in Fig. 7.5a and b. Pressures are 1.9 kb for the MVS ne- trachybasalt and 5.4 kb for the YVS ne-trachybasalt. The results thus corroborate those reported above inasmuch as they indicate that the YVS lavas began to crystallize at higher pressures than the MVS lavas. These pressure estimates are reported in Table 7.7. The pressures of crystallization of the evolved MVS lavas (hy-trachyandesites and Q-trachytes) have been estimated by comparison of temperatures from plagioclase-liquid thermometry, Tl-Fe2+ exchange thermometry for biotites and two-feldspar thermometry. As discussed above, temperatures calculated using the plagioclase-liquid thermometer are strongly pressure dependent, and we assume that temperatures calculated from Ti-Fe24- exchange thermometry for biotites are pressure Independent. Comparison of temperatures calculated at various pressures from the plagioclase-liquid thermometer with the temperature calculated from Ti-Fe2 + exchange thermometry (935*C) thus yields an estimate of the pressure of crystallization of the phenocryst phases in the Q-trachytes (Table 7.8). Stormer (1975) formulated a two-feldspar thermometer which has recently been modified by Brown and Parsons (1981). The latter workers presented a graphical version of the thermometer which Whitney and Stormer (1985) believe to be the most accurate available. The thermometer is appropriate for pressures of 1 kb, but as noted by 21S Whitney and Stormer (op. cit) a correction of 18*C/kb can be used to account for the effects of pressure. We have estimated pressures of crystallization for the hy-trachyandesites and Q- trachytes by comparison of temperatures derived from the two-feldspar thermometer at various pressures with temperatures calculated from plagioclase-liquid thermometry (also pressure dependent) or Ti-Fe2+ exchange thermometry in b lotlte. The following points should be noted: a) the biotites in the hy- trachyandesites do not have compositions appropriate for equilibrium with the whole-rock and therefore have not been used for these pressure estimates; b) the compositions of coexisting felspars in the hy- trachyandesites indicate higher temperatures than can be determined from the graphical version of the thermometer presented by Brown and Parsons (1981). We have, therefore, reformulated an analytical version of the thermometer using the approach of Stormer (1975), but u tilizing the Margules parameters given by Brown and Parsons (op. cit) rather than those given by Waldbaum and Thompson (1969). Thus, T(K) - [2890 + 640Xaf - 9950XAF + 6420XAF + (0.02XAF + 0.079 - 0.277X af + 0.178xJp)P]/[-1.9872 In (XAF/XPF)+0*58 " 4 *0XAF + 3-42xAF1, 219 ip PF where K * XAb^XAb an(* P *s *n ^ars> This equation appears to closely reproduce temperatures determined from the graphical thermometer at temperatures < 1000'C (This work; Barton, in preparation). The feldspar-palrs selected for thermometry satisfy the criteria for equilibrium listed In Brown and Parsons (1981). Application of this method is depicted graphically in Fig. 7.6, and results are given in Table 7.8. The estimated pressure and temperature of equilibration of phenocrysts in the hy- trachyandesites are 3.5 kb and 1031*C. For the Q- trachytes, we estimate P * 3.2 kb and T - 920*C from two feldspar and plagioclase-liquid thermometry, and 4 kb, 935*C from two-feldspar and Ti/Fe2* exchange thermometry. It is not possible to estimate the absolute errors associated with the pressure estimates reported in this section. However, as noted previously, the pressure estimates are subject to errors resulting fromi 1) analytical uncertainty; 2) incorrect formulation of activity-compos it ion relationships. Errors inherent in the geothermometers may compound errors in pressure estimates. For example, an error of ± 50’C in the temperatures estimated from olivine-liquid equilibria results in an uncertainly of i 1-2 kb in pressures estimated from siltca-activity buffers and i 2 kb In pressures estimated from plagioclase-liquid thermometry. While we do not consider our pressure estimates to be 100% accurate, we believe that they provide Important information about the conditions of crystallization of the Patmos magmas. In particular, we suggest that the results indicate that the YVS ne-trachybasalts began to crystallize at substantially higher pressures than the MVS ne-trachybasalts and, most importantly, that the members of the MVS began to crystallize at about the same pressure (3 ± 2 kb). We note in support of the last statement that three independent methods of geobarometry yield surprisingly coherent results. Water Contents For the primitive magmas, pre-eruptive water contents were estimated from the plagioclase-liquid geothermometer of Ghiorso and Carmichael (1980) and Ghiorso et a l., (1983), since the latter is dependent upon water content as well as P water contents of these magmas from the Kudo and Weill (1970) plagioclase-liquid geothermobarometer. Temperatures calculated from the latter are dependent upon and we have calculated P ^ q by comparison of temperatures obtained with this thermometer with those obtained using other techniques (Table 7.9). For the Q-trachyte we have converted the value of PH20 to fH20 using data from Burnham M a I f* et a l., (1969) and have calculated Xi, n from the rt2 U relationship given by Spera (1984). Finally, the gram formula weight of the magma was calculated (69.4) and used to calculate the pre-eruptive water content (5.2-6.0 %). This is much higher than that calculated from the regular solution model for silica te melts of Ghiorso and Carmichael (1980) and Ghiorso et a l., (1983). It.must be emphasized that the differences in the water contents calculated by the two methods cannot be attributed to the degree of crystallin ity, i.e . the lower water content does not apply to liquidus conditions and the higher water content to sub- liquidus conditions when the magma is partially crystallized and the concentration of water in the residual liquid increased. Assuming an initial water content of 0.5 wt.%, the water content in the residual liquid after 50% crystallization may be calculated from the Rayleigh fractionation equation and is only ~ 1.0%. Fifty per cent crystallinity is probably a maximum value for eruptives (March, 1981). 222 At the present time, we cannot explain the discrepancy between the water content of the Q-trachytes estimated by the two methods described above. Water contents calculated using the thermodynamic model of Ghiorso and Carmichael (1980) and Ghiorso et a l . , (1983) for siliceous lavas from Santorini (Barton and Huijsmans, 1986; Huijsmans and Barton, 1987) and Nisyros (Wyers and Barton, 1987b) appear to be rea listic and in broad agreement with water contents calculated from the Kudo and Weill (1970) plagioclase- liquid geothermobarometer. We note that the water contents calculated using the approach of Ghiorso and co-workers are too low to account for the presence of biotlte and apatite in the Q-trachytes and hy-trachyandesites; the amount of biotite and apatite observed petrographlcally require that the magmas contained at least 0.34 - 0.44 wt.% 0 respec tively. We suggest, therefore, that the pre-eruptive water content of the Q-trachyte was probably close to 5 or 6 wt.%. The water content of the hy-trachyandesites can also be roughly estimated as described above and is 4.3 wt.% (note, however, that pre-eruptive temperature is above the range for which Burnham et a l., (1969) reported fugacity data for water so that a small extrapolation of their data is required). 223 Oxygen fugaclties The coexistence of olivine and Ti-magnetIte in the ne- trachybasalts allows calculation of pre-eruptive f02 from the reaction 3Fe2 SiO^ + 02 ♦ 2Fe3 0,, + 3Si02 (12) _ii •. since P, T and are known. Reaction constants were extracted from the data of Helgeson et a l . , (1978) and values of f02 calculated from (12) are listed in Table 7.10. We used Stormer's (1983) method to calculate ar§Cn » and the regular solution model described 3 <+ previously to calculate ap^S108> Values 1°8 ^ 2 are plotted as a function of temperature in Fig. 7.7 and, for comparison, data for various calc-alkaline lavas and basaltic lavas (Carmichael et a l., 1974; Luhr and Carmichael, 1980; Huijsmans, 1985) are also plotted. It Is clear that the ne-trachybasalts evolved under relatively oxidizing conditions—at values of log 0 f 2 greater than those appropriate for the NNO buffer and comparable to those prevallng during crystallization of calc-alkaline volcanics. High f02 is indicated for crystallization of both the phenocryst and microphenocryst phases in the MVS ne-trachybasalts. It appears likely that the YVS ne- trachybasalts crystallized at slightly lower values of f02 than the MVS ne-trachybasalts. An estimate of fCXj during crystallization of the MVS Q-trachytes can be made because 224 these lavas contain phenocrysts of blotite, K-feldspar and Ti-magnetite, the compositions of which are related via the equilibrium: KFe3 AlSi3Ol0 (0H)2 + l- 0 2 j KAlSi3Oa + Hj 0 + Fe3 Q, (13) 2 Wones (1972) has determined AG*/2.303RT for this reaction r experimentally. In order to calculate fQj, we calculated apl^Q as described above, calculated using the Margules parameters reported by Brown and Parsons (1981) and adopted an ideal two-site mixing model for annlte, v iz . aKFe3CAlSl3O10 (0H)2 " (XK>(XFe2+>3 • TaklnS fH20 as 2060 bars (from PH2 O given in Table 7.9), f02 is calculated -12 l as 10 * bars. Changing the values of f 0 (to 2230 bars) and (2.9-4.0 kb) to reflect the range of permissible values does not have a large effect on the calculated value of f02. The range for the latter is -12 1 -12 2 10 to 10 * . These values are considerably lower than expected for liquids generated via fractional crystallization of the mafic MVS magmas (Fig. 7.7) which raises a question about the validity of values of f02 calculated from (13). The factors which influence most strongly the value of f02 are therefore briefly examined. Estimates of f02 using reaction (13) are subject to three sources of error: a) inaccurate values of P and T; b) erroneous values of ffyO, and c) incorrect formulation of activity-composition models for the crystalline phases. 225 a) As noted above, pressure has relatively lit t le influence upon values of f02(or f 0) calculated from reaction (13)(Luhr et a l., 1984). Temperature has considerable influence, but the possible range of pre-eruptive temperatures for the Q-trachytes is too small to have a substantial effect on f02 . Values of f02 change from 10~V^'* to -12 2 10 * over the range of F-T conditions calculated in the preceding sections and we conclude that Inaccuracies in the estimated pre-eruptive values of P and T do not account for the low f02 of the Q- trachytes. b) A decrease in fH20 results in a decrease in calculated values of f02. For example, reduction of fHjO from 2060 to 66.5 bars (appropriate for a water content of 0.61 wt.%) results in a decrease in fO2 from 10"^** to 10“^ **. There is some uncertainty in the estimated pre-eruptive water contents, as discussed previously, but the major question is whether or not the water content was lower than that determined using the Kudo-Weill geothermobarometer. Hence, a revision of the pre- eruptive water content would lead to a reduction of the calculated value of f02 . c) The major sources of uncertainty in the calculation of f02 using (13) are the actlvity-composltion 226 models adopted for the solid phases, or variations in composition of the solid phases* For magnetite and sanidine, we have used established activity- compos it ion models and adoption of other models (e.g. the model proposed by Ghiorso et a l., (1983) for magnetite) does not significantly influence calculated values of f02 • Both Ti-magnetlte and sanidine display a range of compositions in the Q- trachytes, but calculations using the observed range of Ti-magnetite and sanidine compositions demonstrate that unrealistic compositions of Ti-magnetlte and sanidine are required if the value of f0>2 is to be appreciably increased. For example, reduction of XjjjAlSij^Q^ from 0.75 to 0.63 (the most alb Itic rim composition) changes f02 from IQ-12. 2 tQ ^q- 12.4 wjier6as reduction of XpfCn from -12 8 0.268 to 0.13 changes f02 to 10 * . These data imply that the low value of f02 calculated for the Q-trachytes is either due to the solution model for annite or is relatively accurate. Bohlen et a l., (1980) have discussed the problem of formulation of activity-composltion relations for biotite. In order to increase the value of f02 calculated for the Q-trachytes, it is necessary to decrease the D | a |> I | * 0 value of «KFe3AlSl3 Ol0 (OH)2 - Thts ls “osl: aasily 227 accomplished by adopting a three-site mixing model for the activity of annite - (Xk)(Xpe)3 (XqH)2 Our microprobe analyses do not Include values for F and Cl, so that we are unable to formally adopt this model. However, we have adjusted the value of XqH in annite to yield values of fOj whichare appropriate for magmas which are derived (via fractionation) from the mafic MVS lavas. The necessary value of X^is ~ 0.3, and is clearly impossible since EDS spectra of the micas in the Q- trachytes do not Indicate the presence of appreciable F or Cl. An additional factor that should be taken into account is the extent of Al-Si disorder on the tetrahedral sites of annite. We have no way of knowing the degree of Al-Si disorder in the biotltes in the Q-trachytes, but suggest that inclusion of a term for Al-Si distribution into the activity model for annite would Increase calculated values of f02 . However, we do not believe that inclusion of a term for Al-Si distribution would substantially increase the value of fO^ and hence we tentatively conclude that the MVS Q-trachytes crystallized at values of f02 equivalent to, or 228 slightly lower than, QFM. Nevertheless, -we urge that future studies address the problem of Al-Si disorder in blotite and emphasize that the biotite sanidlne-magnetite equilibrium Is a potentially sensitive indicator of fQ2 (i.e. relatively insensitive to variations in P, T, ap|^0 ^ and fj^g) 8tven suitable activity- compos it ion relations for annite. DISCUSSION Comparison with other lavas The temperatures estimated for the Patmos lavas (1192 - 920*C) agree well with those calculated or measured by other workers for lavas of similar composition. Downes (1973) measured liquidus temperatures of 1200*C for the 1971 lavas erupted from Etna. Wolff and Storey (1983) calculated temperatures of 880 - 910*C for trachytic pumices from the Azores. Baker et a l., (1977) estimated temperatures of 1200 - 900*C for a basalt-trachyte lava suite from the Gregory Rift, Kenya. We conclude that our temperature estimates are accurate and are applicable to other suites of lavas of alkali-basaltic to trachytic composition. The relatively high values of f02 calculated for the primitive ne-trachybasalts are comparable to values exhibited by island arc, calc-alkaline lavas (Luhr and Carmichael, 1980, G ill, 1980; Huijsmans, 1985) and are consistent with the tectonic setting of the Patmos lavas. Unfortunately, however, there are few estimates of pre- eruptive f02 for primitive alkaline lavas so that it is not yet possible to determine whether or not arc-related alkaline magmas crystallize at higher f02 than anorogenic continental or intra-oceanic alkaline lavas* There is some evidence (Carmichael et a l . , 1974) that alkaline magmas crystallize at higher f02 than tholeiitic lavas, so that the values of f0^ reported in the present paper could simply reflect the alkaline nature of the Patmos lavas (see Carmichael et a l., op. cit. for further discussion). Water contents estimated for the Patmos ne- trachybasalts are comparable with the average water contents estimated for alkali-basalts (0.9 - 1.0 wt.