VOLCANOLOGY, PETROLOGY AND GEOCHEMISTRY OF LASCAR VOLCANO, NORTHERN CHILE
by
Stephen John Matthews
at
University of London
January 1994 ProQuest Number: 10017777
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ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT.
This is the first comprehensive pétrographie and geochemical study of Lascar Volcano, an active calc-alkaline stratovolcano located in the
Atacama Desert, Northern Chile. The volcano probably initially became active during the last glacial maximum, about 20,000 to 27,000 years ago. It has successively built up three eruptive centres which form a slightly elongated structure trending ENE-WSW. The lavas and pumices produced by this volcano are predominantly 2-pyroxene andésites and dacites. The chemical and pétrographie features of the eruption products are explained here in terms of a fractionating magma chamber at shallow depth which receives periodic influxes of basaltic andésite magma. This magma, which has already crystallised in middle to lower crustal magma chambers, mixes with the resident more evolved magmas to create a variety of disequilibrium textures including mafic inclusions and reaction coronae on olivine phenocrysts. These influxes of hotter magma lead to convective overturn of the magma chamber.
The “primitive” magma which rises from deep levels is relatively rich in dissolved sulphur and chlorine as well a large amounts of water. A model is proposed in which these volatile phases are degassed from the magma on quenching, depressurisation and oxidation of the mafic endmember to form a separate mixed fluid phase in the magma chamber.
This release of volatiles is an important contributor to the violence of the eruptions which frequently occur in such volcanoes. The discovery of anhydrite in a prehistoric dacitic pyroclastic flow, the Soncor Flow, and also in the April 1993 eruption products, indicates that sulphur is an important volatile phase and that the magma chamber is relatively oxidised, stabilising sulphate rather than sulphide in the magmas. This is confirmed by calculations of oxygen fugacity using the compositions of coexisting magnetite-ilmenite pairs. A trend of increasing f02 with decreasing
temperature has been attributed to buffering by the SO 2 -H 2 S equilibrium in a coexisting H20-rich fluid phase. This model explains the common association of oxidised, anhydrite-bearing magmas with excessively high
SO 2 emissions in some volcanoes, notably the 1991 eruption of Mount
Pinatubo, Phillipines. The source of this sulphur is believed to be either the subducted oceanic crust below the volcanic front or the overlying mantle wedge. Important changes in the plumbing system are believed to have occurred in the past, producing magmas which appear to have bypassed the magma chamber on their way to the surface. This is based on whole rock geochemical trends and pétrographie analysis, which shows a lack of evidence for magma mixing in such eruption products. One of these, the andesitic Chaile Flow, lies stratigraphically between dacitic flows with very similar geochemical and pétrographie features indicative of magma mixing and typical of magma chamber derived products. A similar change is believed to have occurred in the present eruption cycle, as the 1986 and
1990 lavas are interpreted as having bypassed the volatile-rich magma chamber which gave rise to the 1993 eruption. This change is thought to be responsible for the switch in eruption style from shallow short-lived vulcanian explosions to a sustained sub-plinian eruptive style producing anhydrite bearing pumices. This petrological monitoring of an active volcano provides a new way of studying the magmatic system and predicting the future eruption style. ACKNOWLEDGEMENTS.
I am indebted to many people for their assistance during this project. In particular, I am grateful to Adrian Jones for his careful supervision and to Claudio Vita-Finzi for advice and support. Moyra Gardeweg, Steve
Sparks, Ana Espinoza, Sergio Manquez, Clive Oppenheimer and Mark
Stasiuk are thanked for assistance in the field. Thanks also to Nena for looking after us in Toconao. I am grateful to Andy Beard for training and assisting me in the use of the electron microprobe at Birkbeck College and to Steve Mirons for carrying out XRD analysis and identification of some difficult minerals. Tony Osborn is thanked for assistance with whole rock analyses and for conducting ICP analyses on a number of samples. Peter
Francis provided some vital samples of the 1993 eruption products and
Marcos Zentilli arranged for sulphur isotope analyses of native sulphur samples. Mathew Thirlwall and Jerry Ingram assisted in the preparation of samples for Nd and Sr isotope analyses and conducted the analyses.
Giselle Mariner helped in the preparation of samples for XRF analysis.
Judith Milledge trained and assisted me in the use of the infrared microscope. Finally, I am grateful to my parents for their support, encouragement and advice over the period of study. This project was funded by Kingston Polytechnic and by the Natural Environment Research Council
Grant no. GT4/90/GS/84. CONTENTS.
PAGE
ABSTRACT. 1 ACKNOWLEDGEMENTS. 3 CONTENTS. 4 TABLE OF FIGURES. 11 TABLES 18 CHAPTER I: INTRODUCTION. 19
1.1 GENERAL. 19 1.2 TECTONIC SETTING. 21 1.2.1 Tectonic Evolution of the Central Andes. 21 1.2.2 Upper Cenozoic and Neogene Tectonic and Voicanic Structure. 23 1.2.2.1 Tectonic Structure. 25 1.2.2.2 Volcanic History of the Central Andean Volcanic Zone. 27 1.3 LOCATION AND ACCESS. 30 1.4 GEOGRAPHICAL FEATURES. 31 1.5 PREVIOUS WORK. 34 1.6 CLIMATE. 35 1.6.1 Modern Climatic Conditions. 35 1.6.2 Pieistocene Ciimatic History. 36 1.6.3 Holocene Climatic History. 38 1.7 BASEMENT GEOLOGY. 38
CHAPTER II: GEOLOGY OF LASCAR VOLCANO. 40 11.1 INTRODUCTION. 40 11.2 CENTRE I. 45 11.2.1 The Early Andésite Lavas. 45 PAGE
11.2.2 The Saltar Flow. 46 11.3 CENTRE II 48 11.3.1 The Piedras Grandes Flow. 48 11.3.2 The Soncor Flow. 48 11.3.3 The Chaile Flow. 51 11.3.4 The Capricorn Lava. 54 11.3.5 The Tumbres Fiow. 54 11.4 CENTRE III. 56 11.4.1 The Talabre Lava. 56 11.4.2 The Collapse Craters. 56 11.5 HISTORIC ACTIVITY. 58
TER III: PETROGRAPHY OF LAVAS AND PUMICES. 66
III.1 GENERAL. 66 III.2 PETROGRAPHY OF ROCKS FROM LASCAR VOLCANO. 66 III.2.1 Plagioclase. 67 III.2.2 Orthopyroxene. 69 III.2.3 Clinopyroxene. 70 III.2.4 Olivine. 70 III.2.5 Amphibole. 73 III.2.6 Biotite. 75 III.2.7 Fe-Ti Oxides. 75 III.2.8 Apatite. 76 III.3 PETROGRAPHY OF INDIVIDUAL FLOWS. 76 III.3.1 Centre 1 Andésite Magmas. 76 III.3.1.1 Orthopyroxene. 77 III.3.1.2 Clinopyroxene. 77 III.3.1.3 Plagioclase. 79 III.3.1.4 Conclusions. 79 PAGE
III.3.2 Chaile Flow. 81 III.3.2.1 Orthopyroxene. 83 III.3.2.2 Clinopyroxene. 83 III.3.2.3 Plagioclase. 83 III.3.2.4 Conclusions. 83 III.3.3 The Soncor Pyroclastic Fiow and Piedras Grandes Flow. 85 III.3.3.1 Orthopyroxenes. 88 III.3.3.2 Clinopyroxenes. 92 III.3.3.3 Amphiboles. 95 III.3.3.4 Plagioclase. 101 III.3.3.5 Spinels. 102 III.3.3.6 Anhydrite. 102 III.3.3.7 Glass compositions. 104 III.3.3.8 Conclusions. 109 III.3.4 The Capricorn Lava. 110 III.3.4.1 Orthopyroxene. 112 III.3.4.2 Clinopyroxenes. 114 III.3.4.3 Amphibole. 117 III.3.4.4 Plagioclase. 119 III.3.4.5 Spinels. 121 III.3.4.6 Glass compositions. 124 III.3.4.7 Conclusions. 124 III.3.5 The 1986-1993 Eruptive Sequence. 125 III.3.5.1 Orthopyroxene. 127 III.3.5.2 Clinopyroxene. 127 III.3.5.3 Plagioclase. 129 III.3.5.4 Glass compositions. 129 Ml.3.3.5 Conclusions. 134 III.4 DISCUSSION. 136 PAGE
CHAPTER IV: GEOTHERMOMETRY, GEOBAROMETRY AND OXYGEN BAROMETRY. 145 IV.1 GENERAL. 145 IV.2 TEMPERATURE AND OXYGEN FUGACITY. 147 IV.2.1 Magnetite-ilmenite Geothermometry. 147 IV.2.2 2-Pyroxene Geothermometry and Oiivine-Spinei Oxygen Barometry. 153 IV.2.3 Melt Inclusion Homogenization Experiments. 155 IV.2.4 Pressure Calculations. 161 IV.4 CONCLUSIONS. 167
CHAPTER V: BULK CHEMICAL AND ISOTOPIC COMPOSITION. 169
V.1 CHEMICAL COMPOSITION. 169 V.2 RADIOGENIC ISOTOPE RATIOS. 185
CHAPTER VI: XENOLITHS AND THEIR RELATION TO KNOWN BASEMENT GEOLOGY. 191
V I.1 GENERAL. 191 VI.2 CENOZOIC LAVAS. 192 VI.3 PARTIALLY MELTED METASEDIMENTARY ROCKS. 193 VI.4 SKARN XENOLITHS. 198 VI.4.1 Petrography of Caic-Silicate Xenoliths. 198 VI.4.1.1 Pyroxenes. 200 VI.4.1.2 Garnets. 202 VI.4.1.3 Wollastonite. 202 VI.4.1.5 Plagioclase. 205 PAGE
VL4.2 Geothermobarometry of Skarn and Buchite Xenoliths. 205 V I.4.3 Glass and Fluid Inclusions. 213 V I.4.4 Stable Isotopes. 220 VI.4.5 Source of Calc-Silicate Xenoliths. 224
CHAPTER VII: VOLATILE COMPONENTS AND THEIR CONTROLS ON MAGMA CHEMISTRY AND ERUPTION STYLE. 226
VII.1 MAIN VOLATILE COMPONENTS. 226 VII.2 WATER. 227 VII.2.1 Direct Evidence for the Presence of Water as a Magmatic Phase. 227 VII.2.2 Solubility Mechanisms of Water in Silicate Melts. 228 VII.2.3 Contentrations and Behaviour of Water in Lascar Magmas. 230 V II.3 CARBON DIOXIDE. 234
V II.3.1 Evidence for CO 2 as a Volatile Phase in Lascar Magmas. 234
VII.3.2 Solubility Mechanisms of CO2 in Silicate Melts. 236 VII.3.3 Behaviour of Carbon Dioxide in Lascar Magmas. 238 V11.3.4 Isotopic Composition of Magmatic Carbon. 238 VII.4 CHLORINE AND FLUORINE. 239 V II.4.1 Evidence for Fluorine and Chlorine in Lascar Magmas. 239 VII.4.2 Solubility Mechanisms of F and Ci in Silicate Melts. 239
8 PAGE
VII.4.3 Origin and Behaviour of Chiorine in Lascar Magmas. 241 VII.5 SULPHUR. 243 VII.5.1 Occurrence of Sulphur in Lascar Magmas. 243 VII.5.2 Solubility Mechanisms of Sulphur in Silicate Melts. 244 VII.5.3 Sources and Behaviour of Sulphur in Lascar Magmas. 248 VII.5.3.1 Possible Sulphur Sources. 248 VII.5.3.2 Behaviour of Sulphur in the Magmatic System. 249 VII.5.3.3 Sulphur Isotopes and Implications for Sources of Magmatic Sulphur. 267 VII.6 VOLATILE TRANSFER TO THE GROUNDWATER SYSTEM. 271 VII.7 DISCUSSION AND CONCLUSIONS. 274
CHAPTER VIII: SUMMARY AND CONCLUSIONS. 281
V II.1 SUMMARY. 281
REFERENCES. 289
APPENDIX 1: SAMPLE DECRIPTIONS. 311 APPENDIX 2: ANALYTICAL TECHNIQUES. 328 Electron Microprobe. 328 Whole Rock Analyses. 328 Isotopes. 329 X-Ray Diffraction. 331 Infrared spectroscopy. 331 PAGE
First-Order Carbon Dating. 331
APPENDIX 3: WHOLE ROCK ANALYSES. 333 APPENDIX 4: SELECTED ELECTRON MICROPROBE ANALYSES. 339 Olivine. 340 Orthopyroxene. 344 Clinopyroxene. 353 Amphibole. 365 Biotite. 368 Plagioclase 372 Garnet. 380 Wollastonite. 383 Spinels. 385 Ilmenite. 391 Apatite Group Minerals. 395 Sulphides. 398 Glasses. 400
10 FIGURES.
FIGURE PAGE 1.1 Map showing immediate area around Volcan Lascar and locations referred to in text. 2 0 1.2 Map showing main plate boundaries of the Nazca and South American Plates. 2 2 1.3 Map showing contours on descending Nazca Plate, main areas of uplift and Cenozoic to Recent volcanic Centres. Slightly modified from I sacks (1988). 2 4 1.4 Map showing main physiographic domains of the Central Andes. Modified from Gardeweg (1991a). 32 11.1 Simplified geological map of Lascar Volcano. See text for details. 4 0 11.2 Aerial photograph of the cones and craters of Lascar Volcano with main features labelled. 4 2 11.3 Geological column showing relative timing of events and limited absolute dating. 4 3 11.4 Centre I andesitic lava flowing into Quebrada de Soncor. Terraces formed of the Centre II Soncor Flow are also visible within the quebrada. 4 7 11.5 Saltar Flow on the south side of Lascar showing multiple flow units and dense andesitic bombs. 4 7 11.6 Large prismatic-jointed boulder of the Piedras Grandes Flow to the West of Lascar, partially buried by alluvium. 5 0 11.7 Terraces of the Soncor Flow filling Quebrada de Talabre near Tumbre. The valley floor is filled with the Tumbres Flow which terminates in the centre of the picture. 5 0 11.8 Chaile cauliflower bomb flow with huge agglutinate block. 5 2 11.9 Tumbres Flow with basal plinian airfall deposit overlying the Soncor Flow at Tumbre. Carbonised “coiron grass" from below the Plinian deposit has been dated at 9,000 yrs (P.W. Francis, pers. comm.). 52 11.10 Tumbre showing the Tumbres flow filling the floor of the valley and overlain by the Centre III Talabre Lava. 5 3 11.11 Correlation diagram showing the stratigraphie relationships of the Soncor and Tumbres flows in the Quebrada de Talabre. 54
11 FIGURE PAGE
11.12 Diagram showing the sequence of events in the
construction of Lascar Volcano. See text for details. 5 7 11.13 Photograph of the collapsed 1990 lava dome with high velocity gas jets. April 1990. 6 2 11.14 A small eruption which occurred on 6 April 1990 ejecting gas and ash to 1,000 metres above the crater rim. 6 2 11.15 Photograph of the collapsed dome in March 1993 with concentric ring fractures and high velocity gas jets. 6 4 111.1 Plagioclase phenocryst (crossed polars) with rounded core rich in devitrified glass inclusions and oscillatory compositional zoning towards the rim. 6 8 111.2 Augite phenocryst (crossed polars) with a rounded, inclusion-rich core and a euhedral overgrowth. 68 111.3 Orthopyroxene-magnetite symplectite replacing olivine phenocryst. 71 111.4 Histograms of olivine compositions from Lascar rocks. 72 II 1.5 Resorbed and reddened hornblende phenocrysts with reaction rims of iron oxides. These textures are the result of post-eruptive oxidation. 7 4 111.6 Apatite inclusions in magnetite phenocryst. 7 4 111.7 Histograms of Mg# of orthopyroxenes from the Centre I andesitic lavas. 77 111.8 Histograms of Mg# of clinopyroxenes from the Centre I andesitic lavas. 7 9 111.9 Histogram of anorthite contents of plagioclase from the Centre I andesitic lavas. 81 111.10 Histograms of Mg# of orthopyroxenes from the Chaile Flow. 8 4 111.11 Histograms of Mg# of clinopyroxenes from the Chaile Flow. 8 4 111.12 Histogram of anorthite contents of plagioclases from the Chaile Flow. 8 4 111.13 Histograms of Mg# of orthopyroxenes in samples from the Soncor and Piedras Grandes Flows. 8 8 111.14 Histograms of Mg# of clinopyroxenes in samples from the Soncor and Piedras G randes Flows. 9 2 111.15 Plots of various elements against Mg (formula units) for amphiboles from the Soncor and Piedras Grandes Flows. 9 5
12 FIGURE PAGE 111.16 Histogram of anorthite contents of plagioclase from the Soncor and Piedras Grandes Flows. 9 8 111.17 “Modified Johnson Spinel Prism” plots (Haggerty, 1976) of spinel compositions from LA-124 and LA-122. 1 02 111.18 Plots of various oxides against Si02 for glass analyses from the Soncor Flow. 1 0 4 111.19 Histograms of Mg# of orthopyroxenes from the Capricorn Lava. 1 1 0 111.20 Histograms of Mg# of clinopyroxenes from the Capricorn Lava. 1 1 2 111.21 Plots of various elements against Mg (formula units) for amphiboles from the Capricorn Lava. 1 1 4 111.22 Histogram of anorthite contents of plagioclase from the Capricorn Lava. 1 1 7 111.23 “Modified Johnson Spinel Prism” plots (Haggerty, 1976) of spinel compositions from the Capricorn Lava. 1 1 9 111.24 Plots of various oxides against Si02 for glass analyses from the Capricorn Lava. 121 111.25 Histograms of Mg# of orthopyroxenes from the 1986- 1993 Lavas. 1 2 5 111.26 Histograms of Mg# of clinopyroxenes from the 1986- 1993 Lavas. 1 2 7 111.27 Histogram of anorthite contents of plagioclase from the 1986-1993 Lavas. 1 2 9 111.28 Plots of various oxides against Si02 for glass analyses from the 1986-1993 Lavas. 131 111.29 Analyses of coexisting Fe-rich and silicate melts from the Soncor Flow. 1 3 9 111.30 Analyses of fresh and altered Fe-rich melts from the Soncor Flow and from the adjacent Volcan Aguas Calientes. The final alteration product is a spongy inclusion-rich magnetite. 1 4 2 IV.1 Graph of log (f02/bar) against temperature (T)) from magnetite-ilmenite and 2-pyroxene-olivine-spinel assemblages. Buffer curves as follows: FMQ = fayalite- magnetite-quartz (Wones & Gilbert 1969), Po/Ah = pyrrhotite-anhydrite (sulphide-sulphate), MH = magnetite-hematite (Eugster & Wones 1962). 151
13 FIGURE PAGE I V.2 Devitrified glass inclusion in plagioclase phenocryst from LA-147, a dacitic pumice from the Soncor Flow, before and after heating at 900°C for 1 hour. The glass inclusion has clearly melted but the vapour bubble has not redissolved. 1 5 7 IV.3 Infrared spectra of a plagioclase phenocryst from LA-147 and an area of the phenocryst containing a devitrified glass inclusion prior to heating experiments. The plagioclase spectrum is subtracted from the plagioclase+ glass spectrum in order to obtain the spectrum of the glass inclusion. 1 5 9 IV.4 Calculated spectra of glass inclusions in a plagioclase phenocryst of LA-147 before and after heating at 900°C for 1 hour. A primary melt inclusion which melted at 900^0 has not lost its dissolved water, whereas an irregular glass inclusion failed to melt due to degassing of all of its water through fractures in the crystal. 1 6 0 IV.5 Silica activity-pressure relationships for quartz-melt and ferrosilite-magnetite-melt in LA-122, a quartz-bearing dacitic pumice from the Soncor Flow. Quartz is in equilibrium with the assemblage at 6-11 kbar. 1 6 4 IV.6 Similar diagram to Figure IV.5 for LAS-23, a partially melted metasedimentary xenolith from the Tumbres Flow. Quartz is never in equilibrium with the orthopyroxene and magnetite in this rock. 1 6 4
V . 1 Variation of K2 Q with SiQ2 for Lascar magmas, showing the Soncor Pyroclastic Flow, mafic inclusions, and magmas of Centres 1, 2 and 3. 1 7 0
V . 2 Variation diagrams for major elements plotted against SiQ 2 for whole rock compositions. Symbols as in Figure 6. 171
V . 3 Plot of whole rock Na20 against SiÛ2 with mixing and fractionation trends. See text for explanation. 1 7 5 V .4 Trace elements plotted against Si02for whole rocks. Symbols as in Figure 6. 181
V . 5 ®^Sr/®®Sr plotted against SiQ 2 , Sr and Rb. 1 8 6
14 FIGURE PAGE V . 6 Comparison of Lascar Sr and Nd Isotopes with volcanic zones of the Andes. Data from this study, Deruelle 1985, James & Murcia 1984, Francis et a! 1977, James et al 1976, James 1982, Coira & Barbieri 1989, Gardeweg 1991a, Davidson et al 1990, Francis, Sparks et al 1989, Futa & Stern 1988, Hickey-Vargas et al 1989, Hickey et al 1986. 1 8 7 V . 7 Comparison of Lascar Sr and Nd isotopes with other volcanic centres of the Central Andes. Data from this study, Deruelle 1985 (Lascar), Gardeweg 1991a (Tumisa), Davidson et al 1990 (Nevados de Payachata), Francis, Sparks et al 1989 (Cerro Galan), Hawkesworth et al 1982 (Purico-Chascon). 188
V . 8 Variation of ®^Sr/®®Sr and ^ ^^Nd/^ '^'^Nd with latitude for late Cenozoic to Holocene volcanic centres along the Andean chain. Values for Lascar are shown for comparison. Data from this study, Coira & Barbieri 1989, Davidson et al 1990, Davidson & deSilva 1992, Deruelle 1985, deSilva et al 1993, Francis et al 1977, Francis, Sparks et al 1989, Futa & Stern 1988, Gardeweg 1991a, Hickey et al 1986, Hickey-Vargas et al 1989, James 1982, James et al 1976, James & Murcia 1984. 1 9 0 V I.1 Plots of major oxides against Si02for partial melt glasses in the buchite xenoliths LA-111 and LAS-23. 1 9 3 V I.2 Projections of pyroxenes from skarn and buchite xenoliths onto the pyroxene quadrilateral using the method of Lindsley (1983). 201 V I.3 Triangular plot (Molar Cr-AI-Fe) of garnet compositions from skarn xenoliths. 2 0 3 VI.4 Trianguar plot of molar Ca-Mg-(Fe+Mn) in wollastonite from skarn xenoliths. 2 0 4 V I.5 Plot of plagioclase from skarn and buchite xenoliths on the anorthite-albite orthoclase triangle. 2 0 8 V I.6 Calculated pressure-temperature relationships of coexisting clinopyroxene, wollastonite, plagioclase and quartz in the wollastonite skarn xenolith LA-108. The limits of magnetite-ilmenite temperatures from magnetite skarn xenoliths are included. 2 1 0
15 FIGURE PAGE VI.7 Diagrammatic representation of the pressure- temperature-time paths of xenoliths which are incorporated into the magma and held at magmatic temperatures before eruption, and xenoliths which are ripped from the conduit walls during an eruption. 2 1 2 VI.8 Calculated temperature-f02 relationships from magnetite- ilmenite pairs in skarn xenoliths and coexisting andradite and hedenbergite in the wollastonite skarn LA-108. 2 1 4 V I.9 Infrared spectrum of apparently inclusion-free quartz grain and portion of grain with colourless glass inclusion from the buchite xenolith LA-111. 2 1 6
V I.10 plotted against of calcite in skarn xenoliths. A calcareous sandstone from the nearby basement is included for reference. 2 2 3
V I.11 plotted against wt% CaCOg in skarn xenoliths and buchite xenolith LA-111. A calculated decarbonation trend from the most carbonate-rich xenolith is included. 2 2 3 V II.1 (=1 GO-TOTAL other oxides) in microprobe
analyses of glasses plotted against wt% Si0 2 from various Lascar rocks. 2 31 V II.2 Calculated viscosities of glasses from microprobe analyses and estimated water contents (Shaw, 1972) plotted against wt% Si02- 2 3 3 V II.3 Whole rock carbon dioxide, sulphur and chlorine contents plotted agains wt% Si02 for Lascar rocks. 2 35 VII.4 Plot of S k alpha wavelength shift relative to pyrite against sulphur valence for various standards. The best-fit line provides an empirical relationship between these variables. 2 51 V II.5 Plot of calculated 8 valence (% sulphate) for sulphur in glasses of the Soncor mafic inclusion LA-124 and the Saltar Flow. The data of Carroll & Rutherford (1988) are included for comparison. 2 53 VI 1.6 Crystallised sulphide melt inclusion included in an ilmenite phenocryst (Im) from the banded Soncor pumice LA-155. The melt inclusion has separated into pyrrhotite (Po) and Chalcopyrite (Ch). 2 55 VII.7 Anhydrite phenocryst from the 1993 scoria. 2 56
16 FIGURE PAGE V II.8 Plot of calculated log(f02/bar) against temperature °C for
Lascar magmas. Lines of constant SO2 /H 2 S are included
to show how this equilibrium reaction might buffer the f 0 2 of the magmas. 1=1:1,10 = 10:1 and 100 = 100:1
isopleths of SO2 /H 2 S respectively. Other buffer curves as in Figure IV.1. 2 59 VII.9 Plot of calculated log(f02/bar) against temperature ®C for 1986,1990 and 1993 magmas. The 1993 samples clearly
have an elevated f 0 2 relative to the earlier magmas. 2 6 6 VII.10 334S plotted against latitude for native sulphur from a number of volcanoes in the Atacama area. The position of the Salar de Atacama is shown. Analyses provided by Marcos Zentilli. 2 6 9
17 LIST OF TABLES.
TABLE PAGE 11.1 Historic Activity of Lascar (From Casertano, 1 9 6 3 ). 58 IV.1 Calculated Temperatures and Oxygen Fugacities of Lascar Magmas. 149 V.1 Compositions of Minerals used in Least- Squares Mixing Calculation on Soncor Pumice. 176 V.2 Result of Least Squares Mixing Calculations on Soncor Pumice. 177 V.3 Least Squares Mixing Calculations on Main Trend: Magma Mixing Model. 179 V.4 Least Squares Mixing Calculations on Main Trend: Fractional Crystallization Model. 180 VI.1 Mineralogy of Skarn Xenoliths from Volcan Lascar and Cerro Lejia. 1 99 VI.2 Isotopic composition of carbonate in skarn xenoliths and CO2 in buchite xenolith. 221 VII.1 Analysis of sulphur wavelength shift relative to pyrite for several standards. 2 50 VII.2 Sulphur fugacities calculated for Lascar magmas from pyrrhotite compositions, compared with results from other silicic magmas. 264 VII.3 Isotopic composition of native sulphur and sulphate minerals from various volcanic centres of the Central Andes. 268 A3.1 Step heating run carried out on mafic inclusion LA-141. 330 A3.2 Step heating run carried out on buchite xenolith LA-111. 330
18 CHAPTER I INTRODUCTION
1.1 GENERAL.
This study of Lascar Volcano is in close collaboration with the
University of Bristol and with the Servicio Nacional de Geologia y Mineria,
Chile (SNGM) who are undertaking a regional program studying the Central
Volcanic Zone of northern Chile.
Lascar is an active calc-alkaline stratovolcano located in the
arid Altiplano of northern Chile (23°22’S; 67°44’W). It consists of three
centres, two of which were probably formed during the last glaciation, and
are now extinct. The recent activity of Lascar including vulcanian eruptions
in 1986, 1990 and 1993 has caused some concern in the region and an
understanding of the eruptive style of the volcano based on a study of the
magma supply system will aid the prediction of potentially dangerous events.
The high output of volcanic gases, notably sulphur dioxide, from Lascar has strong significance for environmental reasons and it is hoped that the models of volatile behaviour in the magma system put forward in this thesis will be enlarged, tested and perhaps considered at other volcanic centres. The Central Andes contains a large number of porphyry/epithermal precious metal deposits developed in subvolcanic intrusions such as the Miocene Volcan Copiapo (Zentilli et al, 1991; Zentilli
19 & Stark, 1992; Stark & Zentilli, 1992). Sections through such systems can be
examined at various levels throughout the region, and it is likely that the volatile output of Lascar represents the surface expression of such a system.
Skarn mineralisation has occurred, as indicated by the presence of many
calc-silicate xenoliths in the lavas and pyroclastic flows and may still be
actively taking place.Using stable isotope studies of xenoliths and
hydrothermal minerals the interaction between the magma, its hydrothermal aureole and the wallrocks can be geochemically investigated.
This chapter includes an outline of the tectonic and magmatic evolution of the Central Andes, and also a discussion of previous work. The main geographic features are described, together with an outline of the regional geology, and the historic activity of Lascar.
FIGURE 1.1: Map showing immediate area around Voican Lascar and locations referred to in text.
20 To San Pedro de Atacama. 68 00 67° 45'
Co. de Rotor Toconao
Hecar Coiachiûr' 4 ; 23° 15 y Co.Laguna 0 Verde ? Scale
Cuyugas 1:313,250. Talabre Nuevo Talabre Viejo
Volcan Aouas Calientes ® Soncor
o„ebraga de c/, Volcan Lascar Cos. de \ Allana
Camar Cerro Corona j# / —— Tropic of Capricorn
Cerro I v Tumisa \ \ Laguna Lejia 23° 30'
To Tilomonte. To Socaire. To Guaitiquina. 1.2 TECTONIC SETTING.
Lascar is situated in the Central Andean Volcanic Zone, a
region of crustal thickening, uplift, and major volcanic activity. This volcanic
zone is as part of the Andean volcanic arc which stretches the entire length
of South America. The tectonic and volcanic activity are related to the
subduction of oceanic crust beneath the western margin of South America.
1.2.1 Tectonic Evolution of the Central Andes.
The Andes Volcanic Chain is the classic example of a
convergent plate margin, in which oceanic lithosphere is subducted beneath
continental lithosphere. It is also the area after which the rock type andésite
is named. The Andes Mountains are constructed on the Western margin of the South American continent, beneath which the Nazca plate is subducted
(Figure 1.2). The history of convergence extends back at least to the
Palaeozoic. Deformation which occurred before the Jurassic is generally
referred to as Pre-Andean, and that which occurred subsequently as part of the Andean orogeny (Jordan & Gardeweg, 1989). During the Palaeozoic the
Central Andes are thought to have formed part of an ensialic basin between the Precambrian Brazilian shield and a hypothetical “South Pacific
Continent” known as “Chilenia" (Ramos et al, 1984). This accreted to
Argentina during the Palaeozoic. More fragments are thought to have accreted further north, forming the Arequipa basement along the southern
Peruvian coast and isolated exposures in northern Chile (Gardeweg,
1991a).
The tectonic and volcanic history of the Central Andes is
21 Northern V.Z.
Galapagos Is. SOUTH AMERICAN PLATE
Central V.Z.
Nazca Ridge
NAZCA San Félix Is. i PLATE Juan Fernandez Is.
Southern V.Z.
FIGURE 1.2: Map showing main plate boundaries of the Nazca and South American Plates.
22 summarized by Jordan and Gardeweg (1989). The Mesozoic “Andean
Cycle” is broadly described as the subduction of oceanic lithosphere below
South America. The magmatic activity has migrated eastwards during this
time. Until the mid-Cretaceous the magmatism was confined to island
volcanic arcs (Cordillera de la Costa), and a back-arc basin. From the late
Cretaceous to Eocene a continental magmatic arc paralleled the coastline
on a trend that is now exposed in the Intermediate depression and the
Cordillera de Domeyko. The Oligocene was a period of subdued tectonic
and volcanic activity, although major porphyry copper systems were
emplaced (e.g. Chuquicamata, a major deposit located near Calama). This
hiatus coincides with a period of low plate convergence between 36 and 26
Ma (Pardo-Casas & Molnar, 1987). Plate convergence became more normal to the plate boundary and accelerated after 26 Ma and magmatism migrated to its present position. About 10 to 12 Ma, major changes occurred, with voluminous magmatism in the north and a cessation of magmatic activity in the south (south of 28°S). At the same time strong deformation occurred, which was responsible for the uplift of the modern Andes. This episode is
known as the Quechua event (Jordan & Gardeweg, 1989).
1.2.2 Upper Cenozoic and Neogene Tectonic and Volcanic
Structure.
The Andean Chain is divided into three main segments. These are the Northern (NVZ; 5°N-2°S), Central (CVZ; 15-27°S), and Southern
(SVZ; 33-52°S) Volcanic Zones (e.g. Thorpe et al, 1982). Futa and Stern
(1988) subdivide the SVZ into northern (NSVZ) and southern (SSVZ) parts.
The same authors define a further zone, the Austral Volcanic Zone (AVZ), at the southern tip of South America. Changes in the volcanic system correlate
23 X '
TRENCH AXIS 3 KM CXEAN DEPTH W-B ZONE DEPTH. KM DRAINAGE DIVIDES ' AVG ELEV > 3 KM VOLCANIC CENTERS
W 78
FIGURE 1.3: Map showing contours on descending Nazca Plate, main areas of uplift and Cenozoic to Recent volcanic Centres. Slightly modified from
Isacks (1988).
24 with changes in the angle of subduction of the Nazca plate, and differences
in the structure and tectonic environment of the overlying continent (e.g.
Barazangi & Isacks, 1976).
1.2.2.1 Tectonic Structure.
The plate convergence history is well known only during the
late Cenozoic, but the general history has been inferred throughout most of
the Cenozoic (Pardo-Casas & Molnar, 1987). Since 26 Ma the convergence
direction has been nearly normal to the plate boundary. The normal
convergence rate increased from less than 5 cm/y to more than 10 cm/y. This
rate has been maintained until the present.
The angle of dip of the subducting plate varies considerably
along the active margin. These changes are reflected in the characteristics of the volcanic arc. From north to south, the following five elements (Figure
1.3) can be discerned (Barazangi & Isacks, 1976); Three regions occur with a
moderately-dipping slab (25-30°). These regions correlate with the three
main volcanic zones. Two regions are underlain by a gently-dipping slab
(10°) which may follow the contours of the lower boundary of the overriding
plate. These areas correlate with the two gaps in the volcanic arc (2-15°S and 27-33°S). The possible absence of the mantle wedge in these regions
is suggested as a reason for the lack of magma generation. The two flat slab
regions coincide with the predicted continuations beneath the continent of two aseismic ridges, the Nazca Ridge in the north, and the Juan Fernandez
Ridge in the south (Pilger, 1981). These buoyant ridges are postulated to have prevented the subducting plate from sinking.
25 The northern termination of the moderately-dipping segment
underlying the Central Volcanic Zone is represented by an abrupt transition to the gently-dipping segment. This was interpreted as a tear by Barazangi &
Isacks (1976), but has since been proposed to be a sharp contortion of the
plate (Hasegawa & Sacks, 1981). The southern transition from the moderately-dipping to gently-dipping slab segments in the Central Andes is gradual, occurring over several hundred kilometres, between 23° and 33°S
(Jordan et al, 1983), by the enlargement of a slab “bench” between the 100 and 125 km contours (Isacks, 1988).
The continental margin is anomalously thick beneath the
Central Volcanic Zone, increasing from a normal thickness of approximately
40 km in the north and south of the area, to a keel of 70 km thick crust in the area underlying northernmost Chile and southern Peru (James, 1971a, b). A region of plateau uplift forming the crest of the Andes known as the Altiplano or Puna lies above this thickened crust.
This apparent slab segmentation is believed to exert a strong control on the deformation style in the Andean belt (Jordan et al, 1983). The area overlying the moderately-dipping segment of the Central Volcanic Zone
(15°-23°S) is characterised by thin-skin tectonics on the western side of the
Andes (Sub-Andean Zone), with east-verging folds and west-dipping thrusts.