%) by Aoki et a l., (1981). Water contents estimated for the evolved MVS lavas are similar to those calculated by Wolff and Storey (1983) for trachytic pumices from Sao Miguel (Azores). The latter authors report pre-eruptive water contents of 6.5 - 6.6 wt.%. The hy-trachyandesites and Q- trachytes were unsaturated with water at depth. We propose that these magmas ascended relatively slowly and that the water boiled off gradually after the magmas became saturated allowing eruption as lava flows rather than as 230 pyroclastics. This scenario is identical to the one envisaged by Barton and Huijsmans (1986) based on a study of recent dacitic lavas erupted at Santorini. Evolution of the MVS lavas The data presented in the preceding section can be used to place constraints upon models for the evolution of the MVS lavas. In Chapters V and VI it was proposed that the MVS magmas evolved via fractionation, mixing and assimilation. New K-At age data confirm that the lavas of this series were erupted over a very short time interval, and our calculations suggest that the phenocrysts in the lavas equilibrated at similar pressures. Given the uncertainties in the pressure estimates, we conclude that the magmas evolved in a single chamber sited at a depth of ~ 9 km, and that our results corroborate the model presented in Chapter V. The large difference in the temperatures calculated from phenocryst core - whole-rock data and from •microphenocryst-groundmass data for the MVS ne-trachybasalt suggest that a substantial amount of crystallization (30 - 50%) occurred prior to eruption. This is also consistent with the model in Chapter V inasmuch as the processes postulated to account for compositional variations of the MVS occurred in the magma chamber. The large crystallization interval is similar to that found for lavas 231 erupted in 1971 from Etna (Downes, 1973) and Is consistent with the porphyritic nature of the ne-trachybasalts. Furthermore, a wide temperature interval suggests that the magmas were initially water-undersaturated, since it is known from experimental studies that the crystallization intervals of magmas can be extremely wide if the magmas are hydrous, but undersaturated with water (Green, 1982). This is consistent with our estimates of pre-eruptive water content for the MVS ne-trachybasalts. The f02-T relations displayed in Fig. 7.7 are unusual since they can be interpreted to Indicate that the oxygen fugaclty decreased during magma evolution. As discussed previously, we do not believe that the low fO^ estimated for the Q-trachytes reflects the method of calculation. Certain workers (e.g. Anderson, 1966; Duchesne, 1972) have interpreted f02-T relations deduced for crystallization of intrusive bodies (especially anorthosltes) to indicate progressive reduction of the melts during evolution. Other interpretations are also possible, however (Ashwal, 1982; Barton, In preparation), and it Is not clear how reduction can accompany progressive fractional crystallization (? periodic degassing of the chamber through eruptive venting). Furthermore, data on f02 - T relations for alkaline magmas, and for other alkaline - sub-alkaline suites such as that which occurs on Patmos, are lacking. Given the model proposed for evolution of the MVS, the 232 possibility that the relatively low f02 of the Q-trachytes results from Interaction with the continental crust should be considered. Assimilation of graphite-bearing schists might change the magma composition substantially and might change the oxygen fugacity in the contaminated magma. This hypothesis must remain untested as there are insufficient data upon the effects of assimilation on fOj . Van Bergen and Barton (1984) have presented evidence that assimilation of graphite-bearing metapelites results in oxidation (due to flux of HjO into the magma) and that the original graphite is oxidized to C02 through interaction with the magma. Further studies of this problem are clearly warranted and one study, involving xenolith-bearing th o leiitic dikes from SW Norway, is already underway (Barton, in prep.). Finally, it may be noted that the amount of heat released during evolution of the MVS over a long period of time may have been sufficient to account for the amount of assimilation necessary to explain observed trends in trace element and isotopic data (cf* Chapters VI and VIII). Implications for the YVS lavas The results presented in this paper suggest that the ne-trachybasalts of the YVS evolved in magma chambers sited at a much greater depth than those from which the MVS magmas were erupted. We infer that the YVS ne- trachybasalts utilized a different plumbing system than the MVS magmas and note that geochemical differences between the YVS and MVS ne-trachybasalts prohibit a mutual relationship via fractionation, assimilation or mixing (Barton and Wyers, 1986; Barton and Wyers in prep.)* . Our results indicate that the YVS ne-trachybasalts began to evolve in a chamber sited at ~ 21 km--possibly at the base of the crust—but there are insufficient geophysical studies to allow accurate definition of the depth to the Moho in this region. Data presented by Makris and Stobbe (1984) suggest that the Moho is sited at a depth of ~ 28 km beneath Patmos. This depth is close to the range of depths (15 - 27km) consistent with uncertainties in the pressures estimated (5 - 9kb) for crystallization of the YVS lavas. Allowing for the uncertainties in the geophysical data used to estimate the depth to the Moho in this region, we suggest that the YVS magmas probably evolved in itially in a chamber sited at the crust-mantle transition. Studies of calc-alkaline lavas from Nisyros (Chapter II) also suggest in itia l magma evolution in chambers sited at the base of the crust. However, we note that our results are applicable only to the early stages of evolution of the YVS magmas (since we have considered only phenocryst-whole rock data for a single composition). Preliminary results for another YVS ne-trachybasalt and for coexisting microphenocrysts and groundtnass In Pac 167 suggest that evolution of this series was polybaric. For a more evolved YVS ne-trachybasalt we estimate T~ 1123*C, P ~ 2 kb, f02 ~ ■7 8 10 * and i^O ~ 1 wt.%. Although we have not completed our work on other YVS ne-trachybasalts, these results lead us to suggest that the later Btages of evolution of the YVS magmas occurred in magma chambers sited at shallow depths. We note in support of this statement that the evolved YVS ne-trachybasalts do not contain xenocrysts of upper mantle olivine and chrome-spinel, and that REE patterns for these lavas are characterized by negative Eu-anomalies (Barton and Wyers in prep.) which implies extensive crystallization at relatively shallow depths. On the basis of the data presented in this paper for the YVS lavas it may be predicted that' these magmas have experienced assimilation in their evolution. This is so because recent work (Barton et a l., 1983; Leeman, 1983) has shown that the amount of assimilation experienced by magmas is related to cruBtal thickness (depth to the magma chamber and hence increased temperature of country rocks or increased opportunity for assimilation to occur as magmas traverse thick continental crust). Indeed, the results presented in Chapter VIII confirm that assimilation played # a role in the evolution of the YVS lavas and, despite the limited compositional range shown by the latter, suggest 235 that assimilation may be extremely important in influencing the compositions of magmas which evolve in deep crustal magma chambers. In contrast to the MVS, fOj approximately paralleled the QFM buffer curve during evolution of the YVS (see above) despite the fact that these magmas evolved via polybarlc fractionation and assimilation. Implications for the intracrustal evolutionary history of primitive, alkali-basaltic magmas. As noted in this paper and elsewhere (Chapter V) the primitive ne-trachybasalts erupted on Patmos contain xenocrysts of olivine and Cr-spinel of upper mantle composition. Petrologic and geochemical data Indicate, however, that the host lavas do not represent primary magmas—that is, pristine mantle melts. Mg #'s of the host lavas (0.64 - 0.56) are too low for them to have been In equilibrium with the likely residual phases in the upper mantle (e.g. O'Hara et al., 1975). Most petrologists and geochemists consider the presence of xenocrysts of upper mantle minerals in primitive magmas to indicate that the latter have not fractionally crystallized since leaving the upper mantle. This argument is based on the fact that olivine, chrome-spinel, diopside etc., are denser than basaltic liquids so that they would settle along with phenocryst phases during fractional crystallization. The 236 preservation of xenocrysts and xenoliths in magmas thus suggests that magma ascent rate was greater than the settling rate. The occurrence of xenocrysts in non-primary magmas is usually attributed to fractional crystallization within the upper mantle—prior to the time of xenocryst incorporation. The data presented in this paper, together with those presented in Chapters V and VI are difficult to reconcile with the concept that the compositions of a ll of the primitive ne-trachybasalts are due to fractional crystallization at upper mantle pressures. The phenocryst assemblage in the MVS ne-trachybasalts clearly equilibrated at low pressures, within the crust. The calculations reported in this paper indicate that these lavas have compositions appropriate for magmas lying along a relatively low-pressure cotectlc, which implies that they are derived from pre-cursor magmas which have evolved via at least polybaric fractionation, and possibly via low- pressure fractionation. This follows from the arguments presented in detail by O'Hara (1968), that It is extremely unlikely that high-pressure cotectics are compos it tonally coincident with low-pressure cotectics, and hence that it is improbable that magmas evolving at high pressures will have compositions similar to those of magmas which ..lie along low-pressure cotectics. In other words, in terms of 237 n-dimensional compositional space, mantle-derived magmas represent a random sampling of compositions relative to the compositions of magmas which lie along low-pressure cotectics. Compositional data for the phenocryst phases in the MVS ne-trachybasalts suggest that these lavas evolved from more primitive lavas via removal of the observed phases. Occasional Mg-rich cores of olivine and Ca-rlch cores of plagloclase, for example, record this earlier stage of magma evolution. The low (relative to primary magmas) Ni and Cr contents of these lavas indicate removal of olivine, spinel and/or cllnopyroxene, whereas the negative Gu anomalies (Chapter VI) require removal of plagloclase. Ti and V contents suggest removal of Ti-Fe oxide (Barton and Wyers, 1986). These data are consistent with evolution at low (~ 3 kb) pressures, and the concept of low-pressure fractionation is in accord with the high An contents occasionally recorded in plagloclase phenocryst cores. We conclude that the data require that the MVS ne- trachybasalts represent magmas which result from intracrustal, most likely low-pressure, fractionation of more primitive magmas so that the occurrence of upper mantle xenocrysts in these lavas requires explanation. The most obvious way to reconcile the occurrence of high-pressure xenocrysts in lavas which contain low- pressure phenocryst phases Is to invoke the mixing of mantle derived magmas with cogenetic magmas which have evolved in intracrustal magma chambers (cf. Brooks and Printzlau, 1978; Duda and Schmincke, 1985). The former would carry the upper mantle xenocrysts whereas the latter would contain the low-pressure phenocryst assemblage. However, despite a diligent search, we have found no evidence for mixing in the MVS ne-trachybasalts although such evidence is normally forthcoming from detailed mineral chemical studies (Barton et a l., 1982). We therefore reject the mixing hypothesis and conclude that our data require that xenocrysts of upper mantle minerals can occur In magmas that have evolved at low-pressures (nb. isotopic data (Chapter VI, VIII) Indicate that assimilation has played lit t le or no role in the evolution of the primitive MVS lavas). On the basis of the above discussion, we suggest that crystal settling is not the only, and possibly not even the major, mechanism of fractional crystallization. Several workers (e.g ., McBirney and Noyes, 1979; Rice, 1981; Barton and Huijsmans, 1986) have suggested that crystal settling may not be as important as has been assumed in the past, and there is evidence (Shaw et a l., 1968; Shaw, 1969) that natural magmas develop a yield-strength--that is, behave as Bingham or pseudoplastic fluids--at sub-liquidus temperatures. Indeed, Sparks et a l., (1977) have argued that the occurrence of upper mantle xenollths In alkalic basalts Is due not to rapid speed of ascent but rather to the yield strength of the host magmas which prohibits settlin g. Sparks et a l., (op. c it.) implied that the. presence of both liquid and crystals are necessary to impart a yield strength to the magma so that xenollths remain in suspension whereas phenocrysts may settle (i.e. the liquid may behave as a Newtonian fluid). Spera (1984) supports this view but doubts that magmas ascending from the upper mantle develop sufficient yield strength to prevent settling of xenollths. Recent studies of Intrusive (evidence summarized by McBirney and Noyes, 1979) bodies as well as lavas (e.g. Barton and Huijsmans, 1986) suggest that crystal settling does not occur in intracrustal magma bodies and have been used to infer that the liquids are non-Newtonian. If this view is correct, and the evidence seems to be convincing that it is, then the ability of a magma to transport xenollths or xenocrysts from the upper mantle does not depend upon the degree of crystallization. We suggest that the xenocrysts in the MVS trachybasalts simply remained in suspension during ascent and residence in the intracrustal magma chamber. Moreover, we postulate that fractional crystallization most probably results from preferential nucleatlon and growth of phenocryst phases in 240 those parts of the magma which experience maximum cooling-- near the margins of Intrusive bodies. In support of this, we note that Irving (1980) has concluded that preferential crystallization occurs along the walls of dikes in the upper mantle whereas McBirney (1980) has concluded that crystallization occurs mainly in thermal boundary layers along the walls of intrusions. An alternative explanation of the data is that crystal settling is inhibited by convection. According to the model in Chapter V, emplacement of hot, primitive basalt along the floor of the magma chamber which contains cooler, more evolved magma triggers intense convection. Rapid heat transfer across the interface between the primitive and evolved magma induces convection in the lower layer of primitive magma as well as in the overlying layer of evolved magma. Convection may inhibit crystal settling, but crystallization could nevertheless occur preferentially along the cooler walls of the magma chamber. Compositional variations in thermal boundary layers induced by wall-rock crystallization may be transmitted relatively rapidly throughout the main magma body via convective mixing. This hypothesis is attractive as it explains why crystal settling does not occur in some mafic, low viscosity magmas. Both of the hypotheses described above provide plausible solutions to the apparent dilemma caused by the occurrence of upper mantle xenocrysts in evolved lavas. It is not, however, possible to discriminate between the hypotheses on the basis of the available data. However, Barton and Huijsmans (1986) have shown that crystal settling does not occur in dacite magmas erupted on Santorini, Greece, and that this is due to the intrinsic physical properties of the melt rather than large-scale convection. We are inclined, therefore, to accept that the primitive MVS ne-trachybasalts behaved as non-Newtonian (Bingham or pseudoplastic) fluids during ascent and evolution. We conclude that the occurrence of upper mantle xenocrysts or xenollths in magmas does not constitute proof that these magmas have not evolved within the crust. The Implications of this conclusion are far-reaching, and detailed discussion is beyond the scope of the present paper. One of the most Important implications is that xenolith/xenocryst-bearing magmas may assimilate crustal material and is thus worthy of note as it may no longer be safe to conclude that the isotopic characteristics of such magmas reflect those of the upper mantle source regions. Detailed petrologic,' geochemical and isotopic data are required to identify magmas with characteristics appropriate for studies of processes occurring within the mantle. There is no evidence that the most primitive YVS lavas have evolved within the crust. These magmas are not characterized by negative Eu anomalies (Barton, et al. in prep.) indicating that plagloclase was not part of the fractionating crystalline assemblage. This, together with the fact that the YVS magmas in itia lly evolved in a high- pressure magma chamber suggests that these primitive magmas may have evolved via fractionation in the upper mantle prior to picking up upper mantle xenocrysts. Since the depth to the Moho beneath Patmos is not accurately known, this hypothesis remains untested. Nevertheless, a plausible scenario is: generation of alkaline magmas within the upper mantle; ascent to the base of the crust where further ascent is arrested by the density difference between the crust and mantle (i.e. loss of gravitational acceleration); fractionation at the base of the crust, incorporation of upper mantle xenocrysts and, finally, eruption. Should. further studies show that the Moho lies at greater depths than that predicted in this study, then the models discussed above for the MVS lavas may apply to the YVS lavas also. The results presented here are difficult to reconcile with evidence that xenolith/xenocryst bearing magmas ascend rapidly from upper mantle depths. It should be obvious from the preceding discussion that ascent rates cannot be 243 confidently predicted from the computation of xenolith/xenocryst settling rates using Stokes' Law (see also Sparks et a l., 1977). However, the preservation of olivine xenocrysts in alkali-basalts and the preservation of Cp2 -rlch fluid inclusions within the olivine xenocrysts (see Spera, 1984, and references therein for a more detailed discussion) places semi-quantitative constraints upon magma ascent rates. Most of the olivine xenocrysts are surrounded by rims with compositions similar to the phenocrysts In the host lavas. The rims presumably grew in the magma chamber and result from heterogenous nucleatlon in the olivine-saturated magmas. The presence of the rims must have inhibited diffusive Interchange between the xenocrysts and the melt, but the lack of zoning profiles at the margins of the xenocrysts prohibits calculation of residence times using appropriate diffusion models. High ascent rates prior to growth of the rims cannot, however, be ruled out, and indeed are suggested by the experimental data on annealing rates of fractures in olivine at high temperatures (Wanamaker et al. 1982). Using these data, we estimate residence times of about one week, and despite the many uncertainties involved, conclude that the netrachybasalts ascended relatively rapidly from the upper mantle. Clearly, evolution is not necessarily inhibited by rapid ascent rates. We suggest that rapid crystallization 244 from supercooled magmas in thermal boundary layers along the walls of dikes or intrusive bodies may influence the compositions of xenolith/xenocryst bearing magmas. This conclusion is controversial, but is consistent with the available data. We suggest that additional data are urgently needed before the physical aspects of magma evolution can be adequately constrained. Such data are essential If models of magma evolution based upon petrological/geochemical evidence are to be tested. CONCLUSIONS Use of established geothermometers and geobarometers, together with application of the methods of classical thermodynamics, allows Internally consistent estimates to be derived for crystallization of the lavas of Patmos. The techniques described here can, in principle, be applied to a variety of lavas of differing composition. Indeed, this approach has been applied to calc-alkaline volcanics from Santorini (Barton and Huijsmans, 1986; Huljsmans and Barton, 1987) and Nlsyros (Chapter II), as well as to anorthositlc, Al-gabbroic and tholelitic magmas Intruded in S.W. Norway (Venhuis and Barton, 1986; Barton, in prep). In detail, the approach adopted here is over-simplified since some of the actlvity-composltlon'relations selected are almost certainly incorrect so that the results presented will be subject to revision. Moreover, errors 245 associated with estimates of Tf P, X^q and £0^ are relatively large, though absolute errors are difficult to * determine, so that interpretation of the results requires caution. In particular, pre-eruptive pressures cannot be estimated to better than ± 2 kb (a greater uncertainty may be applicable to high pressure estimates). In terms of the evaluation of the Intracrustal evolutionary history of magmas, however, our assumptions are probably adequate and the results provide important constraints upon petrogenetlc models. The following conclusions are of general validity: 1. Magmas with compositions ranging from ne- trachybasalt (or alkali basalt) to trachyte crystallize at temperatures between ~ 1200 and ~ 900*C. 2. Relative to buffer curves, f02 may decrease during magma evolution and this may reflect assimilation of (for example) graphite-bearing metamorphic rocks. 3. The presence of xenollths or xenocrysts in a particular magma cannot be taken to indicate that the magma is primary or that it has riot evolved during ascent through the crust. At least some (if not many) magmas exhibit non-Newtonian properties and crystal fractionation reflects preferential crystallization in thermal boundary 246 layers along cooling surfaces rather than crystal settling. 4. Magma ascent rates cannot be acurafjely estimated from the presence of xenollths or xenocrysts and application of Stokes' law; The occurrence of xenollths or xenocrysts of upper mantle material In evolved magmas requires relatively rapid ascent rates from the upper mantle, but the host magmas may nevertheless evolve via crystallization under conditions of supercooling within the magma chamber. 5. The heat supplied by primitive (i.e. basaltic), hot magmas which pond on the floore of magma chambers is necessary for assimilation of crustal material by evolved magmas within the chambers* These additional conclusions are applicable to the evolution of the magmas erupted on Patmos: 6. The lavas of the MVS evolved In a magma chamber sited at ~ 9 km depth, supporting the assertion that these magmas are related via fractionation, assimilation and mixing and hence tepresent a cogenetic magma series (Chapter V) 7. The YVS lavas Initially evolved at higher pressure than the MVS lavas which indicates that a complex plumbing system exists beneath Patnos. Evolution 247 of the YVS magmas is apparently completely independent of that of the MVS magmas (Barton and Wyers, 1986). 8 . The amount of assimilation experienced by magmas may be related to the depth of the magma chamber, e.g. assimilation was relatively more important in evolution of the YVS than in the evolution of the MVS. 9. fyO contents of the Patmos lavas are too low to cause saturation within the magma chamber. High water contents are not ossentlal for the development of trachytic magmas, but are probably necessary for the eruption of trachytic ash-flows, pumices, ignimbrites, etc. Development of more sophisticated thermodynamic models for activlty-composltion relations of silica te minerals and melts is required for a deeper understanding of the processes involved in magmatic evolution. We hope that the approach adopted here, and advocated by other workers (e.g ., Ghiorso and Carmichael, 1980; Ghlorso et al., 1983; Luhr and Carmichael, 1980) will stimulate the additional research necessary to allow more accurate estimates to be made of the conditions under which magmas crystallize. Table 7.1. Whole-rock analysis and CIIV norm of a primitive YVS ne-trachybasal Sample Pat. 167 SlOj 48.85 or 8.73 TioJ 1.68 ab 20.79 19.11 an 27.57 £ 0 8.10 ne 8.86 MnO 0.21 dl 16.19 MgO 5.87 ol 11.83 CaO 10.00 mt 1.96 Na-0 4.36 i l 3.17 V* 1.44 *P 0.97 *2% 0.39 Sun 100.03 Whole-rock analyst* normalised to 100.0 on.a volatile-free basis. CItV norm calculated with Fe»0L ■ 0.15 FeO. *Total Fe as FeO. For Pat 167 Hg/(Mg*F*T) - 0 .5 6 4 . Table 7.2. Representative iniljriei of phtnocryiti and xenocrysts in TVS nc-trachybasalt Fat 167. M ineral 01 .x c 01 .p c 01.p r Cpx. pc Cpx.pc Cpx.pr CpxTxc PI .p c FI . p r Mag.pc Mag.pc C r.x c C r.x c SiOj 40.4 39.8 38.2 51.1 48.7 52.1 44.5 48.3 51.7 - - -- TiO^ - -- 0.38 1.19 0.30 2.15 - - 16.6 16.5 0.37 0.54 4ljO| 0.20 - 0.14 4.18 5.26 1.19 9.02 32.4 30.0 3.72 3.21 29.1 30.1 - 0.15 - 0.75 0.49 -- - - 1.56 0.22 35.1 25.2 FeO 9.77 14.8 23.6 4.89 6.60 8.54 9.23 0.80 1.20 73.3 74.8 21.4 29.6 MnO - 0.23 0.41 -- 0.92 0.13 -- 1.81 0.73 0.31 - MgO 49.2 44.9 37.8 15.26 14.1 13.9 11.0 - 0.34 0.21 0.49 14.3 11.0 CnO 0.24 0.28 0.14 22.40 23.0 22.4 22.2 15.10 13.1 0.29 0.07 0.09 0.18 Ha-0 - -- 0.37 0.42 1.09 0.37 Z.54 3.36 - - - - K,0 ------0.19 0.42 - - - - ZnO ” “ ~~ 0.26 — Total 99.8 100.2 100.3 99.4 99.8 100.5 98.6 99.3 100.1 97.5 96.3 100.6 96.7 FeOb _- - 3.84 2.90 3.10 5.18 -— 44.9 45.0 15.0 19.3 Fc2°l — - — 1.16 4.11 6.05 4.50 -— 31.6 33.1 7.1 11.4 0.90 0.84 0.74 0.848 0.79 0.74 0.68 0.54 0.40 (Hg*TFe ) 0.876 _tU------—— - 0.90 0.89 0.79 —— - — 0.63 0.50 (Mg^Fc2 *)® An I ------0.77 0.68 - - -- Abbreviations; OL - olivine; Cpx - clinopyroxene; FI - plagloclase; Kag - titaniferous aagnetite; Cr - chroae spinel; xc - xenocryst; pc - phenocryst core; pr - phenocryst ria. * - Total Fe as FeO; - FeO and Fe^O, calculated froa stoichioactry atsuaing Icat - 4.00 for pyroxenes and Scat » 3.00 for spinels. Table 7.3. Conparison of predicted and observed phenocryat/aicrophcnocryst coopositions 01 01 Nic. Hica Flag Flag Lava type Pred. Obs. & ■ . » > Obs.uCp* (1) > Fred.1” Obs.” 1 Fred. Obs. MVS - Ne-Tb Fo87-SH Fo#7-fll 0.88 <0.85(0.90)(2) - **87-77 **83-77 Hy-Tb - - 0.81 0.82-0.76 0.81 0.B1<3) **78-73 **78-88An <3> ny-Ta - - - - 0.75 0.69 **65“58 **88”% •* Q'T -- - - 0.67 0.69-0.62 **S3->*6 **5V-V1 TVS Ne-Tb Fo83-79 Fo#H“7H 0.82S 0.8-0.68 (0.9-0.8) - - **75-70 **77-88 Abbreviations! Ne-Tb - nepheline trachybaaalt; Hy-tb ■ hyperathene trachybasatt; Hy-Ta » hypersthene tracbyandeaite; OrT “ quarts trachyte; Fred. “ predicted; Oba. * observed. JIWoig+IFe2*) J*fValue in parentheses corrected for the presence of FejOj. 250 '3,Values for aicrofdicnocrysts (see text for discussion) 251 Table 7 .4 . Sutnaary of calculated condition* of cryatallliatlon of the Patmoa nagtsa*. Lava Type T'C Pkb pH,0kb CH,T* w t.l f02 bar. HVS He-Tb 1192 2.9 0.25 10,“5.9 Hy-Ta 1031 3.5 1.9 4.5 - 12.2 Q-T 920-935 2.9 -4 .0 2 .3 -2 .5 5 .2 -6 .0 10 YVS 1151 7.0 <0.7 10-7 .6 Table 7.5. 252 Pre-eruptLve and eruptive temperatures (In Celsius) calculated from ollvlne- llqutd equilibria for the ne-trachybasolts. Series Assemblage Method Average RE R B L MVS Phcnocryst-Whole Rock 1188 1183 1169 1226 1192 Mlcrophenocryst-groundmass 1051 1018 1011 1075 1064 TVS Phenocryst-Whole Rock 1150 1143 1125 1185 1151 R£ - Roeder and Easlte (1970) equation; R - Roeder (1974) equation; B - Bender et al. (1978) equation; L - Leenan (1978) equation. 253 Table 7.6. Eatl'nate* of pressure and tog a^[^u^d for Che ne-trachybasalt* baaed upon •iuutcaneou* aolucton of Che equation* defining equilibria invotving olivine* cltnopyroxene and anorthlte-CATS and uatng otivine-Hquid tenperature* Serie* Aaaesblage P(kb) i.« •sin,"1'1 MVS Phenocryat-Whole Rock*1* 2.9 - 0.50 Phenocryst-Vhole Rock*2* 2.0 - 0.85 Microphenocryst-Croundnats*2* 3.0 - 0.18 YVS Phenocryat-Vhole Rock*2* 7.0 - 0.65 ^ ^Calculated uilng actlvlty-compoaitlon relation* for MgjSijOg in ...cltnopyroxene given by DePaolo (1979). ' 'Calculated using activicy-conpoaltlon relation* for HgjStgOg in cltnopyroxene given by Bolland et al. (1979). Tibia 7.7. 254 Preaaure eatimatea baaed upon conpariaon of temperaturea calculated at varloua preaaurea uatng the model of Chiorao and Carmichael (1980) and Chtorao et al., (1983) vlth temperaturaa calculated from ol(vine-liquid equilibrium. Lava type MVS Ne-Tb TVS He-Tb Preaaure (kb) 1.9 5.4 Table 7.8. 