The Altiplano and Puna form a distinctive high-altitude plateau upon which the Cenozoic and Neogene volcanoes are constructed. Uplift occurred during the Plio-Pleistocene. The transitional region south of this area (23°-
28°S) is characterised by a transition from thrust-style tectonics in the northern sedimentary sequences to high-angle reverse faulting in
26 Precambrian gneisses and intrusives to the south. The “flat-slab” region to
the south (28°-33°S), in sharp contrast to the thin-skinned thrusting to the
north and west, is dominated by a broad region of crystalline basement
uplifts in the Pampeanas Ranges, thrown up in 4 to 6 km high blocks on
steep reverse faults.
1.2.2.2 Volcanic History of the Central Andean Volcanic
Zone.
The Cenozoic to Neogene Central Andean Volcanic Zone is
one of the most extensive and concentrated volcanic regions in the world. It
involves large-volume dacitic to rhyolitic ignimbrites associated with major
caldera structures, interstratified with the products of andesite-dacite
stratovolcanoes and dome complexes. Small monogenetic basaltic andésite
and andesitic cinder cones and lava flows are less common, as are isolated
dacitic and rhyolitic eruptions which form flat-topped domes. The volcanism
is subdivided into segments, which have differing geological histories and
show variations in magma geochemistries. These changes are thought to be
controlled by the tectonic and compositional structure of the underlying
continental crust. It is well known that locations of Cenozoic and Quaternary volcanic centres are frequently controlled by major faults. N-S faults in the El
Tatio-San Bartolo area control the emplacement of domes and volcanic
cones of Miocene to Pleistocene age (Lahsen, 1982). NW-SE and NE-SW faults cut this system and many volcanoes which lie away from the main N-S trend are located along these. Other photolineaments have been drawn
along lines of cones further south, around Toconao and Saltar (Ramirez &
Gardeweg, 1982), implying the existence of N-S, NE-SW and NW-SE trending faults. Marrett and Emerman (1992) have suggested that mafic lava
27 flows and scoria cones on the Altiplano-Puna plateau may be associated
with Pliocene-Quaternary strike-slip fualts. They propose a mechanism by
which the change in deformation style from thrusting to strike-slip during the
late Pliocene or Quaternary reduced the horizontal compression in the
upper crust, allowing dense mafic magma to rise to the surface.
Neogene (10.5-6.6 Ma) to Pleistocene (0.20-0 Ma) volcanics of
the Nevados de Payachata region (Lat. 18°S) fall into two age-dependent
chemical groups (McMillan et al, 1993). Pleistocene lavas are enriched in
incompatible elements relative to Neogene samples, but have similar Sr, Nd
and Pb isotopic values. The younger lavas are interpreted as having a
stronger lower crustal signature. This is thought to be due to thermal equilibration of the crust following a period of thickening which began at approximately 15 Ma, leading to heating of the base of the crust and
involvement of the lower crust in anatexis by Pleistocene times.
Large-scale changes (in the order Of 500 km) in isotopic composition can be correlated with variations in the composition of the
underlying crust. A geochemical traverse of north Chilean Quaternary centres from 17.5° to 22°S (Worner et al, 1992) crosses the boundary between Proterozoic crust in the north to Palaeozoic crust in the south. This is reflected in changes in isotopic compositions and variabilities, notably an increase in 206pb/204pb from north to south, thought to be due to contamination of the magmas by crust of different characteristics.
The segment of the arc which lies to the east of the Salar de
28 Atacama (23° to 24°S) and contains Lascar Volcano is known as the
Toconao segment. South of this (24° to 25°S) lies the Socompa segment.
The onset of volcanism occurred at different times in these two segments. In
the Socompa segment the activity started at 23 Ma (Gardeweg 1991a), and
is dominated by large composite cones and small-volume ignimbrites. The
volcanic front is dominated by eroded Miocene cones while the Quaternary
Volcanoes lie along the Chilean-Argentinean border. They are about 90-110
km above the Benioff zone and 290-310 km east of the Peru-Chile trench. In
the Toconao segment, and northward to about 21°S, the volcanic activity
resumed during the Late Miocene (<11.7 Ma, Gardeweg 1991a), with the
eruption of large volume ignimbrites and contemporaneous smalier volumes
of lavas from volcanic complexes. An eastward deflection of the arc is
associated with this late inception of volcanic activity in the Toconao
segment. The Quaternary Volcanoes of this region form the volcanic front,
lying about 120 to 140 km above the Benioff zone and 360 to 390 km east of the Peru-Chile trench.
The differences in the structure and magmatic histories of these two segments are believed to be related to the small horizontal bench in the
slab at about 20°S (above). It has been suggested that the curvature of the
slab changes from convex to concave south of 24°S and is nearly horizontal
near 28°S (Isacks, 1988).
Differences in the chemistry of the rocks from the two segments are also discernible. Ignimbrites from the Toconao segment fall systematically in the high-K calc-alkaline series and some show shoshonitic affinities, having high KgO and KgO/NagO (Gardeweg, 1991a), whereas
29 those from the Socompa segment are normal calc-alkaline rocks. The higher
KgO and Rb contents of the rocks of theloconao segment may be compatible
with a deeper source of the parental melts, since the KgO content of
andésites can be correlated with the depth to the top of the Benioff zone in
many areas (Hall, 1987). This consistent with the greater depth of the Benioff
zone below the Quaternary Volcanoes of the Toconao segment. However,
conclusions such as this must be approached with care, since there are a
number of factors which influence the compositions of Central Andean
magmas, including the degree of crustal contamination during fractional
crystallisation.
Within this medium-scale segmentation (around 100 km) the
arc can be subdivided into smaller segments based on geochemical variations. Major geochemical differences can be resolved between volcanic complexes and even individual volcanic centres. These variations reflect differences in the source regions of parental melts, and also differences in their paths to the surface, the age and therefore the tectonic environment of the centre, magma chamber processes and compositions of crustal contaminants. It is one of the purposes of this thesis to identify and quantify these parameters for Lascar Volcano.
1.3 LOCATION AND ACCESS.
Lascar Volcano rises to an altitude of 5,592 m, with its base at about 4,000 m. It lies to the east of the Salar de Atacama, in what is reputed to be the most arid desert on Earth (Messerli et al, 1991). A dirt road approaches close to the west of the volcano, crossing many of its lavas and pyroclastic flows, then crosses through a pass to the south (Figure 1.1). The
30 nearest village is Talabre (70 families), which has recently been relocated to
a more distant position about 17 km WNW of volcano, partly due to the
increased risk since the eruption in 1986. The town of Toconao lies 34 km to
the NW. Four-wheel drive vehicles are required for fieldwork due to the poor
quality of the dirt roads, which are frequently washed away by mudflows
during the rainy summer season (December to February). During the winter
(April through October) the roads are commonly inaccessible due to snow.
Ascent of the volcano on foot is difficult because of its steep rubbly sides and
high altitude. The combined effects of altitude and the poisonous gases
emitted from the crater can conspire to make fieldwork difficult and
sometimes unpleasant.
1.4 GEOGRAPHICAL FEATURES.
In this part of the Andes, a series of elongate physiographic domains are defined, which are a few hundred kilometres long and tens of
kilometres wide. These domains, illustrated in Figure 1.4, parallel the coast and are the result of the deformation history of the area. From west to east these are; the Cordillera de la Costa, the Intermediate Depression or
Longitudinal Valley, the Cordillera de Domeyko, the Intermontane Basins
(including the Salar de Atacama), the Cordillera Principal de Los Andes or
Western Cordillera (in which Lascar Volcano is situated) and the Altiplano or
Puna. To the east lie the Cordillera Oriental or Eastern Cordillera, the
Subandean Ranges and the Brazilian Platform. These features are described by Gardeweg (1991a) as follows;
The Cordillera de la Costa is a 50-60 km wide horst
31 Main Physiographic Domains.
V \ r ^ V VV V Ç V V VV '
^ J lO O km ^
Intermontane Basins Cordillera de la Costa incl. Salar de Atacama
V V V V g : Intermediate Depression V VV V Western or Main Cordillera V V V V
Cordillera de Domeyko Altiplano or Puna
Figure 1.4: Map showing main physiographic domains of the Central Andes. Modified from Gardeweg (1991a).
32 structure parallel to the coast which reaches a maximum height of 3,108 m.
A steep fault escarpment defines its western boundary. The Intermediate
Depression is a low flat area dividing the Cordillera de la Costa and the
Cordillera de Domeyko. It disappears at 25°S where the two Cordilleras
come together. The altitude of the depression ranges frpm 1,000 m in the
west to 2,000 m in the east. The Cordiiiera de Domeyko is a mountain
range consisting of a series of horsts and grabens oriented roughly N-S. It
rises to a maximum of 4,000 m. West of the study area it forms a mountain
range with an average height of 3,000 m. This part of the Cordillera is formed by thrust faulting which verges towards the Salar de Atacama. The
Intermontane Basins are a series of internally-drained basin systems
representing forearc basins. In the study area these basins are represented
by the Salar de Atacama, on whose western boundary lies the Cordillera de
la Sal. The Salar de Atacama has a surface area of approximately 4,200 km2, and an average height of 2,300 m. A salt pan or salt encrusted playa
has been developed over 75% of this area. The Cordillera de la Sal, a
mountain range approximately 4 km wide and about 2,500 m high,
represents Salar de Atacama sediments which have suffered deformation due to thrusting. The Salar has a large central area formed of massive
layered halite, surrounded by a rim with a soft crust, including silty rock salt and moist gypsum, and brine pools close to the outer rim. The C o rd illera de Ids Andes is composed of volcanic cones and pyroclastic flows lying on non-volcanic basement rocks. This area lies immediately to the east of the intermontane basins and adjacent to the Salar de Atacama it forms a gentle westward dipping slope (3-5°) formed of ignimbrites and pyroclastic flows, ejected from cones and calderas mainly located on its eastern border.
33 These ignimbrites abut against a range of low hills which lie on the eastern
border of the Salar de Atacama. The chain of hills is more than 60 km long and composed of Palaeozoic volcanic and sedimentary rocks. This slope is cut by deep E-W gorges, known as quebradas, which channel the sporadic drainage from the Andes to the Salar. The main composite volcanoes rise above 5,000 m in this area. They include, from north to south, Licancabur
(5,916 m), Purico (5,604 m), Cerro Negro (5,025 m), Lascar (5,592 m),
Tumisa (5,614 m), Lejia (5,793 m), Miscanti (5,622 m), Miniques (5,910 m) and Pular-Pajonales (6,233 m). The Altiplano or Puna is a large plateau about 200 km wide and with a basal elevation of about 3,800 m. It consists of
Upper Cenozoic volcanic cones and ignimbrites which rise to over 5,500 m and lie in a non-volcanic basement of Palaeozoic to Tertiary age. Large calderas form internal drainage basins, usually with small saline ponds or salt crusts. The eastern flank of the Altiplano also forms a gently dipping slope (1-4°) mainly formed by ignimbrites that abut against hills of
Palaeozoic sedimentary rocks from the Sub-Andean ranges close to the
Argentinian border.
1.5 PREVIOUS WORK.
The volume of work on this area is large, but is mainly limited to regional studies (e.g. Co ira & Barbieri, 1989; Hildreth & Moorbath, 1988).
However, a number of studies of individual centres has been carried out.
These include the Nevados de Payachata at 18°S (Worner et al, 1988;
Davidson et al, 1990), Volcan Tata Sabaya at 19^8 (de Silva et al, 1993),
Volcan Ollague at 21°S (Feeley et al, 1993), San Pedro-San Pabio at 22°S
(Francis et al, 1974, 1977; O'Callaghan & Francis, 1986), Purico-Chascon at
23°S (Hawkesworth et al, 1982), La Pacana Caldera at 23°S (Gardeweg &
34 Ramirez, 1987), Cerro Tumisa at 23°S (Gardeweg, 1988, 1991a) and the
Cerro Galan Caldera at 26°S (Francis et al, 1980, 1983,1989).
Some previous petrological and geochemical work has been
carried out on Lascar by Garin (1961) and Deruelle (1979, 1985), but recent
studies have concentrated on satellite monitoring of eruptive activity (Glaze
et al, 1989; Francis et al, 1989; Mouginis-Mark et al, 1989; Oppenheimer et
al, 1993) and seismographic studies (Espinoza, 1993).
1.6 CLIMATE.
1.6.1 Modem Climatic Conditions.
The research area lies within the Atacama Desert, thought to
be the most arid desert on Earth (Bowman, 1924; Messerli et al, 1989). The drying effect of the Humboldt Current combined with subsiding anticyclonic air masses of the SE Pacific High Pressure Belt, and the moisture-blocking
Andean mountain chain results in extremely low precipitation between the coast and the Western Cordillera (Messerli et al, 1989). In a transect from the coast to the Altiplano, annual precipitation increases up the western slope of the Andes from 0-2 mm between Antofagasta and San Pedro de
Atacama, to 100-200 mm on the crest of the Andes. Summer precipitation is
limited to occasional storms (“Invierno Boliviano"), sometimes resulting in catastrophic flash-flooding. Winter precipitation falls mainly as snow on the
Andean Mountains. Vegetation cover increases with precipitation from zero in the hyperarid desert to low scrub up to around 4,000 metres. Small oasis communities such as Talabre and Camar (Figure 1.4) rely on stable springs which issue from the foot of the volcanic chain below 4,000 m. Larger water outflows support the towns of Toconao and San Pedro de Atacama.
35 1.6.2 Pleistocene Climatic History.
The Pleistocene period of the Andes is dominated by a series of glacial advances and retreats. In detail the chronology is poorly constrained, with conflicting evidence from different parts of the Andean chain, although an approximate history of Pleistocene glaciations is given in
Clapperton (1983). The maximal Pleistocene advance occurred between 28 and 20 kyr B.P. Later advances occurred from 20 to 19 kyr, 16 to 14.5 kyr, shortly after 13 kyr, and between 11 and 10 kyr B.P. These periods were associated with a lowering of temperatures and an increase in precipitation.
Seltzer (1990) summarises data from the Peruvian-Bolivian Andes. He places the maximum glaciation at between >27 kyr and 16 kyr B.P., with the retreat from the last maximum occurring from 12 to 10 kyr B.P. Clapperton
(1993) suggests that in the Northern and Central Andes the last glacial maximum peaked at 27 kyr B.P., earlier than in the Southern Andes where the peak occurred at around 20 kyr B.P.
The Pleistocene snow-line depression allowed glaciers to form in the North Chilean Andes down to about 4,000 m altitude. Small remanent glaciers in the region occur today only on the three highest peaks of
LIullaillaco (Lat. 24°43’S, altitude 6,723 m), Ojos del Salado (Lat. 27°07’S, altitude 6,885 m) and Tortolas (Lat. 29^56'S, altitude 6,323 m). The glaciers on these mountains occur at altitudes of between 6,600 and 5,323 m. The snow line at these altitudes is now at about 6,000 m, but was depressed to about 5,000 m during the maximum glaciation (Hastenrath, 1971).
Hollingworth and Guest (1967) report cirques around El Tatio and Volcan
Toconce (Lat. 22°10’ to 22°21’S) and a terminal moraine at about 4,280 m.
Poorly developed cirque morphologies have been identified on LIullaillaco
36 at about 5,500 m and in the area between Imilac and Socompa (Lat.
24°19’S, altitude 6,050 m) at about 5,000 m (Hastenrath, 1971). Clapperton
(1993) reports that low-gradient river networks became incised by at least
100 m as global sea level fell by 120 m during the last glacial maximum and
suggests that his may have occurred during the build-up to the maximum,
due to enhanced runoff corresponding to colder and wetter conditions. The
quebradas of the region around Toconao have not been dated accurately,
but they are clearly incised through Pleistocene pyroclastic deposits
including the Cajon Ignimbrite north and east of Toconao, dated at 1.7± 0.3
Ma and 1.3 ± 0.3 Ma (Ramirez & Gardeweg, 1982) and the pyroclastic fan of
Cerro Tumisa, dated at 1.5 ±0.5 Ma (Gardeweg, 1991a). If these gorges were formed at the same time as other incised river networks, during the
build-up to the last glacial maximum, then a minimum age of 27 kyrs can be assigned to them.
According to Messerli et al (1991), the last cold maximum in the
Atacama region was represented by temperatures probably 7°C colder than today, and the lack of widespread valley glaciation which is characteristic south of La Serena indicates that the conditions remained arid. A stratigraphie section through sediments at Laguna Lejia (Messerli et al,
1991) allows reconstruction of local climates during late glacial time (17 to
11 kyr B.P.). From 17 to 15 kyr B.P., lake levels were 5-10 m higher than today, indicating a more humid and colder environment. After 15 kyr B.P., a shift to higher lake levels took place. A short- to medium-term maximum in lake level was reached at 25 m above the present level, indicating a further increase in precipitation.
37 1.6.3 Holocene Climatic History.
Analyses of extinct fauna and pollen (Markgraf, 1989) indicate that the Puna was wetter and colder than today, prior to 10 kyr B.P. Between
10 and 8 or 7 kyr B.P., the temperature increased but moisture remained relatively high. The occurence of fossil soils between San Pedro de
Atacama (2,500 m) and Portezuelo del Cajon (4,800 m) also indicates warmer and wetter climates for the early Holocene, prior to 8.4 kyr (Messerli et al, 1991).
Analysis of the pollen record from Tumbre (Messerli et al,
1991) indicates that warmer conditions prevailed between 6 and 3 kyr B.P.
Markgraf (1989) suggests that the climate became drier between 8 and 4 kyr
B.P. Human impact began at about 2 kyr B.P., as indicated by regional analysis, and by the record at Tumbre.
1.7 BASEMENT GEOLOGY.
The pre-Cenozoic basement (Gardeweg, 1991a) consists of marine Devonian to Early Carboniferous quartz-sandstones (Lila
Formation), a sequence of Permian volcanic and volcaniclastic rocks (Cas
Formation) and a sequence of volcanic and volcaniclastic rocks with fine grained lake sediments (Peine Formation and Cerro Negro Strata) of
Permian to Triassic age, which are intruded by Permo-Triassic granitoids and rhyodacites. These are overlain by volcaniclastic siltstones, sandstones and conglomerates of probable Cenozoic age (Quepe Strata) and a number of large-volume ignimbrites and smaller andesitic and dacitic domes and stratocones. The large 4 m.y. La Pacana Caldera with a diameter of 36 km
38 (Gardeweg & Ramirez, 1987) lies just to the south of Lascar and is one of the nearby sources of these ignimbrites.
39 CHAPTER II GEOLOGY OF LASCAR VOLCANO
11.1 INTRODUCTION.
Lascar Volcano is a slightly elongate structure with a series of
5 craters aligned in an ENE-WSW trend (Figure 11.1). Immediately to the east
lies the extinct Volcan Aguas Calientes and to the north and south are a line of dacitic domes, dated at 5.2 ± 0.8 Ma (Ramirez & Gardeweg, 1982). A major crustal lineament known as the Miscanti Line follows this N-S alignment and passes directly through Lascar (Matthews & Vita-Finzi, 1993).
The volcano probably lies on the intersection between this and a smaller
ENE-trending fracture, which controls the crater alignment and passes through Volcan Aguas Calientes.
The main features and geological history are described briefly in Gardeweg & Sparks (1993), and in Matthews et al (in press). Three recognizeable eruptive centres have been active through the history of the volcano. The first and oldest (Centre I) lies on the eastern side of the complex and is preserved as an eroded cone which is visible as a sequence of lava flows and pyroclastics below an unconformity on the inner walls of the active craters. Centre I erupted a sequence of andesitic lavas which are
FIGURE 11.1: Simplified geoiogical map of Lascar Volcano. See text for d e ta ils .
40 Lascar Volcano. Volcan Aguas Calientes.^/"
—. ^ A A A A A A A A A ^ ~ *^ A AAAAAAAAA A^
5 m.y. Dacite Dome.
5 m.y. Dacite Dome.
Capricorn % Lava. Debris
V Tumbre
Cerro Tumisa.
LEGEND.
Andésite Lava. Soncor Pumice Flow.
Dacite Lava. p .ç .f Moraine. 1 . n ,1
Lava (Unident.). Alluvium.
Tumbres Recent Andésite Pumice Mudflow. Flow. Talabre
Cauliflower h A A Volcanic Bomb Flow. k A A Ejecta.
If I 0 V o lc a n
m
C e n tre II m . -'mm A
5 M a ^ , D o m e • w i
K'A
FIGURE 11.2: Aerial photograph of the cones and craters of Lascar
Volcano with main features labelled.
42 exposed to the west and northwest of the volcano as flat-lying, rather eroded flows. This centre was active during the glaciation and it is thought likely that the original edifice was breached to the NE, leaving a horseshoe-shaped cone, possibly as the result of a sector collapse. Evidence for this is seen on the inner walls of the active crater and in the distribution of glacial deposits around the volcano. Probably at a later date than this collapse, but before the onset of glacial erosion, an andesitic pyroclastic flow, the Saltar Flow, was erupted from Centre I. This deposit is preserved to the south, NW and
NE of the volcano and also mantling the slopes of Volcan Aguas Calientes. It has apparently suffered extensive glacial erosion in the valley to the NE.
Overlying the Saltar flow is a monolithologic deposit consisting of large prismatic-jointed blocks of dacite up to 8 metres in diameter. This deposit, known as the Piedras Grandes Flow, is thought to represent a “jokulhlaup” or glacier-burst event triggered by an eruption during the Pleistocene glaciation. This deposit is preserved to the west of Lascar and also to the
NE, overlying the eroded Saltar Flow, and may represent a precursor to the
Soncor Flow which directly overlies it.
Centre II is visible as a rounded cone on the western side of the complex and is the highest part of the volcano. The first eruption of this
Centre was a large-volume (5-10 km^) dacitic pyroclastic flow, the Soncor
Flow. This eruption was probably associated with a caldera collapse, now buried beneath the later cone. This flow forms a large fan of pyroclastic material extending for 27 km downslope to the west, and 15 km to the SE,
FIGURE 11.3: Geological column showing relative timing of events and limited absolute dating.
43 STRATIGRAPHY. DATING. CLIMATE.
Dome Building 0 and Degassing. .
lU Successive Œ I t - t T Crater Collapse. HOLOCENE. 2 Ü
Talabre Lava.
Cone Building. /7n\ (Pyroclastics).
>✓ N/" o o o o o o o Tumbres Row. 9,000.
Minor EROSION.
Cone Building. LATE (Lavas and pyroclastics). /7FW UJ cc GLACIAL. Capricorn Lava. UJ ooooooo Ü ooooooo Chaiie Row.
Glacial EROSION. 11,700 ±700 ooooooo Soncor Row. Approx. 20,000 ? GLACIAL. V V W V V V Piedras Grandes Row. o o o o o o o (Subglacial Eruption).
EROSION of Centre I by cirque glaciers. o o o o o o o Saltar Row. UJ / \ /N. /X ». » GC Z Andeslte Lavas. UJ INTER Ü GLACIAL?
EROSION. GLACIAL. (Quebradas Formed). > 27,000. with an average thickness of 20 metres (Gardeweg & Sparks, 1993). It also
mantles Volcan Aguas Calientes. The flow has thick extensive lithic-rich lag
breccias and an associated plinian airfall deposit on the SE flank. Overlying the Soncor flow is an andesitic cauliflower-bomb flow, the Chaiie Flow, which is visible on the SW side of the volcano. This flow is composed of
large cauliflower bombs and breadcrusted blocks with glassy rinds, and clastogenic blocks up to 10 metres in diameter. Overlying the Chaiie flow is a dacitic lava flow, the Capricorn Lava (author), which is geologically significant because it is the only known product of Lascar which contains glassy mafic inclusions, similar to those found in most of the surrounding centres. A sequence of andesitic and dacitic lavas and pyroclastics has then built up to form the edifice of Centre II. The last of these, the Tumbres Flow, is an andesitic pumice flow consisting of scoria clasts and cauliform bombs underlain by a plinian pumice fall deposit.
Following the demise of Centre II, activity shifted eastwards and Centre III was constructed on the site of the old Centre I. The Talabre andesitic lava was erupted to the NW, and three successively deeper collapse craters were formed. The youngest of these is still active. The edifice of Centre III is composed mainly of ejected bombs and scoria. The individual flows are dealt with in more detail below;
11.2 CENTRE I.
11.2.1 The Early Andésite Lavas.
The earliest known products of Lascar are a sequence of extensive andesitic lava flows exposed mainly to the west but also to the north of the volcano. The longest flow has reached a distance of 15 km
45 downslope to the west. It can be seen flowing into Quebrada de Soncor
(Figure 11.4), indicating that Lascar postdates the formation of the canyon system. This means that Centre I was probably built up during the last glacial maximum, assuming that the quebradas have a “last glaciation” age. The lack of evidence for subglacial eruptions or glacial erosion of these lavas may indicate a period of glacial retreat between 27 kyr and 20 kyr B.P., or simply that Centre I was not high enough to develop glaciers until the eruption of the Saltar Flow. The lavas are rather eroded, with flow fronts 5 to
10 metres high and consist of rounded blocks with well-developed desert varnish. Zeolites are abundant in vesicles. The lavas are medium-grained and plagioclase-phyric, often with many flow-aligned phenocrysts of the mineral. Lesser amounts of augite and orthopyroxene occur. Olivine is occasionally seen.
11.2.2 The Saltar Flow.
This flow represents the last activity of Centre I and consists of dense to poorly vesicular cauliform and breadcrust bombs. A basal unit is dense and black and an upper unit is reddened and oxidised (Figure 11.5).
An angular lithic-rich fall deposit is occasionally seen below the basal flow
(Gardeweg & Sparks, 1993). The Saltar flow is seen overlying the early andésite lavas to the NW and overlying hornblende dacite lavas of Volcan
Aguas Calientes to the NE of Lascar. It also clearly mantles the western side of Volcan Aguas Calientes to some considerable height (700 to 800 metres estimated) It is exposed as a steeply dipping deposit on the south side of
Lascar. The flow appears to have suffered glacial erosion to the NE, where it has been cut into a series of low elongate ridges, parallel to the valley sides.
The bombs have large plagioclase phenocrysts, and are characterised by
46 FIGURE 11.4: Centre I andesitic lava flowing Into Quebrada de Soncor. Terraces formed of the Centre II Soncor Flow are also visible within the quebrada.
FIGURE 11.5: Saltar Flow on the south side of Lascar showing multiple flow units and dense andesitic bombs.
47 large euhedrai augite phenocrysts up to 1 cm across. Vesicles in the bombs
are zeolitised. The Centre I andesitic lavas and the Saltar Flow are the only
products of Lascar in which zeolites have been observed.
11.3 CENTRE II
11.3.1 The Piedras Grandes Flow.
This deposit consists of large prismatic-jointed boulders of pyroxene dacite with minor hornblende. It is exposed up to 15 km to the west of Lascar, where the boulders are up to 8 m across (Figure 11.6). The boulders often have a quenched glassy rim extending inwards for up to 1 cm. Glassy hornblende-rich mafic inclusions are common. Occasional calc- silicate xenoliths consisting mainly of wollastonite also occur. This flow was originally thought to be associated with Volcan Aguas Calientes, but pétrographie analysis indicates that it is related to the overlying Soncor Flow and therefore it is considered more likely to represent a dome-building precursor to this flow. The presence of such large boulders with very little ash matrix suggests that the boulders may have been transported by water.
Their radial prismatic jointing and glassy rims indicate fairly rapid cooling and it is thought that a sub-glacial dacite eruption led to melting of the glacier and high-energy water transport of the quenched lava. This is similar to an Icelandic “Jokulhlaup” or glacier-burst eruption (Thorarinsson and
Saemundsson, 1977).
11.3.2 The Soncor Flow.
The Soncor eruption was dacitic, involving 5 to 10 km^ of magma. The source was probably a caldera, now buried beneath
48 Centre II. A fan of pyroclastic material extends 27 km westwards downslope
and is concentrated in the quebradas, where it has been eroded to form high
(up to 50 metre) terraces. Three main facies are recognised (Gardeweg &
Sparks, 1993). These are: (1) a lithic-rich massive pumice flow deposit (lithic
contents 20-40%), (2) pumice-rich facies with large pumice clasts set in a fine ashy matrix with minor lithic fragments and (3) lithic-rich lag breccias with interstitial pumiceous matrix. Facies (3) contains a variety of rock types
including Cenozoic lavas, fragments of Piedras Grandes deposits,
hydrothermally altered breccias and green prismatic-jointed blocks of dacitic composition. These blocks are interpreted as fragments of a dome or cryptodome which was present prior to the eruption.
Associated with the flow is a plinian airfall deposit, which is developed mostly to the SE of the volcano. It shows a strong compositional zonation, with a pale dacitic base and a dark andesitic top. A thin layered ash deposit 2 to 3 cm thick underlies the flow at Tumbre to the NW and this is thought to correlate with the 30 metre plinian deposit to the SE.
The Soncor flow shows evidence of some glacial and fluvioglacial erosion, with moraines rich in Soncor pumice developed on the south side of Lascar and extensive erosion of the deposit in the quebradas, which are now dry. The flow is preserved as high (up to 80 m) terraces in the
Quebradas de Talabre and Soncor (Figure 11.7). Later fluvial sediments form low terraces within the deep channel at Talabre Viejo and interbedded with these is a tufa deposit containing carbonate-coated plant remains. Only the upper part of this sequence contains dark pumice clasts from the Tumbres
Flow, indicating that the lower part of the sequence is older than 9 kyr B.P.
49 .a, * - t
FIGURE 11.6: Large prismatic-jointed boulder of the Piedras Grandes Flow to the West of Lascar, partially buried by alluvium.
9 ^
FIGURE 11.7: Terraces of the Soncor Flow filling Quebrada de Talabre near Tumbre. The valley floor is filled with the Tumbres Flow which terminates in the centre of the picture.
50 (see below). First-order carbon-dating (Vita-Finzi, 1983), using HCI to extract
COg from the carbonate (Appendix 2) yields a tentative age of 11.7 ± 7 kyr for the tufa. Bentonite horizons in a sediment core in Laguna Lejia (Messerli et al, 1991) are most likely to be due to activity of Lascar and one may represent the edge of the Soncor plinian fallout. They occur between 16 and
>19 kyr, indicating a possible age of around 15 to 20 kyr for the Soncor flow.
This is consistent with the age obtained from the overlying tufa in Talabre
Viejo, the conjectured glacial erosion of the deposit and the further interpretation of the underlying Piedras Grandes Flow as resulting from a subglacial eruption. This would indicate that these eruptions occurred during the Pleistocene maximum glaciation. However, more complete sampling and further dates are needed in order to test this hypothesis. The flow is extensively indurated around Tumbre and down Quebrada de Talabre and contains degassing pipes at Tumbre, perhaps due to emplacement onto wet ground in the quebrada.
Various types of pumice are present, including hornblende dacite, 2-pyroxene dacite, and banded pumic horizons. A holocrystalline andesitic mafic inclusion has been found in the section at Talabre, containing large domjinantly olivine and clinopyroxene phenocrysts.
11.3.3 The Chaile Flow.
The Chaile Flow directly overlies the Soncor Flow on the SW side of the volcano. It is an andesitic cauliflower bomb flow consisting of dark, dense cauliform bombs and breadcrusted bombs. In the proximal part, large clastogenic welded breccia blocks up to 10 metres in diameter occur
(Figure 11.8), together with rounded blocks of dense lavas and of flow banded
51 ' m
FIGURE 11.8: Chaile cauliflower bomb flow with huge aggiutinate biock.
FIGURE 11.9: Tumbres Flow with basal plinian airfali deposit overlying the Soncor Fiow at Tumbre. Carbonised “coiron grass” from below the Plinian deposit has been dated at 9,000 yrs (P.W. Francis, pers. comm.).
52 FIGURE 11.10: Tumbre showing the Tumbres flow filling the floor of the valley and overlain by the Centre III Talabre Lava.
53 lavas. This is interpreted as synchronous avalanching of lava flow fronts
(Gardeweg & Sparks, 1993).
11.3.4 The Capricorn Lava.
The Chaile Flow is overlain by a sequence of andesitic and dacitic lavas and pyroclastics, which have built up to form the edifice of
Centre II. The lowermost of these, lying directly on top of the Chaile Flow, is a dacitic lava flow, the Capricorn Lava. This lava has well-developed levees and a steep, high flow front (about 30 metres). It is blocky, with large dense fragments up to 2 metres across, commonly with glassy rims. The lava is greyish-blue and coarse grained, often with well-developed flow banding.
The main phenocryst phase is plagioclase, with lesser pyroxenes and hornblende. It contains large numbers of andesitic glassy mafic inclusions, containing many hornblende microlites, and olivine phenocrysts. The mafic inclusions are rounded but irregular shaped, up to 10 cm across, and commonly have a more glassy rim. Also common are green diopside-rich calc-silicate xenoliths ranging from a few mm to 10 cm diameter.
11.3.5 The Tumbres Flow.
This flow large-volume andesitic pumice was the final eruption of Centre II. It is best exposed on the NW and western sides of the volcano, but also to the south. It consists of large pumice clasts and breadcrusted bombs, with a red welded agglutinate close to the vent. It contains clasts of
FIGURE 11.11: Correlation diagram showing the stratigraphie relationships of the Soncor and Tumbres flows in the Quebrada de Talabre.
54 TALABRE VIEJO. TUMBRE.
Mudflows.
Soncor Flow. Pumice-RIch.
v\ en
Sllty Fine laminated Layer. Debris Flows c with Tumbres .2 Pumloe. ______Tumbres Flow 9,000 y I with Plinian Alrlall. Gypsum and I Fluvial Wash. £ Sonoor Flow. a> LIthlc-Rlch. E 11,700 y |_ I t I Tula and Fluvial Wash, g oooc p:p:p:.c Soncor Flow. ppp:( Pumlce-Rlch. p:p:.o:.c ooec p :.o :p :< c p : p : p : ^ 4.5 metre Soncor Flow. section LIthlc-Rlch Matrix- Supported. 10 metre p :p :p :< : section
p :p :p :< c p:p:p:.c highly vesicular rhyodacite pumice, and xenoliths of Cenozoic lavas, hornfels and diopside-rich and wollastonite-rich calc-silicate nodules. It is underlain by a bleached plinian airfall deposit, which only occurs to the west of the volcano (Figure 11.9). A carbon date of 9,000 years b.p. was obtained from burnt vegetation below this deposit (Francis, pers. comm.). This is supported by a date of 7,500 years B.P. for peat directly overlying the deposit
(Messerli et al, 1991).
11.4 CENTRE III.
11.4.1 The Talabre Lava.
The Talabre lava is a young andesitic lava flow which is clearly visible on the NW side of the volcano. Its flow front is close to the springs at
Tumbre (Figure 11.10). It has well-developed levees and a steep, blocky flow front, about 20 metres high. It contains phenocrysts of plagioclase, pyroxenes and hornblende. It appears to be truncated by the earliest of the collapse craters of Centre II, and is therefore thought to predate this collapse.
11.4.2 The Collapse Craters.
Centre III is characterised by three deep collapse craters, which become successively deeper and younger to the west (Figure 11.2).
These are associated with pyroclastic fall material which covers older deposits around Centre III. There seems to be little erupted material associated with these large craters, so they are thought to be the result of removal or contraction of magma within the edifice. This might be achieved through degassing of a subvolcanic intrusion.
56 C: Soncor Pyroclastic Flow. A: Centre 1. 5 m.y. Dome. Volcan Aguas Calientes.
5; Soncor Flow Centre 1 Lavas Cerro Corona. 5 m.y. Dome.
Saltar Flow o o c0> 3 O’0) (0 LO B: Glaciation. Dacitic Dome and Jokulhlaups D: Centres 2 and 3. 0) £ o o O) c 5 o X JC 0) oooooo (/) oooo E o OOOOOOOOOOOQ ra a> ooo L. (/) O) ooo (Q 6 c Lavas and (TJ Pyroclastics of O Piedras Grandes Centres 2 and 3 CM Ô Flow > k.fO UJ o V) OC CO 10 km 3 Ü U. 11.5 HISTORIC ACTIVITY.
There is no systematic record of the historic activity of Lascar prior to 1986. The recent behaviour of the volcano is based mainly on eyewitness accounts of the inhabitants of the region, listed in Gardeweg et al
(1990). This record is in approximate agreement with that of Casertano
(1963). The volcano has been closely watched since 1986 (Gardeweg et al,
1989; Gardeweg et al, 1990; Gardeweg, 1991b), and satellite monitoring has contributed to the prediction of eruptions (Rothery et al, 1988; Glaze et al, 1989; Francis et al, 1989; Mouginis-Mark et al, 1989). The historic activity of Lascar, taken from Casertano (1963), is given in Table 1.
TABLE 1.1: HISTORIC ACTIVITY OF LASCAR (FROM CASERTANO, 1963). 1848 Eruption in central crater. 1854 “Normal explosions". 1858 “Normal explosions”. About 1875 Eruption in central crater. 1 8 8 3 -8 5 “Normal explosions". Last years of 19^^ to first years of 20*^ Century. “Normal explosions" 1933 Eruption in central crater. 1940 “Normal explosions". 1 9 5 1 -5 2 Eruption in central crater. 1959 “Normal explosions". 1960 March 28-April 1. Eruption in central crater. I960 July and December. “Normal explosions".
58 An additional summary has been provided by Gardeweg et a!
(1990) as follows;
During the first years of the 20th Century: Weak column visible above Lascar. 1933: Relatively strong eruption. Visible from
Chuquicamata, 170 km to the NW. November 1951-January 1952:
Intermittent activity. Approximateiy in 1955: At 3.00 pm on an unknown day, an eruption with a small number of large bombs, accompanied by a loud noise. The bombs were ejected towards the north, (Saltar) and caused a fire in the pasture of that area. Approximately in 1965: At 9.00 on an unknown day, strong explosions came from the volcano, heard in Talabre, 11 km west of the crater. The eruption was accompanied by large fumaroles.
Approximately in 1967: An eruption lasting 3 days, with ashfall.
In 1984 and 1985, Landsat Thematic Mapper (TM) images revealed a substantial thermal anomaly in the crater (Glaze et al, 1989). This was originally interpreted as a lava lake, but is more likely to have been due to the presence of a lava dome or high temperature fumarole field. On 16^^
September 1986, ashfall was reported on the Argentine city of Salta, 285 km to the ESE (Gardeweg et al, 1989). This was later found to have originated from an eruption of Lascar, observed from Toconao, 34 km NW of the volcano. Individual explosions occurred on the 14^*^, 15^^ and 16^^
September, and were preceded by the production of unusually strong vapour columns, but with no detectable seismic activity. The eruption of 16^"^
September began at 7.30 am with a strong, prolonged sound, followed by the eruption of a column consisting of two slugs of brownish-tinged ash to an
59 altitude of 10 km above the volcano. This expanded to form a cauliflower which became extended towards the SE and dispersed rapidly. This eruption plume was observed by the Geostationary Operational
Environmental Satellite (GOES). The eruption was seen to consist of two pulses (Glaze et al, 1989), which became extended towards the SE, detached from the volcano and later passed over the town of Salta.
Field studies carried out during April 1987 revealed the presence of numerous bombs in fresh impact craters, probably resulting from the eruption. Landsat TM images of the crater (Glaze et al, 1989), which were acquired in November 1987, showed that the thermal anomaly had increased to an intensity comparable to that before the eruption. On 13^^ and
15^^ July, 1988, plumes 1-3 km high were observed from Toconao, with minor ash production. In February 1989, geologists from MINSAL Co. reported the presence of a lava dome in the active crater, with intense fumarolic activity. A second ascent in April 1989 confirmed this, and the dome was estimated to have a diameter of 200 m and a height of 50 m. It was slightly concave and dark coloured. In October 1989, geologists from the Universidad del Norte (Sergio Espinoza and Luis Baeza) and from the
Universidad de Salta in Argentina (Jose Viramonte) ascended the volcano and monitored the seismic activity. During this period intense fumarolic activity was observed, and the dome was found to have collapsed. It seems likely that the thermal anomaly observed on Landsat TM images is due to these hot fumaroles. On 20^^ February 1990, at 3.45 pm, a column of grey ash was ejected to an altitude of 7-8 km above the volcano (Gardeweg et al,
1990; Gardeweg, 1991b). Three pulses were distinguished on photographs
60 taken by J.R. Gerneck. The first was whitish in colour and composed mainly of water vapour. The second two were dark grey and composed of water vapour and ash. Subsequently, the column was displaced towards the south. No seismic activity was recorded during the eruption, although a loud noise accompanied it, and was associated with the rattling of windows in
Toconao. Fieldwork carried out in March and April 1990 revealed the presence of large blocks of andésite, up to 1 m across, lying in impact craters up to 4 km from the crater. The dome was found to have been almost completely destroyed, with a strongly concave shape and vigorous fumarolic activity(Figure 1.5). It has been calculated that between 10 and 30% of the dome was ejected as bombs. A strong smell of SOg and HCI was detected on the crater rim. Using a infrared radiometer, the fumaroles were found to have temperatures of 700 to 900 °C.
On April a small eruption was observed from Talabre
(Figure 1.6). A pale greyish column of water vapour with minor ash rose to a height of 1 km above the volcano, and was dispersed to the SE during the next 20 minutes. No sound or seismic activity was detected. At 1620-1625 on 21 October 1991 an explosion and a 2000 metre high column were observed from Toconao (Bull. Glob. Vole. Netw. V.16 no. 10, 1991). The explosion was not audible, but was accompanied by a small shock felt at
Toconao. A second, smaller eruption column was observed the following day with no accompanying explosion. Another eruption occurred in January
1992 (Bull. Glob. Vole. Netw. V.17 no.3, 1992). During fieldwork carried out between 26 February and 8 March 1992, a new lava dome was observed in the crater (Gardeweg et al, 1992). This was associated with normal intense
61 FIGURE 11.13: Photograph of the collapsed 1990 lava dome with high velocity gas jets. April 1990.
FIGURE 11.14: A small eruption which occurred on 6 April 1990 ejecting gas and ash to 1,000 metres above the crater rim.
62 fumarolic activity. Aerial photographs obtained by the military on 20 March
1992 revealed that the dome had continued to grow, and covered the base of the crater (Gardeweg, 1992). The volcano was observed between 27
January and 6 February 1993 and again during fieldwork between 22
February and 14 March (Gardeweg et al, 1993a). The dome was found to have collapsed, probably during a series of small eruptions on 21 May, 6
June, 16 July, 4 August and 19 September 1992. The intense fumarolic activity continued with slight emission of non-juvenile ash. Concentric ring fractures occurred around and within the collapsed lava dome and small pit craters were visible on the crater floor.
The largest historical eruption of Lascar began late on 18 April
1993, and lasted until 20 april (Gardeweg et al, 1993b, Bull. Glob. Vole.
Netw. V.18 no.4, 1993). Strombolian explosions ejected spatter over the crater rim, and a Plinian column ejected ash to between 15 and 22 km above the volcano. Pyroclastic flows descended the NE and SE flanks, the longest travelling 7.5 km to Tumbre, covering the swampy ground there.
Eruption products were white and vesicular pumices and dense scoriae.
Bombs were ejected up to 4 km frorn the crater. Ashfall occurred over a large area, reaching Buenos Aires, 1500 km to the SE. The total volume of erupted material was estimated as 0.1 km^. A new lava dome was found occupying the base of the crater during an overflight by the Chilean Air
Force. A preliminary estimate of the volume of the dome was 4.6 X 10® m®.
The new dome was found to have partially collapsed during an ascent of the volcano by Peter Francis on 19 May (Gardeweg & Medina, 1993).
Observations made during May 1993 included seismic monitoring and
63 FIGURE 11.15 Photograph of the collapsed dome In March 1993 with concentric ring fractures and high velocity gas jets.
64 analysis of the water at Tumbre, which had suffered serious contamination due to emplacement of pyroclastic flows onto the springs.
65 CHAPTER III. PETROGRAPHY OF LAVAS AND PUMICES.
111.1 G ENERAL.
The minerals and glasses of the eruptive products of Lascar and some surrounding centres have been analysed primarily by electron microprobe (Appendix), although other techniques such as ion microprobe and infrared spectroscopy have also been tried. The mineral formulae have been calculated from the electron probe analyses and percentages of end members calculated where relevant. From these results, magma chamber conditions and processes have been estimated (Chapter IV) using various geothermometers, geobarometers, oxygen barometers and water barometers. The importance of magma mixing, wall rock assimilation and fractional crystallisation has been recognised from compositional zoning patterns in minerals and disequilibrium mineral assemblages. A model is developed which will then be tested quantitatively using whole-rock analyses to model magma mixing and fractional crystallisation (Chapter 4).
Changing melt compositions are documented using glass inclusions in various minerals.
111.2 PETROGRAPHY OF ROCKS FROM LASCAR
VOLCANO.
The eruptive products of Lascar Volcano are andésites and dacites, which are erupted in the form of lava or pumice. They primarily
66 contain orthopyroxene and augite as the dominant ferronagnesian minerals, although in some crystal-rich pumices of the Soncor Flow, amphibole becomes dominant. The most abundant mineral phase in all of the rocks is plagioclase. Other phases which are always present are magnetite, ilmenite and apatite. Minor quartz, olivine and iron or copper-iron sulphides are common. Anhydrite is present in rocks of the Soncor and Piedras Grandes
Flows. The melt phase is preserved either as a fine-grained dacitic groundmass in the andésite lavas, as a brownish dacitic glass in andesitic pumices, or as a brownish to colourless rhyolitic glass in the dacitic lavas and pumices. The groundmass of the lavas contains the same types of minerals as occur in the phenocryst assemblage. An interstitial brownish rhyolitic glass is usually present. Iron-rich melt globules occur in some samples from the Soncor Flow and are believed to have formed by liquid immiscibility. The individual mineral phases are dealt with below.
III.2.1 Plagioclase.
Plagioclase is the dominant phenocryst phase in all of the eruptive products and the dominant groundmass phase in andesitic lavas.
Phenocrysts commonly show complex oscillatory zoning, often with a more homogeneous core. The boundary between the core and rim is usually rounded and other resorption horizons may occur in the rim region. Some cores contain an interconnected network of brown rhyolitic glass inclusions
(Figure III.1). Other bands of glass and/or opaque oxide inclusions are often seen in the oscillatory zoned rim. In some andesitic pumices the large cores are seive-textured, but the rims are not. Euhedrai, rounded and resorbed and broken crystals are usually present together in any one sample.
Plagioclase compositions range from 32 to 80% anorthite. The
67 FIGURE III.1: 1 mm Plagioclase phenocryst (crossed polars) with rounded core rich In devitrlfied glass Inclusions and oscillatory compositional zoning towards the rim.
FIGURE III.2: 1.5 mm Augite phenocryst (crossed polars) with a rounded, inclusion-rich core and a euhedrai overgrowth.
68 compositional zonation is not usually coherent, with normal (decreasing anorthite) and reverse zoning occurring in the same sample. This, the oscillatory zoning, and the occurrence of resorption horizons is thought to be due to a number of factors, including changing melt compostions, temperatures and water contents within the magma chamber and possibly also to the mixing of multiple populations.
III.2.2 Orthopyroxene.
This is the most abundant ferromagnesian mineral, except in some of the hornblende-rich pumices of the Soncor Flow. Orthopyroxenes are anhedral to euhedrai and usually exhibit pale green to pale brown pleochroism in thin section. Both normal (decreasing Mg#) and reverse zoning occur in all samples. Some orthopyroxenes are unzoned. Core to rim zonation in phenocrysts can sometimes be seen to converge on microphenocryst compositions in pumices of the Soncor Flow. Compositions range from hypersthene to bronzite. The compositional zonation is thought to be due in most cases to mixing of different populations, followed by fractional crytallisation. A more complete story is provided by analyses of orthopyroxenes of the Soncor Flow, which show subtle variations in composition between different pumices, with magma mixing effects superimposed. This may indicate a zonation of the magma chamber, with changes in either magma composition or oxygen fugacity. An increase in oxygen fugacity would result in a higher melt Fe^+/Fe^+ ratio, increasing the
Mg# of ferromagnesian minerals.
69 111.2.3 Clinopyroxene.
Clinopyroxenes have the same types of morphologies as orthopyroxenes, although they are generally less euhedrai. Zoning patterns appear to be chaotic, although the statistics are poorer due to the lesser abundance of clinopyroxene. Figure III.2 shows an augite phenocryst with a rounded, inclusion-rich core and a thick euhedrai overgrowth. All clinopyroxenes in lavas and pumices are augites. The compositional variations are thought to be due to mixing of multiple populations, fractional crystallisation and possibly changes in melt oxygen fugacity. A mafic inclusion from the Soncor Flow (LA-124) contains sub-calcic augite and Mg- pigeonite in addition to augite, indicating a relatively high crystallisation temperature. An olivine phenocryst from this sample contains inclusions of fassaitic (Fe^+AI) augite. Large (up to 1 cm) euhedrai augite phenocrysts have been found in andesitic bombs of the Saltar Flow on the south side of
Lascar. These contain inclusions of orthopyroxene, olivine, Cr-spinel and magnetite.
111.2.4 Olivine.
Olivine is present in small amounts in most andesitic lavas and in some andesitic and dacitic lavas and pumices. Olivine phenocrysts are usually, but not always, corroded with a corona texture. Two types of overgrowths are seen. Either a simple overgrowth of orthopyroxene occurs, or a symplectite of orthopyroxene and magnetite, commonly with a later orthopyroxene overgrowth (Figure III.3). This symplectite can be seen replacing the olivine and in many cases the olivine has completely disappeared, leaving a symplectite core with large radiating orthopyroxene
70 SPINEL (White) and ORTHOPYROXENE (Grey)
OLIVINE
10 microns
FIGURE III.3: Orthopyroxene-magnetite symplectite replacing olivine phenocryst. Backscattered electron image.
71 12
10 OLIVINE COMPOSITIONS.]
8
I «
.72 .74 .78 .8 .82 Mg/Mg+Fe+Mn
Soncor Mafic Inclusion (LA-124).
Capricorn Mafic Inclusions (LA-141, LA-143).
Soncor and Piedras Grandes Dacite (LA-147, SM93/22).
Centre I Andésites (LA-18, LA-19, SM93/43).
Centre III Bombs (LA-2, LA-6).
FIGURE 111,4: Histograms of olivine compositions from Lascar Rocks. crystals surrounding it. Olivines are common in mafic inclusions of the
Soncor Flow and the Capricorn Lava. Cr-spinel inclusions are common in
all olivine phenocrysts. Olivine compositions vary from 74 to 87% forsterite
(Mg/Mg+Fe+Mn), but most analyses have forsterite contents of 78 to 87%.
The Soncor mafic inclusion LA-124 contains olivine phenocrysts which span this compositional range (Figure III.4), but mafic inclusions from the
Capricorn lava contain olivines with a more restricted compositional range
(83 to 87% forsterite). This indicates that the olivines which are found in
other rocks such as andesitic lavas and more evolved pumices have most
likely been derived from the mafic magma represented by the mafic
inclusions.
III.2.5 Amphibole.
Amphiboles are recalculated by the method of Leake (1978).
According to his system of nomenclature, Lascar amphiboles are dominantly
magnesio-hastingsite and magnesio-hastingsitic hornblende, with lesser
tschermakite and tschermakitic hornblende and minor magnesio-
hornblende and edenitic hornblende. Amphiboles are henceforth referred to
as “hornblende”. In andesitic lavas and some andesitic pumices,
hornblendes show various stages of oxidation and dehydration. Virtually
euhedral crystals have an opacitic overgrowth and a patchy red-brown
staining (Figure III.5). This is probably due to oxidation of the original crystal
following eruption, causing a loss of hydrogen from the hydroxyl groups and
conversion of ferrous iron to the ferric state (Deer et al, 1966). The “basaltic
hornblendes” which result are not homogeneous, but a patchy
compositional variation is common. Many hornblendes are partially or
wholly replaced by a fine-grained aggregate of pyroxenes, plagioclase and
73 1 mm
FIGURE III.5: Resorbed and reddened hornblende phenocrysts with reaction rims of iron oxides. These textures are the result of post- eruptive oxidation.
10 microns
FIGURE III.6: Backscattered electron image of apatite inclusions in magnetite phenocryst.
74 Fe-Ti oxides. The original crystal shape is retained, sometimes with the
opacité rim preserved. This dehydration breakdown of amphibole is thought to be due to instability on decompression as the magma rises to the surface.
Hornblende is generally a minor phase in Lascar rocks, but it occasionally
occurs as the main ferromagnesian mineral in crystal-rich pumices of the
Soncor Flow, along with lesser biotite.
111.2.6 Biotite.
Biotite is not present in most Lascar rocks, although common in
surrounding older centres. It has been found along with hornblende and
minor pyroxenes in crystal-rich pumices of the Soncor Flow, in some facies
of the Piedras Grandes Flow and in a 1993 andesitic bomb.
111.2.7 Fe-Ti Oxides.
Titanomagnetite and ilmenite occur in all Lascar rocks. They
are generally present as microphenocrysts, groundmass phases and
inclusions in other minerals. In slowly cooled lavas and indurated parts of
the Soncor Flow, exsolution lamellae are well-developed, with lamellae of
magnetite in ilmenite hosts, and vice versa. In these rocks, individual grains
apparently lacking lamellae have wide ranging compositions, indicating
growth under a variety of temperatures and oxidation states. In other rocks,
such as rapidly-cooled pumices and dacitic lavas, the oxides preserve their
original compositions which are equilibrated under magmatic conditions and
the compositional variations are much narrower. These are suitable for
geothermometry. The compositional zoning which is seen in the plagioclase
and pyroxene phenocrysts is not present in oxide minerals. This is because
oxides can re-equilibrate with changing melt compositions much more
75 rapidly.
Cr-spinel inclusions occur in olivines and in some clinopyroxenes but Cr-spinel is not a phenocryst phase in Lascar rocks
except the Soncor mafic inclusion LA-124. This supports the observation that the olivines are not generally in equilibrium with the magmas in which they occur. The spinel inclusions have been protected from reaction with the
relatively oxidising, silica-rich melts by their host olivines.
III.2.8 Apatite.
Apatite is present in all Lascar rocks (except the Soncor mafic
inclusion LA-124), as small inclusions in other phenocrysts, notably the Fe-
Ti oxides (Figure III.6). It also occurs as subhedral microphenocrysts in many
lavas and pumices. Very long acicular crystals have been found in the glassy mafic inclusions of the Capricorn Lava, indicating rapid quenching of the andesitic magma.
III.3 PETROGRAPHY OF INDIVIDUAL FLOWS.
III.3.1 Centre I Andésite Magmas.
This section deals with the petrography of the early andésite
lavas and the later Saltar andésite bomb flow of Centre I. The lavas, all of which are rather similar, are 2-pyroxene andésites with plagioclase the most abundant phenocryst phase. Olivine is common, usually surrounded by orthopyroxene or replaced by a symplectite of magnetite and orthopyroxene.
Magnetite and ilmenite occur in large quantities and apatite is found as inclusions in other minerals, notably the opaque oxides. Inclusions of
76 pyrrhotite have also been recognised in oxide minerals. Rare resorbed
quartz occurs. Post-eruptive cooling has led to the formation of exsolution
lamellae of ilmenite in magnetite and vice versa. The matrix is a fine-grained
groundmass containing plagioclase, pyroxenes, Fe-Ti oxides and an
interstitial rhyolite glass. The Saltar Flow has a similar mineralogy although
plagioclase phenocrysts commonly have sieve-textured cores, quartz is
absent and the matrix is a brown dacitic glass. Large augite phenocrysts (1
cm) collected from an outcrop on the southern flank of Lascar contain
inclusions of orthopyroxene and olivine with Cr-spinel in the core and
orthopyroxene with magnetite in the rim.
111.3.1.1 Orthopyroxene.
Orthopyroxene in theCentre I andésite lava LA-18 and in a
sample from the Saltar Flow (LAS25) has a restricted compositional range
(Mg# = 0.69-0.77) but another lava sample LA-19 contains a few more
magnesian compositions (Mg# = 0.78-0.85) in addition (Figure 111.23).
Orthopyroxenes included in a large augite megacryst from the Saltar Flow
(SM93/43) are also more magnesian (Mg# = 0.76-0.85).
111.3.1.2 Cllnopyroxene.
Augite compositions are plotted in Figure 111.24. Augites from the lava LA-19 are all normally zoned (decreasing Mg#), cores having Mg#
= 0.78-0.80 and rims having Mg# = 0.73-0.79. This pattern is consistent with
fractional crystallisation deriving the andésite from a more mafic magma.
FIGURE III.7: Histograms of Mg# of orthopyroxenes from the Centre I andesitic lavas. Dark stipple = cores; pale stipple = rims; unstippled = undifferentiated.
77 LA-18 SM93/43 Centre t Andésite Lava. Large Augite Phenocryst. Saltar Flow.
0 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ LA-19 Centre I Andésite Lava. c O3 O
64 .66 .68 .7 .72 ,74 .76 .78 .8 .82 .84 .86 3 LAS25 Saltar Flow.
2
1
0 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ The sample LAS25 from the Saltar Flow contains more magnesian augites with a convergent zoning pattern two groups of core compositions can be discerned with Mg# = 0.74-0.80 and 0.85-0.86. Rim compositions are
intermediate between these with Mg# = 0.80-0.84. The large phenocryst
SM93/43 shows a strong normal zonation, with Mg# = 0.82-0.87 in the core and 0.76-0.78 in the rim. The occurrence of olivines and Cr-spinel in the core and orthopyroxene and magnetite in the rim is consistent with this
pattern which is thought to indicate the early stages of evolution of a more
mafic magma. The convergent zonation of augites in LAS25, however, is
indicative of injection of this mafic magma into a more evolved andésite thus
mixing together two core populations, and subsequent growth of the rims in the mixed magma.
111.3.1.3 P lag ioclase.
Plagioclase compositions are plotted in Figure 111.25. The compositional range of plagioclase phenocrysts in the lavas LA-18 and LA-
19 is relatively restricted (An51-67) although LA-18 contains rare more calcic cores (An75). Plagioclase in LAS25, however, extends to more calcic compositions (An56-85), notably demonstrated by the seive-textured cores.
111.3.1.4 Conclusions.
The pétrographie data indicates a difference between the
Centre I lavas and the overlying Saltar Flow. The lavas exhibit disequilibrium textures which are consistent with injection of a more mafic
FIGURE III.8: Histograms of Mg# of clinopyroxenes from the Centre I andesitic lavas. Dark stipple = cores; pale stipple = rims; unstippled = undifferentiated.
79 LA-19 Centre I Andésite Lava
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9
Saltar Flow
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9 4 SM93/43 Large Augite Phenocryst. Saltar Flow. 3 c 3 O 2 ü
1
0 64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9 Mg/Mg+Fe2+
g o magma into an andesitic magma chamber, resulting in reaction ccrcnae on
olivines and the occurrence of a few magnesian pyroxene cores and calcic
plagioclase cores. Fractionation has occurred since the postulated mixing
event, producing most of the observed mineral compositions. The groundmass has formed during post-eruptive crystallisation although compositions of groundmass phases are not significantly different from
phenocryst compositions. The Saltar Flow is less evolved than these
andésites, containing relatively magnesian pyroxenes and calcic
plagioclase. The augite and plagioclase zonation patterns indicate mixing of two populations of phenocrysts and subsequent fractional crystallisation.
The large augite phenocrysts represented by SM93/43 have originated in the injected mafic magma and have formed a late, relatively iron-rich
overgrowth in the mixed magma. A shorter time period has occurred
between the mixing event and eruption of the magma than in the lavas.
III.3.2 Chaile Flow.
The Chaile Flow is an andesitic cauliflower bomb flow. It is
relatively crystal-poor and contains phenocrysts of orthopyroxene, augite,
plagioclase and magnetite with minor apatite. No olivine has been found in the sample examined. The matrix is a rhyolitic glass. None of the usual
disequilibrium textures have been recognised. Mineral compositions are
discussed below.
FIGURE III.9: Histogram of anorthlte contents of plagioclase from the Centre
I andesitic iavas. Dark stippie = cores; paie stipple = rims; unstippled = undifferentiated.
81 LA-18 Centre I Andésite Lava.
c 3 O O
.25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9
LA-19 Centre I Andésite Lava.
c 3 O ü
.25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9
LAS25 Saltar Flow.
c 3 O O
Anorthlte
gz 111.3.2.1 Orthopyroxene.
Orthopyroxenes have an unusually restricted Mg-poor compositional range for an andésite, with Mg# ranging from 0.64-0.68
(Figure 111.26). One core is slightly more magnesian (Mg# = 0.74). This pattern is comsistent with derivation of the magma by fractional crystallisation of a more mafic magma.
111.3.2.2 Cllnopyroxene.
Augite is relatively scarce in this rock and only nine crystals have been found in one thin section. Augite compositions are, as usual, more variable than orthopyroxene compositions (Mg# = 0.70-0.80; Figure
111.27). Most phenocrysts are normally zoned. Again the pattern is consistent with fractional crystallisation.
111.3.2.3 Plagioclase.
Plagioclase has the usual wide compositional range (An38-
83). Cores have a more restricted compositional range (An64-69) indicating that the compositional zonation may be divergent (Figure 111.28). This might
indicate that conditions such as temperature, melt composition and water content in the magma oscillated for some reason, perhaps due to transport to shallower levels or mixing with another magma. However, the pyroxene zoning patterns do not indicate that magma mixing has occurred.
111.3.2.4 Conclusions.
The mechanism by which this unusual flow was generated is
unknown, but explosive collapse of a lava lake has been tentatively
83 Orthopyroxene. Mg#.
FIGURE 111.10: Histograms of Mg# of orttiopyroxenes from the Chaile Flow.
Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated.
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84
Cllnopyroxene. Mg#. O) ■ f o
FIGURE 111.11: Histograms of Mg# of clinopyroxenes from the Chaile Flow.
Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated.
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9
Plagioclase. Mole Fraction Anorthlte
o
FIGURE 111.12: Histogram of anorthlte contents of plagioclase from the Chaile .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 Flow. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. suggested on the evidence of field studies (R.S.J. Sparks, pens. comm.). This might occur if volatile pressure built up beneath a partly solidified lava lake, eventually leading to an explosion which launched large amounts of
relatively volatile-poor magma over the crater rim. The rhyolitic matrix glass indicates that a large degree of fractionation has occurred in this magma, and the relatively crystal-poor texture could be interpreted as due to settling of phenocrysts in a lava lake. This would also explain the relatively Mg-poor compositions of orthopyroxenes, which would have equilibrated with a
rhyolitic liquid. It might also explain the complex zoning pattern of plagioclase, since the water content of the lava lake would have varied widely in order for the proposed eruption mechanism to have occurred. The
Chaile flow directly overlies the Soncor Flow and underlies the Capricorn
Lava, both of which are dacitic magmas with strongly-developed magma mixing textures. It is suggested that the Chaile Flow represents an andesitic
magma which bypassed the dacitic magma chamber which gave rise to these other two eruptions. This model is supported by the absence of apparent magma mixing textures in the Chaile Flow.
III.3.3 The Soncor Pyroclastic Flow and Piedras Grandes
Flow.
The Soncor deposit is significant since it was formed during the most violent eruption in the history of Lascar. The different types of pumice represent different magma types which existed in the magma chamber prior to the eruption, so that the variation in conditions can be estimated.
Examination of the holocrystalline mafic inclusion LA-124 yields information about the mafic magma which was introduced into a dacitic magma chamber prior to the eruption. Unfortunately no compositional stratification has been
85 recognised within this deposit. On the contrary, it appears that the different
magma types were erupted simultaneously, as a large variation in pumice types can be found at most locations. This may be partially due to the
erosion of earlier flow units by later flows, but the stratigraphy of the deposit
is not well enough known to distinguish between the two mechanisms.
Individual flow lobes do in places concentrate particular types of pumice,
such as the banded pumice at Tumbre. Because of this lack of stratigraphie
order, it is difficult to assign different magmas to different horizons within the
magma chamber prior to the eruption. The Piedras Grandes Flow is
considered to represent a precursor to the Soncor Flow, so it is included in this discussion.
The reason for the lack of vertical compositional stratification of the Soncor flow may be due to overturn of the magma chamber before commencement of the eruption. The obvious cause of this is injection of
hotter mafic magma into the dacite, causing convection to occur. The clearest evidence for the existence of a mafic precursor is the occurrence of
mafic inclusions and parti ally-reacted olivines in the Piedras Grandes Flow
and less commonly in the Soncor Flow. Pétrographie evidence from the dacitic and andesitic pumices is also presented here. In comparison with the
large (probably >1 Okm^) Soncor eruption, the Piedras Grandes Flow
represents a relatively small dome-building phase, probably through a
summit or cirque glacier of the old Centre I. This initial eruption is unlikely to
have resulted in a drawing-up effect within the magma chamber, since no
large pyroclastic flows have been found accompanying it. The magma which
86 was erupted has therefore probably come from the upper part of the magma chamber. However, the prismatic-jointed boulders of this flow show a pronounced compositional variation, with some rich in pyroxenes and others containing predominantly hornblende and biotite. Glassy hornblende-rich
mafic inclusions also occur in some parts. It appears that the mixing together of different parts of the magma chamber occurred well before the initiation of the eruption.
Green prismatic blocks of dacitic composition occur throughout the Soncor Flow. These are thought to represent a cryptodome which was destroyed by the main eruption. This subvolcanic intrusion may have formed at the same time as the Piedras Grandes Flow. It is holocrystalline and has the texture of a hypabyssal rock, with coarsely intergrown orthopyroxene, augite, plagioclase, quartz, titanomagnetite and ilmenite. The lack of
hornblende suggests either that it cooled slowly at shallow depth or that
hornblende was unstable due, for example, to degassing of water.
The hornblende-rich dacitic pumices also contain large phenocrysts of plagioclase. They may represent fragments of wall or roof cumulates which have been dislodged and mixed into the magma chamber.
The presence of biotite seems to indicate that the water content of the
magma was high, and such conditions are thought most likely to have developed towards the top of the magma chamber, as exsolved volatiles are carried upwards as bubbles in a separate gas phase. Water solubility is likely to be lower in the upper levels of the chamber since the pressure is lowest in that region.
87 A holocrystalline mafic inclusion (LA-124) was found in the lithic-rich lower part of the Soncor Flow at Talabre Viejo. It has a pillow-like morphology, with an inner dense region and an outer more vesicle-rich shell. It contains phenocrysts of olivine, cllnopyroxene and plagioclase.
Microphenocrysts of the same minerals plus orthopyroxene, magnetite and ilmenite also occur. Olivines contain inclusions of Cr-spinel and andesitic glass. Clinopyroxenes contain inclusions of plagioclase, magnetite, augite, orthopyroxene, pigeonite and dacitic glass. A fine-grained groundmass is present containing pyroxenes, plagioclase and oxides. Sub-calcic augite has been found along with augite and orthopyroxene in a thin overgrowth on an olivine phenocryst.
The large variation in pumice types is reflected in the compositions and zonation patterns of pyroxenes and plagioclase. Each sample collected from the Soncor Flow and the Piedras Grandes Flow has its own unique patterns. Orthopyroxene, augite, plagioclase and hornblende from this eruption are examined in turn below.
III.3.3.1 Orthopyroxenes.
Orthopyroxene compositions are plotted as histograms of Mg#
(Mg/Mg 4 -Fe^+) in Figure 111.7. There are a number of different types of patterns, but certain features can be discerned. Firstly, the mafic inclusion
LA-124 has a relatively narrow range of compositions towards the Mg-rich
FIGURE 111.13: Histograms of Mg# of ortfiopyroxenes in samples from the
Soncor and Piedras Grandes Flows (Following 3 pages). Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated.
88 Crystal Filch Morritilencie Ruimlc*
CJ
.74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 7 LA122 6 SM93/22 White Crystal-Poor Piedras Grandes Lava. Pumice. 5 c OQ 4 -D 8 3
2
1
0 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 64 .66 .68 .7 .1T2 .74 .76 .78 .8 .82 .84 .86
LAS47 SM93/44 Prismatlc-Jolnted Vltrophyre. Green Block.
3c o
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ ; Mg/Mg+Fe2+ ü
.64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86
LA-155 LA-124 Banded 2-Pyroxene Holocrystalline Pumice. Mafic Inclusion.
- 0 c i 0 o
n n 1 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+
LA-121 Cream 2-Pyroxene Pumice.
.64 .66 .68 .7 .72 .74 5 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ end of the spectrum. The orthopyroxenes from this rock, most of which are microphenocrysts, have Mg# between 0.73 and 0.83, with the majority between 0.78 and 0.82. The one phenocryst which has been analysed shows a strong normal zonation (decreasing Mg#). In contrast, most of the white crystal-poor pyroxene dacites, which are represented by the Soncor pumice LA-122 and the Piedras Grandes lava SM93/10, have relatively Mg- poor orthopyroxenes with narrow compositional ranges. These orthopyroxenes have Mg# between 0.65 and 0.69. The dacitic prismatic jointed blocks, represented by LA-147, have a similar compositional range but slightly shifted towards higher Mg# (0.66-0.72). Both normal and reversed core to rim compositional zonation is observed in these rocks, although the change in Mg# is relatively small.
Other rocks have more complex distributions. Pyroxene dacite pumices which show evidence for magma mixing, such as the cream
coloured pumice LA - 1 2 1 and the banded pumice LA-155, contain a larger compositional range of orthopyroxenes. In addition to the relatively Mg-poor group mentioned above, a second major peak in Mg# is seen between 0.71 and 0.76. This second compositional group is also well represented in the hornblende-bearing crystal-rich pumices LAS29-2 and LAS36-1 and the vitrophyre SM93/44. More Mg-rich orthopyroxenes with Mg# between 0.75 and 0.85 also occur in small numbers in many of these rocks, and these are interpreted as having originated in a mafic magma similar to that represented by the mafic inclusion LA-124.
The orthopyroxenes with intermediate Mg# (0.71-0.76) may have formed in a hybrid magma following mixing between a resident dacite
91 and an injected basalt or basaltic andésite magma. The more Mg-poor orthopyroxenes (Mg# = 0.65-0.72) are thought to have originated in the dacitic magma chamber prior to the mixing event. Slight variations in the compositional range of this group may represent a compositional variation in the original magma. If this model is correct it explains the lack of compositional stratification of the Soncor Flow, since magma mixing, overturn of the magma chamber and subsequent fractional crystallisation would have occurred prior to the eruption.
III.3.3.2 Clinopyroxenes.
Clinopyroxene compositions are generally more variable than
orthopyroxene and more difficult to interpret (Figure III. 8 ). This is partly because augite is less common in the dacitic rocks than orthopyroxene and the compositional groups are less well defined, but also because the compositional zonation patterns are less organised. However, certain features can be recognised. The mafic inclusion LA-124 contains large clinopyroxe phenocrysts and many microphenocrysts. The compositional range is high (Mg# = 0.75-0.92) but the majority of analyses fall in a tighter group (Mg# = 0.79-0.88). The dacites contain augites with relatively Mg-poor compositions and several samples contain a major group with Mg# = 0.72-
0.83. The mixed pumice LA - 1 2 1 also contains two clinopyroxenes with even lower Mg# (0.63 to 0.67). Core to rim zonation patterns are apparently random with both normal (decreasing Mg#) and reverse zoning occurring within individual samples. It appears that the postulated mixing event has
FIGURE 111.14: Histograms of Mg# of clinopyroxenes in samples from the
Soncor and Piedras Grandes Flows. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated.