255 Tenperature* and preaaure* eat{mated for evolved MVS lavaa from plagloclaae- liquid equilibria, tvo-feldapar equilibria and Tl-Fe ratio* of blotltea. Lava type Hy-Ta Q-T Plagloelaae-l'.quid tenperaturea: 1152(0.001) 1087(0.001) (value in parentheae* la preaaure In kb) 1099(1.5) 1038(1) 1066(2.5) 984(2) 1048(3.0) 930(3) 1031(3.5) 903(3.5) 1014(4.0) 822(5) Tvo-feldapar tenperaturea: 897(1) 881(1) (value in parentheae* ia preaaure in kb) 1005(2) 899(2) 1023(3) 917(3) 1041(4) 935(4) Biotlte tenperature: - 935 Condltiona of cryatalllzatlon: Preaaure, tenperature: 3.5, 1031tl) 3.2, 920<>> (kb, *C) 2.9, 935” 4.0, 935” ' Conpariaon of plagloclaac-llquid and tvo-feldapar tenperaturea. ” 'Conpari*on of plagioclaae-liquid and blotite tenperaturea. '^'Conpariaon of tvo-feldapar and biotlte tenperaturea. Table 7.9. Estimates of pre-cruptive water content and kbj Lava type Wt.S p (2) Wt.t HjO(2> HVS Ne-Tb 0.13-0.25 - - Hy-Ta 0.35 1.92 4.5 Q-T 0.56-0.61(3) 2.33-2.50(3) 5.2-6.0 (3} TVS Ne-Tb 0.5-0.7<3) - - ^Estimated from plagloclate-llquld equilibrium '^Estimated from the Kudo-Weill plagloclase-liquld theraobarometer . .with Pu g converted to vt.S HjO ai described In the text. '3'Range S% values reflects different pressures and temperatures calculated by different methods at described In the text. Tho lowest value is the water content appropriate for the highest temperature. 257 Table 7.10. Ratiaatea of pre-eruptive f02 (bare) MV5 TVS Ne-Tb WRtl} Ne-Tb CM'*' Q-T WR*2) Ne-Tb WR{1) log foj -S.85 -8.00 -12.17 -7.61 fj^Calculated from Che coextatence of olivine, magnetite end melt. * ^Calculated from the coexiatence of blotite, magnetite and tanidine attuning pre-eruptive water contenta reported in Table IX. Reported foj La the average of valuea calculated at tenperaturea, preaaurea and water contenta Hated in Tablet VIII and IX. WR - Calculated uatng the whole-rock analytic at the liquid. CM - Caculated uaing the groundnaaa analyata aa Che liquid. 258 BULGARIA i W Rhodopoi W estern SALONIKI Thrace Samotherki Llmnos k Hagios Eustration Lesbos Lokris Corinth Samos ethane cm Poros f t Patm osf . ' fpsarimos Antimilos Antiparos KosX ?*' MHos n *. SaglorlnlSantorini Christiana & S O 1 0 0 k m Trench Fig.7.1. Hap of the Aegean Sea, showing the location of volcanic rocks (in black). The dashed lines nark the location of the Hellenic trench system and the Hellenic arc. 259 ISLE OF PATMOS (SIMPLIFIED FROM ROBERT 1973) 2 Km itmiv CG H tU N O U CKOUMANAS N » . Tr**r h*un Mf • Tndv bcufc M» • Twdv tndmw Q.Tr*Ch»* CAHEMXJMS Tndwi* liWWI Flwnofcii E223 Fig.7.2. Simplified geologic map of Patmos after Robert (1973). 260 1300 1200 U 1200 1150 1100 -0.70 -0.60 -0.40 -0.30 matt lop aSIO, B 1300 1250 O 1200 1150 1100 •O -0.70 -0.60 -0.50 •0.40 -0.30 mall lag a SI0„ Fig.7.3. Curves showing log Sgio2 versus T calculated from the equilibria Fo + SiO, ■ En ana CaTs + SiO, ■ An at various pressures for the MVS ne-trachybasalts. Curves calculated from the latter equilibrium are denoted by filled circles. Intersection of the two sets of curves yields a single line from which P and &si02 can be estimated by comparison with independently determined temperatures. The arrow on the temperature axis indicates the average temperature determined from olivine-1iquid thermometry, a: En-activity in clinopyroxene was calculated using the model of DePaolo (1979); b; En-activity was calculated using the model of Holland et al. (1979). 261 6.5 6.5 4.5 2.5 0.5 1300 1250 1200 eO h 1150 1100 0.5 2.5 4.5 6.5 8.5 0.9 0.8 -0.7 0.6 0.5 0.4 0.3 melt log a SIO„ Fig.7.4. Diagram similar to Fig. 7.3, but for a YVS ne- trachybasalt. Open circles are calculated values of the equilibrium Fo + Si02 ■ En using the model of Holland et al. (1979) for the activity of Mg2Si206 in clinopyroxene. a 262 5.0 6.0 A aM ; 3.0 2.0 1.0 1100 1150 1200 1250 1300 T*C b 7.0 6.0 5.0 4.0 M a. 3.0 2.0 1.0 1100 1150 1200 1250 1300 T*C Fig.7.5. PlagioclaBe-liquid temperatures calculated by the method of Ghiorso and Carmichael (1980) plotted as a function of pressure, a: MVS ne-trachybasalt; b: YVS ne- trachybasalt. Arrows indicate temperatures derived from olivine-liquid thermometry. 263 a. Hy»trachyande«1te 4 3 j 1 960 1000 1050 1100 1160 1200 T#C 5 b. Q-trachyte 4 3 Ptag.-llq 2 1 .850 900 950 1000 1050 1100 T*C Fig.7.6. Pressure-temperature estimates for the evolved lavas, a: hy-trachyaridesite; b: Q-trachyte. Squares - temperatures from two-feldspar thermometry. Circles - temperatures from plaqioclase-liquid thermometry. Intersection of the two lines yields an estimate of the pressure and temperature of equilibration. The arrow on the temperature axis of b indicates temperatures estimated from biotite thermometry. Comparison of this temperature with that indicated by the coexistence of two feldspars or plagioclase and liquid yields an estimate of the pressure of phenocryst formation. 264 CM O o> o T 13 800 900 1000 1100 1200 T°C Fig.7.7. Estimates of f02 plotted against temperature. Symbols: filled circle - estimate based upon microphenocryst and groundmass data, MVS ne-trachybasalt; - Filled square - estimate based upon phenocryst and whole- rock data, YVS ne-trachybasalt; Triangle - estimate for Q- trachyte of the MVS. Abbreviations: B - field of oxygen fugacities recorded by mineral assemblages in basalts (Carmichael et al., 1974); S - field of oxygen fugacities recorded by mineral* assemblages in lavas from Santorini (Huijsmans, 1985); C - f02-T trend recorded by lavas from Colima (Luhr and Carmichael, 1980); HM - hematite-magnetite buffer; QFM - quartz-fayalite-magnetite buffer. CHAPTER VIII PETROLOGY, ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY OF THE VOLCANIC COMPLEX OF PATMOS, DODECANESOS, GREECE INTRODUCTION Patmos is located in the eastern Aegean Sea, some 100 km to the north of the Hellenic arc (Fig. 1.2.). The lavas on Patmos are strongly alkaline to sub-alkaline, whereas the Hellenic arc is characterized by calc-alkaline volcanics. Alkaline and sub-alkaline lavas also occur on the Turkish peninsula Bodrum and on other islands in the eastern' Aegean Sea (Samos, Kos, Lesbos; see Robert and Cantagrel, 1977; Innocenti et al., 1982; Keller, 1982). These islands roughly define a N-S trending chain approximately parallel to the inferred plate boundary between the Aegean and Anatolian plates (Fytikas et al., 1976). Fault plane solutions for recent earthquakes indicate that the eastern Aegean is an area of horizontal tensional stress (Papazachos and Comninakis, 1978) which resulted in the development of a series of E-W grabens in western Turkey that extend into the eastern Aegean Sea (Mckenzie, 1978). The Aegean Graben system originated during the Middle Miocene and is related to movement along the North 265 Anatolian Transform and resulting uplift of parts of Turkey and the Aegean (Sengor and Canitez, 1982). Hence alkaline volcanism in the eastern Aegean Sea may well be related to this type of structural deformation. However, body wave tomography (Spakman, 1986) has shown the presence of a north-dipping high velocity slab down to a depth of at least 600 km, which indicates that subducted material must be present in the upper mantle beneath Patmos. Furthermore, Barton and Wyers (1986) recognized a distinct subduction signature in the chemistry of some of the lavas from Patmos. Hence a relation between this volcanism and the calc-alkaline volcanism of the Hellenic arc cannot be ruled out (see Barton and Wyers, in preparation). The petrology and geochemistry of the Main Volcanic Series (MVS) on Patmos have been described elsewhere in detail (Chapters V and VZ). The lavas of the MVS range in composition from ne-trachybasalt through hy-trachybasalt and hy-trachyandesite to Q-trachyte. The MVS experienced a complex evolutionary history which involved simultaneous operation of fractional crystallization, magma mixing and crustal contamination (see Chapter V). The other rock types that occur on Patmos have not been previously described in any detail. These include trachybasalts, phonolites, trachytes, and rhyolites that represent other periods of volcanic activity. The objectives of the present paper are to present new isotopic and age data for the different 267 volcanic units on Patinos and to discuss the temporal and petrologic relationships between these volcanics. g e o l o g y The geology of Patmos has been described previously by Robert (1973) and only a summary is given here. The rock names used here are different from those used by Robert, and are based upon a classification scheme (see Fig. 8.3) which utilizes Differentiation Index and (CIPW) normative mineralogy (see Chapter V). The structure of Patmos is dominated by a NW-SE trending horst-graben structure, with a central graben bounded by N70°W faults, and structural highs in the north and south of the island. The older volcanic units occur predominantly in the northern and southern parts of the island, whereas the younger lavas are exposed in the central part. Block faulting has also occurred, with fault orientations of N70°w, N30°w and N70°E (Robert, 1973), and this makes it difficult to confidently reconstruct the detailed stratigraphy. The distribution of the major volcanic units was shown in Fig. 5.3, which is modified from Robert (1973). Individual eruptive centers are rarely recognizable, although outcrops of volcanic breccias may indicate the sites of former vents. Some small explosion craters from which volcanic bombs of basaltic composition were erupted occur east of Grikou. 268 A volcanic series, consisting of alternating flows of trachyte and rhyolite with some basaltic flows, tuff layers and breccias, occurs approximately 1 km SE of c. Ghenoupas. This series, which was named the Old Volcanic Series by Robert (1973), has been tilted into a sub-vertical position and probably represents the earliest period of volcanic activity on Patmos. The Old Volcanic Series is overlain by trachytes and pyroclastics, the latter being locally intruded by volcanic breccias. Only two small outcrops of phonolite are known on Patmos, one near C.Gheranou and one near Lambi. The units listed above (trachytes, pyroclastics, volcanic breccias and phonolites) also make up most of the northern part of Patmos. The central part of the island is covered predominantly with deposits from the Main Volcanic Series (MVS). Hy-trachyandesites and Q-trachytes are volumetrically the most important units of the MVS, whereas ne-trachybasalts and hy-trachybasalts are relatively rare. Ne-trachybasalts belonging to the MVS occur exclusively as volcanic bombs in or near small craters E of Grikou. Lava flows of hy-trachybasalt are found near C. Koumanas, S of the village Chora and SW of the village Skala. They appear to be younger than the more evolved hy-trachyandesites and Q-trachytes. Rhyolites are fairly abundant in the central part of Patmos, but are often severely affected by hydrothermal alteration. Field observations suggest that 269 the rhyolites are closely related to the MVS deposits, and that they are stratigraphically older than the hy- trachyandesites and Q-trachytes. Ne-trachybasalts occur on Patmos as dikes that have intruded the MVS and several other units (marbles, old Volcanic Series, trachytes), and as lava flows on the island of Chiliomodi (Fig. 5.3). These younger basalts were referred to as the Young Volcanic Series (YVS) in Chapter VII. The only non-volcanic rocks on Patmos are marbles which occur on the southwestern edge of the island, near c. Ghenoupas. The contact between the marbles and adjacent volcanic units is a thrust fault, and deformed blocks of lava are found in the fault breccia. This implies that these marbles were emplaced relatively recently, after the onset of volcanism on Patmos. ANALYTICAL METHODS Mineral chemical studies were performed at the State University of Utrecht using a TPD microprobe equipped with a Tracor Northern Energy Dispersive system. Operating conditions were: 15 kV accelerating voltage, 3-4 nA sample current and 60-100 s counting time. Major oxides and Rb, Sr, Ni, Zr, Nb and Ba were measured at Utrecht by automated XRF. All other trace elements were done by INAA at IRI, Delft. Precision of the 270 major and trace element analyses is given in Huijsmans et al. (1988). Potassium concentrations for the K/Ar determinations were measured by flame photometry. Argon concentrations were measured by isotope dilution using a 38Ar-enriched spike. The ages for three samples were determined using the 40Ar/39Ar technique. These samples were irradiated for 100 h in the H-5 position of the Ford Nuclear Reactor at the University of Michigan. The details of the irradiation procedure and geometry may be found in Foland et al. (1984). The irradiated samples were fused by RF induction heating in a molybdenum crucible. Argon isotopic compositions were analyzed with a 15 cm, 60° Nuclide gas source mass spectrometer. All ages were calculated using the decay constants of Steiger and Jager (1977). Sr and Nd isotopic compositions were analyzed with a Finnigan MAT 261 multicoliector mass spectrometer operated in the dynamic mode. Sr and Nd isotope ratios were normalized to 86Sr/88Sr*0.1194 and 146Nd/144Nd-0.7219 respectively. The average measured isotope ratios for the NBS-987 and the La Jolla standards are 0.710244+/“0»000011 and 0.511852+/-0.000005 respectively. The procedure for' chemical separation of Sr and Nd is summarized in Chapter 111 . 271 AGE RELATIONSHIPS OF THE PATMOS VOLCANICS The results of the K/Ar determinations are listed in Table 8.1 for five lavas. Three other samples were analyzed for *°Ar, but the reproducibility was extremely poor, apparently because of incomplete fusion of the samples. These samples were analyzed using the 40Ar/39Ar technique and the results are listed in Table 8.2. The ages reported in Tables 8.1 and 8.2 indicate that there were at least three periods of volcanic activity on Patmos. The samples that yielded the oldest dates are the trachytes and phonolites at 6.14-6.20 Ma. The MVS lavas are slightly younger, with ages ranging from 5.47-5.67 Ma. The rhyolite sample yielded an age which is indistinguishable from the ages of the MVS lavas suggesting that the rhyolites should be considered part of the MVS. The entire MVS sequence apparently formed within a time-span of approximately 200,000 years. The ages of the different members of the MVS are within analytical uncertainty. The most recent volcanics are the ne-trachybasaltB from the YVS which were erupted/intruded at 4.49-4.64 Ma. The age we obtained on a Chiliomodi basalt (4.49 +/“ 0*06 Ma) agrees with the age reported by Fytikas et al. (1976) (4.38 +/“ 0.15 Ma). However, these ages are much older than the 3.50 Ma age given by Robert and Cantagrel (1977). Robert and Cantagrel (op.cit.) do not give a detailed description of their K/Ar analytical techniques, and at the present time 272 we do not know why the age obtained by these workers is too young. Volcanism on Patmos could have started as early as 7 m.y. ago, since two trachytic lavas dated by Fytikas et al.