92 SM93/22 LA-122 Piedras Grandes. White Pyroxene- Pumice.
1
0 .6 .65 .7 .75 .85 .95 ,6 .65 7 .75 ,8 .85 .9 .95
SM93/44 LAS47 Vitrophyre. Prismatic Jointed Green Block. S)
.6 .65 .7 .75 .85 .9 .95 .6 .65 .7 .75 .85 .9 .95
LAS29-2 LAS36-1 Crystal-Rich Crystal-Rich Hornblende Pumice. Hornblende Pumice.
.6 .65 .7 .75 .8 .85 .9 .95 .6 .65 .7 .75 .8 .85 .9 .95 Mg/Mg+Fe2+ Mg/Mg+Fe2+ LA-155 Banded Pyroxene Pumice.
.6 .65 .7 .75 .85 .9 .95
LA-121 Cream Pyroxene Pumice.
-D f
,6 .65 7 .75 .8 .85 ,9 .95
6 LA-124 5 Holocrystalline Mafic Inclusion. 4
3
2
1
0 .6 .65 .7 .75 .8 .85 .9 .95 Mg/Mg+Fe2+ not significantly affected augite ccmpcsiticns in the dacite, which were already rather variable (as seen in the pumice LA-122). The main effect has been tc introduce a small number of more magnesian augites into the dacitic magmas, as can be seen with the orthcpyrcxenes. Two groups of core compositions are thought to be present in the crystal-rich pumice LAS29-2 although the small number of analyses makes this interpretation tentative.
The first group (Mg# = 0.78-0.80), which is believed to have originated in the dacitic magma, coincides with the rim compositions (Mg# = 0.78-0.83) and the second group is more magnesian (Mg# = 0.86) and is thought to have originated in the injected mafic magma.
Two small (20|i) grains of fassaitic (Fe^+AI) augite have been found included in an olivine phenocryst in the mafic inclusion LA-124. The paragenesis of fassaite in this andésite is uncertain, but may indicate crystallisation under elevated pressure, since the subsitution of AI into clinopyroxene is strongly pressure-dependent (Grove et al, 1989).
III.3.3.3 Amphiboles.
There is a fairly large variation in amphibole compositions within the Soncor and Piedras Grandes Flows. Mg, for example, varies from
2.7 to 3.4 formula units, and Fe^+ from <0.1 to 1.1 formula units. This variation is thought to be the result of a number of processes including magma mixing, re-equilibration of phenocrysts during transport to the surface and post-eruptive oxidation. One notable feature is the difference in
FIGURE 111.15: Plots of various elements against Mg (formula units) for amphiboles from the Soncor and Piedras Grandes Flows.
95 0.4 • • □ Crystal Poor A Crystal-Rich 0.3 - 0.8 A □ . O Vltrophyre • Piedras Grandes 0 6 -
0.2 - 0.4 -
0.2 -
0.0 2.6 2.8 3.0 3.2 3.4 3.6 2.6 2.8 3.0 3.2 3 4 3.6 Mg Mg
G\ 2.2
2.0 - 1.0
1.8 -
0.5
1.6 -
0.0
2.6 2.8 3.0 3.2 3.4 3.6 2.6 2.8 3.23.0 3.4 3.6 Mg Mg
0.03 1.85
1.80
0.02 -
1.75 o Ü
0.01 1.70
/ 1 t J ^l— LJ 0.00 -Ù-WMÏM 1.65 2.6 2.8 3.0 3.4 3.6 2.6 2.8 3.0 3.2 3.4 3.6 3.2 i\ig ivig << o s g e N 1 0 4 7 - morphologies between the euhedral amphiboles of the crystal-rich pumices and the subhedral to anhedral phenocrysts in the crystal-poor pumices, the vitrophyres and the Piedras Grandes lavas. The hornblendes which occur in the crystal-rich pumices are clearly close to equilibrium with the melt phase, unlike those in the other Soncor and Piedras Grandes magmas. Discrete compositional groups of amphiboles as defined by the type of host rock can be discerned when elements are plotted as formula units against Mg (Figure III.9). The following variations are clear; 1 ) Amphiboles from the Soncor vitrophyre fall within the compositional range of those from the crystal-poor pumices. This is consistent with the view that the vitrophyres were formed during the same eruption as the pumices. 2) Amphiboles from the crystal-rich pumices form a continuous compositional trend with those from the crystal-poor pumices, but are slightly shifted towards higher Al, FeZ+ Ca and K and lower Mg and Fe^+. 3) Amphiboles from the Piedras Grandes Flow form a group of compositions which are distinct from those of the Soncor Flow, having higher Mg, Al, Fe^+ and Na and lower Ti, Fe^+, Ca and K. The reasons for these variations are difficult to define. The FIGURE 111.16: Histogram of anorthite contents of plagioclase from the Soncor and Piedras Grandes Flows. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. 98 SM93/44 SM93,1Ü 4 Vitrophyre. Piedras Grandes Lava. 3 2 1 0 25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 LA-155 LA-122 Banded Pyroxene White Pyroxene _ 0 Pumice. Pumice. .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 LA-121 SM93/22 Cream Pyroxene Piedras Grandes Lava. Pumice. .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 Anorthite Anorthite LAS30 Dark Andesitic Pumice. 0 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 LA-124 Holocrystalline Mafic Inclusion. o o .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 Anorthite lavas of the Piedras Grandes Flow were presumably extruded at a slower rate than the pumices of the Soncor Flow, possibly allowing the amphiboles to re-equilibrate at shallower levels. Post-eruptive oxidation of the Piedras Grandes amphiboles might have occurred within the postulated lava dome, thus explaining their higher Fe^+ and lower Fe 2 +. III.3.3.4 Plagioclase. Plagioclase compositions reflect to a certain degree the effects of magma mixing (Figure 111.10). The most anorthite-rich plagioclase (An60- 85) originates in the mafic magma as demonstrated by the compositions of grains in the mafic inclusion LA-124. The dark andesitic pumice LASSO and the vitrophyre SM93/44 also contain plagioclase which falls into this group (An55-80). The dacitic endmember as represented by the crystal-poor pumice LAI 2 2 and the Piedras Grandes lava SM93/10 contains more albitic plagioclase (An35-55). A small number of analyses from these samples also fall into the more anorthite-rich group and may also have originated in the mafic endmember. Other samples contain a wide spread of plagioclase compositions varying from AnSO to An80. Distinct peaks at An40-50 and An60-70 can be seen in the mixed pumices LAI 21 and LAI 55, but the Piedras Grandes lava SM93/22 shows no distinct peaks. Intermediate compositions (An50-60) are thought to represent post-mixing growth of plagioclase. The wide variation in compositions may be due to a combination of factors including mixing of populations from different magmas followed by further fractional crystallisation as well as variations in temperature and water content of the magmas during overturn of the magma chamber and transport to the surface. 101 111.3.3.5 Spinels. Spinel compositions from the mafic inclusion LA-124 are plotted on projections of a modified Johnson spinel prism in Figure 111.11 (Haggerty, 1976). “Cr-spinel” inclusions in olivines and augites and “magnetite” phenocrysts and groundmass grains form a continous trend of increasing Fe^+/Fe^++IVIg, increasing Fe3+/%R3+ and constant Cr/Cr+AI. This compositional evolution is typical of calc-alkaline basaltic spinel trends (Haggerty, 1976). The magnetite phenocrysts of the dacitic pumice LA-122 represent a change in this trend, continuing the increase in Fe^+ZFe^^-i-Mg and Fe^+/ZR^+ but having very low Cr/Cr+AI (<0.1 ). Magnetite from other Lascar dacitic rocks is compositionally very similar to this. The continuous calc-alkaline basaltic spinel trend of the mafic inclusion LA-124 supports the evidence for gradual uninterrupted fractionation provided by the glass inclusion analyses (below) and the mineral crystallisation sequence. The magnetites of this rock are dissimilar to those of the Soncor pumices indicating that crystallisation had ceased before the mafic magma was subjected to the lower temperatures of the dacitic magma chamber. 111.3.3.6 Anhydrite. Anhydrite (pure CaSO^) has been found as inclusions in magnetite, ilmenite and orthopyroxene phenocrysts in dacitic and banded FIGURE 111.17: “Modified Johnson Spinel Prism” plots (Haggerty, 1976) of spinel compositions from LA-124 and LA-122. 102 LA124 Spinels LAI22 Spinels 0 .8 - 0 .8 - o u + + < 0 .6 - < 0 .6 - + A Incl. in Olivine + O Titanomagnetite □ inci. in Augite 0) 0) u. 0.4- O Phenocr., G mass. u. 0 .4 - 0) u. 0 .2 - LL 0 .2 - 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Fell/FelUMg Fell/Fell-I-Mg 0 .8 - 0 .8 - < 0 .6 - < 0 .6 - + + o Ü 0.4 - 0 .4 - o O 0 .2 - 0.2 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Fell/Fell + Mg Fell/Fell + Mg pumices of the Soncor Flow and dacitic lavas of the Piedras Grandes Flow. Anhydrite inclusions have also been found in large hornblende phenocrysts of a crystal-rich pumice sample. Any anhydrite phenocrysts which might have been present are likely to have been leached from the rock in the estimated 2 0 kyrs since the eruption, despite the extreme aridity of the region. Anhydrite is an unusual phenocryst phase in calc-alkaline magmas, but has been found in four recent eruptions of evolved calc-alkaline magmas. These are El Chichon, Mexico in 1982 (Rose et al, 1984), Nevado del Ruiz, Colombia in 1985 (Fournelle, 1990), Pinatubo, Phillipines in 1991 (Bernard et al, 1991 ; Fournelle, 1991 ) and Lascar in 1993 (below). In the first three examples and possibly the fourth the anhydrite was associated with unusually high emissions of SO 2 . Since the melt phase in the Soncor dacitic magmas contained very little sulphur (undetectable by electron microprobe) it is likely that the anhydrite formed in equilibrium with a sulphur-rich hydrous volatile phase. The source of the sulphur in the case of Lascar is probably the injected mafic magma, since the andesitic glass inclusions in olivines in the mafic inclusion LA-124 contain up to 0.5 wt% SO 2 . A similar model has been proposed for Pinatubo (Matthews et al, 1992). This is discussed further in a Chapter VII. III.3.3.7 Glass compositions. Glass matrices and inclusions in minerals which are thought to represent melt compositions have been analysed in dacitic and andesitic pumices and in the mafic inclusion LA-124. Glasses in dacitic pumices are FIGURE 111.18; Plots of various oxides against Si02 for glass analyses from tfie Soncor Flow. 104 2.5 2 2.0- À LA-124 # Andésite Pumice 1 O Dacite pumice 0.5 - 0.0 0 50 60 70 80 50 60 70 80 SI02 S102 3 22 - 0 20 - 2 18 - 16 - 14 - 12 - 0 50 60 70 80 6050 70 80 SI02 SI02 5 4 3 2 50 60 70 80 SI02 SIÜ2 6 0.4 5 0.3 - p o 4 0.2- 3 0.1 - 2 0.0 6050 70 60 70 8050 SI02 S'02 o (T\ 1.0 0.5 •^O 0.0 50 60 SI02 0.5 0.4 - a a 0 .3 - 0.2- 0.1 - 0.0 50 60 70 80 S I0 2 entirely rhyolitic (S 1O 2 = 71-76 wt%). No systematic variations between glass inclusions and matrix glasses have been found in any sample. There is no detectable compositional difference between the brown and colourless glasses in the banded pumice LA-155. The brown glass contains many tiny plagioclase microlites, which are so densely concentrated that it is difficult to analyse the glass between them. A crystal-rich andesitic pumice (LAS30) has a brown dacitic glass matrix (Si 0 2 = 69-70 wt%) and contains rounded globules of iron-rich glass (52 wt% FeO, 28-30 wt% SI 0 2 , 0 .6-0.7 wt% Ti 0 2 ) both in the matrix glass and as inclusions in large hornblende phenocrysts. These have suffered desilicification to varying degrees, which is first manifested as an iron-enriched rim around the globule and culminates in the formation of a Ti-poor iron oxide grain with a spongey texture. Iron-rich melt globules associated with anhydrite and sulphides have also been found associated with hornblende crystal clots in a dacitic block-and-ash flow of the nearby Volcan Aguas Calientes and are interpreted as having formed by liquid immiscibility in both cases (Matthews, in press) Most magnetites in LAS30 have fine exsolution lamellae of ilmenite suggesting that the magnetites have experienced a drop in temperature. Ilmenite phenocrysts do not occur. The mafic inclusion LA-124 has no glass matrix, but olivine and clinopyroxene phenocrysts contain a number of brown glass inclusions. The inclusions in the olivines are andesitic (Si 0 2 = 55-59 wt%) and those in the dinopyroxenes are dacitic (Si 0 2 = 62-68 wt%). Major and some minor element contents of Soncor glasses are plotted against Si 0 2 in Figure 111. 1 2 . It is clear from the plots of CaO and K 2 O 107 that those from the mafic inclusion form a separate trend to those in the pumices. This supports the suggestion that LA-124 represents a magma which underwent fractional crystallisation prior to mixing with the Soncor dacite. A number of other conclusions can be drawn from the observed trends. AI 2 O3 decreases steadily from 23 wt% in the andesitic glasses to 12 wt% in the rhyolitic glasses. MgO similarly decreases from 1.8 wt% to around 0.3 wt%. This is consistent with the fractionation of plagioclase and ferromagnesian minerals. CaO forms two separate decreasing trends in the LA-124 and pumice glasses due to fractionation of calcic plagioclase and clinopyroxene and K 2 O forms two separate increasing trends due to its incompatibility in both types of magmas. Na^C forms a flat trend in the LA- 124 glasses but decreases strongly in the pumice glasses. This is probably due to a difference in the composition of plagioclase in the two magmas. It has already been demonstrated (above) that the plagioclase in the mafic magma was relatively calcic and that which formed in dacitic and mixed magmas was relatively sodic. Ti 0 2 and FeO reach maxima of 2 wt% and 5.5 wt% respectively in the LA-124 glasses when Si 0 2 reaches approximately 64 wt%. This correlates with the appearance of Fe-Ti oxides in the dacitic glass inclusions. Strongly decreasing trends follow and continue in the pumice glasses. P 2 O5 increases from approximately 0 . 2 wt% to 1 . 2 wt% in the LA-124 glasses due to the instability of apatite in the mafic magma. The pumice glasses, in contrast, have relatively low (<0.5 wt%) P 2 O5 contents. Apatite is common in all Soncor Pumices. Cl has a flat trend with high variablity (0.1-0.3 wt%) in LA-124 glasses and is slightly lower (<0.1 to 0.2 wt%) in the pumice glasses. S decreases rapidly from 0.5 to <0.1 wt% in the LA-124 glasses and remains undetectable in the pumice glasses. This is thought to be due to the separation of a sulphur-rich gas phase from the 108 magma during fractional crystallisation. The separation of the melt phase compositional trends for the mafic magma and the andesitic to dacitic pumices is probably due to a combination of different parent magmas and different physical conditions such as temperature, pressure and oxygen fugacity during fractional crystallisation. This would result in differences in the phenocryst phases that would separate from a given melt. ill.3.3.8 Conclusions. From the combined evidence of large-scale textures such as mafic inclusions and mixed andesitic-dacitic pumice flows and pétrographie data on mineral compositional zonation and melt inclusion chemistry it is concluded that the Soncor Flow represents a dacitic magma chamber, probably zoned, into which was injected a basaltic andésite magma. Mingling and mixing of the two magmas was combined with convective overturn of the magma chamber and fractional crystallisation occurred subsequently. Olivine is rare in the dacitic pumices but small clots of orthopyroxene crystals surround areas of orthopyroxene-magnetite symplectite which is commonly seen replacing olivine in other rocks. There was clearly a time interval between the injection of the mafic magma and the eruption of the Piedras Grandes and Soncor Flows which was sufficiently long to allow the complete digestion of olivine phenocrysts by the dacitic magma as well as a significant amount of fractional crystallisation of orthopyroxene, augite and plagioclase. It is possible that the basaltic andésite ponded at the base of the magma chamber and fractionated separately, forming a magma represented by the mafic inclusion LA-124. 109 The hornblende-plagioclase bearing pumices are thought to represent cumulates which were detached and mixed into the dacite magmas by the overturn of the magma chamber. The presence of large amounts of hornblende and minor biotite in the cumulates compared with the paucity of these minerals in the other Soncor magmas may indicate that water levels at the margins of the magma chamber were elevated. These cumulates may have formed at the walls or roof of the chamber. III.3.4 The Capricorn Lava. This Centre II lava has many petrological features in common with the Soncor and Piedras Grandes magmas. The lava itself is a dacite containing orthopyroxene and augite as the dominant ferromagnesian phenocryst phase together with plagioclase and minor quartz. Hornblende and olivine occur as slightly resorbed phenocrysts. The matrix is a colourless rhyolitic glass. Strongly folded colour banding is caused by the alternation of pyroxene-rich and plagioclase-rich layers. This is partly due to an original inhomogeneity and partly due to post-eruptive flow. The lava contains large numbers of olivine-rich andesitic inclusions which are up to 1 0 cm in diameter and have rounded, irregular chilled margins. As well as olivine, pyroxenes and plagioclase occur as phenocrysts in these inclusions. The matrix is a brownish rhyolitic glass with an interlocking network of acicular hornblende and minor plagioclase. The hornblende forms overgrowths on olivine and clinopyroxene, but little resorption of these minerals is evident. Apatite also occurs as elongate acicular crystals in the FIGURE 111.19: Histograms of Mg# of ortfiopyroxenes from the Capricorn Lava. Dark stipple = cores; pale stipple = rims; unstippled = undifferentiated. 110 / 3 LA-140 LA-143 G Host Dacite Lava. Mafic Inclusion. 5 2 4 3 1 2 1 0 0 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg'Mg+Fe2+ LA-143HL Host Dacite Lava. o .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 LA-141 Mafic Inclusion .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ mafic inclusions. This texture is thought to be the result of injection of the andésite into the dacitic magma and rapid quenching to form a rhyolitic melt and acicular mineral phases. Small partially-disaggregated fragments of the mafic inclusions a few mm across occur in the host dacite. These consist most commonly of olivine or clinopyroxene phenocrysts with an overgrowth of acicular hornblende. Calc-silicate xenoliths up to 1 0 cm in diameter occur in large numbers in the Capricorn lava. These commonly contain augite, plagioclase, sphene and hemoilmenite. They have clearly disaggregated in many cases, since the hemoilmenite occurs together with magnetite and ilmenite in the lava. The petrology of calc-silicate xenoliths is described separately in a Chapter VI. The compositional variations of the major phenocryst and melt phases in the lava and mafic inclusions are described and discussed below. III.3.4.1 Orthopyroxene. Orthopyroxenes from two samples of the host lava (LA-140 and LA-143HL) and from two mafic inclusions (LA-141 and LA-143) have been analysed (Figure 111.13). All show the same bimodal distribution of Mg# with a major group between 0.68 and 0.74 and a minor group between 0.75 and 0.78. Two samples also contain a small number of more Mg-rich orthopyroxenes (Mg# = 0.79-0.85). Core compositions tend to cover the entire range of Mg#, whereas rim compositions fall into the two major FIGURE 111.20: Histograms of Mg# of cllnopyroxenes from tfie Capricorn Lava. Dark stipple = cores; pale stipple = rims; unstippled = undifferentiated. 112 LA-143HL Host Dacite Lava. .7 .75 . 8 .85 .95 LA-141 Mafic Inclusion. .7 .8 .85 LA-143 Mafic Inclusion. .7 .75 .8 .85 .9 .95 Mg/Mg+Fe2+ 13 groups. The most Mg-poor pyroxenes (the main group) are interpreted as having originated in the dacite prior to mixing. The few Mg-rich cores are thought to have originated in the injected andésite, and the minor group dominated by rim compositions (Mg# = 0.75-0.78) is believed to represent orthopyroxenes which formed after the mixing event. This means that cores from the mafic inclusions have been transferred to the host dacite and vice versa. III.3.4.2 Cllnopyroxenes. There are few pyroxenes in the host lava and they have a relatively small compositional range (Mg# = 0.78-0.89). The analyses from LA-143HL show predominantly normal zonation (decreasing Mg#; Figure 111.14). In contrast the mafic inclusions contain many clinopyroxenes with a wider compositional range. Those analysed from LA-143 have Mg# ranging from 0.73 to 0.84, but analyses from LA-141 have a much wider range (Mg# = 0.74-0.99). Most of the rim compositions lie within a much narrower compositional range (Mg# = 0.78-0.87) which is similar to the analyses from the host lavas. It is concluded that the wide range of clinopyroxene core compositions from this rock represent mixing of two populations, the most Mg-rich originating in the mafic magma and the relatively Mg-poor analyses originating in the dacitic magma. The relatively narrow range of rim compositions from LA-141 which coincides with the compositional range from the host lavas is thought to represent post-mixing crystallisation of clinopyroxene. This must have occurred after the quench crystallisation of FIGURE 111.21: Plots Of various elements against Mg (formula units) for amphiboles from the Capricorn Lava. 114 ( f 0.3 - % 0.8 - + / r?o 0.6 - 0.2 - • Host Lava 0.4 - O Mafic Incl. 0.1 H------1------1----1------1------1--1-----1------1------1--1------,------1------0.2 2 .9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 2 .9 3.0 3.1 3.2 3.3 3 .4 3.5 3.6 Mg Mg 2.2 1.0 oco 0.8 - 2.0 - 0.6 - n • s 1.9 - 0.4 - 1.7 0.2 2.9 3.0 3.1 3.2 3 .3 3.4 3.5 3.6 2 .9 3 .0 3.1 3.2 3 .3 3 .4 3.5 3.6 Mg Mg 0.06 1.90 0.05 1.85 - 0.04 1.80 - <0 0.03 Ü U 1.75 - 0.02 1.70 - 0.01 r:*/ 0.00 1.65 2 .9 3.0 3.1 3.2 3 .3 3 .4 3 .5 3 .6 1 3 .2 3.3 3.5 Mg Mg 0.80 0.75 - 0.70 - 0.65 - 0.60 - 0.55 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Mg 0.11 0.10 o\ 0.09 • • 0.08 0.07 0.06 0.05 3.3 Mg 0.025 0.020 0.015 o 0.010 • • 0.005 0.000 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Mg amphibole in the mafic inclusions and again there is evidence that phenocrysts were transferred across the inclusion boundaries. III.3.4.3 Amphibole. Amphibole is common in the mafic inclusions but rarer in the host dacite. The larger more stubby crystals from the dacite are chemically distinct from the acicular quench amphiboles in the mafic inclusions. Amphibole phenocrysts in the mafic inclusions are thought to be crystals from the host dacite which have been transferred across the inclusion boundaries. The phenocrysts from the dacite have relatively low Mg#, Al, and Fe3+ and relatively high Ti and Fe^+compared to the quench crystals in the mafic inclusions (Figure 111.15). The quench crystals have a wide range of compositions (Mg# = 0.78-0.93) whereas the analysed crystals from the host dacite have a relatively restricted range of compositions (Mg# = 0.77-0.78). With increasing Mg there are clear trends of decreasing Ti and Fe^+. An inclusion from a clinopyroxene in a mafic inclusion has the highest analysed Mg, Cr and Ca and the lowest Ti and Fe^+. This may be a xenocryst. The relatively wide range of quench amphibole compositions may be due to disequilibrium during rapid growth which has resulted in a major change in the associated melt composition. The matrix glass is rhyolitic but the original melt phase was probably a dacite such as occurs in many of the andésite magmas including the Soncor mafic inclusiion LA-124. FIGURE 111.22: Histogram of anorthite contents of plagioclase from the Capricorn Lava. Dark stipple = cores; pale stipple = rims; unstippled = undifferentiated. 117 LA-143HL Host Dacite Lava. c 3 O ü .2 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 6 LA-141 5 Mafic Inclusion. 4 C 3 O ü 3 2 1 0 .2 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 6 LA-143 5 Mafic Inclusion. 4 O ü 3 2 1 0 .2 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 Anorthite g III.3.4.4 Plagioclase. There is a wide range of plagioclase compositions in both the dacitic host lava and the andesitic mafic inclusions (Figure 111.16). The mafic inclusion LA-143 contains plagioclase with compositions ranging from An40 to An69 and those in the adjacent host lava are very similar (An41 to An71). Plagioclase compositions from the mafic inclusion LA-141 range from An27 to An71. There is little structure to the bar charts of mole fraction anorthite although two peaks appear to occur in LA-141 and LA-143HL. These occur at An35-45 and An50-65. Most of the plagioclase analyses from LA-143 fall into the An50-65 range. In the case of LA-143HL all of the rim analyses fall into these two groups. None of the more calcic plagioclase (An75-85) which is typical of the mafic inclusion LA-124 from the Soncor Flow (Section III.3.1.4) has been found in any of these samples. This may indicate that the andésite of the Capricorn Lava was injected into a dacitic magma chamber and quenched before calcic plagioclase had begun to fractionate in the andésite. This means that the phenocryst phases in the andésite prior to mixing were olivine, augite, orthopyroxene and Cr-spinel. Pétrographie evidence from the Soncor mafic inclusion indicates that plagioclase and magnetite formed at a relatively late stage in the fractionation sequence as these phases are not found as inclusions in the olivine phenocrysts. The wide range of plagioclase compositions in the Capricorn Lava can be explained as the result of a combination of several processes. Some of the cores must have originated in the host dacite prior to mixing with the andésite. Most of the overgrowths on these cores have probably formed FIGURE 111.23: “Modified Johnson Spinel Prism” plots (Haggerty, 1976) of spinel compositions from the Capricorn Lava. 119 LA141 Spinels LA140 Spinels w \ o o + A Incl. in Olivine + < < + □ Incl. in Augite + A Incl. in Hbl. o Titanomagnetite o> 0) Li. ■ Incl. in Flag. u. O Phenocryst «) «> U_ u. O 0.0 i " I I I > I—r—I—r 0.0 I I 1 'I " I" I " r r I 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.8 0.8 1.0 F e 11 / F e 11 + M g Fell/Fell + Mg 0 .8 - < 0 . 6 - < + + o O 0.4- o o 0 .2 - 0 . 0 I ' I ' 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 F e ll/F e ll + Mg Fell/Fell + Mg during the mixing event(s) and also during subsequent fractional crystallisation. Depending on the time interval between magma mixing and eruption of the mixed magmas it is possible that many of the large euhedral phenocrysts which are found within both the host lava and its mafic inclusions have grown entirely after the injection of the andésite. In order for the mafic inclusions to have been erupted within this relatively small lava flow the magma chamber must have been overturned prior to the eruption. Strong fluctuations in temperature, pressure and water content of the magmas are likely to have occurred during this overturn, possibly having a large effect on plagioclase compositions (e.g. Housh & Luhr, 1991). III.3.4.5 Spinels. The spinels of the mafic inclusion LA-141 do not form the continuous trend shown by those in the Soncor mafic inclusion LA-124. Instead, the early part of the trend (increasing Fe^+/Fe^++IVIg, increasing Fe3+/]2R3+ and constant Cr/Cr+AI) is described by Cr-spinel inclusions in olivines and augite and then a jump to typical dacitic magnetite compositions with low Cr/Cr+AI occurs in the phenocrysts and inclusions in plagioclase (Figure 111.17). This break in the trend is thought to be due to an interruption of the normal calc-alkaline basaltic fractionation sequence by quenching of the mafic magma as it was injected into the dacitic magma. A spinel inclusion from a hornblende lath lies away from the normal trend, having low Cr/Cr+AI and high Fe^+/IR3+ at relatively low values of FIGURE 111.24: Plots Of various oxides against Si02 for glass analyses from tfie Capricorn Lava. 121 0.9 0.8 0.7 0.6 O Host Lava 0.5 • • • Mafic Incl. 0.5 0.4 0.3 0.2 0.0 6 6 6 8 70 72 74 76 78 6 6 6 8 70 72 74 76 78 Si02 SI02 ro N 13 - 12 - 0.8 • • 0.6 0.4 Oo o 0.2 6 6 6 8 70 72 74 76 78 6 6 6 8 70 72 74 76 78 Si02 SI02 4 3 1.0 - 2 0.8- 0.6 1 6 6 6 8 70 72 74 76 78 6 6 6 8 70 72 74 76 78 SI02 SI02 -WO . O OZX 1 2 3 Fe2+/Fe2++Mg. The reason for this is unknown, but may be related to the rapid change in melt composition during quench-crystallisation of the hornblende. 111.3.4.6 Glass compositions. Matrix glasses from both the host lava and the mafic inclusions have been analysed. The host lava contains only rhyolitic glass with a narrow compositional range (SiOg = 75-77 wt%). The glass matrix in the mafic inclusions is dacitic to (dominantly) rhyolitic (Si 0 2 = 68-76 wt%). Many major elements show clear trends when plotted against Si 0 2 on a Marker diagram (Figure 111.18). TO 2 is scattered and Na^D and K 2 O have flat trends. AI2 O3 , FeO and CaO all decrease steadily with increasing Si 0 2 . MgO decreases rapidly in the dacitic glasses to below detection level and remains low in the rhyolitic glasses. The wide compositional range in the matrix glasses of the mafic inclusions is probably due to the rapid quench- crystallisation of amphibole and plagioclase. The inhomogeneity is thought to have developed as crystal faces advanced into the melt and created boundary zones depleted in the major elements of which the quench crystals were composed. 111.3.4.7 Conclusions. The evidence from pétrographie analysis of the host dacite and its mafic inclusions indicates that the Capricorn Lava has formed by the mingling and mixing of andesitic (or basaltic andésite) and dacitic endmembers. The andésite has suffered rapid quenching on injection into the dacite, resulting in the formation of glassy hornblende-rich mafic 124 inclusions. Transfer of phenocrysts across the inclusion boundaries in both directions is indicated by the compositional zonation of orthopyroxene, augite and plagioclase crystals. A significant amount of fractionation involving these minerals as well as magnetite and ilmenite has taken place since the mixing event and prior to eruption of the magma, accompanied by convective overturn of the magma chamber and disaggregation of many of the mafic inclusions. III.3.5 The 1986-1993 Eruptive Sequence. The recent activity of Lascar has involved at least three stages of dome extrusion, punctuated by periods of collapse (Section 1.8). Three vulcanian eruptions have occurred, in September 1986, February 1990 and April 1993, which have ejected material from the active crater and allowed samples of the dome and deeper magmatic system to be collected. The products of the 1986 and 1990 eruptions were dense to vesicular bluish- grey to greenish blocks of 2 -pyroxene andésite, containing also plagioclase, titanomagnetite, ilmenite, apatite and rare olivine, amphibole and sulphides in a fine-grained holocrystalline matrix with interstitial brown rhyolite glass. These blocks represent fragments of the solid outer part and the gassy more fluid inner part of the lava dome. The 1993 eruption ejected dense vesicular bombs of dark scoria and pyroclastic flows with white pumice. The scoria contains phenocrysts of orthopyroxene, augite, plagioclase, anhydrite, rare reacted biotite, magnetite, ilmenite and apatite. Chalcopyrite has also been found. The matrix is a dacitic glass. The white pumice, which is reported to FIGURE 111.25: Histograms of Mg# of orthopyroxenes from the 1986-1993 Lavas. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. 125 PWF1 LA-6 1993 White Pumice. 1986 Andésite. <5 2 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ LA-100 1990 Andésite. c O3 o .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 4 PWF2 1993 Dark Scoria. 3 2 1 0 .64 .66 .68 .7 .72 .74 .76 .78 .8 .82 .84 .86 Mg/Mg+Fe2+ have an andesitic composition with around 60 wt% Si02 (Smithsonian Institution, Bull. G.V.N. vol.18 no.4, 1993), contains the same assemblage of minerals but biotite and sulphides have not been found in the samples available. The matrix is a rhyolitic glass. The presence of olivine has been reported (Smithsonian Institution, Bull. G.V.N. vol.18 no.4, 1993). The occurrence of anhydrite and biotite in the samples correlates with the appearance of more evolved magma and the initiation of pyroclastic flows and may indicate that the magma is more volatile-rich than in 1986-1990. The evolution of mineral compositions with time is examined in order to monitor the changing magma chamber conditions. 111.3.5.1 Orthopyroxene. Orthopyroxene compositions in the andésites have not changed significantly from 1986 to 1993 (Figure 111.19). Mg# lies consistently between 0.69 and 0.81. There is a peak at Mg# = 0.72-74 in the three samples LA - 6 (1986 bomb), LA-100 (1990 bomb) and PWF2 (1993 scoria). The orthopyroxene in the 1993 white pumice PWF1 has a more restricted compositional range (Mg# = 0.68-0.73) with a well-constrained peak at Mg# = 0.71-0.72. The andesitic magmas from the three eruptions are clearly closely related. 111.3.5.2 Ciinopyroxene. Augite compositions are more variable but consistent between the 1986 and 1990 samples (Mg# = 0.77-0.92; Figure 111.20). The 1993 FIGURE 111.26: Histograms of Mg# of clinopyroxenes from tfie 1986-1993 Lavas. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. 127 LA-6 PWF1 1986 Andésite. 1993 White Pumice. 3 2 1 0 ,72 .74 .76 .78 .8 .82 .84 .86 .88 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9 .92 .94 Mg/Mg+Fe2+ LA-100 1990 Andésite. c O3 ü 1 .72 .74 .76 .78 .8 .82 .84 .86 .88 .9 .92 .94 PWF2 1993 Dark Scoria. m .72 .74 .76 .78 .8 .82 .84 .86 .88 .9 .92 .94 Mg/Mg+Fe2+ scoria and pumice contain more Mg-poor augites with a distinct compositional range (Mg# - 0.73-0.82), suggesting that a change in conditions has occurred between the 1990 and 1993 eruptions and that the 1993 magmas may be petrogenetically related. 111.3.5.3 P lag ioclase. Plagioclase compositions (Figure 111.21) show the wide variation typical of Lascar magmas in both LA - 6 (An40-AnS5) and LA-100 (An50-An85). A well-developed peak occurs at An55-65. The 1993 samples contain plagioclase with a more restricted compositional range (An45- An70), although the same peak in numbers of analyses occurs in the white pumice. In the 1993 andésite this peak is slightly shifted to a more calcic composition (An65-70). [11.3.5.4 Glass compositions. The matrix glasses of the 1993 andésite scoria and dacitic pumice are considered to represent true magma chamber melt compositions since they contain relatively few microphenocrysts. The andésite PWF2 has a brownish dacitic matrix glass (62-65 wt% Si 0 2 ) and the white pumice PWF 1 has a colourless rhyolitic matrix glass (72-73 wt% SI 0 2 ). However, in the case of the 1986 and 1990 bombs the melt has crystallised during extrusion of the magma to form a dome, leaving an interstitial rhyolitic glass (71-76 wt% Si 0 2 ). If the 1993 more evolved magma has formed by fractional crystallisation of an andesitic magma similar to the 1993 andésite then it FIGURE 111.27: Histogram of anorthite contents of plagioclase from the 1986- 1993 Lavas. 129 PWF1 LA-6 1993 White Pumice. 1986 Andésite. c 3 O3 O .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 Anorthite LA-100 1990 Andésite 0 ll-'i'! ‘rfl I 25 .3 .35 .4 .45 .5 ,7 .75 7 PWF2 6 1993 Dark Scoria. 5 4 3 2 1 0 .25 .3 .35 .4 .45 .5 .55 .6 .65 .7 .75 .8 .85 .9 Anorthite follows that the matrix glass compositions of the two 1993 magmas are related by simple fractional crystallisation. The phenocryst assemblage, particularly in the case of augite and plagioclase, suggests a relationship between the two magmas. However, the similarity of the orthopyroxenes of the 1993 andésite with those of the 1986 and 1990 magmas also indicates a possible petrogenetic relationship. It is considered likely that the 1986 and 1990 andésite lavas are derived from a relatively crystal-poor magma represented by the 1993 andésite by crystal fractionation within the subvolcanic plumbing system and in the surface lava dome. The similarity between the residual rhyolite glass of these holocrystalline lavas and the matrix glass of the 1993 pumice is striking. In plots of major elements against Si0 2 the rhyolitic glasses are essentially the same except for a lower Ti 0 2 concentration in the matrix of the 1993 dacite (Figure 111.22). With increasing Si0 2 a steady decrease of AI 2 O3 , FeO, MgO and CaO occurs. Na20 remains constant or increases slightly at first and then decreases sharply after Si 0 2 = 71 wt%. K 2 O increases and Ti 0 2 shows a scattered but fairly constant trend. It appears that both the deep and shallow fractionation sequences lead to the derivation of very similar melt compositions except for a difference in Ti 0 2 - The change in the Na^O trend is thought to represent an increase in the importance of sodic plagioclase in the fractionation sequence. The other trends are consistent with the crystallisation of plagioclase, pyroxenes and Fe-Ti oxides. FIGURE 111.28: Plots of various oxides against SIO2 for glass analyses from the 1986-1993 Lavas. 