(1976) yielded ages of 7*03-7.20 Ha. Samples from these outcrops have not been dated in the present study, but these trachytes may have formed during the same period as to the Old Volcanic Series. PETROGRAPHY AND MINERAL CHEMISTRY Representative mineral analyses are listed in Appendix C. The compositions of clinopyroxene and plagioclase in the different volcanic formations are plotted in Figs. 8.1 and 8.2 respectively. Main Volcanic Series The petrography and mineral chemistry of the MVS lavas have been described in detail in Chapter V. We therefore do not report data for these lavas. However, we do compare mineral chemical data for the YVS lavas with those for the MVS lavas. The MVS ne-trachybasalts contain phenocrysts of olivine, plagioclase, clinopyroxene and microphenocrysts of Fe-Ti oxides. The phenocryst assemblage in the more evolved lavas (hy-trachyandesites and Q-trachytes) consists of plagioclase, clinopyroxene, phlogopite, alkali feldspar and apatite. 273 YVS ne-trachvbasalts The cores of olivine crystals in the dikes range in composition from F<>89-74’ The most forsteritic olivine cores have euhedral inclusions of Cr-spinel and contain up to 0.5 wt.% MiO. Similar Mg-rich olivines with spinel inclusions are found in the MVS ne-trachybasalts and are thought to represent xenocrysts of upper mantle origin (Chapter V). Fhenocryst cores in the Chiliomodi basalts are more Fe-rich and range from Fo81„63. The rims of olivine crystals in the dikes and Chiliomodi flows are Fo81-70 and F°69_57 respectively. Clinopyroxene in the YVS basalts is slightly more Al-rich (Al-o.040-0.408 afu) than, but otherwise similar in composition to, clinopyroxene in the MVS basalts. The range in composition in terms of Mg/(Mg+ Fe2+) is 0.85-0.68. Ti ranges from 0.005-0.077 afu and Na contents are less than 0.080 afu. Plagioclase in the basaltia dikes is similar in composition to that in the MVS basalts (coreB: An87_71; rims: An76„65) but plagioclase in the Chiliomodi lavas is more sodic (cores: An78-48; rims: Aft66-5o)* The most albitic cores in these lavas are mantled by alkali feldspar (Or48An4-Or32An9). Tl Emqn$tlt9 in the YVS is much more common than in the MVS basalts, and contains up to 27 wt.% Ti02, 3.3 wt.% A1203 and 2.6 wt.% MgO. Apatite is a common accessory phase in the YVS and occurs as inclusions in phenocrysts of plagioclase and clinopyroxene. The main phases in the groundmasses are 274 plagioclase, sanidine and Fe-Ti oxide, together with minor olivine and clinopyroxene. # Phonolites The phonolites contain phenocrysts of clinopyroxene, alkali feldspar and, sometimes, amphibole. The clinopyroxene phenocryst cores are salitic in composition, Na-rich (Na-0.082-0.290 afu) and with high Fe3+ contents (Fe3+/Fe2+-0.4-1.1, calculated assuming cation sum-4.000 on a 6 oxygen basis). These pyroxenes become acmitic towards the rim. Some microphenocrysts are nearly pure acmite, and contain up to 1.5 wt.% Zr02. The pyroxenes, when plotted in the quadrilateral (Fig. 8.1), define a continuous trend from salite to Fe-rich compositions. This trend is similar to that shown by pyroxenes in shonkinites and syenites from the Shonkin Sag laccolith (Nash and Wilkinson, 1971) and indicates that there is complete solid solution between Ca-rich and Na-rich pyroxenes. Alkali feldspar ranges in composition from Or5gAn0 to Or2gAn3. Strongly resorbed crystals with a hexagonal outline are frequently found in the phonolites. This mineral was identified as analcite by XRD, and is probably a pseudomorph after nepheline. The groundraass consists of sanidine, acmite and Fe-Ti oxide. The latter mineral is frequently mantled by acmite. Patches of analcite have been observed in the groundmass and are probably of a secondary 275 origin* Trachytes The phenocrysts in the trachytes are sanidine (Or5BAn1-Or32An5), phlogopite and titaniferous magnetite. In contrast, the Q-trachytes of the MVS also contain plagioclase and clinopyroxene. Ti-contents of phlogopite are high (0.26-0.78 afu) but are on average lower than Ti- contents in the Q-trachyteB of the MVS. Ti-magnetite is abundant and contains 12-18 wt.% Ti02, 0.56-0.86 wt.% Al2o3 and up to 0.31 wt% MgO. Igneous phases in the groundmass are sanidine and Ti-magnetite. Most trachytes on Patmos have been affected by hydrothermal alteration, as indicated by the occurrence of Fe-hydroxides and of cracks filled with quartz. Rhvolites The rhyolites contain phenocrysts of alkali feldspar and phlogopite. This phenocryst assemblage is not characteristic for rhyolites, but we will continue to use the name rhyolite for these highly siliceous rocks to be consistent with the classification scheme described earlier. The phenocrysts are set in a glassy groundmass that has extensively devitrified and partly crystallized to an assemblage of alkali feldspar, quartz and Fe-Ti oxide. The rhyolites are banded on a mm scale, which is 276 caused by the presence of thin, closely spaced quartz veinlets. The alkali feldspar in these rocks is extremely potassium-rich (Or95An1-Or100). Alkali-feldspar in rhyolite usually contains less than 80 mol% Or (e.g. Carmichael et al., 1974) and we attribute these unusual feldspar compositions to alteration and sub-solidus recrystallization (see below). WHOLE ROCK GEOCHEMISTRY Major element compositions and CIPW norms are listed in Table 8.3 for representative samples of the YVS, and for some phonolites, trachytes and rhyolites. In Fig. 8.3 normative nepheline or quartz is plotted against the Thornton-Tuttle Differentiation Index. Johnson et al. (1976) have previously used this classification scheme for lavas from Papua New Guinea. The field for the MVS lavas (from Chapter V) is outlined in this diagram and shows the transitional nature of this series, from Si02- undersaturated trachybasalts to Q-normative trachytes. Most of the analysed samples of the YVS are mildly Si02- undersaturated (<9 wt.% ne), but one of the dikes is saturated with Si02 (3.2 wt.% hy). The phonolites are strongly ne-normative (16.3-21.5 wt.% ne). The older trachytes plot in the same field as the Q-trachytes of the MVS, but we shall refer to the older lavas as "trachytes", i.e. without the prefix Q. The rhyolites cannot be 277 represented in Fig. 8.3 but would plot at Q-21.5-26.1 wt.% and DI**95.4-96.6, i.e. to the left of the Q-trachyte field. The variations in major oxide compositions are illustrated in Harker variation diagrams in Fig. 8.4. The ne-trachybasalts of the YVS are enriched in Ti02, FeO and Na20, but relatively depleted in MgO, K20 and P2O5 in comparison with the MVS basalts. Also, the K20/Na20 ratios of the YVS are considerably lower than those for the MVS (0.34-0.64 and 0.74-1.44 respectively for YVS and MVS). The major element compositions of the phonolites are fairly similar to those of a phonolite cone in the Pleiades, N. Victorialand, Antarctica, which has been described by Kyle (1982). Phonolites from other localities (e.g. Laacher See, Schmincke et al., 1983; Mt. Erebus, Kyle, 1977; Mt. Kenya, Price et al., 1985} are much less siliceous than the phonolites on Patmos. The trachytes and rhyolites are high in sio2 and alkalies, and low in all other major oxides. On most variation diagrams (except that for K20 versus Si02) these lavas plot on the high-silica extension of the MVS trend. The trachytes and rhyolites are characterized by extremely high K20/Na20 ratios (2.9-5.2 and 4.0-6.6 respectively), whereas K20/Na20 ratios in the MVS Q-trachytes do not exceed 2. The trachytes and rhyolites are also more potassic than trachytic lavas from ocean islands (e.g. Carmichael et al., 1974) and lavas from the Tuscan and 278 Roman districts in Italy (s.9 . Appleton, 1972; Dupuy and Allegre, 1972). Orendites from the Leucite Hills, Wyoming are also characterized by very high K20/Na20 ratios (Carmichael, 1967), but these lavas are considerably more mafic than the trachytes and rhyolites from Patmos. The strongly potassic nature of the latter lavas is thus unusual and is discussed in a later section of the chapter* Trace element compositions of representative samples are listed in Table 8.4. Eight trace elements are plotted against Th as a differentiation index (cf. Chapter VI) in Fig. 8.5. The ne-trachybasalts of the YVS are very different in trace element composition from the MVS trachybasalts. The YVS are relatively depleted in several compatible (Cr, Ni, Sr) and most incompatible (Rb, Ba, Th, U and REE) trace elements. Both generations of basalt contain similar concentrations of S * . V and HFSE. The large range in Sc and V contents in the YVS is undoubtedly due to crystallization and removal of Fe-Ti oxides, which are very abundant in these lavas and dikes. Chondrite normalized REE patterns (Fig. 8 .6) for the YVS show that these basalts are considerably less LREE enriched than the MVS basalts (see Chapter VI). LaN/YbN values range from 4.8 to 8.8 in the YVS and from 17 to 23 in the MVS trachybasalts. The YVS dikes show small negative Eu anomalies, whereas the Chiliomodi flows (the upper two patterns in Fig. 8.6) show small negative or positive Eu anomalies. Petrographic 279 studies suggest that the positive anomalies are caused by plagioclase accumulation. The phonolites are depleted in compatible elements and enriched in incompatibles, especially in the HFSE (Zr, Mb, Ta, Hf), relative to the other lavas on Patmos. They are IAEE enriched (Lajf/Yb^l^lS, see Fig. 8 .6) and are characterized by pronounced negative Eu anomalies, which indicate that fractionation of plagioclase was important at some stage in the evolution of these magmas, even though this mineral is not present in the lavas. The phonolites are characterized by flat HREE patterns, which has also been observed in phonolites from other localities (cf. Price et al., 1985). The trachytes contain very low concentrations of compatible trace elements. Relative to the Q-trachytes from the MVS they are depleted in some incompatibles (Ba, Th, U) but enriched in many others (Rb, HFSE, REE). The rhyolites show a similar depletion in compatible trace elements and Ba, but are moderately enriched in Zr, Ta and Hf and strongly enriched in Rb, Nb, Th and U. The rhyolites are characterized by relatively low Zr/Nb ratios (3.6-4.4) compared to the other lavas on Patmos which have ratios ranging from 9 to 15 (see Chapter VI). These low Zr/Nb values are probably a result of crystallization of zircon, since no other common mineral can cause significant fractionation in the ratio of these two elements. The REE 280 patterns of the trachytes and rhyolites show LREE enrichment (LaN/YbN«12-13 and 12-19 respectively) and contain large negative Eu anomalies, again indicating that plagioclase fractionation must have played a major role in the evolution of these lavas. ISOTOPE GEOCHEMISTRY Sr and Nd isotope ratios for 14 whole rock samples are listed in Table 8.5. The Sr isotope ratios reported are initial ratios. The correction for decay of 87Rb since solidification was made with the K/Ar and 40Ar/39Ar ages listed in Tables 8.1 and 8.2. The age correction for 143Nd/144Nd is insignificant. Sr isotope ratios (of relatively low precision) have been measured previously in several samples from Patmos at the ZWO Laboratory for Isotope Research in Amsterdam (Chapter VI). Most of these samples were taken from the same outcrops as the samples listed in Table 8.5, but the 87Sr/86Sr ratios are much higher (by several digits in the fourth decimal place) than the present values. We have no explanation for this discrepancy between the two data sets, but the new Sr- isotope ratios are preferred because the precision of the analyses is much bettter, and because the 87sr/88sr of the NBS-987 at the Ohio State University is well known, and similar to the values reported by most other laboratories. 281 87Sr/86Sr is plotted versus 143Nd/144Nd in Fig. 8.7 and the variations in 87Sr/86Sr and 143Nd/144Nd with whole rock Si02 are illustrated in Figs. 8.8 and 8.9 respectively. The MVS lavas show a very large overall range in isotopic composition. In the mafic lavas of the MVS Sr isotope ratios increase, and Nd-isotope ratios decrease with increasing differentiation. In the more evolved compositions (Sio2 > 58 wt.%) the isotope ratios remain near constant (87Sr/86Sr«0.707125-0.707383 and 143Nd/144Nd-0.512416-0.512422). Sr and Nd isotope ratios in the YVS are very different from those of the MVS ne-trachybasalts. They show a substantial variation, considering the small range in major oxide composition of the YVS. Surprisingly, the more siliceous dike (Pat-177) has a higher 143Nd/144Nd and a lower 87sr/86Sr than the more mafic dike (Pat-167). The Chiliomodi trachybasalt is characterized by lower 143Nd/144Nd and higher 87Sr/86Sr than the dikes from the main island. Only one trachyte and one phonolite sample have been analysed for Sr and Nd isotopes. They have identical 143Nd/144Nd and slightly different 87Sr/86Sr, which are higher (Nd) and lower (Sr) than the isotope ratios in the evolved lavas of the MVS. The rhyolite also has lower 87Sr/86Sr and higher 143Nd/144Nd ratios than the MVS Q- trachytes, although field observations and ages imply that 282 the rhyolites are closely related to the MVS. The rhyolites are hydrothermally altered, but in general alteration will cause an increase in 87Sr/88Sr and a (probably) smaller decrease in 143Nd/144Nd. PETROGENESIS Main Volcanic Series The evolution of the MVS has been discussed in detail in Chapters V and VI, where it was concluded that the hy- trachyandesites and Q-trachytes have evolved from a ne- trachybasalt magma by coupled assimilation and fractional crystallization (perhaps with some magma mixing) in a relatively shallow magma chamber (P*3-4 kbar). AFC- calculations, based on preliminary, low-precision Sr- isotope ratios, showed that the Sr-isotopic composition and trace element concentrations of the Q-trachytes can be explained by assimilation of a crustal end member with a composition similar to that of average crust, as estimated by Taylor and McLennan (1981), but more LREE enriched (see Chapter VI). However, the new isotopic data show that 143Nd/144Nd for the hy-trachyandesites and Q-trachytes does not change as a function of bulk composition, but remains constant within analytical uncertainty (Fig. 8.9). The Sr-isotopic composition of these lavas is also relatively constant, but variations in 87Sr/86Sr exceed analytical uncertainty. Most 283 of these lavas show slight effects of hydrothermal alteration, and it is feasible that alteration caused the scatter in ®7Sr/06Sr ratios but left the 143Nd/144Nd ratios unaffected. The relative constancy in isotopic composition of the hy-trachyandesites and Q-trachytes indicates that assimilation was not Important in the evolution of the evolved lavas of the MVS, and suggests that these magmas are related by fractional crystallization, possibly accompanied by magma mixing (Chapter V, VI). The isotopic composition changes considerably in the mafic part of the HVS, as a result of coupled assimilation and fractional crystallization (hy-trachyandesites), or simultaneous operation of AFC and magma mixing (hy- trachybasalts). In Chapter III the isotopic composition of a metamorphic xenolith from Santorini was used to model AFC for Santorini and Nisyros. However, the 143Nd/144Nd of this xenolith (0.512420) is similar to that of the hy- trachyandesites and Q-trachytes, and obviously the contaminant involved in AFC must have been characterized by a lower 143Nd/144Nd ratio. Briqueu et al. (1986) analyzed several xenoliths and basement samples from Milos and Santorini for Sr- and Nd-isotope ratios. One of these samples, a metavolcanic rock, haB 07Sr/86Sr-O.70991 and 143Nd/144Nd-0.512320, which seems to be a more appropriate isotopic composition for the contaminant involved in AFC on Patmos. Calculations with the new isotopic data indicate 284 that the ne-trachybasalts and hy-trachyandesites could be related by AFC involving average crust (from Taylor and McLennan, 1981) at Ma/Mc-0.8. This value is higher than that reported in Chapter VI. However, the new data do not affect any of the major conclusions of Chapter VI about the evolution of the MVS. Further work is in progress on the modeling of the simultaneous operation of fractionation, assimilation and magma mixing, which was important in the evolution of the hy-trachybasalts (Barton, in prep.) and the results of the AFC modeling are therefore not reported in detail here. X9vmq-Y9lgflnis-.s.9risB As was shown in a previous section, the most mafic dike of the YVS is characterized by higher Sr- and lower Nd-isotope ratios than the most evolved dike. Magmas forming these two dikes cannot be related simply by low- pressure differentiation processes, on the basis of major oxide and trace element data it is appropriate to consider the most mafic dike (Pat-167) as the parent for the Chiliomodi lavas (Pat-97), rather than dike Pat-177. Dike Pat-177 cannot have been the parental magma of the Chiliomodi lavas since it has higher concentrations of Si02, ?2°5f ^ and and lower concentrations of Cr than Pat-97. The major element least-sguares mixing calculations for the step Pat-167 to Pat-97 are shown in Table 8 .6. The overall agreement between estimated and observed major element compositions is good, which implies that fractional crystallization alone may have caused the observed major element variations in the YVS. However, the variation in isotopic compositions of Sr, Nd and 0 (see also Wyers and Pietersen, 1983) suggest that the YVS has been contaminated with continental crust. The equations from DePaolo (1981) were used to calculate lava compositions resulting from AFC. The bulk solid-liquid distribution coefficient for each trace element was determined by summation of the products (weight fraction of mineral in bulk solid)* (mineral-liquid partition coefficient). The composition of the bulk solid is known from major element least-squares mixing calculations (Table 8 .6). We used mineral-liquid partition coefficients for calc-alkaline and tholeiitic lava compositions from the compilation in Chapter II (Table 2.10), and partition coefficients for silica-undersaturated lava compositions from Roden (1981) and Baker et al. (1977). The exact procedure for calculation and selection of the best results was described in Chapter III, and the results, using average crust and upper crust from Taylor and McLennan (1981) as contaminants, are listed in Table 8.7. The isotope ratios of the crustal end members are those measured in a metavolcanic rock by Briqueu et al.(l986), that were also used to model AFC for the MVS (see above). 286 The calculations with upper crust show that the predicted concentrations of Sc, Rb and Ba are significantly higher than the measured concentrations of these elements in the Chiliomodi basalts (Table 8.7). Better results are obtained with average crust as contaminant. In that case all trace elements (except Sc) are predicted within 20 % (maximum analytical uncertainty) of the measured concentrations, and the calculated 87Sr/86Sr agrees well with the analysed value. The calculated 143Nd/144Nd is too high (by 5*10“5, see Table 8.7), but otherwise the AFC calculations with average crust yield a composition that closely, but not exactly, matches the composition of the Chiliomodi ne- trachybasalts. There are large uncertainties involved in the calculations described above. The composition of the crust in the eastern Aegean is poorly known, and no metamorphic xenoliths have been found on Patmos, so average,crustal compositions had to be used as contaminants. It was further assumed that the Sr and Nd isotope ratios of the crustal contaminant involved in AFC on Patmos are the same as those measured in metamorphic xenoliths from Santorini. However, the isotope ratios measured in different xenoliths (see above) indicate that the isotopic composition of the basement of Santorini is not uniform (see also Briqueu et al., 1986). Nevertheless, the match between predicted and analysed lava compositions is very good, and overall it can 287 be concluded that the Chiliomodi lavas and the mafic dike (Pat-167) are related by coupled assimilation and fractional crystallization, although the exact composition of the crustal contaminant is not known. Phonolites and Trachytes Phonolites are generally interpreted as residual liquids of basanitic magmas (e.g. Sun and Hanson, 1976; Schmincke et al., 1983), although some workers have suggested that phonolites originate by partial melting of basaltic material in the lower crust or upper mantle (see Price et al., 1985, for a summary of current hypotheses). The CIPW normative compositions of the phonolites from Patmos are plotted in the system NaAlSi04-KAlSi04-Si02 in Fig. 8.10. Phase relationships are shown for Ph20-1 H°ar (Hamilton and MacKenzie, 1965) and for the dry system (Schairer, 1950). The phonolites plot in the leucite field (dry system) or in the field for sanidine ss (hydrous sytem), near the temperature minimum on the field boundary between sanidine ss and nepheline ss. Most phonolites from other localities plot near this minimum, or in the thermal trough that extends between this minimum and the temperature minimum on the feldspar join (Schairer, 1950; Hamilton and McKenzie, 1965). A suitable parent magma composition of the same age as the phonolites and trachytes (6 .1-6.2 Ma) is not present on Patmos, but least-squares 288 nixing calculations (Table 8 .6) show that the najor oxide conposition of the phonolites is consistent with derivation from an sio2-undersaturated nafic magma such as ne- trachybasalt Pat-26 of the HVS by fractional crystallization of olivine, clinopyroxene, plagioclase, titaniferous magnetite and apatite. However, the high concentrations of incompatible elements in the phonolites (especially Zr and Kb, see Fig. 8.5) indicate that these lavas cannot be related to a ne-trachybasalt magma by fractional crystallization. The phonolites must have been derived from a magma that was considerably more enriched in incompatible elements than the HVS ne-trachybasalts. It is more likely that the phonolites evolved from a basanite magma, which has also been suggested for phonolites from other areas (e.g. the Bifel, Schmincke, 1983). The occurrence of phonolites on Patmos indicates that there has been a period of volcanic activity characterized by strongly alkaline magmas. In a previous section it was shown that the phonolites and trachytes have nearly identical isotopic compositions, which suggest that they originated from a common parent. However, the trachytes are Si02-oversaturated, and they can therefore not be directly related to the phonolites by low-pressure fractional crystallization since these lava compositions are separated by the Ab-Or thermal divide. The trachytes show evidence of silicification (see above) and are characterized by 289 relatively high K20/Na20 ratios, which could also be a result of alteration (discussed below). The relationship between the trachytes and phonolites is not understood, but it is possible that the trachytes were originally Si02- undersaturated lavas that became Si02-saturated by secondary silicification. KhYPlitCg The elevated K20/Na20 ratios and the Sr and Nd isotope ratios of the rhyolites suggest that they are not simply late-stage differentiation products of the HVS. Rhyolites from the Esterel, France, are characterized by high K20/Na20 ratios and very Or-rich alkali feldspars (Terzaghi, 1948). Vitrous obsidian from this area has K20/Na20-0.8, whereas devitrified obsidian and altered rhyolites are characterized by much higher K20/Na20 ratios (1.9-12) and by the occurrence of extremely Or-rich alkali feldspar near cracks and veinlets in the lavas. These features were suggested to have resulted from metasomatism and sub-solidus recrystallization (Terzaghi, 1948). Stewart (1979) and Curling et al (1985) concluded on the basis of stable isotope fractionation that highly potassic glasses form by hydration involving meteoric water. The glass-water exchange constants for H* and K+ are very high, whereas Na+ , Ca2+ and Mg2* have low exchange constants and will preferentially enter the aqueous phase. Eventually, at high 290 water/rock ratios, H+ and K+ will have replaced the other cations in perlites. We therefore conclude that the rhyolites on Patmos have been modified in an analogous way by interaction with fluids. If the rhyolites were indeed erupted during the same period as the HVS lavas (as is suggested by field relationships), then hydrothermal alteration must have occurred during or shortly after extrusion of the rhyolites; otherwise the K/Ar age would probably be different from that of the HVS. Alteration will probably result in an increase in the K/Ar ratio, because Ar will be lost from the glass during devitrification (or is immobile relative to K, see Curling et al., 1985), and K will be enriched in the glass (see above). Therefore the K/Ar age of the rhyolites would have been younger than that of the HVS if a significant period of time has elapsed between extrusion of the rhyolites and alteration. Compared to the Q-trachytes of the HVS, the 87Sr/86Sr of the rhyolites is relatively low and 143Nd/144Nd is relatively high. This suggests that the rhyolites are not simply the most evolved products of the HVS, since it is unlikely that alteration caused a decrease in 87sr/86Sr and an increase in *43Nd/144Nd of the rhyolites. If the Sr and Nd isotope ratios of the rhyolites have not been changed significantly by alteration and still reflect the isotopic composition of the original lavas (i.e. prior to alteration), then it is 291 possible that the rhyolites represent silicified hy- trachybasalts or hy-trachyandesites. Another possibility is that the rhyolites are older than the MVS, and that the' similarity in age is an artifact of hydrothermal alteration. CONCLUSIONS There have been at least three periods of volcanic activity on Patmos, with each period characterized by eruptives that are chemically and isotopically distinct. The oldest lavas we have studied are phonolites and trachytes which were formed between 6.14 and 6.20 Ma. The phonolites probably evolved from a basanite magma, although a suitable parent magma composition (i.e. with an age of 6 .1-6.2 Ma) is not known from Patmos. The trachytes are isotopically similar to the phonolites but modified by alteration, and the exact relationship between these two lava types is not understood. Prior to silicification the trachytes may have had Si02-undersaturated compositions. The MVS lavas comprise the volumetrically most abundant series on Patmos. They were erupted at about 5.6 +/- o.i Ma. This series experienced a complex evolutionary history that involved the concurrent operation of fractional crystallization, assimilation and magma mixing. The evolutionary model for the MVS as proposed in Chapter V is essentially correct, despite the availability of new 292 isotopic and age data. The composition of the rhyolites is partly the result of strong hydrothermal alteration, but it is possible that these lavas are silicified hy- trachybasalts or hy-trachyandesites. The YVS were erupted/intruded at 4.55 +/“ 0.1 Ha, and evolved through coupled assimilation and fractional crystallization, although the composition of the crustal end member is not precisely known. The ne-trachybasalts of the YVS are chemically and isotopically distinct from those of the HVS, and are characterized by lower 87Sr/86Sr, higher 143Nd/144Nd and lower concentrations of incompatible elements. This suggests that the composition of the upper mantle source region changed with time and became progressively depleted. The evolution of the upper mantle source region in terms of spatial-time relationships has been briefly discussed by Barton and Wyers (1986) and is the subject of a paper in preparation by Barton and Wyers. Table 8.1. K/Ar ages of whole-rock saaples froa Patmos. 293 Saaple rock type** K ••OAr* "»Ar** (w t.S) (10“^aol/g) (2 of total Ar) ( a . y . ) Pat-180 KVS Hy-Tb 3.729 71.3 3.804 68.1 3.746 67.6 Av:3.914 3.760 5.5310.08 Pat-105 MVS Hy-Ta 5.729 64.9 5.609 72.8 5.826 71.0 Av:5.812 5.721 5.6710.12 Pat-220 YVS He-Tb 1.263 21.9 1.257 21.5 1.270 25.2 Av:1.621 1.263 4.4910.06 Pat-201 Phonollte 5.729 59.0 5.431 54.7 5.582 54.8 5.717 56.3 Av:5.269 5.615 6.1410.17 Pat-148 Khyolite 8.622 22.2 8.647 19.2 8.532 23.8 Av:8.8S9 8.600 5.5910.08 a i, radiogenic argon b Ke-Tb ■ ne-trachybasalt; Hy-Tb ■ Ky-trachybasalt; Hy-Ta « hy-trschyandeslte; Q-T ■ Q-trachyte Table 6.2. t>0Ar/19Ar analytical data and agea for whole-rock saaplca fro* Fitaos 37 ar/)9 Ar Saaple rock type* *»Ar/»Arae„. ' " c o r r . “ Ar/J’ A r ^ ^ r” '®Ar*(Z) J Age (« .y .) Pat-167 HVS, He-Tb 3.465 1.356 0.00711 0.440 17.85 0.006921 5.4910.16 2.224 1.339 0.00626 0.444 19.93 0.006833 5.4610.29 Aw.5.47 Pat-179 TVS, Ke-Tb 1.174 2.095 0.00314 0.382 32.50 0.006775 4.6610.22 2.085 2.264 0.00632 0.369 17.66 0.006960 4.6210.31 Aw:464 Pat-136 Trachyte 1.469 0.0184 0.00317 0.497 33.89 0.006938 6.2210.10 1.743 0.0248 0.00407 0.503 28.85 0.006815 6.1710.10 Aw:620 * Abbreviatlona aa in Table 1 F ia the ratio o f radiogenic *°Ar to K-deriwed J*Ar c Fat-167 Pat-177 Pat-97 Pat-19 Pat-13 Pat-71 TVS TVS TVS Phonolite Traehy.e Rhyol it r Sid, 48.63 51.12 51.01 59.85 68.04 72.96 TiQj •.67 1.77 1.71 0.38 0.29 0.28 18.94 18.97 18.91 20.94 16.05 13.59 8.01 7.22 8.39 2.32 2.12 1.67 HnO 0.45 0.12 0.15 0.21 0.07 0.02 MrO 5.90 5.27 5.03 0.30 0.21 0.18 CaO 10.11 9.65 8.68 1.55 0.04 0.15 Na,0 4.39 3.27 4.15 7.77 3.39 1.47 * 2 ° 1.48 2v04 2.19 6.63 9.74 9.63 *2°S 0.41 0.58 0.53 0.03 0.05 0.03 Q -- - - 9.48 26.09 or 8.75 12.06 12.94 39.18 57.56 56.91 ab 20.47 27.67 28.4B 34.32 28.33 12.44 an 27.60 31.06 24.54 2.68 - 0.55 ne 9.03 - 3.59 17.02 -- d i 16.25 10.62 12.37 4.17 -- by - 3.18 -- 3.46 2.54 ot 12.34 9.44 12.13 1.41 -- mt 1.54 1.38 1.61 0.45 0.25 0.32 il 3.17 3.36 3.25 0.72 0.55 0.53 ap 0.95 1.34 1.23 0.07 0.07 0.07 295 Total Fe as FeO. Norms calcu lated with Fe^Oj “ 0.15 FeO Table 8.4. Trace element concentrations of selected Patmos lavas Pat-167 Pat-177 Pat-97 Pat-19 Pat-13 Pat-71 TVS TVS TVS Phonol it e Trachyte Rhyolite Sc 28 21 22 1 4 4 V 206 194 179 17 7 19 Cr 47 18 28 U 7 8 Ni 21 23 9 4 10 1 1 Zn 51 60 63 88 46 11 Rb 17 57 21 349 511 600 Sr 800 976 771 180 34.4 45.4 Zr 184 240 244 979 539 248 Mb 16 35 24 89 52 57 Ba 682 628 710 25 16 205 La 22 34 33 132 84 98 Ce 41 38 62 210 153 138 Sm 5.02 5.94 6.14 8.81 9.68 5.61 Eu 1.64 2.41 1.95 0.71 0.49 0.76 Tb 0.89 0.B6 0.87 0.98 1.02 0.66 Yb 2.08 1.21 2.42 5.25 3.76 2.96 Lu 0.40 0.63 0.44 0.89 0.66 0.57 Hf 3.50 5.10 5.15 21 15 8.62 Ta b .d . b .d . 