131 A O 1986 A • 1990 4 A 1993 Andesiie CD' A 1993 Dacite 0 7 0 8 060 60 70 80 S I02 SI02 N : ▲ 16 - AA ^ A 14 - >oo o oo 12 - 1 60 70 80 60 70 80 S I02 S i02 6 5 4 - % ° o 3 2 ------— 0 60 70 80 60 70 80 S i0 2 S i0 2 133 III.3.5.5 Conclusions. The 1986-1993 eruptive sequence implies a rather complex plumbing system since relatively volatile-poor andésite is erupted before more volatile-rich mixed andesitic-dacitic magma. The simplest explanation is that the andésite magma has at first bypassed a magma chamber containing dacite, rising to the surface relatively slowly and degassing through an open vent system. This would account for the intense degassing in the active crater since at least 1984. Fractional crystallisation at shallow depth in the feeder system has resulted in a crystal-rich magma which has been extruded to form a series of lava domes. The 1993 eruption clearly involves a different mechanism. The mixed magmas are relatively rich in volatiles, containing phenocrysts of both biotite and anhydrite and erupting violently to form pyroclastic flows. They appear to have risen rapidly from depth since they are both glassy. A model is proposed in which the andésite has been injected into a pre-existing dacitic magma chamber at depth whilst also bypassing this magma chamber and rising to the surface via a separate plumbing system. Quenching and oxidation of the andésite on mixing with the dacite has caused it to exsolve its volatile phases, notably water and sulphur compounds. Biotite is altered to orthopyroxene and magnetite and anhydrite has crystallised in equilibrium with the mixed volatile phase. An increase in hydrostatic pressure within this magma chamber has caused fracturing of the wall rocks and opened a conduit into the nearby plumbing system. The April 1993 eruption resulted from injection of the mixed magmas into this conduit until the hydrostatic pressure had decreased sufficiently in the magma chamber to close the feeder system. The origin of the dacite may be explained by 134 fractionation of a body of andésite magma similar to that which was erupted in 1993. This may have been emplaced below the volcano during a previous phase of activity. The magma chamber may have been compositionally zoned prior to the 1993 eruption but further samples are needed in order to investigate this. The similarity of phenocryst phases between the 1993 magmas may indicate rather efficient mixing between the two magmas. This recent eruptive phase shows a number of similarities with the prehistoric Soncor Flow, notably the presence of anhydrite and biotite and the eruption of dacitic pumice. 135 FIGURE 111.10: Histograms of Mg# of ortfiopyroxenes from tfie Cfialle Flow. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. FIGURE 111.11: Histograms of Mg# of clinopyroxenes from tfie Cfialle Flow. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. FIGURE 111.12: Histogram of anortfilte contents of plagioclase from tfie Cfialle Flow. Dark stipple = cores; pale stipple = rims; unstlppled = undifferentiated. III.4 DISCUSSION. Almost all the lavas and pyroclastic flows of Lascar (except the Chaile Flow) show disequilibrium textures consistent with magma mixing. On the macroscopic scale these include mafic inclusions in dacitic magmas, mixed andesite-dacite pumice flows and banded pumice clasts. On the microscopic scale olivine occurs in most magmas from basaltic andésite to dacite composition and has commonly developed a thick reaction corona. Olivine and quartz occur together in many rocks, but the olivine can be seen to have originated in the basaltic andésite magma represented by mafic inclusions. Mineral compositions are extremely variable in most rocks, as is demonstrated by the Mg# of orthopyroxene and augite, and the mole fraction anorthite in plagioclase. Both normal (decreasing Mg# or An) and reverse compositional zoning can be discerned in any one sample and in some cases the zonation is convergent from widely variable core compositions to more restricted rim compositions. In most cases fractional crystallisation can be demonstrated to have occurred after the postulated mixing event. Individual samples from the large Soncor dacitic pyroclastic flow contain unique mineral compositional patterns, indicating extreme heterogeneity in the magma chamber prior to eruption. The mixing does not appear to be due to a “drawing-up” effect caused by large eruptions, since the same textures occur in small dome and lava flow eruptions, and fractional crystallisation has occurred between mixing and eruption. In the case of andesitic magmas the injection of mafic magma into the magma chamber appears to have been more frequent, producing more mafic magmas without such advanced fractional crystallisation between mixing events. This results in less well- developed but still recognisable disequilibrium textures due to a lower 136 compositional contrast between the two end members. In single flows of the Centre I andésites one can find olivines in various stages of corrosion, from almost pristine to entirely replaced by orthopyroxene-magnetite symplectite. This would appear to indicate several phases of injection into the magma chamber prior to eruption and would explain the complex and apparently random compositional zoning of pyroxenes and plagioclase in these magmas. Assimilation of wall rock material appears to have occurred in most Lascar magmas. Xenoliths of various sizes, from 20 cm to a few mm, can be found in many samples. These include calc-silicates and Palaeozoic and Cenozoic lavas. Disaggregation of smaller xenoliths can be demonstrated, notably in the case of glassy lavas whose matrix glass melts and releases phenocryst phases into the host magma. It is difficult to distinguish such xenocrysts from true phenocrysts owing to the large compositional range of mineral phases in these magmas. Quartz in andésites, which is usually rounded and embayed, is probably usually xenocrystic. The differences between individual flows and between the characteristics of the three centres seems to indicate major changes in the subvolcanic plumbing system. Centres I and III have erupted dominantly, and possibly entirely, magmas of andesitic composition. These flows contain evidence of frequent injection of mafic magma into the subvolcanic system resulting in eruptions of relatively unfractionated magmas. Centre II, on the other hand, has erupted magmas of andesitic to dacitic composition. Some of these, such as the Soncor Flow, have originated in a zoned magma 137 chamber which has been highly disturbed by the injection of mafic magma. In order for a compositionally zoned magma chamber to develop it must presumably be relatively undisturbed, indicating a longer time period between injection events. The dacitic magma chamber which gave rise to the Soncor Flow has apparently formed during a time of reorganisation of the plumbing system, since it represents the extinction of Centre I and the initiation of Centre II. Eruptions of andesitic magma have occurred between such dacitic eruptions. An example is the Chaile bomb flow, which lies stratigraphically above the Soncor Flow and below the Capricorn Lava. It is possible that such flows represent magma which has bypassed a long-term dacitic magma chamber having crystallised at greater depth, possibly in another magma chamber. A similar model is proposed for the 1986-93 eruption, with the early andesitic lava having crystallised at shallow levels and the 1993 mixed magmas having formed in a magma chamber. The mafic inclusions from the Soncor Flow and the Capricorn Lava show evidence for deeper level fractionation of the andesitic magma prior to injection into the shallow magma chamber. The stable phases are olivine, ciinopyroxene, Cr-spinel and orthopyroxene. Plagioclase and magnetite do not form until a later stage and in the case of the Capricorn mafic inclusions they do not occur until the andésite is quenched on mixing with the dacite. The large augite phenocrysts of the Saltar flow have also formed initially in a magma in which olivine, Cr-spinel and orthopyroxene but not plagioclase or magnetite are also stable. Davidson & de Silva (1992) have also suggested from the evidence of whole-rock and isotopic compositions that primitive rocks from the Bolivian Altiplano have formed initially by deep plagioclase- absent fractionation. The presence of fassaitic augite inclusions in an olivine phenocryst from the Soncor mafic inclusion LA-124 supports the model of 138 deep crustal fractionation, since fassaite is stable in magmas only at relatively high pressures. Iron-rich melts occur as immiscible globules in sveral hornblende-rich andesitic pumice samples from the Soncor Flow (LAS29-2, LAS 30 and LAS 36-1) and in hornblende crystal clots in a dacitic block- and-ash flow of the adjacent Volcan Aguas Calientes (SM93/36, SM93/37). Analyses of coexisting iron oxide and silicate melts are presented in Figure 111.29. Both samples contain sulphides and anhydrite. LAS30 contains anhydrite, pyrrhotite and chalcopyrite as inclusions in hornblende. SM93/37 contains pyrrhotite and pyrite as inclusions in magnetite and hornblende. A mafic inclusion from this flow (SM93/38) contains coexisting anhydrite and chalcopyrite in a glass inclusion in orthopyroxene. Exsolution lamellae in magnetites of LAS30 indicates that they have experienced a drop in temperature. This is thought to have occurred when the andésite was injected into the dacitic magma chamber. Iron oxide-silicate liquid immiscibility has been found to occur in certain experimental systems at relatively high temperatures and high f 0 2 (Naslund, 1983). Silicate glass inclusions in quartz in an ignimbrite from Karuma Hills, Central Asia, contain very similar iron oxide-rich immiscible globules (Naumov et al, 1991) which were found to homogenize at 1050-1100°C. A similar iron oxide melt inclusion in a kaersutite megacryst from Mount Etna, Sicily (S.J. Matthews, unpub. data) has partially crystallised to a fine grained mixture of several unidentified phases and contains a vapour bubble. A common feature of FIGURE 111.29: Analyses of coexisting Fe-rich and silicate melts from the Soncor Flow. 139 80 3 ■ Fe-Rich Melt 70 te) O Silicate Melt 60 2 CM O O 50 O) w S 40 1 30 20 —r- — 1— —T— — r - 0 10 20 30 40 50 60 0 10 20 30 40 50 60 FeO FeO 0.8 3 f 0.7 o 2 0.6 - M O o (0 0.5 - O 1 0.4 - 0.3 0 0 10 20 30 40 50 60 20 30 40 50 60 FeO FeO 5 4 14 - co O 3 o 12 - CM CM CO z 2 10 - 1 0 0 1 0 20 30 40 50 60 0 1 0 20 30 40 50 60 FeO FeO 0.2 ■ Fe-Rich Melt O Silicate Melt 3 - © o CM ü 0.1 0.0 0 1 0 20 30 40 50 60 FeO 0.4 0.3 - tn O 0.2 - CM Ou 0.1 - 0.0 4 0 0 10 20 30 40 50 60 FeO 0.3 0.2 - co o (/> 0.1 0.0 - 1 0 2 0i 40 50 60 eO these liquids is their low Ti content. The samples from Volcan Aguas Calientes and Etna are enriched in phosphorus, sulphur and chlorine. The low totals of analyses from Aguas Calientes are thought to indicate high water contents which may have lowered the temperature of the two-liquid field. The immiscible liquids are thought to have separated in the andésite magma prior to or during magma mixing. A decrease in temperature on contact with the dacitic magma destabilised the iron oxide melt globules causing them to quench to glass and then gradually desilicify, losing Si, A! and volatiles and crystallising as a spongy Ti-poor magnetite (Figure 111.30). Hornblende became stable due to an increase in fHgO on quenching of the mafic magma, overgrowing the altered globules. A Late Pliocene (2.1 ±0.1 Ma; Maksaev et al, 1988) iron oxide-phosphate lava flow occurs at El Laco (23°48’S, 67°30’W). It consists mainly of magnetite, hematite and apatite with minor iron phosphates, anhydrite, monazite, xenotime, quartz and orthopyroxene (Park, 1961 ; Haggerty, 1970a, b; Frutos & Oyarzun, 1975; Henriquez & Martin, 1978; Matthews, 1993). The low Ti content of magnetite and the presence of phosphate and sulphate minerals suggests a genetic link between the immiscible liquids observed at Lascar and Aguas Calientes and the extrusive deposit at El Laco. It is possible that such magmas are quite common in similar calc-alkaline environments. FIGURE 111.30: Analyses of fresh and altered Fe-rich melts from the Soncor Flow and from the adjacent Volcan Aguas Calientes. The final alteration product is a spongey inclusion-rich magnetite. 142 4 o Aguas Calientes • Lascar 30 - 3 0 20 - 2 1 10 - 1 0 2 0 4 0 6 0 1 0 0 2 0 4 0 6 0 8 0 1 0 0 FeO FeO 2.0 1.5 1.0 0.5 0.0 ■O 2 0 4 0 6 0 8 0 1 0 0 2 0 4 0 G 0 8 0 1 00 FeO FeO 12 O- 1.5 10 - 1.0 - # O 6o o 0.5 •o 0.0 2 0 4 0 6 0 8 0 1 0 0 2 0 4 0 6 0 8 0 1 00 FeO FeO o Aguas Calientes Lascar 1.0 - Oo 0.5 0.0 100 20 40 60 80 100 FeO 10 - ■r 8 - So 2 0 4 0 6 0 8 0 1 00 FeO 6 5 4 CO O 3 2 1 0 2 0 4 0 6 0 8 0 100 FeO CHAPTER IV GEOTHERMOMETRY, GEOBAROMETRY AND OXYGEN BAROMETRY. IV.1 GENERAL. Mineral compositions in magmas are affected by physical as well as chemical constraints. If thermodynamic properties of mineral phases are known then the temperature, pressure and oxygen fugacity (fOg) can be calculated for the magma. Since all three variables as well as melt composition can control the compositions of mineral phases it is more common to examine the partitioning of certain elements between two or more mineral phases. The most useful geothermometers for Lascar rocks are the magnetite-ilmenite and 2-pyroxene methods. The first method can be used to calculate the temperature and fOg conditions in the magma from the partitioning of Ti between titanomagnetite and hemoilmenite (Andersen & Lindsley, 1988) and the second to calculate magma temperature from the Mg-Fe partitioning between augite and orthopyroxene (Lindsley, 1983). The reactions are as follows; 1 ) FeTiOg + FogO^ = FOgOg + FogTiO^ Ilmenite + Magnetite = Hematite + Ulvospinel 2 ) CaFeSigOg + MgSiOg = CaMgSigOg + FeSiOg Hedenbergite + Enstatite = Diopside + Ferrosilite 145 These techniques have been expanded so that pressure, temperature and fOg can be calculated for magmas containing part or all of the assemblage titanomagnetite, ilmenite, augite, orthopyroxene, olivine and quartz (Lindsley & Frost, 1992; Frost & Lindsley, 1992). However, this method has not been found useful for the calculation of pressures due to the large uncertainties involved. If the temperature and pressure of the system is known from one of these techniques then the fOg can also be calculated using olivine-spinel pairs in equilibrium with orthopyroxene (Ballhaus et al, 1990). The reaction is given below. 3) GFOgSiO^ + Og = SFOgSigOg + 2Fe30^ Olivine + <3g = Orthopyroxene + Spinel This method was devised for spinel peridotites and the calculated temperatures are not highly sensitive to uncertainty in pressure at crustal levels, allowing a good estimate of fOg to be made without accurate knowledge of the pressure. Pressure can be estimated in several ways. The AI content of hornblende in equilibrium with melt, fluid, biotite, quartz, sanidine, plagioclase, sphene and magnetite or ilmenite has been calibrated as a geobarometer (Johnson & Rutherford, 1989). However, Lascar magmas do not contain stable quartz, sanidine or sphene, so this method cannot be used. It has also been suggested that temperature and melt composition exert controls on hornblende composition, leading to an amphibole-plagioclase geothermobarometer (Blundy & Holland, 1990). Unfortunately amphibole and plagioclase in any one sample are highly variable in composition so that equilibrium pairings cannot be identified. Another method of calculating pressure is to use equilibria which buffer 146 silica activity with varying temperature and pressure (Nicholls et al, 1971). Three of these reactions may be applicable to Lascar magmas. These are listed below. 4) SiOg = SiOg Glass = Quartz 5) + SiOg = FOgSiO^ + VgOg Magnetite + Glass = Fayalite + Og 6 ) ^/sFe^O^ + SiOg = FeSiOg + VgOg Magnetite + Glass = Ferrosilite + Og Melt inclusions in phenocrysts are commonly devitrified causing them to become brownish in colour. Pressure-sealed melt inclusions also contain a contraction bubble due to a decrease in volume on glass formation. Heating of the sample will eventually re-melt the inclusions provided they remain pressure-sealed within the phenocryst, providing a direct measurement of the melt temperature at the time of entrapment. IV.2 TEMPERATURE AND OXYGEN FUGACITY. IV.2.1 Magnetite-llmenite Geothermometry. Titanomagnetite and ilmenite analyses from Lascar rocks have been used to calculate temperatures and oxygen fugacities of the original magmas. Oxide populations, recalculated to provide ulvospinel content of 147 magnetite and ilmenite content of hemoilmenite (Stormer, 1983) were examined for compositional variation and “flyers” (analyses of abnormal composition) rejected. These populations were then checked for equilibrium using Mg/Mn partitioning between spinel and rhombohedral phases (Bacon & Hirschmann, 1988). The most useful samples containing oxide populations with the lowest variability are rapidly-cooled magmas represented by pumice, cauliflower bomb flows and some dacitic lavas. Andesitic lavas contain highly variable oxide populations due to re equilibration at lower temperatures following eruption. It is common to find exsolution textures in such rocks, with lamellae of ilmenite in Ti-poor magnetite grains and vice versa. Where these textures are coarse grained the lamellae and host grains have been analysed in order to examine the post-eruptive cooling sequence. It is notable that these exsolution pairs do not fall in the equilibrium range on the Mg/Mn diagram, but this test is not thought to be applicable to exsolved grains. Certain lava flows such as the 1986 and 1990 lavas clearly demonstrate a variation in magnetite composition from phenocrysts to groundmass. The groundmass magnetites of the 1990 lava are more Ti-rich than the phenocrysts and no groundmass ilmenite has been found. The instability of ilmenite following eruption has resulted in incorporation of excess Ti into titanomagnetite. Xenocrystic oxides have been recognised in certain cases. The Capricorn lava contains magnetite and ilmenite within a relatively tight compositional range, except for a few Ti-poor hemoilmenite grains, which are compositionally identical to hemoilmenite from a partially diaggregated skarn xenolith. Once equilibrium between spinel and rhombohedral phases has been demonstrated and flyers eliminated from populations, average 148 Andersen & Lindsley 1988. QUILF (Lindsley & Frost 1992). Ballhaus et al 1990 Magnetite-llmenite. Magnetite-llmenite. 2-Pyroxene. Olivine-Spinel. SAMPLE. Description. Temp. °C. Log f02. Temp. °C Log f02 Temp. °C Logf02. LA-3 Unknown. 971 -9.81 LA-14 Unknown. 926 -9.71 LA-15 Unknown. 914 -9.88 SM93/43 Saltar Flow. Augite Core. 1068 ± 55 -6 .7 ± 0 .7 SM93/43 Saltar Flow. Augite Rim. 1061 ± 7 7 SM93/10 Piedras Grandes Flow. 910 -9.92 f SM93/22 Piedras Grandes Flow. 905 -9.89 LA-103 Soncor Dark Pumice. 900 -10.21 LA-122 Soncor Pumice. 920 -9.81 902 -10.09 929 ± 78 LA-155 Soncor Pumice. 906 -9.97 LAS-29-1 Soncr Crystal Rich Pumice 940 -9.48 LAS-36-1 Soncor Andesitic Pumice. 899 -10.74 SM93/44 Soncor Vitrophyre. 887 -10.52 LAS-47 Soncor Prismatic Block. 748 -12.88 740 -13.15 973 ± 65 LA-124 Soncor Mafic Incl. 1025 ± 65 -9.4 ± 1.5 LA-139 Capricorn Lava. 899 -10.14 LA-140 Capricorn Lava. 913 -9.81 LA-143(HL) Capricorn Lava. 908 -10.06 LA-141 Capricorn Mafic Incl. 891 -10.26 LAS-23 Tumbres Acid Clast. 953 ± 29 LA-6 1986 Lava. 923 -10.64 LA-100 1990 Lava. 906 -10.91 PWF1 1993 White Pumice 907 -10.36 PWF2 1993 Dark Scoria. (1) 1002 -8.96 LA-I(XEN) Skarn Xenolith (2) 779 -12.11 LA-I(XEN) Skarn Xenolith (2) 679 -13.54 LA-140(XEN) Skarn Xenolith (2) 657 -13.72 LAS-15 Skarn Xenolith 360 ± 220 analyses have been used to calculated temperature and oxygen fugacity using the equations of Andersen and Lindsley (1988). For a few samples the “QUILF” equations (Lindsley & Frost, 1992; Frost & Lindsley, 1992; Andersen, in press) were also used. Good agreement between the two methods was achieved in all cases (<20°C difference). For most Lascar rocks the calculated temperatures lie in the range 887-923°C and log fOg in the range -10.52 to -9.81 (2.67 to 2.89 log units above the fayalite- magnetite-quartz buffer). This temperature range illustrates the bias towards dacitic samples which are more commonly rapidly-cooled than andesitic samples. Higher temperatures (up to 970°C) and lower fOg (down to FMQ + 1.88) are obtained for a few andesitic samples. The prismatic jointed block LAS-47 from the Soncor Flow yields a rather low temperature of 740-748°C. The holocrystalline nature of this dacitic sample indicates slow cooling at hypabyssal levels and so the oxide temperature is probably due to re equilibration. Exsolution pairs from skarn xenoliths have also been used to calculate temperature and fOg, yielding metamorphic temperatures of 657- 779°C. These lower temperature samples have successively higher oxygen fugacities (FMQ + 3.35 at 779°C and FMQ -f- 4.96 at 657°C). For rocks with multiple populations of oxides due to magma mixing, coarsely intergrown oxide pairs were used. An example of this is the 1993 andesitic scoria sample PWF2, for which an oxide pair included in a pyroxene phenocryst yielded a relatively high temperature of 1000°C. The oxygen fugacities of the 1986 and 1990 lavas are unusually low (FMQ 4 - 1.9) but the 1993 anhydrite- bearing samples have higher fOg (FMQ + 2.45 at 907°C and FMQ + 2 . 2 2 at 1000°C). It is thought likely that the elevated fOg of Lascar magmas is due to buffering by sulphur gas reactions. This is discussed further in a later chapter. Results of these calculations are plotted in Figure IV. 1. 150 Lascar Volcano SM93/43 -6 - -7- - 8 - (S MH Ji -9- LA-124 o -10 - O) -11 o -12 -13 - FMQ □ □ -14 “ Po/Ah -15 600 700 800 900 1000 1 100 Tem perature °C FIGURE IV.1: Graph of log (f02/bar) against temperature (°C) from magnetite-llmenite and 2-pyroxene-olivine-spinel assemblages. Buffer curves as follows: FMQ = fayalite- magnetite-quartz (Wones & Gilbert 1969), Po/Ah = pyrrhotite- anhydrite (sulphide-sulphate), MH = magnetite-hematite (Eugster & Wones 1962), open circles = Lascar magmas from magnetite-llmenite, open squares = xenoliths from magnetite- llmenite, boxed areas = mafic magmas from 2-pyroxene-olivine- spinel. 151 The temperature at which liquid immiscibility occurred in the andesitic magma of the Soncor Flow cannot be determined due to subsequent cooling of the andesitic magma, as indicated by exsolution lamellae in magnetite phenocrysts. However, the heating effect on the coexisting dacite due to injection of this magma can clearly be seen in the sample LAS 29-1 which originates from a mixed andesitic-dacitic flow lobe and is associated with an andesitic pumice sample with liquid immiscibility textures (LAS 29-2). This dacitic crystal-rich pumice has a magnetite- ilmenite temperature of 940°C and a slightly lower fOg relative to FMQ than cooler Soncor dacitic samples (-9.48 log units or FMQ + 2.38). Since the iron oxide melt globules in the associated andesitic samples have suffered alteration at magma chamber temperatures it is reasonable to assume that liquid immiscibility occurred at a temperature higher than 940°C and lower than the original temperature of the incoming andésite magma (1025°C). A dacitic block-and-ash flow from Volcan Aguas Calientes with similar immiscibility textures and hornblende crystal clots has magnetite-ilmenite temperatures of 878°C (SM93/36) and 872°C (SM93/37) with log fOg from - 10.46 to -10.48. A coarse grained intergrowth of magnetite and ilmenite from SM93/37 yields a temperature of 1027°C (log fQg = -8.70). This is nearly identical to the calculated temperature of the primary Soncor andésite and this crystal intergrowth has probably originated in a similar magma. Again the liquid immiscibility seems to have occurred at less than 1025°C. 152 IV.2.2 2-Pyroxene Geothermometry and Olivine-Spinel Oxygen Barometry. It is difficult to find samples from Lascar in which the silicate phases are demonstrably in equilibrium, due to the high compositional variability resulting from magma mixing. Oxide phases have re-equilibrated rapidly following magma mixing events in most cases, but pyroxenes are strongly zoned and it is difficult to identify equilibrium pairs. However, in a small number of cases the pyroxene populations exhibit a limited compositional variability consistent with equilibrium. Two samples from the Soncor flow contain relatively unvaried pyroxene populations. These are the white pumice LA-122 and the prismatic jointed block LAS-47 and temperatures have been obtained for pyroxene pairs from these two samples using QUILF. The uncertainties in calculated temperatures are partly the result of varying the input pressure from 1 to 3 kbar for dacitic samples and from 1 to 5 kbar for mafic samples. The 2-pyroxene temperature obtained for LA-122 (929°C) is in good agreement with the magnetite-ilmenite temperatures from the same sample (920 and 902°C) confirming that the pyroxenes are in equilibrium. For LAS-47 the 2-pyroxene temperature (973°C) is much higher than the magnetite-ilmenite temperatures (748 and 740°C), consistent with the view that the oxide phases have re-equilibrated at lower temperatures. One advantage of the “QUILF” program (Andersen et al, in press) is that it can restore such re equilibrated oxides to their original compositions in equilibrium with associated silicate phases (Frost & Lindsley, 1992). When the oxides in LAS-47 are restored in this way an equilibrium temperature of 887 °C ± 23 °C is obtained for the assemblage magnetite-ilmenite-augite- 153 orthopyroxene and the compositions of the oxides become very similar to those in the acid pumice LAI22; LAS-47 nTi (re-equilibrated) 0.12 nTi (original) 0.22 XHem (re-equilibrated) 0.19 XHem (original) 0.33 LAI 22 nTi 0.22 XHem 0.39 where nTi no. of Ti atoms in the magnetite formula M 3 O 4 . XHem mole fraction hematite in ilmenite. This temperature is consistent with the magnetite-ilmenite temperatures obtained from other dacitic samples from the Soncor and Piedras Grandes flows (887-920 °C) and indicates that this type of recalculation may yield meaningful results if used carefully. An acid clast from the Tumbres Flow (LAS-23) yields a 2- pyroxene temperature of 953°C. This acid clast contains large amounts of fine grained quartz and feldspar (10-40|i) in a glassy matrix and may be a partially melted metasedimentary xenolith or buchite (Spry, 1969). Two samples representing the primitive mafic magma have relatively high 2 - pyroxene temperatures. The Soncor mafic inclusion LA-124 yields a 154 temperature of 1025°C and a large augite phenocryst from the Saltar flow (SM93/43) containing orthopyroxene and olivine phenocrysts yields temperatures of 1068°C in the more magnesian core and 1061°C in the more iron-rich rim region. Olivine-spinel equilibria yield f02 values of -9.4 log units (FMQ + 1 .43) for LA-124 and -6.7 log units (FMQ 4- 3.49) for the core of SM93/43. The unusually high fOg obtained for this sample is difficult to explain. It may be a true figure or due to disequilibrium between the olivines and their spinel inclusions. Temperatures and oxygen fugacities calculated using orthopyroxene-clinopyroxene-olivine-spinel assemblages are plotted in Figure IV.1. IV.2.3 Melt Inclusion Homogenization Experiments. A glass inclusion melting experiment has been carried out on a white pumice sample from the Soncor Flow, LA-147. Large plagioclase phenocrysts in this sample contain four types of brownish devitrified melt inclusions. The simplest type are primary melt inclusions which are small (1- 40 |i in diameter) and contain a single contraction bubble. These inclusions represent melt which was trapped during growth of the phenocryst and retained under pressure during eruption of the magma. Hourglass inclusions are similar to primary melt inclusions but have a very thin capillary of glass connecting them to the surface of the crystal. The vapour bubble is either absent or rather small, indicating that the inclusions have degassed on depressurisation. Irregular and network inclusions are large irregular bodies of glass up to several hundred microns across. They are often connected throughout the crystal and are commonly associated with cracks and planes of secondary fluid inclusions representing healed cracks. Many small vapour bubbles are present. Ruptured inclusions are irregular and have amoeba 155 like projections spreading out along fractures. These have no vapour bubble and are thought to have exploded into an adjacent fracture on depressurisation of the magma. All types of melt inclusions contain small inclusions (1-2 p) of a transparent colourless acicular mineral. Electron microprobe analyses indicate that this is apatite, which is thought to have crystallised due to phosphorus saturation in a boundary layer adjacent to the growing phenocryst (e.g. Bacon, 1989). Magnetite is also a common inclusion in melt inclusions, often growing from the walls of the inclusion. The textures described above also occur in pyroxenes in LA-147 and are common in all other samples studied. Heating experiments were carried out on thin slices of large (5mm) plagioclase phenocrysts, polished on both sides. The samples were placed in a furnace and heated for 1 hour at 800, 850 and 900°C respectively. After each heating experiment the glass inclusions were examined under a microscope. No changes were observed at 800 or 850°C but at 950°C some of the primary glass inclusions became colourless, indicating that they had melted. The derived melt inclusion formation temperature is therefore 875 ± 25°C for LA-147. No changes in the size of vapour bubbles were observed and apatite needles failed to dissolve. This temperature is in good agreement with the magnetite-ilmenite temperatures of 887-920°C for Soncor flow samples. Irregular melt inclusions failed to melt indicating that they had lost volatiles on heating due to a poor pressure seal. Photographs of a devitrified melt inclusion before and after melting are shown in Figure IV.2. Infrared spectra of glass inclusions were obtained before and 156 20 microns \ 20 microns FIGURE IV.2: Devitrified glass inclusion in plagioclase phenocryst from LA-147, a dacitic pumice from the Soncor Flow, before and after heating at 900°C for 1 hour. The glass inclusion has clearly melted but the vapour bubble has not redissolved. 157 after the heating experiments using an FTIR microscope in order to study the content and behaviour of water and carbon in the glass. Since the inclusions were so small and it was necessary to keep them contained in the “pressure vessel” of the host plagioclase, these spectra had to be collected through the plagioclase. The spectrum of an inclusion was calculated by subtracting the spectrum of an adjacent inclusion-free area of the plagioclase from the plagioclase+glass spectrum. This technique was met with variable success due to a number of factors including a difference in the path length of the beam through plagioclase between inclusion-free and inclusion-bearing areas, the presence of fluid inclusions in apparently clear areas of plagioclase, and the apparent presence of hydrous alteration phases and possibly structurally bound water in the plagioclase. Figure IV.3 shows typical infrared spectra of plagioclase and plagioclase+glass from which the glass inclusion spectra were derived. The initial spectra are dominated by two strong absorbance peaks. A broad peak at circa 3550 cm '' which tails off to lower wavenumbers is interpreted as the fundamental 0-H stretching vibration of either hydroxyl groups or molecular water (Ernsberger, 1977; Stolper, 1982a; McMillan & Remmele, 1986) dissolved in the glass and a sharp peak at 1635 cm-'' is interpreted as the bending mode of the water molecule (Ernsberger, 1977). This confirms that at least some of the water is present in the molecular form. The two sharp 0-H stretch peaks at lower wavenumbers in the plagioclase spectra (Figure IV.3) are thought to represent either water-bearing fluid inclusions or structurally-bound water in hydrous alteration minerals. The absence of significant peaks at 2340 and 2360 cm-'' (or a single peak at 2353 cm-''; Fine & Stolper, 1985/86) apart from a small doublet due to atmospheric COg indicates that no detectable 158 Plagioclase with glass inclusion and adjacent plagioclase which Is subtracted w to obtain glass Inclusion spectrum. c 3 - I (0 < - Plagioclase + Glass Inclusion Plagioclase Only 4000 3500 3000 2500 2000 1500 Wavenumt>ers cm-i FIGURE IV.3: Infrared spectra of a plagioclase phenocryst from LA-147 and an area of the phenocryst containing a devltrlfied glass Inclusion prior to heating experiments. The plagioclase spectrum Is subtracted from the plagioclase + glass spectrum In order to obtain the spectrum of the glass inclusion. $9 Glass inclusion which melted after heating for 1 hour at 900X. g c 3 3550 S O-H Stretch z 1635 k . Water Bending < cs Initial g k . o < J 900X 1 hour T T 4000 3500 3000 2500 2000 1500 Wavenumbers cm-i Glass inclusion which failed to melt after heating for 1 hour at 9 0 0 X 3 _ >% € Initial < - n o - (/) n 900°C 1 hour 40003500 3000 2500 2000 1500 Wavenumbers cm-i FIGURE IV.4: Calculated spectra of glass Inclusions In a plagioclase phenocryst of LA-147 before and after heating at SOOX for 1 hour. A primary melt Inclusion which melted at 900X has not lost Its dissolved water, whereas an Irregular glass Inclusion failed to melt due to degassing of all of its water through fractures In the crystal. 160 COg is present. In the glass inclusion which melted at 900°C the two water peaks are preserved indicating that little or no water loss has occurred during the experiment. However, a glass inclusion which failed to melt has clearly lost nearly all of its dissolved water since the two peaks have disappeared during the experiment. Since the irregular glass inclusions are usually associated with cracks in the crystal it is reasonable to suppose that volatile loss occurs along these fracture planes. IV.2.4 Pressure Calculations. The calculation of pressure from temperature, fOg and silica activity requires specific assemblages containing olivine-magnetite quartz, orthopyroxene-magnetite-quartz or olivine-orthopyroxene-magnetite. Unfortunately, quartz is not a stable phase in most Lascar magmas and in the more mafic rocks containing olivine, magnetite is not stable, so the simultaneous equations required for pressure calculations cannot normally be derived. Two samples containing the assemblage orthopyroxene- magnetite-quartz have been used in these calculations. LA-122, a 2-pyroxene dacitic pumice from the Soncor Flow, contains plagioclase, magnetite, ilmenite and rounded quartz grains in addition to orthopyroxene and augite. The temperature and oxygen fugacity are calculated as 900°C and -10.1 log units from magnetite-ilmenite equilibria. The equations describing the relationship between silica activity and pressure for the assemblage ferrosilite-magnetite-quartz are described by Nicholls et al (1971) as follows; 161 Quartz-liquid. log asiOg = -309/T + 0.183 - 0.0239(P-1)/T = -0.080 - 2.038 X 10-5 P Ferrosilite-magnetite-liquid. (Xfs = 0.323, XMt = 0.770) log asiOg = 4467/T - 1.63 - 1.1479(P-1)/T + log SFs + Vglog fOg - Vglog 8 Mt Where asiOg = silica activity in melt. XFs = mole fraction FeSiOg in orthopyroxene. XMt = mole fraction Fe^O^ in spinel, aFs = activity of FeSiOg in orthopyroxene. aMt = activity of Fe^O^ in spinel. T = temperature in K, P = pressure in bar. Magnetite is assumed to be ideal (aMt = XMt). Two models are used for ferrosilite. The first assumes ideality (aFs = X fs ) and the second (Williams, 1971) is decribed by the equation; aFs = 0.22(1-Xfs )2 + log XFs Two equations are derived using these two models; 162 log asiOg = 0.0418 - 4.084 X 10'^ P (Ideal model) and log asiOg = 0.143 - 4.084 x 10'^ P (Williams model) When log asiOg is plotted against P for these relationships the activity of silica is found to decrease with increasing pressure (Figure IV.5). The ferrosilite-magnetite-liquid lines have a steeper slope than the quartz- liquid line and intersect it at 6 kbar (ideal model) and 1 1 kbar (Williams model). The observed phenocryst assemblage is thus estimated to be in equilibrium with quartz at 6-11 kbar. The Williams model is considered to provide a better approximation to the activity-composition relationship for ferrosilite (Nicholls et al, 1971) but these authors have found that the ideal mixing relationship provides more realistic results in these calculations when combined with an ideal mixing model for magnetite. The rounded and embayed nature of the quartz in this and other samples from the Soncor Flow may be due to depressurisation on transport to the surface which would undersat urate the melt in silica and result in resorption of quartz. Other factors might be an increase in temperature, which would raise the quartz-melt equilibrium line to higher silica acticity at given pressure, or fHgO, which would decrease the silica activity of the melt. LAS-23 is a white clast from the andesitic Tumbres Flow. It contains large amounts of quartz with minor plagioclase, pyroxenes and Ti- rich magnetite in a glassy matrix. The 2-pyroxene temperature is approximately 950°C. Because ilmenite is absent the f 0 2 cannot be 163 LA122. T=900°C, logfO2=-10.1 0.2 Fs 0.323 Mt 0.770 0. 0 - CNJ Wiliiams O (/> Q uartz CB - 0.2 - O) O Ideal -0.4 - - 0.6 0 10000 150005000 P (bar) FIGURE IV.5: Silica activity-pressure relationships for quartz-melt and ferrosilite-magnetite-melt in LA-122, a quartz-bearing dacitic pumice from the Soncor Flow. Quartz is in equilibrium with the assemblage at 6-11 kbar. LAS23. T=950°C, logf02=-9.35. 0.0 - 0.1 - Q uartz - 0.2 - CM p -0.3 - Williams 5) <0 Fs 0.116 -0.4 D> Mt 0.596 O -0.5 - Ideal - 0.6 - -0.7 0 1000 2000 3000 4000 5000 6000 P (bar) FIGURE IV.6: Similar diagram to Figure IV.5 for LAS-23, a partially-melted metasedimentary xenolith from the Tumbres Flow. Quartz Is never in equilibrium with the orthopyroxene and magnetite in this rock. i6 m- calculated but has been estimated as -9.35 by comparison with other Lascar magmas at the same temperature. This may not be correct. The texture of this rock indicates that it may represent a partially melted metasedimentary xenolith. The equations relating silica activity to pressure at this temperature and fOg are as follows; Quartz-liquid. log asiOg = -0.070 - 1.954 X 10'^ P Ferrosilite-magnetite-liquid. log asiOg = -0.397 - 3.917 X 10'^ P (Ideal model) log asiOg = -0.224 - 3.917 xIO'^P (Williams model). In this assemblage the ferrosilite-magnetite-liquid lines do not intersect the quartz-liquid line at any pressure but indicate that the melt is undersaturated with respect to silica (Figure IV. 6 ). This may be interpreted in a number of ways. Firstly, silica may not be a stable phase but insufficient time has passed for all of the quartz to have dissolved. Secondly, the assumed oxygen fugacity may be incorrect for this rock, although altering fOg within reasonable values will not shift the ferrosilite-magnetite-liquid lines significantly. Thirdly, the magnetite may not be in equilibrium with the pyroxenes. The partially molten nature of the rock indicates that it may have 165 experienced a large increase in temperature following metamorphism, presumably due to incorporation into the andesitic host. Temporal variations in temperature, fOg and water content are likely to have occurred, producing a disequilibrium assemblage. If this is the case then the pyroxene temperature may also be incorrect. Seismic data collected shortly after the April 1993 eruption (Gardeweg et al, 1993) indicate that volcanotectonic events associated with injection of magma or fluids into fractures occurred at depths of less than 2 0 km, corresponding to pressures of less than 5.6 kbar. Exactly what this represents is unknown. It may indicate stoping at the top of the magma chamber, or magma movement in the subvolcanic feeder system or in an adjacent fissure system. The absence of plagioclase in the mafic magma prior to injection as represented by the Soncor mafic inclusion (LA-124) and the large augite phenocrysts of the Saltar flow (SM93/43) is probably due to elevated pressure. The stability of plagioclase extends to higher pressures with decreasing water content in andésites. In water-free magmas it remains until 15 kbar, but is absent at 150 to 700 bar in water-saturated magma (Sekine et al, 1979), at 6 kbar in a Paricutin andésite containing 2 wt% water (Gill, 1981), and at 10 kbar in a Cascade andésite containing 5 wt% water (Eggler & Burnham, 1973). The andesitic glass inclusions in olivines of LA- 124 contain about 2-4 wt% water as calculated from the totals of microprobe analyses. The pressure under which this magma first crystallised may have been above 5 to 10 kbar, corresponding to depths of 18 to 35 km. This is simply an estimate, but it indicates that a deep magma chamber is 166 necessary for the generation of the primary magma prior to injection into the shallower magma chamber beneath Lascar. A basaltic eruption from a small maar crater at Cerro Overo 20 km SSW of Lascar provides samples of one of these primary magmas which contains small phenocrysts of augite, olivine and Cr-spinel. Orthopyroxene does not occur and plagioclase and magnetite are found only in the groundmass. This eruption overlies the Cajon Ignimbrite indicating an age of less than 1.3-1.7 Ma (Ramirez & Gardeweg, 1982). Primitive magmas from the Bolivian Altiplano (Davidson & deSilva, 1992) demonstrate a fractionation sequence interpreted as plagioclase-absent crystallisation in deep crustal reservoirs followed by plagioclase-dominated fractionation at shallower levels. The sequence recognised at Lascar is clearly not unusual. IV.4 CONCLUSIONS. The following conlusions can be drawn from the results of calculations carried out above. 1 ) The primary mafic magma supply is a basalt or basaltic andésite which has crystallised at temperatures of 1025-1068°C and at pressures above the stability field of plagioclase. This implies that lower or mid-crustal reservoirs are present. The oxygen fugacity of this magma is about 1.4 log units above FMQ prior to injection into the shallow magma plumbing system. 2 ) The subvolcanic plumbing system consists of a shallow magma chamber at unknown depth and probably an associated fissure 167 system associated with one of the major lineaments which pass through the volcanic edifice. Magma either passes directly up the fissure system or is injected into the magma chamber. Temperatures in the magma chamber(s) have not varied greatly throughout the history of Lascar. Dacitic magmas resided at temperatures of 887-920°C and andesitic magmas at 900-970°C. The temperature contrast between resident and injected magmas is responsible for the quench textures in mafic inclusions and is likely to promote vésiculation of the mafic magma and mobilisation of the resident magma as indicated by the slightly higher (940°C) temperature calculated for a crystal-rich dacitic pumice from a mixed andesitic-dacitic flow lobe of the Soncor flow. These conditions are responsible for the efficient magma mixing which is observed. 3) Oxygen fugacity is elevated relative to FMQ with decreasing temperature in both the magmas and in calc-silicate xenoliths. This is thought to be due to buffering of melt oxygen fugacity by sulphur redox reactions in the gas phase. This will be discussed further in a later chapter. The unusually high fOg of the assemblage included in large augite phenocrysts of the Saltar flow has not been explained. Magmas which are thought to have bypassed the magma chamber such as the 1986 and 1990 lavas have a relatively low fOg, supporting the suggestion that conditions in the magma chamber are responsible for the elevation of fOg. 4) The temperature and oxygen fugacity conditions under which liquid immiscibility occurs in the andesitic magma of the Soncor flow is not accurately known, but is assumed to have occurred at a temperature between 940 and 1025°C and fOg between 1.43 and 2.38 log units above FMQ. 168 CHAPTER V. BULK CHEMICAL AND ISOTOPIC COMPOSITION. V.1 CHEMICAL COMPOSITION. From careful pétrographie analyses, three processes have been identified. These are magma mixing, fractional crystallization, and country rock assimilation. Some or all of these processes may occur simultaneously. Whole rock analyses of major and trace elements, and of strontium and neodymium isotopes, have been carried out in an attempt to quantify these processes and are listed in Apppendix 3. A plot of KgO against SiOg (Figure V.1) shows that Lascar magmas are medium- to high-K andésites and dacites. Si 0 2 contents vary from 55.5 to 67.8 wt%. Major elements have been plotted against SiOg in Figure V.2. Most form narrow straight trends, which might indicate either magma mixing or fractional crystallization. The exception is Na, which plots in a scattered fashion, and Al, which shows some scatter in the andésites. TiOg, AlgO^, FegOg*, MnO, MgO, CaO and PgOg all decrease with increasing silica, and KgO increases. This is consistent with the fractionation of plagioclase, pyroxenes and Fe-Ti oxides, with minor apatite. Some of the scatter in AlgO^ can be attributed to 2 analyses of Centre 1 andésites, which have anomalously high aluminium contents. The same samples have high sodium and low iron and magnesium contents, with respect to the main trend. This could be attributed to plagioclase 169 ANDESITESDACITES High-K # Soncor Pumice ^ Soncor Mafic Incl. □ Centre 1 0 Centre 2 Medium K 8 1 Centre 2 Mafic Incl. T A Centre 3 70 SI02 FIGURE V.1: Variation of K2 O with SI02 for Lascar magmas, showing the Soncor Pyroclastic Flow, mafic Inclusions, and magmas of Centres 1, Il and III. rfo accumulation, but it is thought more likely to be the result of zeolite formation. Zeolites are common within vesicles of Centre 1 andésite lavas and pyroclastic flows. A simple mixing line can be drawn through the data in most of the plots, but when least-squares mixing calculations between compositional end-members are carried out, high residuals are obtained for sodium and aluminium. The sodium plot is examined more closely in Figure V.3. A mixing line, drawn from the mafic inclusions to an acid pumice of the Tumbres Flow of Centre 2 , fits some of the data. However, two trend lines can be seen which depart from this line at different points, towards lower sodium contents. The first occurs at low silica, and involves some andésites of Centre 2 and all of Centre 3. The second involves the pumices of the Soncor Flow. These trends are extrapolated backwards and pass through average plagioclase analyses from these two sets of rocks. The trends are thus interpreted as the result of plagioclase-dominated fractionation. Pétrographie analysis of pumices from the Soncor Flow indicates that fractional crystallization followed mixing between acid and mafic endmembers. This is deduced from zoning patterns in plagioclase and pyroxenes, as well as from the presence of mafic inclusions and other disequilibrium textures. The fractionation was dominated by plagioclase, with lesser pyroxene and minor oxides. Least-squares mixing calculations FIGURE V.2: Variation diagrams for major elements plotted against Si02 for whoie rock compositions. Symbois as in Figure 6. 171 0.14 0 . 1 2 - 0 . 1 0 - 0.08 54 56 58 60 62 64 66 68 70 SI02 SI02 19 6 - 18 5 - 17 4 - O O) 16 ^ 3- o Q o ^ e e 15 2 - • o 14 4— 1— I—’— 1— I— I— 1—1— 1—1— 1— 1— 1—1— 1— 1 - — 1— 1—'— 1— '— 1— '— 1— 1— r —1— 1— '— 1— '— 54 56 58 60 62 64 66 68 70 54 56 58 60 62 64 66 68 70 Si02 Si02 8 8 7 7 6 6 5 5 4 4 3 54 56 58 60 62 64 66 68 70 54 56 58 60 62 64 66 68 70 SI02 SI02 COIS OZ 89 9 9 »9 2 9 0 9 8S 9S tS SIC 020 S20 oeo SCO 20IS OZ 99 99 tr9 29 09 8S 9S VS G 20!S OZ 89 99 t'S 2 9 09 8S 9S ^S OC 2G 9G 0> 2 > have been carried out on these pumices, using mean analyses of minerals from two pumices from the section at Talabre. Non-reequilibrated oxide analyses were obtained from a pumice at the top of the flow, and a wide variety of plagioclase and pyroxene analyses were averaged from a pumice at the base of the flow. These mineral compositions are listed in Table V.1. The results of the calculations are presented in Table V.2. The most mafic Soncor pumice analysis was selected as a parent, and all others were derived from it using the mineral compositions. The residual errors from these calculations are relatively low (Table V.2), and indicate that the model is viable. Fractionation is dominated by plagioclase and clinopyroxene, with plagioclase increasing relative to clinopyroxene as fractionation proceeds. Lesser orthopyroxene and minor magnetite and ilmenite are also involved. The other plagioclase fractionation line, involving andésites of Centres 2 and 3, has also been investigated by this method. It is significant that lavas of the 1986 and 1990 eruptions span the entire trend. Modelling indicates that the trend is produced by low levels of fractional crystallization (up to 11.5 %), involving mainly plagioclase, with minor growth of pyroxenes and oxides. Since this trend originates from the most mafic compositions found in Lascar rocks, represented by the mafic inclusions of the Soncor and Capricorn flows, it is considered likely that the magmas involved have not fractionated in the postulated magma chamber, but have crystallised in a separate feeder system during transport to the surface. This has already been suggested on the basis of pétrographie analysis of the 1986-90 lavas 174 4 .8 ^ D Plagioclase 4.6 - 4.4 - • Soncor, P.Grandes 4 .2 - A Soncor M.l. Mixlng" Line □ Centre 1 4.0 - o Centre II CM O (0 3.8 - ■ Centre II M.l. A Centre III 3.6 - ® Plagioclase 3.4 - Plagioclase 3.2 - Fractionation Lines 3.0 5 4 5 6 58 60 6 2 64 66 68 7 0 S i02 FIGURE V.3: Plot of whole rock Na 2 Û against SiOg with mixing and fractionation trends. See text for explanation. n s - TABLE V.1. Compositions of Minerals used in Least-Squares Mixing Calculation on Soncor Pumice. (n = 15) (n = 18) (n = 9) (n = 9) (n = 12) Parent (Tu-69) Plagioclase Orthopyroxene Clinopyroxene M agnetite Ilm enite 81 0 2 61.40 54.16 53.44 52.20 0 . 2 0 0.24 T I0 2 0.76 0 . 0 0 0.22 0.41 7.12 28.86 A I2 0 3 15.49 28.26 1.75 1.75 2.15 0.46 -w F e 2 0 3 * 6.05 0.52 18.36 9.96 83.06 63.85 MnO 0 . 1 0 0 . 0 0 0.47 0.30 0.07 0.02 MgO 3.32 0.03 25.23 13.84 1.59 1.45 CaO 5.39 11.59 1.30 21.49 0 . 0 0 0 . 0 0 N a 2 0 3.80 4.69 0.13 0.38 0.41 0.23 K 2 0 2.17 0.31 0 . 0 0 0.00 0 . 0 0 0 . 0 0 TOTAL 9 8 .4 8 99.56 100.90 100.33 94.60 95.11 Note: Low total in whole rock analysis is due to the omission of PgOg and H20+, high totals in pyroxene analyses are due to recalculation of iron as FegO^) TABLE V.2. Result of Least Squares Mixing Calculations on Soncor Pumice. ...... Percentage of Parent...... Sample SI02 Wh. Rock Plag. Opx. Opx. Mtite. llmte. Residuals^ T U -85 62.23 94.02 2.32 0.34 2.82 -0.08 0.40 0.0847 tJ TU 8 6 62.92 89.02 5.08 1.40 3.64 0.13 0.47 0.0581 •W TU - 6 6 63.14 90.04 3.38 1.85 3.35 0.38 0.36 0.1818 TU 65 63.87 86.48 6 . 0 2 2.80 2.72 0 . 6 8 0.43 0.0817 LA122 63.88 85.69 6.94 2.33 3.22 0.94 0.44 0.2622 T U -87 64.08 81.27 10.72 3.68 2.99 0.47 0.70 0.1058 LA121 64.94 78.60 11.76 2.84 4.75 1.09 0 72 0.1356 and other flows {Chapter III). An intermediate magma storage system may be present or plagioclase-dominated fractionation may simply be occurring on the walls of the conduit. Fractional crystallisation within the growing lava dome at the surface has been discounted due to the absence of plagioclase cumulates or plagioclase-enriched compositions in the erupted rocks. In contrast, the April 1993 eruption products define a trend which appears to indicate mixing between the Centre III type magmas and a more silica rich composition lying on the main trend line. This is interpreted as indicating that the 1993 eruption involved magma from the magma chamber which mixed with rocks already in the conduit, suggesting that a major change in the plumbing system had occurred. The main trend, represented by the “mixing line" in Figure V.3, could be the product of either fractional crystallization or magma mixing. Two sets of least-squares mixing calculations have been carried out on analyses which lie on this trend. Firstly, magma mixing is investigated by deriving intermediate compositions from the two compositional end members (LAI 43 and LA117). Secondly, the magmas are derived from a mafic parent (LAI 43) using the mineral compositions in Table V.1. These compositions are used because the host rock lies roughly midway along the trend, and the average mineral compositions do not vary greatly from one rock to another. Both methods are successful, although the fractional crystallization model produces lower residual errors because the program has more phases to manipulate. The results of these calculations are presented in Tables V.3 and V.4. The fractional crystallization trend predicts that plagioclase is again dominant, but orthopyroxene is also very important, with minor contributions from clinopyroxene, magnetite and ilmenite. This prediction agrees with 178 Table V.3. Least Squares Mixing Calculations on Main Trend: Magma Mixing Model S am ple. S I0 2 % Mafic Parent (LA-143) % Acid Parent (LA-117) Residuals^ 4Ü LA124 56.19 97.52 2 . 8 8 0.6714 S> LA141 57.17 83.91 15.68 0.1283 T U -90 57.48 8 6 . 2 0 14.09 0.8152 LA138 57.76 88.14 13.09 0.8897 LA130 60.28 59.89 40.00 0.3064 T U -69 61.40 46.64 52.24 0.5444 TABLE V.4. Least Squares Mixing Calculations on Main Trend: Fractional Crystallization Model. Percentage of Parent.- S am ple SI02 Wh. Rock Plag. Opx. Cpx. Mtite. llm te. R esidi oo LA124 56.19 91.84 6.27 -0.06 1.08 0.23 0 . 0 0 0.0050 O LA141 57.17 77.19 13.28 5.70 2.23 0.57 0.84 0.0316 T U -90 57.48 71.55 18.05 7.46 1.26 -0 . 0 1 0.59 0.0631 LA138 57.76 73.61 14.86 8.92 0.40 -0.48 1 . 1 0 0.0922 LA130 60.28 53.95 27.07 11.37 4.38 0.58 1.60 0.1359 T U -69 61.40 46.45 33.48 13.32 3.41 0.87 1.63 0.1579 LA117 67.79 32.07 40.91 16.27 5.67 1.28 2.17 0.3118 pétrographie observations, as orthopyroxene is generally the dominant ferromagnesian mineral. A third possibility, considered the most likely, is that this trend is the result of combined magma mixing and fractional crystallization. This would be in agreement with pétrographie analysis. Indeed, in order for the compositional variation to be set up in the first place, fractionation of the magma would have to occur. Injections of the mafic parent into the magma chamber would then produce mixing trends with compositions along the fractionation trend. In the case of Lascar, the fractionation trend is approximately linear, so that it is parallel to the mixing lines. Trace element trends (Figure V.4) are also investigated. Of the K- group, Rb and Ba show increasing trends with increasing SiOg. Some Centre 3 lavas lie slightly below the Ba trend, probably due to Ba incorporation into plagioclase. Sr decreases steadily due to partitioning into plagioclase. The compatible group elements Sc, V, Or, Co and Ni all show decreasing trends. V decrease is interpreted as evidence for magnetite fractionation (Garcia and Jacobson, 1979). The decrease in Ni and Cr is due mainly to pyroxene fractionation. The chalcophile group elements Cu and Zn decrease due to incorporation into sulphides. Cu is anomalously low in the mafic inclusion LA-124, presumably due to the separation of chalcopyrite. The Ti-group elements Nb and Zr both have increasing trends, except for the Soncor dacite pumice and the Capricorn lava of Centre 2, both of which FIGURE V.4: Trace elements plotted against SIO 2 for whole rocks. Symbols as in Figure 6. 181 ZOIS ZOIS OZ 99 09 99 OZ 89 99 t'9 29 09 89 99 ^9 J— I— I— I— I— I— I— I— L_i— I— I— I— 1_4_ 000 o o - OOP - 001 - 009 - 009 - 002 OT - OOZ ooe ZOIS ZOIS OZ 89 99 P9 29 09 89 99 P9 OZ 89 99 P9 29 09 89 99 P9 ___1___1___1___1___1-__l— . — 1— .— 1 _ L _ I , — 1— ^ ------o1----L__l_____1__1__1__1__1__l_J__l_J__1__ - 09 - 000 / v - 001 C55^7 - OOP * □ < • “ çO WO 091 - 009 — • a • 8 # o ZOIS ZOIS OZ 89 99 P9 29 09 89 99 P9 OZ 99 09 99 01 - 09 91 - 08 02 e • - 001 92 021 40 100 30 - 60 20 - 54 56 58 60 62 64 66 68 70 54 56 58 60 62 64 66 68 70 Si02 SI02 120 14 13 100 12 11 10 9 8 7 54 56 58 60 62 64 66 68 70 54 56 58 60 62 64 66 68 70 SI02 SI02 200 180 - 160 - 140 120 56 58 60 62 64 66 68 70 60 65 7055 SI02 SI02 ZOIS ZOIS OZ 59 09 55 - 02 - 52 ZOIS ZOIS OZ S9 09 SS OZ 59 09 55 O t ov O V J- Oo O e 8Z ZOIS ZOIS OZ 59 09 55 OZ 59 09 55 - 01 - 05 - 21 have Zr values well below the main trend. Both of these flows contain mafic inclusions, with Zr concentrations similar to the host rock values. It is possible that the dacitic magmas in both cases have been enriched in Zr by fluid transport from the injected mafic magma. The thorium-group elements Th and Pb both increase with fractionation, except for some Soncor Pumices which have suffered slight hydrothermal alteration and have depleted Pb concentrations. Of the rare earths. La and Ce increase, and Nd has a flat trend, with some scatter. Y has a scattered trend. 01 levels are fairly constant at a few hundred ppm but are anomalously high in the altered pumices of the Soncor flow where it fills Quebrada Talabre. These trace element variations support the conclusions drawn from petrography and major element geochemistry, and are similar to most orogenic andesitic trends (Gill, 1981). V.2 RADIOGENIC ISOTOPE RATIOS. 87Sr/86Sr increases from 0.7057 to a maximum of 0.7068 with increasing Si 0 2 and Rb, and decreasing Sr (Figure V.5), but flattens off at about 60 wt% SiOg. ^^^Nd/ shows no apparent correlation with major element or Sr isotopic variation. The same lack of correlation is observed in lavas from Cerro Tumisa (Gardeweg, 1991a). The Sr isotopic variation is interpreted as evidence for assimilation of some radiogenic continental crust during differentiation. The Sr and Nd isotopic variation is compared with trends for the various volcanic zones (Figure V. 6 ) and with other centres of the Central Volcanic zone (Figure V.7). The variations of Sr and Nd isotope 185 0.7075 0.7070 ■ Deruelle 1985 O) # Soncor Pumice CD 00 A Soncor Mafic Incl. 0.7065 en □ Centre 1 Centre 2 00 O 0.7060 B Centre 2 Mafic Incl. A Centre 3 0.7055 ■ I " ' I 55 60 65 70 S i0 2 0.7075 0.7070 ■ en (D 00 □ ■ □ 0.7065 ■ en 00 0.7060 ■ 0.7055 0 20 40 60 80 100 120 Rb 0.7075 k_ 0.7070 ■ en CD 00 0.7065 ■ en 00 0.7060 ■ 0.7055 300 400 500 600 700 800 Sr FIGURE V.5: ®^Sr/®®Sr plotted against SiOg, Sr and Rb. \26 0.5132 O Lascar Volcano AVZ 0.5130 ilombia T3 0.5128 ssvz Z 5 T— 0.5126 •O Z NSVZ co ^ 0.5124 CVZ 0.5122 0.5120 0.702 0.704 0.706 0.708 0.710 0.712 87Sr / 86Sr 0.725 O Lascar Volcano 0.720 c/) C£> 0 0 0.715 en E5 CVZ 0.710 S.Peru 0.705 NSVZ SSVZ AVZ 0.700 50 60 70 80 Si02 FIGURE V.6: Comparison of Lascar Sr and Nd Isotopes with volcanic zones of the Andes. Data from this study, Deruelle 1985, James & Murcia 1984, Francis et al 1977, James et al 1976, James 1982, Co Ira & Barblerl 1989, Gardeweg 1991a, Davidson et al 1990, Francis, Sparks et al 1989, Futa & Stern 1988, HIckey-Vargas et al 1989, HIckey et al 1986. l? 7 0.5128 O Lascar Volcano 'Mantle Array 0.5127 0.5126 ■O Z Tumisa 5 0.5125 N. de Payachata Z 0.5124 CO Chascon ^ 0.5123 Cerro Galan Puricoi 0.5122 0.5121 0.704 0.706 0.708 0.710 0.712 87Sr / 86Sr 0.712 O Lascar Volcano 0.711 Cerro Galan 0.710 C/) 0.709 CO Purico 0 0 k . 0.708 co Chasran 00 0.707 N. de Payachata 0.706 Tumisa 0.705 0.704 50 55 60 65 70 75 Si02 FIGURE V.7: Comparison of Lascar Sr and Nd isotopes with other volcanic centres of the Centrai andes. Data from this study, Deruelle 1985 (Lascar), Gardeweg 1991a (Tumisa), Davidson et al 1990 (Nevados de Payachata), Francis, Sparks et al 1989 (Cerro Galan), Hawkesworth et al 1982 (Purico- C hascon). ratios with latitude from the Northern Andes to the Austral Volcanic Zone in Southern Chile are plotted for Cenozoic to Holocene volcanoes in Figure V.8 . The large degree of crustal contamination in Central Andean magmas relative to the rest of the Andes is clearly visible. Lascar is typical of the C.V.Z., but does not show the extreme crustal contamination of large centres such as Cerro Galan and Purico-Chascon. In fact, the Sr and Nd isotopic variation is almost identical to that shown by the nearby Cerro Tumisa (Gardeweg, 1991a). It is possible that these two centres, which are adjacent, are closely related in terms of primary magma source and magma evolution. In order for the magma to assimilate its wallrocks, it is only necessary for the least refractory component of the wallrock to melt. Disaggregation will then release the remaining phases into the magma as xenocrysts. It is interesting that the transition from A.F.C. to fractional crystallization as the magma evolves also occurs in other centres (e.g. Villarica-Lanin; Hickey-Vargas et al, 1989). This is probably related to the temperature drop during differentiation. As the magma cools it eventually becomes unable to assimilate the wallrocks and so evolves by pure fractional crystallization. Alternatively, a melt-depleted buffer zone may form on the walls of the magma chamber. 189 0.725 o o □ Colombia 0.720 - A Ecuador « S.Peru CO 0.715 - CO O O C.V.Z 00 &_ ■ o 8 N.S.V.Z CO 0.710 - o AS.S.V.Z CO • A.V.Z 1 ■ a. 0.705 - % J 0 o □ LASCAR 0.700 -10 1 0 20 30 40 50 60 Latitude°S 0.5132 0.5130 - 0.5128 - 0.5126 - CO 0.5124 - 0.5122 - 0.5120 -10 0 1 0 20 40 50 6030 Latitude °S FIGURE V.8: Variation of ®^Sr/®®Sr and with latitude for late Cenozoic to Holocene volcanic centres along the Andean chain. Values for Lascar are shown for comparison. Data from this study, Co ira & Barbieri 1989, Davidson et al 1990, Davidson & deSilva 1992, Deruelle 1985, deSilva et al 1993, Francis et al 1977, Francis, Sparks et al 1989, Futa & Stern 1988, Gardeweg 1991a, Hickey et al 1986, Hickey-Vargas et al 1989, James 1982, James et al 1976, James & Murcia 1984. m o CHAPTER VI. XENOLITHS AND THEIR RELATION TO KNOWN BASEMENT GEOLOGY. VI.1 GENERAL. The basement rocks beneath the volcanic front are very poorly known since they are covered by a continuous blanket of large-volume ignimbrites and other volcanic rocks. The discovery of xenoliths of various rock types in many of the eruption products of Lascar allows an indirect study of these basement rocks. Skarn xenoliths are indicative of past and probably active contact metasomatism around the magma chamber underlying the volcano. From this study various types of xenoliths are known from Lascar and surrounding volcanic centres. These are of four main types; 1 ) Cenozoic lava fragments, consisting of biotite and amphibole- bearing dacites with strongly developed dehydration textures and partial melting of matrix glasses. 2) Partially-melted hornfels xenoliths, or buchites, with a grain boundary or web-like melt phase. 3) Skarn xenoliths of various types indicating contact metasomatism of carbonate rocks. 4) Fragments of the near-surface hydrothermal alteration zone consisting of altered lavas and pumices with sulphur and sulphate minerals. 191 Granitic xenoliths have also been reported from blocks ejected during the 20 February 1990 eruption (Gardeweg et al, 1990) but have all been found to be partially melted metasedimentary fragments. These xenoliths are obviously derived from a variety of sources at different depths within the subvolcanic system. Since the basement rocks in the area are very poorly exposed such xenoliths may provide an indication of basement geology and help to correlate between isolated exposures. The parageneses and significance of the xenoliths are dealt with according to the above classification. V I.2 CENOZOIC LAVAS. Hornblende and biotite-bearing dacite xenoliths occur in the Soncor and Tumbres flows in large numbers, and are particularly common in the lithic-rich lag breccias of the Soncor Flow. These are similar in appearance to samples from the lava domes to the north and south of Lascar and are thought to represent fragments of Cenozoic volcanics ripped from the conduit walls at shallow depths during explosive volcanism. Smaller xenoliths of this type only a few cm across are found in a number of lava flows. They usually exhibit features indicative of reheating at shallow depths including dehydration alteration of biotite and hornblende and disaggregation due to melting of the matrix glasses. It is common to find hydrous phenocrysts either partially or completely altered to a fine-grained aggregate of pyroxenes, plagioclase and Fe-Ti oxides. The pyroxene and plagioclase phenocrysts from these xenoliths are similar in compositional range to those in the host magmas, so that it is difficult or impossible to distinguish xenocrysts from phenocrysts in Lascar magmas, particularly when disequilibrium textures due to magma mixing are so common. Much of 192 the quartz found in Lascar rocks, especially in andesitic lavas, may therefore be xenocrystic, since calculations of the pressure stability of quartz in Lascar magmas seems to indicate that it has not grown in equilibrium with the melt and other phenocryst phases. The disaggregation textures illustrate the ease with which assimilation of such rocks can occur, since the host magma need only melt the matrix glass to release the phenocryst phases. VI.3 PARTIALLY MELTED METASEDIMENTARY ROCKS. Quartz-rich partially melted contact metamorphic rocks occur in small numbers in the Tumbres Flow, along with large concentrations of skarn xenoliths. A continuous compositional trend exists between partially melted metaquartzites and calcic skarn xenoliths. Only two samples of these partially melted rocks have been examined by optical petrology and electron microprobe analysis. LA-111 consists mostly of quartz with a granular texture and 120° grain boundary triple points. Salite, anorthite, apatite and sphene occur in smaller amounts between quartz grains along with a brownish devitrified glass. Zircon and wollastonite occur as rare inclusions in quartz. This rock is thought to have originally been an impure carbonate-cemented sandstone which has undergone decarbonation reactions producing pyroxene, wollastonite and anorthite followed by melting of hydrous phases at grain boundaries to produce an interstitial glass. The glass is dacitic in composition (63-66 wt% SiOg) and contains high CaO (9-10 wt%), low Na^Q (0.43-0.66 wt%) and high 01 (1.0-1.3 wt%). High temperatures (approximately 900°G) are required to produce this texture (Spry, 1969) so FIGURE VI.1 Plots of major oxides against Si02 for partial melt glasses in the buchite xenoliths LA-111 and LAS-23. 193 FeO AI203 Ti02 o p o lO o o> o> o " (g eg 5 5 f ro ro ► > ►► ► N a 2 0 C aO MgO IS9 CA 8 ro > > at at o “ o " o “ (g eg eg o 5 5 A ro ro ro o ” o " o “ o CM A LA111 A LAS23 Si02 0.15 0.10 - CO 0.05 - 0.00 50 60 70 80 Si02 1.5 1.0 - ü 0.5 - 0.0 5 06 0 7 0 8 0 Si02 that it probably formed following the incorporation of the xenolith into the andesitic Tumbres magma. Glass inclusions within quartz phenocrysts are more mafic (51-52 wt% Si 0 2 ) with lower Cl (0.45-0.51 wt%), higher Na20 ( 1 . 2 wt%) and very high Ca (24 wt%). Glasses of different compositions may indicate melting of different phases or growth of quartz during the early stages of melting to incorporate earlier, more mafic partial melts. LAS-23 is a white clast from the Tumbres Flow. It contains large amounts of quartz (70-80 %) with lesser plagioclase, orthopyroxene, augite, magnetite and apatite in a glass matrix. Small (10-20 p) euhedral crystals of zircon and thorite have also been found. The quartz is rounded due to partial melting and the glass is rhyolitic (74-75 wt% SiOg). Other phases are believed to have formed by crystallisation from this partial melt. Calculations of the pressure-silica activity conditions in this assemblage (Chapter IV) indicate that the melt was silica undersaturated during crystallisation of these phases, confirming that quartz was being dissolved whilst pyroxenes and magnetite were forming. The appearance of this rock is very similar to that of a buchite figured in Spry (1969; Plate XIX (e)). This type of texture commonly occurs at temperatures of around 950°C, in agreement with the 2 - pyroxene temperature obtained for this rock (Chapter IV). The final result of this process is a crustal melt and the buchites found in the Tumbres Flow may represent the source of the crustal contaminant indicated by Sr isotope analysis (Chapter V). Glass compositions from LA- 1 11 and LAS-23 are plotted in Figure VI. 1. 197 VI.4 SKARN XENOLITHS. VI.4.1 Petrography of Calc-Silicate Xenoliths. Several types of calc-silicate xenoliths are found in the Lascar lavas and pyroclastic flows. These are broadly divided on the basis of mineralogy into wollastonite skarn, diopside-garnet skarn and magnetite skarn. Wollastonite skarn typically consists mainly of wollastonite with lesser anorthite and quartz and minor diopside-hedenbergite, fassaitic augite, grossular-andradite garnet and apatite. Fine veins of calcite-quartz or wilkeite-quartz occur. Small relicts of calcite are included in wollastonite. These xenoliths often exhibit concentric compositional zonation due to reaction with the host magma. They are commonly either rimmed with a thin layer rich in diopside or partially digested with a thick friable outer zone and a fresh white core, due to digestion of the wollastonite by the host magma. One sample (LA-108) has a rim zone with fine-grained quartz, anorthite, diopside-hedenbergite, grossular-andradite garnet, fassaitic augite, apatite, wilkeite, monazite and barite. Many diopside-rich rims on wollastonite skarn xenoliths also contain magnetite. Diopside skarn (e.g. LA-115) is generally fine grained and contains quartz, anorthite, diopside-hedenbergite and large poikiloblastic grains of grossular-andradite garnet. Zircon and sphene occasionally occur. Magnetite skarn (e.g. LA-144) is a fine-grained rock containing augite, grossular-andradite garnet, anorthite, quartz, magnetite, apatite and sphene. A small xenolith ( 1 cm) in the Capricorn Lava (LA-140) contains diopside-hedenbergite, plagioclase, quartz, hemoilmenite, magnetite, ilmenite and sphene. These magnetite and hemoilmenite bearing rocks are thought to have formed under progressively more oxidising conditions. 