1.26 3.92 3.04 2.89 Th 2.61 3.99 3.71 53 22 66 U b.d. 5.06 b.d. 6.45 5.33 14 296 Rb and Sr for Fat-19, Fat-13 and Fat-71 analysed by isotope d ilu tio n . Table 8.5. Sr and Nd isotope ratios in lavas from Patinos 297 Sample rock type SiOj (67 5r/96 gr)a t 2 a lUM'l^Nd t 2 a Pat-26 HVS, Ne-Tb 50.03 0.704593'± 9 0.512642 t 5 Pat-27 MVS, Ne-Tb 50.55 0.704661 t 10 0.512663 t 6 Pat-181 MVS, Hy-Tb 56.77 0.705941 t 14 0.512647 t 5 Pat-108 MVS, Hy-Ta 57.60 0.707051 t 12 0.512618 i 8 Pat-117 MVS, Hy-Ta 60.38 0.707282 t 12 0.512620 1 7 Pat-159 HVS, Q-T 62.40 0.707100 t 15 0.512417 1 7 Pat-53 MVS, q-T 63.92 0.707167 t 14 0.512616 t 7 Pat-58 MVS, Q-T 65.86 0.707174 ± 13 0.512422 t 6 Pat-167 YVS, Ne-Tb 48.63 0.704121 t 11 0.512793 t 7 Pat-177 YVS, Ne-Tb 51.12 0.703580 t 12 0.512888 t 6 Pat-97 YVS, Ne-Tb 51.01 0.706345 t 10 0.512713 t 8 Pat-19 Phonolite 59.85 0.706264 t 19 0.5125S1 t 6 Pat-13 Trachyte 68.04 0.706081 t 68 0.512552 t 7 Pat-71 Rhyolite 72.96 0.706542 t 60 0.512452 t 8 ^Uncertainty in Initial Sr-lsotope ratios includes uncertainty in Rb/Sr and in K/Ar or l*®Ar/, , Ar ago Table 8.6. Remits of least-squares dsss balance calculations 298 Pat-167 ♦ Pat-97 Pat-26 + Pat -19 Pat-167 Pat-26 Obs. Calc. D iff. Obs. Calc. D iff. SiO, 48.86 48.94 -0.0910 50.04 49.96 0.0807 TtOU 1.68 1.52 0.1636 1.17 0.92 0.2533 aiA 19.11 19.11 -0.0030 17.73 17.75 -0.0231 FeO 8.10 8.14 -0.0381 7.29 7.35 -0.0566 MnO 0.21 0.15 0.0553 0.13 0.22 -0.0871 MgO 5.87 5.77 0.0987 7.14 7.19 -0.0504 CaO 10.00 9.86 0.1375 10.06 10.18 -0.1215 Na.0 4.36 3.55 0.8108 3.03 3.59 -0.5568 Kjl 1.44 1.59 -0.1523 2.71 2.67 0.0355 0.39 0.37 0.0207 0.70 0.54 0.1600 Hix Propor Hlx Propor Variable tions Variable tions Pat-97 0.6969 Pat-19 0.3904 ol 0.0380 ol 0.1061 Cpx 0.0456 Cpx 0.1911 Plsg 0.1937 Plag 0.2675 Ti-msg 0.0157 Tl-nag 0.0363 Ap 0.0122 SSR ■ 0.7492 SSR - 0.4362 Pat-26 ■ HVS ne-trachybasalt; Pat-167, Pat-97 ■ YVS ne-trachyhasslt; Pat-19 " phonolite Table 8.7. Re«ult» of AFC calculation* for the Toung Volcanic Series 299 P«t-970bi. Pat“97caic. Fat-97caie, Contaminant: Average Crutt Upper Crutt Sc 22 29 - 37 27 - 35 V 179 158 - 261 168 - 26« Cr 28 1 - 63 1 - 60 Hi 9 9-28 9-27 Zn 63 - 52 - 67 Rb 21 25 - 26 32 - 33 Sr 771 763 759 Zr 2 44 236 - 260 250 - 255 Va 710 852 - 890 888 - 928 U 33 28 - 29 29 - 30 Ce 62 53 - 55 56 - 58 Nd 32 30 31 So 6.1 6.6 - 6.7 6.5 - 6.7 Eu 1.9 1.9 - 2.2 1.9 - 2.1 Yb 2.6 2.7 - 2.8 2.7 - 2,8 Hf 5.1 6.6 - 6.7 6.9 - 5.0 Th 3.7 6.1 - 6.3 6.7 - 6.9 ®7 Sr/®6 Sr 0.706365 0.70660 0.70636 INlM/UtiM 0.512713 0.512766 0.512751 AFC parameter*: Ha/Hc - 0.3 0.3 F - 0.8 0.8 °i'L - 1.0 1.0 •IT- - 0.05 0.05 Fig.8.1. Compositions of clinopyroxenes plotted in the conventional Ca-Mg-Fe quadrilateral. Abbreviations: MVS Main Volcanic Series; YVS - Young Volcanic Series; PHO - phonolites. 301 ,e**ia • • mo Fig.8.1, cont 302 YVl M1 ■OK Fig.8.2. Feldspar compositions plotted in terms of the components An-Ab-Or. Abbreviations! MVS - Main Volcanic Series; YVS - Young Volcanic Series; PHO - phonolites; TRA - trachytes; RHY - rhyolites. 303 in ■on Fig,8.2, cont. 304 AN ANY AB Fig.8.2, cont. 305 too ?s ao as ao 10 10 ao I |tHIKh)ll| j *»-l<»dlju M Fig.8.3. Classification of Patmos lavas in terms of CIPW normative Ne or Q and Thornton-Tuttle differentiation index. Classification scheme after Johnson et al. (1976). Symbols: squares - Young Volcanic Series; triangles - phonolites; circles - trachytes. Dashed curve indicates field for Main Volcanic Series lavas. 306 1.3- oCM p J- 5045 53 60 70 73 Si02 o uvs v PHO 20 IB- n o CD CM 5? IB- 14- 63 SI02 Fig.8.4. Major oxides plotted against Si02. Abbreviations as in Fig. 8.2. i.., cont.Fig.8.4, F eO 45 xo 50 56 65 60 55 Si02 5i02 60 D 0 ED □o' □ • RHY O PHO V YVS X MVS □ 307 308 oa o CO Si02 10 C MVS X YVS V PHO • TRA 8 o RWlf 6 o No z 4 2 0 45 50 55 60 65 SI02 Fig.8.4, cont. 309 K20 o m 4* LM U o» ©• Ui o s:o2 X D < i m i * • P205 □ □ □ U) s; a IA s; O ofO Fig.8.4, cont. 310 W □ □ 9 0 10 20 30 40 50 60 70 60 Th 250 a mvs x YVS v PHO • IRA O RHV oa > 0 10 20 30 40 50 Th Fig.8.5. Variation diagrams for selected trace elements with Th as a differentiation index. Abbreviations as in Fig. 8.2. 311 250 200 O 150 100 50 0 10 20 30 50 60 80 Th 700 D MVS X YVS V PHO 600 * TRA © RHY 500 <400 OZ 300 200 100 2 0 30 40 60 Fig.8.5, cont. 312 1B00 1600 Be 1400 1200 1000 □ a t/ti . BOO 600 400 2 0 0 - ** 0 20 30 40 SO 60 BO Th 2000 O MVS X YVS v PHO • TRA 1600 0 RKT o a Do 1200 t P o ma BOO 0 1 0 2 0 30 40 SOBO Th Fig.8.5, cont. 313 1 2 0 0 1000 800 200 0 10 20 30 40 50 60 80 Th 100 O MVS X YV5 v PHO • TRA 20 0 20 30 40 7060 Fig.8.5, cont. 200 •00 100 M «0 10 YVS PHO la ct »M CU ra T0 tU ni n to in Fig.8*6* Chondrite~normalized plots of REE concentrations 314 in representative samples. Abbreviations as in Fig. 8.2. 100 100 M K> TRA RHY t o T O U l 10 Cl M CU to tlf 10 Fig,8*6, cont 315 316 .5129 .51273* *o CO z to .51257* • A .5124 .7035 .7043 .7051 .7059 .7067 .7075 87Sr/86Sr Fig.8.7. 143Nd/144Nd plotted against 87Sr/86Sr. Symbols: filled squares - Young Volcanic Series; open squares - Main Volcanic Series; triangle - phonolite; filled circle - trachyte; open circle - rhyolite. i... 87Sr/86Sr Symbols Fig.8.8.plotted againstSio2contents. as in 8.7.asFig. 87Sr/86Sr .7035 .7045 .7055 .7065 ,7075 53 8634858 SI02 68 317 i... 143Nd/144NdFig.8.9. plotted Symbolsagainst Si02contents. as in 8.7.asFig. 143Nd/l44Nd .5125 ,5127 .5128 .5129 5126 853 48 00 58 SI02 63 68 318 73 319 ao Silica SOj JO 40j Sonidine SS NoAISijCfe •*o. i*QI no— — to Nepheline SS ^ to 30 4 0 SO TO 00 $0 Fig.8.10. The CIPW normative composition of the phonolites (triangles) plotted in the system NaAlSi04-KAlSi04-Si02”H20 at Pu2p “ 1 Jcbar (fro® Hamilton and MacKenzie, 1965). The field boundaries for the dry system (after Schairer, 1950) are indicated by the dashed curves. APPENDIX A WHOLE-ROCK MAJOR OXIDE AND TRACE ELEMENT ANALYSES OF LAVAS FROM NISYROS 320 321 Tibli (Cl *■» ta9 H fc*n Ml ►11 ►6 ►i Ml ta2 ta a ta ll ta7» sioe s i s 5141 s i n S i l l 59.19 59.21 st* s S t 94 *117 1151 (ID 7111 7141 Tioe .9* 1.11 i.it i.n i.U 1.11 i . u 1.17 .*] .*4 .(2 .» .14 11201 1147 17.14 17,85 17.52 17.17 17.51 17.11 17.17 I ll* i i a I ll* 1152 15.45 F*0 111 154 157 147 i a 147 i n 174 1*1 t » 177 142 141 M .11 .14 .14 .11 .14 .11 .15 .14 .19 .n . a .a . a W 4.U i n tM 111 1M LU t5 9 149 t . a i . u !.£! .71 . a CtO 9.(7 177 i n 151 t a 151 IK 5.51 1.4* 142 141 I N L4I tag) 171 111 1*7 4 .a 4,54 i a 4.41 4.51 171 4.S9 4.44 4.N 4.M K2Q i . a I.M i . a 1.71 1.73 1.71 t i l i n t i l t a i a 11* 117 nos .» . a . a .14 . a . a . a .17 .15 .17 .15 M .14 tot 1.4* -.14 .41 - . a .11 .i i -.a -.a . a . a t a .12 .U Ec a 21 21 a 21 21 19 19 7 9 1 4 4 V 211 197 a i ta 214 217 IN III (1 74 (1 a 24 Ml 17 1 i s 7 * 1 7 4 4 4 3 3 Ci 14 a 54 (2 K 52 a » 11 21 12 9 11 In 71 71 74 74 t l 74 73 a M 41 M S 43 a * 52 51 52 47 4* u ii n 17 19 92 a Gr S I 411 417 411 419 411 4a 421 317 14* S7 219 w r II a a 24 a a 24 a 24 a 24 a a If 141 191 194 192 194 191 a* m 241 242 241 ca B4 ta 11 IS II 15 14 14 15 15 11 1* 11 i* !( Ct 1.19 t a 1.(1 1.E5 N/A i.a 1.94 t s 131 t i s IS 114 171 ta 22* 43J 421 411 471 451 492 sis 715 714 a t 715 TU U 17 27 a a a a a K 1* a 42 S 17 b a 47 41 42 49 52 s a 5* *9 (4 » 89 ta 111 4.42 4,11 4.a 111 I S 4.71 in I S 4.H 4,11 t u 111 Ei 1.12 i.a 1.41 1.49 l.S 1.41 1.54 1.42 i.a l.M 1.12 .77 .M Th 1.12 N/A .91 1.21 1.49 1.19 1.39 1.47 .45 .a .a .14 .(1 Yb ISIS 174 t u ts tM ts i t a t a t i t t!9 1.91 1.(9 l i .41 .41 .51 .47 .a .47 .52 .52 .32 .44 l.M .» .11 Nf 111 4.41 4.4* 197 4.11 4.S 4.79 4.19 4.M 5.(4 1(2 3. II 1(9 7i .(2 .a i.a .K N/A N/A .79 1.14 i.a 1.12 1.11 1.1* l.S Th i a s. a I S 1 U 114 149 (.77 7.15 1114 11.31 11.49 9.95 11.41 322 ►12 N-21 11-22 ►14 sice 716* 9E.lt 17.31 3172 T102 ,34 .71 .41 1.19 A12Q3 13. £1 1114 1117 1171 FtO 149 3.22 191 7.S3 M .M .tl .17 .13 "SO .12 3.31 1.73 143 CiO 143 9.33 114 7.44 ma 4.72 2.49 129 117 «0 3. K 1.14 171 1.44 P2SB .M .19 .19 .19 LOI 1.17 .IE .34 1.24 Ge 4 11 7 22 V 29 144 44 213 HI 1 44 3 1 b I 41 14 M/A I* 42 44 41 N/A «b 92 14 W 31 Er tu 1194 444 474 T 22 14 21 N/A I r 119 144 144 141 Mb 14 4 12 11 Ci IN M/A 124 N/A h 711 219 447 334 u 31 14 33 21 Ct 33 31 41 44 Ei 131 143 124 147 E« .71 1.12 .77 1.34 Tb .41 .44 .33 Tb 1.71 1.39 1.77 139 li .34 .31 .24 .41 Hf 191 142 113 144 Ti .93 N/A .93 N/A Th 1123 131 9.(4 133 APPENDIX B WHOLE-ROCK MAJOR OXIDE AND TRACE ELEMENT ANALYSES OF LAVAS FROM PATMOS 323 324 Tibl*61 Pit-» Pit*117 Pil*l27 Pit-27 P«*l2t Pit*12J Pit-IU Pit*181 Pit *54 Pit-Ill Pit-IM P»t*ll4 Pat-163 IM tw IM p m p m p m i m tM tM IM PM PM PM site 3613 3621 3611 3655 3615 51. IS 34.15 36K 34.31 34.77 57.0 57.M 11.21 7102 1.17 1.21 1.11 1.16 1.13 1.16 1.19 .19 I.U 1.11 . 0 .11 .M A1201 17.71 17.11 1611 1 6 0 17.71 17.11 17.21 13.71 17.21 1621 1619 1617 17.79 r*o 7.29 7.61 7.11 7.21 7.17 7.11 617 617 672 611 6 0 679 639 M .13 .13 .41 .14 .14 .14 .11 .17 .14 .a . 4 .44 .14 HgO 7.14 i n 631 6 0 7.K 7.11 137 4.31 113 671 111 631 I.U bo 16 K 677 634 617 611 611 6U 612 7.11 619 691 614 643 Ki20 113 144 147 117 631 111 619 1 4 692 113 119 117 177 K20 671 634 699 691 172 111 621 614 611 633 673 671 671 PSOS .71 .71 .74 .73 .11 .12 .73 .11 .71 .M .31 .49 .a LAI 621 613 1.34 639 6 4 1.11 1.19 i n 694 1.92 611 614 Gc 21 21 22 22 21 22 II it 19 II 17 11 9 V OS 2K 111 111 217 191 111 in t92 in 14 133 m fr 243 214 192 IBS 232 244 II 114 123 19 121 IM ti w 91 •1 t l 73 in 117 29 43 4 O 42 4 7 Cu SI 33 52 31 53 51 44 4 a a a 47 ii to 57 12 11 59 II 11 12 33 u 13 4 49 32 Rb 111 72 91 IK 142 111 191 177 113 ESI ia in tu 6r 1311 1554 1373 1371 147 1411 911 391 917 974 714 174 117 r 21 22 23 21 29 27 31 34 17 4 4 a 44 to 221 224 233 231 224 213 291 20 07 111 294 Ul 3M Kb 11 t9 21 21 » 21 11 12 O U a 27 41 Bi 1173 1721 1171 1144 I1U 1711 1494 IKI 22U 1321 1191 1121 1275 U (1 19 19 19 71 71 11 31 12 II 19 II U Ci 113 114 119 111 117 III 113 91 III 117 114 IM 14 Si 9.41 627 9.14 631 671 611 1621 7.42 60 9.11 612 64 1631 Ei 641 627 639 671 6 0 641 611 1.11 629 629 1.71 1.19 I.U Tb t.17 1.24 .11 1.21 1.14 .94 .94 .74 .a .U .74 .a .17 Yb 1.12 611 621 623 1.71 612 6 0 631 637 614 647 629 611 u .32 .31 .» .33 . » .41 .17 . a .19 .a . 4 .a .a Hr 4.M 112 4.11 4.17 631 4.33 7.32 7.31 7.42 7.32 7.U 657 9.95 Ti .73 N/A 1.13 N/A N/A .32 1.31 1.11 1.19 614 1.33 1.24 60 Tb 691 692 649 639 11 11 31 27 21 a 11 21 a U 4.11 N/A N/A 1.71 131 114 631 613 7.11 691 64 691 671 325 Pit-117 Pit-115 Pit-IU Pit-74 Pit-31 P»t-75 Pit-159 Pit-IU Pit *49 Pit-53 Pit-144 Ptt-21 Pit-151 WG M m tM MS MS MS MS MVS MS IM MS MS G102 4XX 6X92 tl.U 41. U 41.49 4X11 4X44 4X12 4Xtl 6X92 £4.21 U .4I 44.42 TIC2 .H .ts .79 .17 .45 .U .44 .U .47 .62 .99 .49 , n At SI I t 79 ILN 17.0 17. M 14.36 17.19 14.49 IXM 14.79 1X75 IXU 17.M 1X75 FtO 4.X 4.27 4.45 4.M X5I XS9 XI? X43 XU X52 XU XM XM M .39 .U .14 .11 .37 .11 .42 .37 .34 .K .37 .15 .« NjO XU t i e XU 1.U 2.26 1.12 1.22 I.U l.4« 1.47 1.39 1.17 1.N CiO XU 2.79 X23 4.33 4.45 4.39 X79 X4I X14 X II X39 XU XU K*a XU 4 .a x n X II X9I XK X94 XU XU XB XM X36 X61 K20 XU £.17 7.44 X42 4.45 XS9 7.45 4.29 £.33 X74 X!4 XII XU P2Q3 .49 .39 .57 .49 .35 .44 .37 .X .36 .31 .27 .X .29 uot 1.79 1.77 X*4 1.73 X57 1.73 1.76 I.U I.U X 43 1.51 X49 I.SI Ge 13 9 9 II 4 11 II 9 1 1 6 9 9 V til 95 111 99 75 112 IS U 99 74 IIB K Cr 15 N/A II 11 19 11 14 II 11 9 1 12 N/A Nl U 4 U 4 11 4 9 4 7 4 5 3 4 Cq 12 14 11 13 21 11 1 9 11 11 9 12 14 2a 52 77 54 S3 41 54 51 51 51 45 44 44 51 Rb 217 225 176 2U 2U 221 IB t u t u 241 261 217 229 Sr 495 612 171 459 479 US 326 s u 546 SB 465 511 539 7 42 43 41 41 42 19 41 42 44 42 U 42 44 2r 162 413 326 311 3M 117 327 314 347 347 359 in 334 Nb 39 43 39 X 17 35 41 XX 17 » u 37 Bi HU 1491 1112 13U 547 1EU 1224 1261 1191 1174 915 1251 I4M Li 74 156 UU 57 U 71 45 U U U 71 U C* 122 154 135 113 X 116 t u 113 IK 112 1U IB 141 fit X91 1X51 XU X35 X1S XX XU 7.41 7.57 4.44 7.11 X14 1X79 E« XG2 XU X22 1.49 I.U 1.95 1.75 I.U 1.44 1.55 1.31 I.U X1S Tb 1.41 l.t l 1.11 .46 .65 .91 .74 .79 .71 .77 .74 .U .42 Yb X12 XI2 X65 XSI 1.45 XS4 XU X6I XSXKXU X65 N/A L« .51 .22 .56 .44 .42 .51 .95 .51 .46 .42 .44 . a N/A Hf 9.34 1X59 XII XI2 7.64 X55 XB X7I XK X44 XU X41 XU Ti I.UXU XU 1.45 1.U 1.77 X35 XU X17 X19 X32 XI9 XU Th 31 34 41 34 41 34 41 41 39 19 41 41 41 U 7.U 11.99 XS4 XI7 ll.M 7.44 IXU XX IXU XII X7I XU X97 326 Pat-63 Pit-64 P lt-tl Pit-61 P«-9 Pit-39 Pit*U Pit-1 Pit-34 PH-16 Pit-16 Ptl'33 Plt-214 IM PVS IM IM IM IM IM IM IM IM IM IM IM sice (A, ST 66,91 63.19 H44 63.66 63.54 63.32 63.36 6126 6193 66.61 66.51 67.11 n e .(1 .37 .61 .66 .66 .66 .39 .64 .39 .57 .39 .36 .39 m en 1171 16,66 16.96 16.71 iT.ei 16.33 16.76 17.22 16.lt 16.71 I1W 11U 16.34 F*0 1 57 3.26 136 3.31 2.63 3.66 3.37 133 161 132 166 136 161 M .N .17 .S3 .K '.63 .67 .63 .66 .17 .12 . r .69 .13 RgO .1) 1.31 .76 .73 .63 .91 .66 .36 .67 1.12 .69 .96 .23 CjO £.13 3.11 171 it s 2.71 2.36 2.33 I.U 2.91 1.93 199 l i t 1.61 k*2o 3.96 3.66 3.39 3.39 2.66 133 167 199 166 136 131 136 166 K20 6.22 6.61 6.39 6.39 6.69 6.17 6.23 6.96 6.19 6.66 IB 6.66 6.67 PKH .23 .26 .27 .26 .26 .27 .26 .21 .26 .23 .29 .26 .67 UJI 1.29 2.31 1.39 1.26 1.62 1.71 I. II I.U 1.63 121 1.36 1.13 .31 Sc 6 6 7 7 7 7 7 6 7 6 7 7 3 V 62 61 63 63 65 61 73 37 61 31 36 SI £1 Or 11 16 11 11 11 9 6 11 16 13 12 11 6 HI 6 6 6 4 1 2 4 I 4 4 2 3 3 Cu 9 11 16 It 6 i t 11 6 12 9 II 9 1 Zn 61 63 62 66 £1 66 31 23 43 37 62 47 47 Rb 233 266 237 236 236 236 253 273 236 271 226 224 239 Sr 336 666 326 336 331 333 362 494 537 419 363 316 111 V 66 61 62 46 49 62 43 34 66 66 43 37 46 Ir 363 331 371 363 363 366 367 377 366 366 361 363 646 Rb 39 66 66 66 66 31 62 63 61 43 41 31 41 Ba 1162 1126 1163 1131 1117 1691 962 1311 1167 1126 1219 1197 112 Li 69 (6 36 66 66 73 69 166 72 69. 