198 SAMPLE TYPE Wol End DIM Fas Opx GrA Pig Qaz Mtt llm Hel Apt WII Sph Zrn Tho Moz Bar Cac Volcan Lascar. LAI 08 Well. Skarn XX XX XX XX An XX XX XX XX XX XX LA111 Buchite XX XX An XX XX XX XX LA112 IZ Woll. Skarn XX XX An XX XX LA112 0 Z Mtt. Skarn XX XX XX An XX XX XX LA113 Woll. Skarn XX XX An XX XX XX XX XX LA115 Di-Gt Skarn XX XX An XX L A I4 0 (X) Mtt. Skarn XX XX XX XX XX XX XX XX LA 144 Mtt. Skarn XX XX XX XX XX XX XX LAI 48 Diop. Skarn XX An XX XX XX XX JD LAS15(A) Mtt. Skarn XX XX XX XX XX XX XX XX - 0 LASIS(B) Woll. Skarn XX XX An XX XX LAS23 Buchite XX XX XX XX XX XX XX XX Cerro Lejla. SM93/55 Mtt. Skarn XX XX XX XX XX XX SM93/56 (A) Mtt. Skarn XX XX XX XX XX SM93/56 (B) Diop. Skarn XX XX XX XX SM93/56 (C) Mtt. Skarn XX XX XX XX XX KEY: Woi=Wollastonite; End=Endiopside; DiH=Diopside-Hedenbergite; Fas=Fassaitic Augite; Opx=Orthopyroxene; GrA= Grossular-Andradlte; Plg=Plagloclase (An=Anorthlte); Qaz=Quartz; Mtt=Magnetlte; llm=llmenite; Hel=Hemoilmenite; Apt= Apatite; Wii=Wilkelte; Sph=Sphene; Zrn=Zircon; Tho=Thorlte;Moz=Monazlte; Bar=Barlte; Cac=Calclte. IZ and OZ denote Inner and outer zones In concentrically zoned xenoliths. A, B, and C denote compositional bands In layered xenoliths. The compositional variation exhibited by skarn xenoliths is a product of varying initial composition, position in the metasomatic aureole and alteration following incorporation into the magma. One sample (LAS-15) is banded with alternating wollastonite-rich and diopside-rich layers. It has an outer alteration zone containing augite, orthopyroxene, magnetite and ilmenite. The banding is probably due to a relict compositional variation in the original rock. The mineral assemblage and textures are indicative of formation by contact metasomatism rather than regional metamorphism which would be unlikely to produce the anorthite, apatite and wilkeite. Whole rock analyses of a wollastonite skarn and a diopside skarn are listed in Appendix 3. Calcite-quartz veins may have formed from COg-rich fluids percolating through fractures and altering wollastonite by the reaction; CaSiOo + COo = CaCOo + SiO 2 Quartz-wilkeite [ Ca 5 (P0 4 ,S 0 4 ,Si0 4 )3 (0 H,F,CI) ] veins indicate that fluids rich in phosphate and sulphate are also present. The presence of wilkeite and absence of sulphides suggest that the oxygen fugacity in the skarns lies above the sulphide-sulphate reaction curve (NNO + 1). VI.4.1.1 Pyroxenes. Pyroxene analyses have been recalculated and projected onto the Di-Hd-En-Fs quadrilateral (Figure VI.2) after Lindsley (1983). The wollastonite skarns examined contain diopside and the diopside skarns 200 LAS-23 LAS-15 En FIGURE VI.2: Projections of pyroxenes from skarn and buchite xenoliths onto the pyroxene quadrilateral using the method of Lindsley (1983). Open triangles = magnetite skarn; Open circles = diopside skarn; Filled circles = wollastonite skarn; Filled squares = buchite. contain salite. In the magnetite skarns the compositions are more variable, falling in the diopside, endiopside and augite fields. The banded wollastonite/diopside skarn xenolith LAS-15 contains ferrosalite and coexisting ferrohypersthene. The buchite LAS-23 contains coexisting endiopside and orthopyroxene on the enstatite-bronzite boundary. There is thus good correlation between the Mg# of coexisting clinopyroxenes and orthopyroxenes in these rocks, with tie lines radiating from Ca on a En-Fs- Wo triplot. It is notable that the magnetite skarns, which are thought to have equilibrated under near-magmatic conditions, contain more Ca-poor clinopyroxene than the diopside and wollastonite skarns, suggesting that they formed at a higher temperature (Lindsley, 1983). The field overlaps that of the buchite LAS-23. VI.4.1.2 Garnets. Garnet compositions lie along the grossular-andradite join with minor amounts of other components (Figure VI.3). Most are andradite-rich but the wollastonite skarn LA-108 contains a bimodal population, some of which are andradite-rich and some of which are more grossular-rich. This has been related to an increase in temperature on digestion of the xenolith by the magma. VI.4.1.3 Wollastonite. Wollastonite is usually nearly pure CaSiO^ but the banded skarn xenolith LAS-15 contains two generations of this mineral. Pre-existing rather altered looking wollastonite crystals contain a significant amount of Mg (0.5-0.9 wt%), Mn ( 1 .9-2.1 wt%) and Fe (7.3-7.7 wt%). Secondary more euhedral wollastonite contains smaller amounts of these impurities (Mg = 202 Cr rO a AI 30 40 50 60 70 80 90 Fe3+ FIGURE VI.3: Triangular plot (Molar Cr-AI-Fe) of garnet compositions from skarn xenoliths. Open squares = LA108; Open triangles = LA113; Open diamonds = LA112; Filled circles = LA115; Filled squares = LA114. Ca 10 20 20 30 30 40 50 50 Mg Fe+Mn FIGURE YI.4: Triangular plot of molar Ca-Mg-(Fe+Mn) in wollastonite from skarn xenoliths. Open circles = wollastonite skarns; Filled squares = banded skarn LAS-15. 0.0 to 0.2 wt%, Mn = 0.8 to 1.0 wt%, Fe = 1.5 to 1 . 8 wt%) Wollastonite from contact-metamorphosed impure limestones contains up to 9 wt% FeO and 1 . 2 wt% MnO (Deer et al, 1966). This altered, impure wollastonite in LAS-15 may have formed at an early stage before the iron, manganese and magnesium were scavenged by growing garnet and pyroxenes. Wollastonite compositions are plotted on a triangular plot of molar Ca-Mg- (Fe+Mn) in Figure VI.4. VI.4.1.5 Plagioclase. In all wollastonite and diopside skarns the plagioclase is very anorthite rich (An90 to 100) with very little or no orthoclase component (Figure VI.5). This implies that the rock is low in alkalis and not in equilibrium with the magma. In magnetite skarns and buchites a second population appears with the compositional range An33 to 55 and a higher orthoclase content (up to 5 mol%). Therefore these rocks have at least partially equilibrated with the magma. VI.4.2 Geothermobarometry of Skarn and Buchite Xenoliths. Magnetite-ilmenite geothermometry of magnetite-bearing skarn xenoliths yields temperatures of 657-779°C (Chapter IV). Variable temperatures obtained from different mineral pairs are probably the result of heating of the sample from contact metamorphic to magmatic temperatures on incorporation into the magma. The buchite xenolith LAS-23 has a 2 - pyroxene temperature of 950°G, consistent with estimates of the temperature required to cause such extensive melting. For samples which do not contain magnetite-ilmenite or augite-orthopyroxene pairs it is more difficult to obtain temperature estimates. Coexisting ferrosalite and ferrohypersthene in the 205 banded xenolith LAS-15 yields a temperature range of 360°C ± 220°C (Andersen, 1992). A fluid inclusion homogenisation experiment was attempted on a wollastonite skarn xenolith containing quartz grains with 2 - phase (liquid 4-gas) fluid inclusions, but the inclusions decrepitated at temperatures of 250-300°C, probably due to fracturing of the grains during thin section preparation. Geothermobarometric calculations have been carried out on a sample (LA-108) containing the assemblage grossular-quartz-anorthite- wollastonite. The temperature-pressure dependent equilibrium reaction for this assemblage is; CagAl2Si20^2 4- Si02 = CaAl2Si20g 4- 2CaSiOg Grossular + Quartz = Anorthite + 2 Wollastonite Mole fractions of grossular in garnet and anorthite in plagioclase were calculated from microprobe analyses. Activity-composition relationships for grossular were estimated from Engi & Wersin (1987; Figure 5) and anorthite- rich plagioclase was assumed to behave ideally (Carpenter & Ferry, 1984). Equilibrium constants for the system were calculated using the equation; K = (8An . awo) / (aor. aSi 0 2 ) Where K is the equilibrium constant for the reaction, aAn is the activity of anorthite in plagioclase, awo is the activity of wollastonite in wollastonite, aoris the activity of grossular in garnet and asi 0 2 is the activity 206 of SiOg in quartz. Since quartz and wollastonite are essentially pure phases the equilibrium constant for the reaction is simply; K = aAn/aGr Two compositional populations of garnets are present in the rock. These have the following compositional ranges. 1 ) XGr= 0.624 to 0.697 (n = 4) 2) XGr= 0.304 to 0.365 (n = 3) Plagioclase is anorthite-rich, with XAn = 0.962 to 0.993. Two sets of equilibrium constants are derived, assuming ideality and using the model of Engi and Wersin (1987) to estimate grossular activities; Ideal Model. (i) K = 1.38-1.59 (Group 1 Garnets) (ii) K = 2.64 - 3.27 (Group 2 Garnets) Engi & Wersin, 1987. (i) K= 1.23-1.51 (Group 1 Garnets) (ii) K = 9.6 - 19.8 (Group 2 Garnets) 207 An Ab Or FIGURE YI.5: Plot of plagioclase from skarn and buchite xenoliths on the a north ite-albite orthoclase friangle. Symbols as in Figure VI.2. ZOS Application of the non-idea! solution model has the largest effect on the Group 2 grossular-poor garnets since the near-end member compositions exhibit the greatest non-ideality. Equilibrium constants at varying pressure and temperature were calculated using the program THERMO (Perkins et al, 1987) and the equilibrium constants obtained from the above calculations were fitted to pressure-temperature lines. These lines have a positive slope on plots of pressure against temperature and at the same pressure the more grossular poor garnets would be in equilibrium with the anorthite at higher equilibrium temperatures (Figure VI. 6 ) The formation pressures have been estimated using the temperature range from magnetite-ilmenite compositions in other xenoliths as limits on formation conditions. For the Group 1 garnets the derived pressure at 650°C is 3.5 to 3.8 kbar (ideal model) and 3.6 to 4.1 kbar (Engi & Wersin, 1987) and at 780 °C is 6.5 to 6.7 kbar (ideal model) and 6.5 to 7.0 kbar (Engi & Wersin, 1987). For the Group 2 garnets the derived pressure at 650°C is 1.8 to 2.3 kbar (ideal model) and -2.29 to -0.9 kbar (Engi & Wersin, 1987) and at 780°C is 4.4 to 5.0 kbar (ideal model) and -0.5 to 1.4 kbar (Engi & Wersin, 1987). The grossular poor garnets are therefore unlikely to be in equilibrium with the rest of the assemblage at lower temperatures. This may indicate that these Group 2 garnets have formed following a heating event. In order for the xenoliths to become entrained in the magma some expansion of the magma chamber at the expense of wall rocks must occur, presumably by some a mechanism like stoping. This will result in an outward migration of isotherms, slowly heating the wall rocks from contact metamorphic to magmatic temperatures. This temperature rise will effectively occur at constant pressure. The xenoliths will then be held at magmatic 209 Ideal Model 8 7 6 n 5 0 ) V. 3 4 (0 (0 3 An99.3 An99.3 0 ) G r62,4' G r30.4 2 An96.2 1 G r36.5 0 450 500 550 600 650 700 750 800 850 Temperature °C Engi & Wersin 1987 5 4 n An99.3 3 o> G r62.4 3 V) (/} 2 o An96.2 / 1 Gr36.5>^ An99.3 0 450 500 550 600 650 700 750 800 850 Temperature °C FIGURE VI.6: Calculated pressure-temperature relationships of coexisting clinopyroxene, wollastonite, plagioclase and quartz In the wollastonite skarn xenolith LA-108. The limits of magnetite-ilmenite temperatures from magnetite skarn xenoliths are included. ZlO temperatures until the magma rises to the surface during an eruption. Decompression of the xenoliths will occur at approximately constant high temperature. Following eruption the xenoliths will either cool slowly within a lava flow or be quenched in a pyroclastic eruption at atmospheric pressure. If the lower magnetite-ilmenite temperature of 650°C is taken as a metamorphic formation temperature of LA-108 then it will have suffered isobaric heating at around 3.5 to 4 kbar. The pressure of the magma chamber is thought to be below 6 kbar since this is the lower pressure limit for silica saturation in the Soncor pumice LA-122 (Chapter IV). If the Group 2 garnets in LA-108 are assumed to be the product of such isobaric heating they indicate temperatures of 700 to 760°C (ideal model) and 840 to 1 1 0 0 °C (Engi & Wersin, 1987). The upper temperature limit is the temperature of the magma, which in the case of the Tumbres Flow is 950°C. The actual pressure-temperature-time path is dependent upon the length of time between incorporation of the xenoliths into the magma and its eruption. If they are ripped from the conduit walls during an eruption they will suffer heating during decompression, whereas if they are incorporated into the magma by stoping of the wallrocks they will be heated to magmatic temperatures at constant pressure (Figure VI.7). The formation of thick reaction rims around many of the xenoliths and the partially-melted textures of the buchites supports the latter interpretation. In the case of the buchites it is clear that the xenoliths have experienced magmatic temperatures. The actual melting of the xenoliths may have occurred during decompression. Using an estimated temperature range of 650 to 750°C and a pressure range of 3.5 to 4.5 kbar, coexisting andradite and hedenbergite populations in LA-108 have been used to calculate the oxygen fugacity 211 5 Metamorphic incorporation conditions into magma 4 Heating by expansion of magma chamber s3 £ Transport to surface I 2 Heating during decompression 1 _ Xenolith incorporated ' into magma chamber. Xenolith ripped ^ — «k mm* 0 500 600 700 800 900 1000 Temperature X FIGURE Vi.7: Diagrammatic representation of the pressure-temperature time paths of xenoliths which are incorporated into the magma and held at magmatic temperatures before eruption, and xenoliths which are ripped from the conduit walls during an eruption. ZIZ conditions under which the assemblage formed (Zhang & Saxena, 1991). The f02 buffering reaction is; 4CaFeSi20g + ZCaSiOg + O 2 = 2 C a g F e 2S i30^2 + 4 S i0 2 4Hedenbergite + 2Woilastonite + 0 2 = 2Andradite + 40uartz The calculations are insensitive to the input pressure (varying the pressure from 3.5 to 4.5 kbar produced an f 0 2 change of 0.01 log units) so that the main controlling variables are mineral compositions and temperature. Only the more grossular-rich Group 1 garnets have been used since the temperature of formation of the other garnets is highly uncertain. The mole fraction of andradite in these garnets varies from 0.300 to 0.372 and the mole fraction hedenbergite in the clinopyroxenes varies from 0.027 to 0.083. This assemblage produces an estimated f 0 2 range of -17.26 to - 14.95 log units (FMQ + 1.63 to 3.95) at 650°C and -13.99 to -11 . 6 6 log units (FMQ 4 - 2.17 to 3.35) at 750°C. These calculations place the f 0 2 of the wollastonite skarn dominantly above the sulphide-sulphate reaction curve, confirming that sulphate is the stable form of sulphur. Magnetite skarn xenoliths have generally higher f 0 2 values (Chapter IV) confirming that more elevated f 0 2 is necessary for magnetite to form in these rocks (Figure VI. 8 ). VI.4.3 Glass and Fluid Inclusions. Three types of fluid inclusions have been found in quartz grains in both skarn and buchite samples. All three types occur in any one grain and there appears to be little morphological difference between fluid inclusions in the skarns and those in the buchites. They are described below: 213 Skarn Xenoliths -10 MH -12 - (Q Po/Ah a -14 - L A -1 0 8 o -16 “ o> FMQ o -18 - -20 600 650 700 750 800 850 Temperature X FIGURE VI.8: Calculated temperature-f02 relationships from magnetite- ilmenite pairs In skarn xenoliths and coexisting andradite and hedenbergite In the wollastonite skarn LA-108. 2«+ 1) Low-relief inclusions with one or more small vapour bubbles and a spherical shape. When in contact with each other they have a planar contact surface which causes them to resemble bubbles. Small crystals are sometimes present attached to the vapour bubble. These are thought to be glass inclusions with contraction bubbles. 2 ) High relief inclusions, often with a very small vapour bubble, which are commonly faceted to form negative crystals. These are thought to be liquid COg inclusions (below). 3) Multiphase inclusions which contain colourless material, vapour and apparently opaque material. These are thought to contain glass, water, COg and possible hydrocarbons. Cavities which are interpreted as exposed fluid inclusions are visible in polished thin section. These are commonly lined with tiny crystals of anhydrite, suggesting the presence of sulphate-bearing fluids. Infrared spectroscopic analysis of the host quartz grain and its fluid inclusions in the buchite LA-111 reveals a variety of phases. Apparently inclusion-free quartz has absorption peaks at 3597, 3382, 3319, 2676, 2607, 2500, 2248 and 2147 cm-"" (Figure VI.9). A series of large absorption peaks due to aluminosilicate network vibrations begins at 2 0 0 0 cm \ continuing to lower wavenumbers and masking any other peaks due to volatile phases. Since quartz does not possess any absorption peaks above 1 1 0 0 cm-1 (Liese, 1975) these bands are suggested to indicate the presence 215 o o a E 0 k_ 0 ) 1n ins co o o Absorbance (Arbitrary Units) § FIGURE VI.9 Infrared spectrum of apparently Inclusion-free quartz grain and portion of grain with colourless glass inclusion from the buchite xenolith LA- 111. l i é of glass in the analyses. The peaks listed above have been attributed to various phases as follows. 3597 cm '' COg. 3382, 3319 cm*'' 0-H stretching vibration of hydroxyl groups in specific sites, probably in hydrous minerals such as clays. 2676, 2607, 2500 cm'"' Unknown origin. 2248 cm-'' Possibly NgO. 2147 cm-'' Unknown origin. No peak representing the combination of v., and Vg modes at 3710 cm-1 (Stolper et al, 1987) has been found but an additional peak at approximately 3600 cm-'' occurs in the spectrum of pure COg gas at 0.996 bar (Scutari et al, 1993) and the peak at 3590 cm-'' has been tentatively assigned to this mode. The two small peaks at 2368 and 2342 cm-'' are attributed to the Vg vibrations of atmospheric COg. The peak at 2248 cm '' is not thought to be a COg peak since it is unusual for the single peak attributed to the Vg vibration of liquid COg to be shifted to lower wavenumbers. Molecular COg dissolved in glass (Fogel & Rutherford, 1990) has a large peak at 2350 cm-'' due to ^^COg and a small peak at 2287 cm-'' due to ''^COg. At elevated pressure in liquid COg these peaks tend to shift towards higher wavenumbers (H.J. Milledge, pers. comm.). The Vg NgO line at very low pressure lies at 2248 cm-'' (Hamdouni et al, 1993) and is tentatively suggested as the source of this peak. 217 Colourless inclusions (Type 1 ) have been analysed through the host quartz and the quartz spectrum subtracted from the quartz + inclusion spectra. These have a number of additional absorbance peaks (Figure VI.9). A wide peak with a maximum absorbance at 3382 cm'"' and tailing off to lower wavenumbers is attributed to the 0-H stretching vibration of water or hydroxyl dissolved in the glass with a continuum of hydrogen bond lengths. Other important peaks occur at 3067-3036 cm’"' and 2966, 29235, 2878 and 2840 cm T These are typical of C-H stretch bands of hydrocarbons containing CH^ groups (Barres et al, 1987; Pironon, 1990) and in this rock are most likely to indicate the presence of methane. The two peaks at 2248 and 2147 cm-^ are still present although this may be due to imperfect spectral subtraction. The coexistence of water, COg and hydrocarbons can be explained by the breakup of water at high temperatures to form hydrogen. This might occur under conditions of low fOg, perhaps in the presence of carbon; 2HgO 4- C = 2 Hg 4- COg The hydrogen thus produced would then react with either COg or more carbon to produce hydrocarbons such as methane; 2 Hg 4- C = CH 4 4Hg 4- COg = C H 4 4- 2HgO 218 These reactions are likely to proceed until any carbon or other reducing agent has been consumed to a point where equilibrium is reached between the fluid phases. In the closed system graphite-HgO, COg and CH 4 are present in almost equal quantities (Labotka, 1991) but if the fOg is buffered by an external reaction such as FMQ the fluid becomes HgO-rich at intermediate temperature and COg-rich at high temperature. Graphite has not been recognised in LA-111 but may be present in the fluid inclusions. This implies that the rock was initially metamorphosed under reduced fOg conditions, perhaps because it contained organic carbon. With increasing temperature the graphite-COg and graphite-HgO curves drop towards lower fOg relative to FMQ so that the metamorphic fluids become steadily more reducing. The possible occurrence of NgQ in the quartz is problematic and no previous reference to the occurrence of this gas in metamorphic rocks has been found in the literature although Ng is known in granulites from Norway (T. Anderson, pers. comm.). If the limestone precursor contained organic material as suggested by the presence of methane then it is possible that nitrogen bearing compounds were produced in the early stages of metamorphism. Further work is necessary before the presence of NgQ can be confirmed. The presence of anhydrite crystals lining cavities in quartz crystals is at odds with the evidence for low fQg in this rock. The assemblage appears to indicate strong disequilibrium between different fluid inclusions, perhaps developed during the change from metamorphic to magmatic conditions. Fluid inclusion homogenisation experiments have been carried out on a quartz grain frm LA-1 1 1 containing all three types of inclusions. The heating stage reached 750°C at its maximum power output with no 219 observable changes in the inclusions, confirming that they had formed at a high temperature. Cracks in the quartz crystal around some of the colourless glass inclusions healed by annealing between 600 and 700°C but reformed on cooling to room temperature over a period of 1 hour. These cracks are thought to be the result of contraction of the quartz during the beta to alpha transition at 573°C (Deer et al, 1966). Therefore the quartz was raised to temperatures exceeding 573°C during metamorphism. VI.4.4 Stable Isotopes. The carbon and oxygen isotopic composition of calcite in skarn xenoliths has been determined by dissolving the samples in phosphoric acid and analysing the evolved CO 2 using a mass spectrometer. A sandstone sample from the Lila Formation exposed in a small hill near the Cerros Cuyugas which contains carbonate cement has also been analysed for comparison. COg has been liberated from the buchite sample LA-111 by step heating in vacuo and analysed using a mass spectrometer. The carbon in this sample is considerably lighter {d^^C (P.D.B.) = -17) than that in calcite in the skarn xenoliths (below) confirming that major decarbonation has occurred. The step heating method does not allow determination of 3‘'®0. The of carbonates in wollastonite skarns varies from -10.06 to 4-3 . 1 0 permil (PDB) and the 3‘'®0 varies from 4-22.25 to 4-26.74 permil (SMOW). The basement sandstone SM93/67 contains carbonate with a similar isotopic composition to that in the skarns = 4-0 .3 5 3 , = 4-24.77) but is not considered a likely parental rock as discussed below. Two trends are visible in the data (Figures VI. 10, 11). Firstly, 220 TABLE VI.2 Isotopic composition of carbonate in skarn xenoliths and COg in buchite xenolith. SAMPLE a‘‘3C (PDB) a'*®0 (PDB) a'lSQ (SMOW) wt% Cal CARBONATES IN CALC-SILICATES. LA105 (IZ) +1.95 -6.53 +24.09 6.60 LA105 (IZ) +1.95 -6.49 +24.14 6.84 LA107 +2.86 -5.60 +25.05 3.17 LA108 -9.52 -8.00 +22.58 1.89 LA108 -9.00 -7.84 +22.75 LA108 -10.06 -8.32 +22.25 LA109 (IZ) +2.84 -5.66 +25.00 6.62 LA112 (IZ) -5.01 -5.25 +25.42 1.36 LA113 (IZ) -2.19 -3.97 +26.74 4.50 LA116 +3.10 -5.27 +25.39 6.54 LAS15 -4.65 +6.59 +24.03 LAS15 -3.81 -5.64 +25.01 LIQUID COg IN BUCHITE. LA111 -17.04 0.00 CARBONATE IN BASEMENT SANDSTONE. SM93/67 +0.35 -5.88 +24.77 221 there is some scatter but a weak positive correlation between 3''®0 and Secondly, decreases rapidly in samples with lower CaCO^ contents from an initial cluster of analyses with _ 2 . 8 to 3.1 andCaCO^ = 6.5-6.S wt% to 9 1 ^ 0 = -17.1 in the carbonate-free buchite LA-111. Both of these trends might be interpreted as the result of decarbonation reactions since heavy carbon and oxygen tend to partition into CO 2 in equilibrium with carbonate at high temperatures (Bottinga, 1969; Mattey et al, 1990; Rosenbaum & Sheppard, 1986). The fractionation factor ^^C/^^C{CO^ I becomes positive at temperatures exceeding 192°C and the fractionation factor ^^0 1 ^ ^ 0 (0 0 I ^^OI^^O(C0iCO^ is positive at all temperatures to at least 1000°C (Friedman & O’Neil, 1977). A calculated decarbonation trend is plotted (Figure VI.11) using the samples with the highest CaCOg and as a parent and assuming an isotopic fractionation factor (COg/CaCOa) for carbon of +2 permil (Mattey et al, 1990) and open- system Rayleigh distillation, using the equation; d - 0Q ~ A In f (Allard et al, 1991) where d and 3q are the present and original of the carbonate, respectively, A is the COg/carbonate isotopic fractionation factor, and f is the fraction of the original carbon remaining. The intermediate data points lie somewhat below the curve but it predicts quite well the strong depletion in as the last carbonate is lost as seen in LA-111. However, although some of the calcite in these samples is relict from the parent rock, most of it occurs in veins and is interpreted as 222 o o CO Q a -5 - O Skarns Ü • Basement CO ro - 1 0 - -15 22 23 24 25 26 27 ai80 (SMOW) FIGURE VI.10 ai3c plotted against of calcite In skarn xenollths. A calcareous sandstone from the nearby basement Is included for reference. OQ Q Q. □ Calc-Silicates O Model CO ro - 1 0 - -20 0 2 4 6 8 wt% CaCOS FIGURE VI.11 ai3c plotted against wt% CaCOg In skarn xenollths and buchite xenollth LA-111. A calculated decarbonation trend from the most carbonate-rich xenollth Is Included. it-i the result of interaction of woilastonite with COg-rich fluids fluxing through the rock (above), so that a simple decarbonation trend is not expected to explain the observed isotopic variation. A complex interaction between fluids from various sources and any remaining carbonate from the parent rock is therefore considered the most likely scenario and analyses of unaltered limestone and COg from the magma would be neccessary in order to elucidate the exact processes. Unfortunately the lavas and pyroclastics contain little carbon and have all suffered extensive degassing of COg which will have strongly altered the carbon isotopic composition so that no estimate of the magmatic carbon isotopic signature is possible without, for example, direct sampling of magmatic gases. VI.4.5 Source of Calc-Silicate Xenollths. The parent rocks of skarns and buchites are most likely to be a sequence of calcareous sediments consisting of carbonate-cemented sandstones, limestones and possibly dolomitic limestones. They have suffered extensive metasomatic alteration adjacent to the subvolcanic magma chambers of Volcan Lascar and Cerro Lejia and active skarn formation is almost certainly taking place at present. Previously the presence of limestone underlying the area was not known and no such rocks occur in the basement outcrops along the eastern margin of the Salar de Atacama. The only carbonate-bearing rocks nearby are carbonate-cemented terrestrial sandstones of the Lila Formation (Ramirez & Gardeweg, 1982) which outcrop in the Cerros Cuyugas 25 km WNW of Lascar. A sample from a small hill about 3 km NE of the Cerros Cuyugas contains carbonate with similar carbon and oxygen isotopic ratios to the carbonate in the skarn xenoliths. This could conceivably represent a source rock for the quartz-rich 224 buchites but not the other skarns. Two marine carbonate sequences occur in adjacent areas. In the Cordillera de Domeyko to the west is a sequence of Jurassic to Lower Cretaceous marine limestones (Flint et al, 1993) deposited in what was then the Domeyko Basin. However, this sequence does not extend eastwards of the Cordillera de Domeyko since a basement high was present on the site of the present Atacama Basin. A more likely candidate is the Yacoraite Formation, part of the Cretaceous-Eocene Salta Group of Northwest Argentina (Marquillas et al, 1993). This formation consists of a post-rift sequence of high-energy light coloured limestones with bands of arenaceous and calcareous-arenaceous rocks which was deposited within and around the Salta Rift. It is known to lap onto the basement high of Northern Chile to the east of the basin. The alternation of pure limestones and calcareous sandstones is consistent with the postulated rock types from which the skarns are derived. No such xenoliths have been found in Cerro Tumisa (Gardeweg, 1991a) suggesting either that the sequence does not extend this far west or that the magma chamber beneath the complex was at a different depth and not in contact with the deposit. The N-S trending Miscanti Line which passes through Volcan Lascar and Cerro Lejia may represent a basement structure which limits the western extent of the Yacoraite Formation. A study of basement xenoliths in other volcanic centres might improve the knowledge of the extent of this group, since the skarn xenoliths are easily recogniseable. 225 CHAPTER VII. VOLATILE COMPONENTS AND THEIR CONTROLS ON MAGMA CHEMISTRY AND ERUPTION STYLE. VII.1 MAIN VOLATILE COMPONENTS. This chapter addresses the problem of volatile components in magmatic and volcanic systems. Since fluid pressure is thought to be a major driving mechanism in explosive volcanic eruptions A careful study has been made of the behaviour of these components and put forward a number of new models explaining the controls which they exert on the magma both within the magma chamber and on its way to the surface. Current thinking on the solubility mechanisms of these components in magmas has been applied in the production of these models. The solubilities of volatile components in magmas are strongly dependent upon temperature, pressure, melt composition and oxygen fugacity. When a volatile component becomes saturated in the melt it will separate either into phenocryst phases or as a coexisting gas. The important volatile components in Lascar magmas are water, sulphur dioxide, hydrogen sulphide, chlorine and fluorine. Carbon dioxide may also be a major component, particularly in parts of the magma chamber in contact with skarn zones. Complex relationships occur between these components dissolved in the magma, incorporated into phenocryst phases and in coexisting gas phases. Since the fumaroles in the active crater are too inaccessible and dangerous for gas phases to be sampled directly, these components must be studied by careful analysis of the 226 eruption products and the formulation of models based on their known behaviour in calc-alkaline magmas.The various volatiles are dealt with individually below. VII.2 WATER. VII.2.1 Direct Evidence for the Presence of Water as a Magmatic Phase. H2 O is the main volatile component of Lascar magmas. A steam dominated plume has been emitted from the active crater since at least as long ago as 1984 (Gardeweg, 1991b) and its source is clearly the high-temperature fumaroles in the active crater. Infrared spectroscopy of glass inclusions in plagioclase phenocrysts in a Soncor pumice clast (Chapter IV) has confirmed the presence of molecular water in the melt. Most glass inclusions in phenocrysts contain a contraction bubble, produced by a differential volume change between the melt and the host phenocryst in cooling. Low totals in most electron microprobe analyses of glass inclusions and matrices provide empirical evidence consistent with the presence of dissolved water. Amphibole and biotite phenocrysts require the presence of magmatic water or halides in order to become stable phases. The common occurrence of low-density pumices is the result of vésiculation due to exsolution of water during rapid decompression of the magma. This is one of the mechanisms which drives violent eruptions of calc-alkaline magmas such as the 1993 Lascar eruption (Gardeweg et al, 1993b). The cyclic extrusion and withdrawal of the lava dome may reflect volume changes in the magma column due to vésiculation and subsequent rise to the surface and degassing of bubbles. 227 VII.2.2 Solubility Mechanisms of Water in Silicate Melts. Models for the dissolution of water in melts depend on certain assumptions about the structure of silicate melts. The "quasi crystalline" model of melt structure (Burnham, 1979) assumes that structural units in a melt mimic those of the corresponding solid, and that electrical neutrality is preserved on a structural unit scale. Melting occurs by the disorientation of (Si, Al) 0 4 tetrahedra. In a pure Si 0 2 melt the water molecule reacts with bridging oxygens to produce OH" groups; HgO + Q2- = 20H" (Reaction 1) Experiments on albite melts show that this reaction is not responsible for water dissolution, as modal solubility of water in NaAISigOg melts is greater than in SiOg melts even with equal modal numbers of O^" ions. Instead, the exchange of protons for Na’’" ions occurs; HgO + Q2- + Na+ = OH" + ONa" + H+ (Reaction 2) This causes depolymerization of the melt. The network breaks into sheets held together by Na - O bonds. In the presence of excess HgO, reaction 1 occurs. NMR and near - infrared studies of albite and rhyolitic glasses have shown that molecular water is present in significant quantities. An equilibrium has been proposed (Stolper, 228 1982b) between molecular water and OH groups; HgO + QO = 20H (Reaction 3) More recent NMR studies of hydrous silicate glasses (Kohn et al, 1989) have led to the discovery of a number of structural groups. In Si0 2 glasses, a water molecule is suggested to be bonded to a Si - OH group and two bridging oxygens. Reaction 3 is postulated for the formation of the OH groups. Spectra from albite glasses indicate the interaction of Si - OH and "O - Si groups, possibly with both groups bonded to the same silicon atom. In all the glasses studied the percentage of the total water dissolved in the form of Si - OH appears to be small (< 2 0 %). Since the solubility of water in silicate melts increases with pressure (Hamilton et al, 1964), a drop in pressure, for example during transport of the magma to the surface, will encourage the formation of a coexisting hydrous vapour phase by bubble growth in the magma. A drop in temperature may also encourage water to exsolve due to the growth of water-poor phenocryst phases which will increase the amount of water remaining in the melt. This might not occur if hydrous minerals such as amphibole or biotite are important phases. 229 VII.2.3 Contentrations and Behaviour of Water In Lascar Magmas. Direct measurements of whole-rock water content do not provide a reliable estimate of original magmatic water concentrations because much of the water is degassed on decompression. Evidence for this comes from the vésiculation of all of the magmas to various degrees and from the strong degassing of the active crater. Water contents of glasses have been estimated by the difference method in electron microprobe analyses of glasses (wt% HgO (diff) = 100 - total other oxides). There are a number of problems associated with this technique. If the analysis total is not known then it is difficult to distinguish between the deficiency due to the presence of unanalysed components and that which is due to a poor analysis. Also the relatively low density of rhyolitic glasses can in itself result in a low analytical total. However, if a large number of analyses are examined then a general trend in water contents with increasing Si 0 2 is visible (Figure VII. 1). H 2 O (diff) follows a flat trend with high scatter (2-6 wt%) until the melt becomes rhyolitic (Si 0 2 = 72-73 wt%) at which point it drops rapidly, reaching 0-2 wt% in the most evolved glasses (SI 0 2 = 75- 77 wt%). This is taken as evidence for degassing of water from the most evolved melts, probably during transport of the magma to the surface and subsequent eruption. This degassing will result in polymerization of the melt and increase the viscosity of the magma. Viscosities of melts have been calculated from glass compositions and estimated water contents by the method of Shaw (1972). Calculated viscosities increase steadily with increasing Si 0 2 and then increase sharply in the rhyolitic glasses (Si 0 2 = 72-77 wt%) due to the drop in water contents (Figure 230 6 5 OO 4 3 o CM X 2 O o 1 0 50 60 70 80 Si02 FIGURE VII.1 H 2 O (=100-TOTAL other oxides) In microprobe analyses of glasses plotted against wt% SIO2 from various Lascar rocks. 