72 63 96 b 116 111 III 116 169 121 117 218 112 113 122 166 162 b 169 199 6.99 6.63 11.51 6.73 166 1162 4.17 7.63 4.99 196 1169 Eti 1.66 1.67 1.36 1.72 133 1.71 1.56 116 1.73 1.61 1.79 1.39 1.92 Tb 1.62 .72 .12 .96 1.12 .63 .11 1.31 .77 .72 .69 .73 1.63 Yb 132 173 186 114 169 112 IM 4.27 166 167 159 136 117 U .63 .66 .69 .66 .67 .33 .52 .73 .69 .31 .61 .47 .64 Hf 9.99 9.37 1119 162 1161 9.49 1122 t i l l 161 176 147 197 11U Ti 116 121 162 126 144 122 161 163 137 131 116 131 121 Th 61 62 62 62 43 66 66 63 62 66 41 66 36 U 169 166 1154 131 1116 1166 147 9.62 186 1149 1131 11.11 171 327 PaM67 Pal-13 Pat-17* P it-in P»t*97 Pat-91 Pat-99 Pat-221 Pit-93 Pat-219 Pat-2 Pat-26 Pat-191 TVS TVS TVS TVS TVS TVS TVS TVS TVS TVS Plfl pm M sioe 46.63 41.96 49.31 si. ie 31.11 51.26 31.13 31.47 51.49 31.36 36.91 39.32 39.61 Ttoe 1.17 l .t t 1.44 1.77 1.71 1.64 1.34 1.76 1.67 1.71 .36 .38 .42 A1201 16.94 19. U 11.68 16.97 16.19 11.16 16.31 17.69 16.17 17.73 26.97 21.11 26.74 FiO (.11 M l 7.21 7.(2 1.19 1.31 7.66 6.26 (.12 6.27 2.34 2.15 2.71 M .49 .14 .41 .12 .13 .13 .14 .13 .14 .14 .21 .21 .22 W> 3. SI 3.74 6.39 3.27 3. U 3.21 3.11 9.16 4.93 3.66 .11 .23 .26 CiO 11.11 11.64 9.54 9.63 1.66 6.47 6.64 6.49 6.37 (.23 1.61 1.19 1.56 Na20 4.19 in J.74 3.27 4.13 4,46 4.27 4.22 4.34 4.43 9.36 6.29 7.76 m 1.41 1.36 (.41 2.14 2.19 2.(3 2.14 2.N 2.29 2.23 6.31 6.63 6.51 tea .41 .17 .47 .31 .33 .36 .36 .31 .36 .33 .63 .64 .11 LSI 1.99 1.(7 3.16 3.26 l.U M l 1.66 1.36 2.11 4.63 2.37 2.16 2.66 Sc 28 29 26 21 22 21 21 21 21 21 1 1 1 V 266 262 79 194 179 119 133 177 166 161 12 IS N/A Cr 47 32 56 16 26 31 27 36 25 23 9 11 4 Nt 21 21 34 21 9 11 9 9 7 6 7 7 11 Co 46 46 41 31 24 23 26 24 19 23 4 6 9 In 31 31 36 61 61 61 66 67 39 64 96 91 92 Rb 17 41 63 37 21 46 47 44 31 49 392 179 337 6r 666 757 736 976 771 616 626 726 761 727 11 31 76 T 21 26 21 21 21 22 22 22 23 21 63 61 66 Zr 164 166 161 246 244 219 212 219 251 241 966 1111 926 Mb 16 16 21 33 24 24 21 23 24 24 91 91 81 •a 662 363 696 628 711 766 639 762 611 747 26 33 N/A U 22 22 26 34 31 11 31 33 14 13 116 114 136 c* 41 42 31 36 62 56 36 66 64 62 213 266 226 fii 3,(2 126 4.96 194 6.14 161 6.66 6.66 6.11 6.83 172 6.67 166 2a 1.64 1.79 1.61 2.41 1.93 1.66 1.91 2.22 2.62 1.86 .56 .72 .71 lb .69 .67 .66 .66 .67 1.31 1.11 .66 .63 .96 1.61 .96 .91 Tb 2.(6 2.57 1.79 1.21 142 2.31 2.46 131 133 2.21 3.11 4.73 4.93 Li .46 .33 .H .63 .44 .34 .46 .49 .31 .46 .91 .19 .93 W 3.36 172 166 111 113 4.33 ' 4.62 121 1(6 4.91 22.71 21.36 22.66 la m .66 1.24 m 1.26 H/A N/A 1.47 1.12 1.16 4.66 1.97 3.96 lb 2.61 169 123 199 171 136 143 193 166 169 39 53 34 U N/A N/A N/A 116 N/A N/A N/A N/A N/A N/A 15.23 6.91 9,16 328 Pit-19 tot-211 Pit-136 tot-13 Pat-142 Pit-131 tot-71 PW . M TAA TAA WT tor MT S102 59. SI 59.91 67. a 61A4 71.11 72.94 72.96 7102 •3S .41 .M .29 .a .21 .a Aiau a .94 21.19 17. U t i n 1119 14.11 1159 toO t s 2. IS 1.91 lie 1.51 1.21 1.67 NnO .21 .21 .S3 .17 .17 .*9 .12 mo .» .24 .*2 .21 .12 .a .11 toO 1.SS 1.41 .M .14 .17 .11 .13 Na20 7.77 7.13 116 139 I.7S 2.26 1.47 wo 6.63 142 11.13 9.74 1117 9.12 9.63 P205 .S3 .M .11 .a .11 .11 .13 UOI 117 1.71 .a .54 1.51 .57 1.29 Sc t 1 4 4 2 2 4 V 17 15 N/A ■7 9 7 19 Dr U I* N/A 7 S 6 1 hi 4 5 9 IS 12 12 11 Co B II 6 4 4 2 4 In 11 94 32 46 a 31 32 to 353 345 444 5N 672 US 593 Sr BMS U 41 34 24 • 11 47 Y U 59 71 72 12 •4 77 Zr *79 914 4S5 S39 255 £33 241 to S9 19 4* 52 64 71 57 to S3 N/A 22 16 2K 27 2 a U 132 141 91 S* 71 12 a to US 2*4 161 153 132 121 131 to lit 9.25 14.19 9.61 4.26 1 H 163 E« .7t l.lt !.«7 .49 .32 urn .76 Tb .91 .11 1.71 I.S2 .41 .13 . a YU i a 4.94 1S9 176 119 166 2.96 Ltt .19 .a .a .66 .51 .67 .52 Hi H.N 21.39 1196 14,11 9.86 9.31 ' 162 T* 192 4.36 2.6! I M 2.* 174 i n Th S3 53 M 22 71 61 66 U 145 N/A 172 133 11.41 12.91 1179 APPENDIX C REPRESENTATIVE MINERAL ANALYSES FOR THE MVS LAVAS OF PATMOS 329 330 Table C.l, Representative analyte* of ottvtnea and ipinel Xenn- Xenn- Phcno- Fheno- Pheno- Hlcro- Hlcro- rryat cryit crjrit cryat crytt pheno- pheno- Spinel core rim eore core rim erytt cryat SiOj A l. 10 39.45 40.60 39.70 38.23 39.16 37.66 0.10 TlOj ------0.65 AljOj ------29.90 CrjOj ------33.10 FeO* 9.45 17.55 11.75 15.55 20.60 17.15 24.31 18.50 HnO 0.15 0.30 0.20 0.20 0.63 - - 0.05 MrO 48.70 42.85 46.80 44.15 39.42 42.20 36.09 16.20 H(0 0.35 0.05 0.15 0.25 - - - 0.20 CnO 0.20 0.30 0.20 0.25 0.20 0.28 0.34 - ZnO -- " -“ - 0.10 Total 99,95 100.50 99.70 100.10 99.08 98.79 98.60 98.60 Atonic proportion! on the baiit of 4 oxy»*ni Si 1.005 1.000 1.005 1.000 0.998 1.006 1.005 0.003 Ti ------0.010 Al ------1.066 ------0.791 Fe24 0.195 0.370 0.245 0.330 0.450 0.368 0.542 0.66R Mn 0.005 0.005 0.005 0.005 0.014 -- 0.000 Hr 1.775 1.615 1.730 1.655 1.534 1.613 1.635 0.730 Hi 0.010 <.005 0.005 0.005 --- 0.005 Co 0.005 0.010 0,005 0.005 0.006 0.008 0,010 - Zn ------0.002 I cationi 2.995 3.000 2.995 3.000 3.002 2.993 2.992 3.076 Fn 0.901 0.AI4 0.876 0.836 0.773 0.814 0.726 - *Tntnl Fe reported a* PnO Table C.2. Representative analyses of ptagiaclaac* l » n type He-Tb He-Tb Ife-Tb He-Tb Ily-Tb lly-Tb Uy-tb Hy-Tb I*-Tb tly-Tb ty-T* Hr-Te Hy-Ta Oy-Ta Q-T 0-T crystal fh.corc ph.rta ph.core ph.ria ph.core ph.ria ph.core ph.vmtle ph.rta ■ph. Ph.core Ph.core Ph.core Ph.rim Ph.core fh.rim SiOj 46.77 47.58 47.72 50.12 46.51 50X0 56.95 58.99 51.13 50.53 48.93 51.12 57.89 57.63 54.75 58.04 31.78 36.51 31.79 30.99 32.90 30.04 27.50 24.24 30.71 30.24 32.37 29.96 25.97 •25X3 27.7*1 26.41 FeO 0.65 0.S2 0.48 0.69 0.64 0.69 0.31 0.63 0 X 6 1.18 0 X 1 0 X 8 0.22 0.41 0.55 0.4K Crf) 17.51 16.47 16.88 14.36 16.65 12.91 8.36 6.70 13.79 13.22 15.61 14.33 8.86 8.42 10.10 8.14 N*jO 1.06 1.45 1.86 3.34 2.0t 4 X 1 6.17 4.51 4.01 3.39 2.19 3 X 7 5.63 5X7 5.317 6.16 KjO 0.22 0.79 0.16 0.45 0.10 0.50 1.24 5.50 0.20 0.54 0.08 0.36 1.02 1.08 0.60 0.7H Total im.oi 101.65 (00.92 99.97 90.09 99.77 100.55 100.57 100.72 99.12 99.63 99X5 99.60 99.26 99.05 100.71 Atmic pnportions at the basis of caygens St 2.151 2.157 2.174 2.297 2.161 2.325 2.551 2.674 2.323 2.312 2.243 2.339 2.608 2.607 2.500 2.592 Al I.B11 1.R45 1.015 1.675 1.806 1.664 1.453 1.296 1,645 1.646 1.750 1.617 1.380 1.178 1.490 I.T1I Fe 0.025 0.011 0.018 0.027 0.025 0.026 0.013 0.024 0X1 3 0X46 0X15 0.020 0.0TB 0.014 0X20 O.OW Ca 0.065 0.000 0.024 0.705 0.030 0.633 0.402 0.326 0.672 0.654 0.767 0.703 0.428 0.408 0.495 0.401 Ha ' 0.095 0.128 0.165 0.297 0.181 -0.356 0.516 0.397 0.354 0.304 0.195 0X08 0.492 0.515 0X70 0 .5 Vi K 0.011 0.046 0.010 0.026 0.006 0.010 0.071 0.318 0.012 0.032 0.005 0.021 0.059 0.061 0.035 O.IV/i 1 Mtlou 4.984 5.007 5.006 5.027 5.013 5.035 5X26 5.035 5.038 5.013 4.975 5.008 4.975 4.984 5.010 A.9RI1 An 0.889 0.822 0.825 0.686 0.816 0.622 0.398 0.313 0.647 0.661 0.792 0.681 0.437 0.414 0X95 0,409 Ah 0.090 0.131 0.165 0.289 0.178 0.349 0.532 0.302 0.341 0.307 * 0.202 0.298 0.501 0.521 0.470 0.545 Or 0.011 0.047 0.010 0.025 0.006 0.029 0.070 0.305 0.012 0.012 0.006 0.021 0.060 0.061 0.015 0.044 M im iitioni: Ifc-Tb “ nH nriijfem lt; fly-Tb ■ hy-trschjbasalt, UrTj * tradiyamfesite Q-T “ Q-trachyte; (h * fhwacty tt; aph “ a?ern|hfnocryst Table C.3. fermentative arul/ncs of cliraiqrrmcnca fe-Tb f e -lh fe-Tb Ifr -lb Ily-Tb Ily-Tb Ifr-Tb iy -T » iy -T a Uy-Ta iyr Q-T ph.core (A.rta *p9t. ph.core p h .rta ph.core ph.ria ph.core p h .ria J im ) ph.core p h .rta SiOj 51.15 46.02 48.72 49.99 47.75 48.29 47.98 53.21 52.89 50.67 53.28 52.94 H O j 0 .8 0 2.08 1.70 0.69 1.93 1.57 1.87 0.26 0.26 1.09 0.27 0.X A ljO j 3.7 9 8.9 5 7.08 5.4 3 6 .8 3 6.61 7.02 0.90 0.5 6 4 .6 0 0.71 0.9 9 0.1 7 O.t? - 1.16 -- 0.1 5 ----- FWt* 5.8 4 7.21 8.3 0 4 .8 2 7.4 6 8 .6 5 7.06 8.26 9.29 6.85 9 .2 2 9.0 3 HO 0.2 5 - 0.47 0.1 6 0.1 4 -- 0.94 0.80 0.46 0.68 0.7 8 ftfl 15.09 13.16 12.51 15.56 13.47 13.23 1 4 .X 13.24 12.84 14.63 13.61 13.35 r.o Z3.02 22.39 20.65 22.18 22.56 22.22 22.66 22.32 22.53 21.05 21.91 2 2 .X n*fl 0 .2 5 0.46 0.6 6 — - — - 0.4 6 0.29 — 0.5 8 0.6 5 Total 101(36 100.42 100.11 99.99 100.14 100.57 101.04 99.59 99.46 99.35 100.26 100.54 Atrnur proportion* on the boaia of 6 oaynem Si 1.883 1.712 1.812 1.838 1.778 1.795 1.767 1.992 1.993 1.881 1.987 1.972 T i 0.022 0.058 0.048 0.019 0.054 0.044 0.052 0.008 0.008 0.031 0.008 0.007 Al 0.165 0.393 0.311 0.235 0.300 0.290 0.305 0.040 0.025 0.202 0.032 0.044 Cr 0.005 0.005 - 0.034 -- 0.004 - - - -- Fe 0.180 0.224 0.258 0.149 0.232 0.269 0.218 0.259 0.293 0.213 0.288 0.281 H« 0.008 - 0.015 0.005 0.004 - ~ O .O X 0.026 0.015 0.022 0.025 0.828 0.729 0.694 0.853 0.748 0.733 0.785 0.739 0.722 0.810 0.757 0.753 Ca 0.908 0.892 0.823 0.875 0.900 0.885 0.894 0.896 0.910 0.837 0.876 0.R90 fta 0.018 0.034 0.050 —— — - 0.034 0.022 — 0.042 0.047 r. C nticn* 4.017 4.047 4.011 4.008 4.016 4.016 4.025 3.998 3.999 3.989 4.012 4.019 H; 0.821 0.765 0.729 0.851 0.763 0.703 0.783 0.740 0.711 0.792 0.724 0.728 Ins4 I F e **) •Total R: reported at FeO. 332 333 Table C.4. Representative analyse* of nica* and Fe-Tl oxide* Mica* Fe-Tl oxide* Hy-Tb Hy-Ta Hy-Ta Q-T q-T Ne-Tb tty-Ta q-T RlOj 39.45 36.45 34.69 36.05 38.09 _ TlOj 6.05 6.60 5.67 6.85 6.30 16.87 13.24 4.40 AlI°J 12.75 14.95 12.80 13.95 13.96 2.46 3.07 2.35 CrjOj ------0.27 - FeO* 7.85 10.90 18.7B 14.90 11.51 70.92 75.49 85.55 KnO 0.10 0.10 - 0.20 0.37 1.25 1.63 0.80 H*0 18.55 16.30 13.39 13.40 16.29 0.27 0.56 1.00 CaO • 0.05 to -- to 0.67 0.50 - H*jO 0.75 0.55 0.38 0.50 0.69 - -- k2o 9.40 9.05 9.08 9.25 8.98 -- - naO - 1.55 - 0.20 --- - Total 94.95 96.50 94.79 94.30 96.17 92.64 94.56 94.10 Atonic proportion* on the bnsls of 22 or 12 oKypnna. fit 5.725 5.360 5.360 5.485 5.551 to Tl 0.660 0.730 0.659 0.670 0.691 1.669 1.30B 0.469 Al 2.175 2.590 2.333 2.500 2.395 0.383 0.675 0.394 Cr --- to - - 0.018 - Fe 0.955 1.340 2.626 1.895 1.603 7.800 8.291 10.162 Hn 0.010 0.010 - 0.030 0.046 0,140 0.160 0.096 Mr 4.010 3.565 3.0R4 3.035 3.560 0.056 0.107 0.212 Ca 0.005 ---- 0.095 0.071 - Na 0.215 0.155 0.114 0.145 0.196 --- K 1.760 1.700 1.790 1.800 1.671 - - - Ra to 0.090 - 0.015 --- - I cation* 15.495 15.550 15.766 15.575 15.493 10.141 10.640 11.333 H* 808 0.727 0.560 0.616 0.716 -- ( h* ♦ I Fe2*) *Totnl Fa reported ai FeO. 334 Table C.5. Represents!ive analytes of K-feldipsr Hy-Tb Hy-Ta Q-T Core Bin Core Hia Core Ria SiOj 65.AS 63.82 64.95 66.90 66.50 65.15 AljOj 18.89 21.07 19.10 19.25 19.15 18.90 FeO 0.27 - 0.50 0.20 0.15 •0.15 0.15 CaO 0.38 1.45 0.40 0.55 0.60 0.55 HajO 3.24 6.36 3.15 3.40 2.50 3.85 k2o 11.55 9.80 11.25 11.05 12.50 10.65 BaO - — 1.00 0.65 0.75 0.65 Total 99.81 100.78 100.05 99.95 99.95 99.70 Atoaic proportions on the basis of 8 oxygens Si 2.986 2.882 2.965 2.965 2.960 2.975 Al 1.015 1.122 1.025 1.035 1.035 1.015 Fe 0.011 0.011 0.010 0.005 0.005 0.005 Ca 0.019 0.070 0.020 0.025 0.020 0.025 Na 0.286 0.380 0.280 0.300 0.225 0.360 K 0.672 0.565 0.655 0.645 0.730 0.620 Ba - - 0.020 0.010 0.010 0.005 I cations 6.987 3.030 6.975 6.985 4.985 6.985 aol X An 1.9 6.9 2.1 2.6 2.1 2.5 Ab 29.3 37.4 29.3 30.9 23.1 34.5 Or 68.8 55.7 68.6 66.5 76.8 63.0 APPENDIX D THE EQUILIBRIUM COMPOSITION OF PHLOGOPITE (ADDENDUM TO CHAPTER VII) 335 336 The expression xjjjg - xj^x/K was extracted from the published compositions of coexisting phlogopite and clinopyroxene in K-rich alkaline lavas and from the results of melting experiments on K-rich volcanics. The data for natural rocks were carefully screened, and analyses were rejected if there was any evidence of disequilibrium resulting from magma mixing etc. Data for intrusive rocks and xenoliths were also rejected as it is often impossible to be certain of the crystallization sequence from published petrographic descriptions. In all, 25 phlogopite-clinopyroxene pairs were selected from papers by: Carmichael (1967), Barton (1979), Barton and Van Bergen (1981), Kuehner et a l., (1981) and Luhr and Carmichael (1983). Unpublished results by Barton for samples from the Leucite H ills, Jumilla (S. G. Spain) and the Toro-Ankole Province (Uganda) were also used. These data indicate that the value of K, defined as above, is 0.994 ± 0.027. The compositions of phlogopites and clinopyroxenes synthesized in high-pressure melting experiments have been reported by Edgar et al, (1976), Ryabchikov and Green (1978), Barton and Hamilton (1979), Edgar et a l., (1980) and Barton and Hamilton (1982). The value of K calculated from these data (20 pairs) is 0.993 ± 0.05 and, considering the fact that iron loss may have occurred during many of Che experiments, the agreement of this value with that derived from natural rocks is encouraging. However, the effects of temperature and composition (Mg-Fe replacement) remain to be determined. APPENDIX E ACTIVITIES OP ENSTATITE AND CATS IN CLINOPYROXENE (ADDENDUM TO CHAPTER VII) 338 339 Several models for Che activity of MgjSijOg have been proposed in the recent literature (e.g ., Wood and Banno, 1973; DePaolo, 1979; Holland et a l., 1979). In order to make a choice between them, we utilized the compositions of olivines, cllnopyroxenes and orthopyroxenes synthesized at known pressure-temperature conditions in high-pressure experiments on basanlte and ultramaflc compositions (Green, 1973a, b; Mysen and Boettcher, 1975) and calculated silica activities from olivine-clinopyroxene and from olivine- orthopyroxene equilibria. For the latter we used the methods and data of Bacon and Carmichael (1973). At temperatures above 1150*C, s ilic a activities calculated from olivine-clinopyroxene equilibria using the solution model of DePaolo (1979) showed slighly better agreement with silica activities defined by coexisting olivine and orthopyroxene than did those calculated using the solution model of Holland et a l., (1979) and we thus adopted the DePaolo model for use at high temperatures and the Holland et a l., model for use. at T < 1150*C. The results reported here complement those reported by DePaolo (op. cit) but, as he stated, do not prove the accuracy of the solution model for compositions which do not lie on the pyroxene solvus. As noted below, however, the model does yield results which are consistent with those calculated from clinopyroxene- plagioclase equilibria. 339 340 Wood (1976), Carmichael et al. (1977), Newton et al. (1977), Herzberg (1978) and Gasparik and Lindsley (1980) have suggested models for the activity of CaAl(AlSl)06 In clinopyroxene, and we evaluated these models using an analogous procedure to that described above, but using experimental data for basaltic compositions (Bender et al. 1978; Walker et al. 1979; Grove et al. 1982). Silica activities calculated using various solution models for CATS in clinopyroxene were compared with those calculated from coexisting olivine and low-Ca pyroxene (if present), olivine and clinopyroxene (using DePaolo's or Holland et a l.'s solution model depending upon temperature) and plagioclase and liquid (Ghiorso and Carmichael 1980). The results led us to adopt the solution model of Herzberg (1978) and, furthermore, indicated that silica activities calculated from coexisting olivine and clinopyroxene are similar to those calculated from plagioclase-iiquid equilibria. 340 LIST OF REFERENCES Anderson, A.T.Jr., 1966. 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