231 VII.2). These calculations provide minimum viscosity estimates for the magmas since the presence of phenocrysts and gas bubbles will increase the effective viscosity. Original melt water contents can be estimated using the plagioclase-melt equilibrium equations of Housh & Luhr (1991). Plagioclase must be demonstrably in equilibrium with the melt (phenocryst rims or microphenocrysts with consistent compositions, glass inclusions in plagioclase), and the temperature of equilibration must be known. Calculation of equilibrium water contents using this method is difficult because of the large spread in mineral compositions in mixed magmas. It has to be stressed that attainment of near-equilibrium is unusual in these magmas. However, the Soncor dacite pumice sample LA-122 contains plagioclase phenocrysts with a narrow range of compositions. These are assumed to be in approximate equilibrium with their matrix glass, and the temperature of equilibration can be estimated using magnetite-ilmenite geothermometry. Melt water contents, calculated using plagioclase rims and matrix glass compositions, and assuming pressure-temperature conditions of 3 kb and 900 °G, are 0.9 to 1.1 wt% (Anorthite model) and 1.9 to 2 . 1 wt% (Albite model). This relatively low water content may indicate that the degassing suggested by the data in Figure VII.1 occurred whilst the magma resided in the magma chamber in the case of the Soncor Flow. The eruption may therefore have been driven by a coexisting hydrous vapour phase which had already separated from the magma. Quenching of the mafic magma during magma mixing appears to have caused it to exsolve much of its water. This can be seen in andesitic 232 LT) O) P oo o 65 70 75 Si02 (melt). FIGURE VII.2: Calculated viscosities of glasses from microprobe analyses and estimated water contents (Shaw, 1972) plotted against wt% 3102. 2 ^ 5 mafic inclusions from the Capricorn Lava, which are strongly vesiculated and contain large amounts of quench-textured hornblende, and in andesitic pumices of the Soncor Flow, many of which are rich in hornblende and biotite. The formation of vesicles in the andesitic mafic inclusions may have reduced their densities to the point where they could rise through the host dacitic magma, thus increasing the efficiency with which the two magmas became mixed. VII.3 CARBON DIOXIDE. VII.3.1 Evidence for CO 2 as a Volatile Phase in Lascar M agm as. There is little CO 2 present in lava and pumice samples from Lascar. Analyses of whole rock carbon using a LEGO 08-125 Carbon- Sulphur Determinator yield between 0 and 2500 ppm (Figure VII.3). No detectable CO 2 peaks were present in infrared absorbance spectra of melt inclusions in phenocrysts (Chapter IV). This implies either that CO 2 is not present in any large quantities in the magma or that degassing of almost all of the CO 2 has occurred prior to melt entrapment in the phenocrysts. If this is the case then it is difficult to assess the importance of CO 2 as a volatile component. The occurrence of skarn xenoliths in many lavas and pyroclastic flows implies that metasomatic alteration of carbonate wallrocks should be releasing CO 2 into the magma. 234 0.3 # Soncor Flow 0.2 - I Soncor M.l. O n Centre 1 CM O o Centre II ü ■ Centre II M.l. o ^ A Centre III 0.1 - o% 0.0 -- 1-- I 0 1 ------^ 1— '— r 54 56 58 80 62 64 66 68 70 Si 02 2000 . . .Q ... 54 56 58 60 62 64 66 68 70 S i0 2 2500 2000 - 1500 - ü 1000 - 500 - 55 80 65 70 Si02 FIGURE VII.3: Whole rock carbon dioxide, sulphur and chlorine contents plotted agains wt% Si02 for Lascar rocks. VII.3.2 Solubility Mechanisms of CO2 in Siiicate Meits. The CO 2 molecule is not hydrolyzeable, so cannot break Si - O - Si bonds and thus CO 2 has a low solubility in both Si 0 2 and NaAISigOg melts (Burnham, 1979). CO 2 dissolves in silicate melts by the reaction; CO2 + 02- = CO32- Solubility is slightly higher in albite melts because AP+ - repulsion is lower than Sl^""" - 0^+ repulsion. Increased (Na + Al)/Si increases CO 2 solubility by depolymerizing the melt. At higher pressures AP+ changes to edge - shared octahedral coordination, disrupting the structure and creating larger holes, which may be able to accept molecular CO 2 . CO 2 solubility is much higher in the presence of H2 O, which breaks Si - O - Si bonds, increasing the expansivity of the melt and allowing the dissolution of molecular CO 2 . Water also depolymerises the melt (as a result of KT^ - Na"^ exchange) increasing the number of non - bridging oxygens with which CO 2 can react. The spéciation of CO 2 in synthetic Ca±Mg-bearing and natural basaltic glasses has been studied using infrared spectroscopy (Fine & Stolper, 1985/86). The authors conclude that CO 2 dissolves in such melts by forming distorted Ca or Mg carbonate ionic complexes, in contrast to C 0 2 -bearing sodium aluminosilicate melts which contain 236 both molecular CO 2 and dissolved Na carbonate ionic complexes. The solubility of CO 2 in these melts varied between 62 and 347 ppm. Rhyolitic melts dissolve CO 2 almost entirely in the molecular form (Fogel & Rutherford, 1990) and this results in a higher solubility of CO 2 in rhyolitic melts than in basaltic melts under the same conditions. The mole ratio of carbonate to molecular CO 2 dissolved in melts generally decreases in the sequence basalt, andésite - rhyodacite - rhyolite. The solubility of molecular CO 2 in rhyolitic melts increases with increasing pressure and decreases with increasing temperature. Because of its low solubility in silicate melts CO 2 is one of the first volatile phases to evolve in magmatic plumbing systems. In a study of Kilauea Volcano, Greenland et al (1985) conclude that the initial magma contained 0.32 wt% H 2 O, 0.32 wt% CO 2 and 0.09 wt% S. It degassed most of its CO 2 in a shallow magma reservoir whilst retaining most of its water and sulphur. Gerlach & Graeber (1985) suggest that most of the volatiles in the primary magma supply were present as dissolved volatiles but CO 2 was transported from depths of circa 40 km as a separate fluid phase before being degassed through the summit caldera. A similar model is proposed for the 1984 eruption of Mauna Loa Volcano (Greenland, 1986). Because of its low solubility in silicate melts at upper crustal levels CO 2 tends to leak continuously from volcanoes through their flanks and from fumaroles (Gerlach, 1991). Emissions of CO 2 tend to increase prior to eruptions in many volcanoes due to an influx of fresh magma. Most subaerially erupted magmas are likely to have degassed almost all of their CO 2 and measurements of whole rock CO 2 are not likely to indicate the original magmatic values. 237 Vil.3.3 Behaviour of Carbon Dioxide in Lascar Magmas. Since the solubility of CO 2 in silicate melts is very low at upper crustal levels the incoming primary magma is likely to have exsolved most of its CO 2 on arrival at the subvolcanic plumbing system. This gas is most likely to accompany the basaltic magma until it is injected into the magma chamber and then be degassed through the summit fumaroles. Any CO 2 that is evolved as a result of skarnification of carbonate wallrocks will be degassed in the same fashion. Because of this it is impossible to estimate the amount of CO 2 present except by either analysing the gas emissions or conducting solubility experiments at elevated temperatures and pressures on known magma compositions. V11.3.4 Isotopic Composition of Magmatic Carbon. The of carbon dioxide evolved from a mafic inclusion between 1100 and 1200°C by stepped heating was found to be -26.4 permil (P.D.B.). This very light carbon reflects the almost complete degassing of the magmatic CO 2 since the average mantle isotopic value is estimated as -3.6 permil (Javoy & Pineau, 1991). The isotopic fractionation factor between CO 2 gas and carbon dissolved in silicate melts is measured as +4 permil (Javoy et al, 1978) and +2.4±0.2 permil (Mattey et al, 1990). Assuming an initial mantle isotopic value and a fractionation factor of +2.4 permil this magma only contains 0.0074% of its original carbon. Similar light carbon has been found in the 1950 lava flows of Mauna Loa, Hawaii (Naughton & Terada, 1954) with= _ig to -24 permil. In contrast, CO 2 collected from fumaroles on Kilauea in 238 1984-85 (Friedman et al, 1986) has a very constant of -3.5±0.3 permil. VII.4 CHLORINE AND FLUORINE. VII.4.1 Evidence for Fluorine and Chlorine in Lascar Magmas. Chlorine is often detectable in electron microprobe analyses of the Lascar andesitic and dacitic glasses (up to 0.3 wt%). It is also present in hornblende, biotite and apatite analyses and in the immiscible iron-rich melt globules from the Soncor Flow. Fluorine has not been analysed in glasses or phenocryst phases since the F peak is overlapped by the Fe escape peak in X-Ray energy spectra. Fluorine is, however, clearly an important volatile constituent since water samples from the springs at Tumbre which had been covered by pyroclastic flows of the 19-20 April 1993 eruption contained greatly increased levels of fluoride. VII.4.2 Solubility Mechanisms of F and 01 in Silicate Melts. According to Burnham (1979) FICI and FIF react with silicate melts by breaking Si - O - Si bonds in the same way as the water molecule; 239 MCI + 02- = c r + OH' HF + 02- = P- + OH' The presence of halite and sylvite in volcanic gases (Shinohara, 1989) indicates ion exchange reactions between the melt and hydrothermal solutions; NaCI + K+ = KOI + Na+ Experiments on chlorine solubility in hydrous silicate melts have been carried out by Metrich & Rutherford (1992). 01 is found to partition strongly into an associated hydrous vapor phase and reaches a plateau in the melt when the 0 1 molality in the vapour phase (moles 0 1 per kg fluid) exceeds approximately 2. This corresponds to an immiscibility field between aqueous and 01-rich fluids. The same effect has been reported by Webster & Holloway (1990). The 01 solubility in silicate liquids is also dependent upon melt composition, temperature and pressure. There is a positive correlation between 01 solubility and Al/Si molar ratio and (Na+K 4-Fe+ 0 a)/AI molar ratio in the melt. A pronounced 01 solubility minimum occurs when the (Na+K)/AI molar ratio is 1. This may indicate that 01 dissolves in metaluminous and peralkaline melts by forming alkali chloride complexes. The fluid/melt distribution coefficient for 01 decreases as the F content of the fluid and melt increase, partitioning in favour of haplogranitic melts that contain >7 wt% F relative to coexisting fluid (Webster & Holloway, 1990). This 240 may be due to the depolymerizing effect of F on silicate liquids (below). Cl solubility decreases in hydrous silicate melts with increasing pressure (Webster & Holloway, 1990; Metrich & Rutherford, 1992). With increasing temperature the fluid/melt distribution coefficient for 0 1 decreases markedly, resulting in increased 0 1 dissolved in the melt (Iwasaki & Katsura, 1967; Webster & Holloway, 1990; Webster, 1992). in a nuclear magnetic resonance (NMR) study of F-bearing nepheline, albite and jadeite glasses, Schaller et al (1992) found that F preferentially coordinates with Al to form octahedral AIFg^- complexes. This explains the decrease in viscosity of F-bearing silicates by the removal of AIO 4 units from the network and depolymerizing the melt. F partitions in favour of the melt in the presence of an associated hydrous vapour phase. F contents of natural glasses are thus likely to provide a good indicator of the pre-eruptive melt content of this element. VII.4.3 Origin and Behaviour of Chlorine in Lascar Magmas. The volatiles which are present in subduction-related volcanics are likely to have originated from a variety of different sources. Potential sources are subducted seawater-altered oceanic crust and sediments, the mantle wedge and the overlying continental crust. Anderson (1974, 1975) estimates that arc volcanism releases 10 times more HgO than, and approximately the same amount of 0 1 as, the ocean 241 ridge magmatism. He suggests that H 2 O and Cl in igneous rocks produced at the ocean ridges may be refluxed during subduction zone magmatism. Ito et al (1983) calculate that subducted altered oceanic crust contains on average 25-75 ppm 01 and 1-2 wt% H 2 O, subducted sediments contain 430-2500 ppm 01 and 2.5-20 wt% H 2 O and average arc magmas contain 900 ppm 01 and 2 wt% H 2 O. Whole rock chlorine contents, analysed by XRF, are consistently low in Lascar magmas (130-710 ppm) except in two samples of pumice from the Soncor Flow in Quebrada de Talabre (LAI 21 and LAI 22), where 01 contents are 1070 and 2230 ppm respectively (Figure VII.3). These elevated 01 levels are thought to be the result of reactions between the pumice and magmatic gases in the quebrada. The most mafic glasses of Lascar (basaltic andesitic to dacitic) contain the highest levels of 01 (mostly between 0.1 and 0.25 wt%), suggesting that the mafic magma is the source of this element. 0 1 levels drop abruptly in the rhyolitic glasses (Si02=72-78 wt%) to between 0 and 0.15 wt%. This decrease coincides with the rapid decrease in H 2 O (diff) in the glasses and indicate that 0 1 partitions into a coexisting vapour phase. The relatively low melt water content calculated from plagioclase-melt equilibria in a Soncor pumice sample indicates that this phase was exsolved whilst the magma was still growing phenocrysts in the magma chamber so that the water-rich vapour remained in contact with the melt under elevated pressure and temperature rather than degassing during ascaent to the surface. The eruption style of the volcano indicates that such shallow degassing is also important but a large proportion of the water in the magma is lost at 242 depth. Cl also partitions into apatite, hornblende and biotite but since these are not major phenocryst phases in many of the rocks it is thought likely that most of the 01 lost by the melt enters the vapour phase. An important consequence of the separation of a Cl-rich fluid is depletion of the melt in copper. A study of melt and fluid inclusions in quartz from a pantellerite (Lowenstern et al, 1991) indicates that extreme partitioning of copper into such Cl-rich fluids can occur. Fluid/melt partition coefficients for Cu as high as 1600 have been calculated. This probably by the formation of chloride complexes of copper. The depletion of copper with fractionation in the Lascar rocks (Chapter V) might be partly due to this process. VII.5 SULPHUR. VII.5.1 Occurrence of Sulphur in Lascar Magmas. Sulphur is a very important volatile element in the magmatic system of Lascar. High levels of sulphur have been detected in andesitic glass inclusions in olivines of a mafic inclusion from the Soncor Flow. Dacitic pumices from the indurated parts of the flow contain vesicle surfaces which are coated in tiny crystals of an iron sulphide mineral, indicating that a sulphur bearing gas was present which reacted with the vesicle walls. Some of the Centre I andesitic lavas contain vesicles which are decorated with iron sulphide spherules, indicating that liquid iron sulphide was produced by reaction of the gas with the liquid magma. Thermodynamic calculations (below) indicate that the gas phase consisted of mixed SO 2 and H 2 S. A crystallised Cu-Fe sulphide melt globule has been found included in an ilmenite phenocryst from the Soncor Flow. Pyrrhotite and chalcopyrite 243 are common in many rocks as inclusions in other phases. Anhydrite occurs in the Soncor and Piedras Grandes Flows and in the April 1993 eruption products and apatite often contains detectable sulphur. Sulphur dioxide is being degassed in large quantities from the summit crater (2.3 ± 1 . 1 2 tonnes per day; Andres et al, 1991 ) and the plume is often yellowish due to condensed sulphur. There are yellow sulphur deposits around fumaroles in the crater and altered pumice flows from an exposure on the western side of Centre I contain native sulphur and sulphate minerals. VII.5.2 Solubility Mechanisms of Sulphur in Silicate Melts. Sulphur solubility in silicate melts is strongly dependent upon temperature, pressure, oxygen fugacity, partial pressures of H 2 S and SO 2 . and melt composition. According to Burnham (1979) hydrogen sulphide reacts with bridging oxygens in a similar manner to water; H2 S + 02- = SH" + OH' SO 2 , produced by the “oxidation of H 2 S at low pressure”, is suggested to dissolve in the same way as CO 2 , and so is less soluble than H 2 S. More recent studies have shown that the solubility mechanisms of sulphur are more complex. Katsura and Nagashima (1974) have conducted experiments on sulphur solubility in silicate melts at varying temperature, pressure and oxygen partial pressure (PO2 ). There is a pronounced solubility minimum at PO 2 = NNO+0.0 to 244 0.5 which corresponds to the sulphide-sulphate reaction. A similar solubility minimum has been found at log pÛ 2 = -4 (NNO-0.2) in calcium ferrite melts equilibrated with sulphur at 1620°C (Turkdogan & Darken, 1961). Sulphide solubility decreases and sulphate solubility increases with increasing pOg. At constant pÛ 2 and temperature the sulphur solubility is positively correlated with the concentration of SO 2 in the vapour phase. Buchanan & Nolan (1979) and Wendlandt (1982) have demonstrated that sulphide solubility in basaltic and andesitic melts increases with increasing FeO, indicating that sulphide dissolves in these melts by the formation of iron sulphide complexes; V2 S 2 + FeO = V 2 O2 + FeS Gas Melt Gas Melt This reaction is balanced by re-equilibration of the Fe^+/Fe^+ ratio to give the reaction; V2 S 2 + 3FeO = FeS + Fe 2 0 g Gas Melt Melt Melt Dissolution of sulphur in a melt to form sulphide therefore tends to increase the f 0 2 of the melt. The relationships between sulphur spéciation and f 0 2 are complex and influence one another in sulphur- rich systems. The positive correlation between FeO and S has also been found in natural basaltic glasses with a coexisting Fe-O-S melt (Wallace & Carmichael, 1992). From the slope of the line on a graph of 245 log Xg against log Xp^o (mole fractions), when log Xpeo> -1, Poulson & Ohmoto (1990) suggest that sulphide dissolves in the silicate melts by forming F 6 3 S 0 2 groups; 2FeO + FeS = FegS 0 2 Silicate Sulphide Silicate Melt Melt Melt Sulphide solubility in basaltic and andesitic melts with a coexisting iron oxide-sulphide melt phase is inversely correlated with pressure and positively correlated with temperature (Wendlandt, 1982). When a sulphur-bearing vapour phase is also present the sulphide solubility of the silicate melt increases with increasing pressure (Carroll & Rutherford, 1985). At low fOg (< NNO -1 ) the sulphide content of the silicate melt is controlled by sulphide-silicate melt equilibrium and increases with increasing fOg (Nilsson & Peach, 1993). At f 0 2 > NNO-1 the sulphide melt becomes destabilised and sulphide saturation in the silicate melt is controlled by vapour-melt equilibrium, so that the sulphur content of the melt is negatively correlated with f 0 2 . Carroll & Rutherford (1987, 1988) have shown that sulphate is the dominant sulphur species at f 0 2 above NNO + 1 to 1.5. Sulphur solubility increases with increasing fÜ 2 in these more oxidised melts. Under these conditions anhydrite becomes a stable magmatic phase and sulphate solubility is significantly higher than the sulphide solubility of melts with equivalent FeO contents under reducing conditions (Carroll & Rutherford, 1985, 1987; Luhr, 1990). There is strong evidence 246 that coordination of Ca^+ with SO/' groups occurs in these melts (Katsura & Nagashima, 1974; Carroll & Rutherford, 1985). However, coordination with other metal ions, such as alkalis, may also occur. This is demonstrated by the positive correletion between SO 3 solubility in, and Na content of, Ga-Na silicate melts (Papadopoulos, 1973) because of the greater thermal stability of Na 2 S 0 ^ relative to CaSO^. A competing effect in this system is demonstrated by Na-poor compositions, because Ca is more able to create non bridging oxygens since it has a stronger polarising power, thus increasing the solubility of SO 3 . The positive correlation between sulphate solubility and f 0 2 can therefore be related to an increase in non bridging oxygens in the melt. Since SO 3 is not an important fluid phase in magmatic systems an equilibrium reaction can be written between sulphate in the melt and SO 2 in the vapour phase; SO2 + CaSi0 3 + V2 O2 = CaS 0 4 + Si02 Gas Melt Melt Melt Melt The dissolution of SO 2 to form sulphatewill therefore tend to reduce the melt f 0 2 . Sulfate solubility is also positively correlated with temperature and pressure (Carroll & Rutherford, 1985, 1987; Luhr, 1990). 247 VII.5.3 Sources and Behaviour of Sulphur In Lascar Magmas. V II.5.3.1 Possible Sulphur Sources. Whole rock sulphur contents are relatively low in most samples (15-600 ppm: Figure VII.3). The mafic inclusions from the Soncor Flow (LAI 24) and Capricorn Lava (LAI 41 and LAI 43) contain consistently low sulphur (58-93 ppm). The two pumice samples from the Soncor Flow in Quebrada de Talabre (LAI21 and LAI 22) contain the highest sulphur contents (1810 and 1430 ppm respectively) and this, like their high chlorine contents, is thought to be the result of reactions between the pumice and magmatic gases derived from the flow. Most of the sulphur in these samples is present as iron sulphide minerals encrusting vesicle surfaces. There is no increase in sulphur contents in anhydrite-bearing samples from the Soncor, Piedras Grandes and 1993 eruptions. The sulphur content of most dacitic and rhyolitic glasses in Lascar rocks is below the electron microprobe detection level (approx. < 0 . 1 wt%) indicating that the sulphur does not reside in the melt in most of the magmas. However, sulphur contents of andesitic glass inclusions in olivines from the Soncor mafic inclusion LA-124 reach a maximum of 0.5 wt%, indicating that the mafic magma represented by this rock is at least one source of the sulphur in the system. Another possibility is that the magma is assimilating large quantities of evaporites such as are found in the nearby Salar de Atacama, thus releasing SO 2 by the decomposition of sulphate minerals. Anhydrite could react with silica in the magma by the following reaction; 248 CaS 0 4 + SiOg = CaSiOg + SOg + '/gOj Anhydrite Melt Melt Gas Melt Isotopic evidence (below) indicates that this process is unlikely to provide a significant amount of the sulphur in the magmatic system of Lascar and other neighbouring volcanoes andthat the main source is the primary mafic magma. VII.5.3.2 Behaviour of Sulphur In the Magmatic System. Glass inclusions in olivines and augites from LA-124 contain variable amounts of sulphur, decreasing rapidly from 0.5 wt% S at SiOg = 58 wt% to less than 0 . 1 wt% 8 at SiOg = 65 wt% (Figure 111.12). This is interpreted as the result of degassing of a sulphur-bearing gas phase during fractional crystallisation. It is notable that maximum concentrations of FeO, TiOg and PgOg occur when the sulphur content reaches this low value. Calculations of temperature and f 0 2 using the 2 - pyroxene-olivine-spinel assemblage indicate that the magma lay astride the sulphide-sulphate reaction curve (Chapter IV). The sulphur in the melt is therefore likely to have been present as mixed sulphide and sulphate and it is possible that the degassing was related to this oxidation, since a solubility minimum has been demonstrated for sulphur at this transition point (above). The valence state of sulphur can be determined by measuring the wavelength shift of 8 k alpha X-rays relative to a standard. Using an electron microprobe equipped with a wavelength dispersive detector various standards containing sulphur of known valence state were analysed and the sulphur peak fitted to a gaussian curve in order to locate it accurately. The results are tabulated 249 Table VII.1 : Analysis of sulphur wavelength shift relative to pyrite for several standards. Standard. 8 Valence. AeV (Pyrite). Error. Pyrite FeSg -1 0.0 0.1 Pyrrhotite (Minas Gerais). Fe^-xS -1.910 -0.25 0.1 Galena PbS -2 -0.35 0.1 Potassium Sulphite K2 SO 3 -h4 +0.7 0.1 Celestine S rS O j +6 +1.15 0.1 Barite BaSO j +6 + 1.1 0.1 Anhydrite CaS 0 4 +6 + 1.2 0.1 250 A n h y d rite C e le s tin e B a rite >« Q. P otassium P y r ite S u lp h ite > < 0 - P y r r h o tite Galena 2 1 0 1 2 3 4 5 6 S Valence FIGURE VII.4: Plot of S k alpha wavelength shift relative to pyrite against sulphur valence for various standards. The best-fit line provides an empirical relationship between these variables. Z ^ ( in Table VII.1 and plotted in Figure VII.4. The derived empirical relationship between S valence and S k alpha wavelength shift relative to pyrite is; AeV (SK alpha) = 0.17609 V + 0.00724 (R2=0.990) Where AeV (S K alpha) is the wavelength shift of S k alpha X-rays relative to pyrite (electron volts) and V is the sulphur valence. The wavelength shift of S^+ relative to S^- is calculated as +1.41 eV by this formula. This compares well with previous work. It has been measured by XRF as 1.33 eV (Faesler & Goehring 1952), 1.37 eV (Lehr et al 1980), and by EPMA as 1.43 eV (Kucha et al 1989) and 1.47 eV (Kucha & Stumpfl 1992). Using this formula the sulphur in the andesitic glasses of LA-124 was found to contain both sulphide and sulphate, in agreement with predictions from the calculated temperature-f 0 2 conditions. Calculated sulphur valence (mol% sulphate) is plotted against calculated oxygen fugacity in Figure VII.5 for sulphur in glasses of the Soncor mafic inclusion and the matrix glasses of the Saltar Flow. The results are in agreement with those of Carroll & Rutherford (1988) and show that the transition from sulphide to sulphate occurs at approximately NNO+ 1 . The partly crystallised sulphide melt inclusions which occur in some Fe-Ti oxide phenocrysts in the Soncor flow are interpreted as having originated in the mafic magma (Figure VII. 6 ). For such immiscible melts to be stable the fOg must probably be lower than NNO- 252 100 O O Carroll & Rutherford O 80 # This study O 60 “ o tn 40 - 20 - 1 1 1 1— - 4 -2 0 2 d Iogf02 (FMQ) FIGURE VII.5: Plot Of calculated S valence (% sulphate) for sulphur in glasses of the Soncor mafic Inclusion LA-124 and the Saltar Flow. The data of Carroll & Rutherford (1988) are included for comparison. 1. A model is proposed in which the melt became more oxidised during fractional crystallisation, perhaps due to assimilation of crustal material or to redox buffering by a mixed sulphur gas phase (below). The fractionation observed in this magma is interpreted as having occurred before its injection into the shallow magma chamber which underlay Lascar, so that oxidation could not have occurred due to magma mixing. The sulphide which was dissolved in the melt phase became partly oxidised, thus reducing its solubility. A decrease in pressure and temperature further reduced the sulphur solubility and a separate sulphur-rich gas phase began to exsolve. During the separation of this gas the FeO and TiOg content of the melt steadily increased, suggesting that titanomagnetite and apatite were destabilised. Apatite is not found in this rock and magnetite does not appear in melt inclusions until the sulphur content drops below microprobe detection levels. The simplest explanation of this is that the oxidation of dissolved sulphide to form SO 2 gas inhibited the conversion of ferrous iron to the ferric state, thus preventing magnetite from forming; FeS + 3 Fe 2 0 3 = 7FeO + SO 2 Melt Melt Melt Gas In fact the gas phase is likely to contain both H 2 S and SO 2 as well as large quantities of HgO. This gas will probably remain in the andesitic magma until it enters the magma chamber when it will migrate upwards into the overlying cooler, more evolved magma. The presence of anhydrite in the Soncor and Piedras 254 20 microns FIGURE VII.6 Crystallised sulphide melt inclusion included in an llmenite phenocryst (Im) from the banded Soncor pumice LA-155. The melt inclusion has separated into pyrrhotite (Po) and Chalcopyrite (Oh). Backscattered electron image. 255 % 20 microns FIGURE Vli.7 Anhydrite phenocryst from the 1993 scoria. Backscattered electron image. 256 Grandes Flows and in the April 1993 magmas (Figure VII.7) indicates that conditions in these magmas were more oxidising. This is confirmed by calculations of f 0 2 using magnetite-ilmenite pairs (Chapter IV). Since the sulphur contents of the matrix glasses are very low it is assumed that the sulphur in these magmas was present primarily in an associated gas phase. The source of this gas is most likely to be the injected mafic magma, as discussed above. Anhydrite has probably formed by a reaction between the melt and this gas phase; OaSiO^ + Fe 2 0 3 + SO 2 = CaSO^ + 2FeO + Si 0 2 Melt Melt Gas Anhydrite Melt Melt The reason for the increase in f 0 2 with decreasing temperature in these magmas may be this mixed SO 2 /H 2 S gas phase (Matthews et al, 2nd in press). In order to test this idea isopleths of constant SO 2 /H 2 S with varying temperature and f 0 2 have been calculated from thermodynamic data. The equilibrium reaction between coexisting SO 2 , H2 S and water is given by the equation; H2S + ^^2^2 “ SO2 + H2O In order to calculate the variation of SO 2 /FI2 S with changing temperature and f 0 2 the thermodynamic equation of Ohmoto & Rye (1979) is used, with Gibbs free energies taken from JANAF Thermochemical Tables (1985), fugacity coefficients of water from Burnham et al (1969), and fugacity coefficients of SO 2 and H 2 S 257 calculated from Shi & Saxena (1992). The equation relating molar SO 2 /H 2 S to temperature and f 0 2 is given below. XSO 2 [fS 0 2 /Pf • (VSO 2 )] X H 2 S [ f H 2 S/Pf • (VH 2S)] K- VH2 S • (f02)3/2 Pj • VH2O XH 2O • VSO 2 Where K = the equilibrium constant of SO 2 /H 2 S reaction, Pf = total fluid pressure, I = temperature, kelvin, fi = fugacity of species i in the mixture Xi = mole fraction of species i in the mixture, Vi = fugacity coefficient of species i at T and Pf. In order for such calculations to be valid it must be demonstrated that the gas phase consists almost entirely of water, with SO 2 and H 2 S present in minor amounts. Westrich and Gerlach (1992) have calculated that the gas phase in the 1991 Pinatubo magma contained approximately 98% water. The andesitic melt phase in the primary mafic magma of Lascar contains approximately 4-5 wt% water and only 0.5 wt% S so that on mixing into the magma chamber it will have exsolved an overall gas phase which is very water-rich. The reaction is shifted to higher f 0 2 by increasing pressure, so that an 258 Lascar Volcano -6 - -7 - -8 - MH (0 -9 - S3 O - 1 0 ” en -11 - o -12 - 100 -13 - □ □ 1 -14 - Po/Ah FMQ -15 600 700 800 900 1000 1100 Temperature °C FIGURE VII.8 Plot of calculated log(f02/bar) against temperature ®C for Lascar magmas. Lines of constant SO2 /H 2 S are included to show how this equilibrium reaction might buffer the f 0 2 of the magmas. 1 = 1:1, 10 = 10:1 and 100 = 100:1 isopleths of SO2 /H 2 S respectively. Other buffer curves as in Figure IV.1. Open circles: magmas from magnetite-ilmenite, open squares: xenoliths from magnetite-ilmenite, boxed area; mafic inclusion LA-124 from 2-pyroxene-olivine-spinel. estimate of the pressure is necessary. A nominal pressure of 4 kbar is thought reasonable for these thermodynamic calculations from seismic data (above). Lines of constant SO 2 /H 2 S at 4 kbar are plotted on a temperature-f02 plot in Figure VII. 8 . The Lascar data lie roughly between the 1 ;1 and 10:1 curves at 4 kbar, but the important point is that the trend lies parallel to these curves. At lower pressures higher ratios of SO 2 /H 2 S would be obtained, but the isopleths would still parallel the data. Sulphur redox reactions are potentially a powerful f 0 2 buffer because of the greater valence change from sulphide to sulphate (+ 8 ) relative to the ferrous to ferric iron reaction ( + 1 ). This is demonstrated by the inhibition of magnetite crystallisation during degassing of SO 2 from the mafic magma during fractionation prior to its injection into the magma chamber. A high sulphur fugacity is required to override the iron buffer reactions in the magma. A model is proposed in which a water- rich gas phase containing H 2 S and SO 2 is released into the magma chamber by the incoming basaltic andésite. The initial SO 2 /H 2 S ratio is determined by equilibration of this gas with the mafic magma. As the gas rises into the overlying cooler more evolved magma chamber the progressive conversion of SO 2 into H 2 S provides oxygen which is dissolved in the melt by oxidation of ferrous iron to the ferric state. This results in an increase in the f 0 2 of the system, stabilizing sulphate in the magma and leading to the crystallisation of anhydrite. This process should result in a decrease in the SO 2 /H 2 S ratio with decreasing temperature. Such a decrease is not detectable in the data and this may be due either to insufficiently precise calculations of temperature and 260 f0 2 in the magma or to the fact that the sulphur content of the magma chamber is so large that it can buffer the f 0 2 without a major change in SO 2 /H 2 S ratios. In this model the sulphur does not behave simply as an accessory component but has a profound effect on the physiochemical conditions in the magma chamber. The actual equilibrium is controlled by a rather complex interaction between sulphur species in the gas and ferrous and ferric iron in the melt. A simple reaction representing this can be written; 3 Fe 2 0 g + H 2 S = 6 FeO + SO 2 + FI2 O Melt Gas Melt Gas Gas A number of further predictions arise from this model. Westrich & Gerlach (1992) calculated that the large volume of SO 2 gas associated with the 1990 eruption of Mount Pinatubo, Phillipines ( 2 0 Mt) resided in the magma chamber as a separate gas and was not dissolved in the melt. The above model is in agreement with this since it predicts that the erupted magma was not the source of the gas, which was actually derived from a more mafic magma injected into the base of the magma chamber. Similarly, Andres et al (1991) calculated the volume of magma which resided in the magma chamber of Lascar by comparing the measured emissions of SO 2 with the calculated loss of sulphur from the erupted magma. This petrological method of estimating the emissions associated with an eruption is likely to result in a serious underestimate since the glass inclusions and matrix in the rock will contain little evidence for the associated gas phase. 261 Several knock-on effects of the increase in f 0 2 are likely to occur due to the increase in Fe^+/Fe^+ in the melt. Firstly, magnetite stability will increase with respect to ferromagnesian minerals, increasing the silica activity in the melt, and the Mg# (Mg/Mg-hFe^+) of ferromagnesian minerals will increase due to an increase in the Mg# of the melt. This may explain some of the complex compositional zoning found in many pyroxenes in Lascar magmas (Chapter III). The addition of oxygen to the melt is likely to depolymerize the melt structure by decreasing the number of bridging oxygens, thus decreasing its viscosity: Fe,2+ + Si-O-Si -n i/gOg = Fe3+ -f- 2(Si-0) Melt B. O. Gas Melt N. B. O. The increase in melt Fe^+ will also tend to increase the solubility of P 2 O5 since phosphorus tends to dissolve in silicate melts by forming (P04)3-complexes with Pe3+ (Toplis & Libourel, 1992). Apatite crystallisation will thus be inhibited. The formation of two immiscible liquids in some andesitic magmas of the Soncor Flow requires a high f0 2 - This can be achieved by the above process which not only creates the right f 0 2 conditions but provides the sulphur and phosphorus required to flux the iron oxide melt. Chalcopyrite and pyrrhotite are common in Lascar magmas, usually as inclusions in magnetite or ilmenite, and occasionally in pyroxenes. The occurrence of sulphides in a magma which lies in the 262 sulphate stability field is at first sight paradoxical. However, it is likely that a boundary layer is set up around growing phenocrysts, particularly in a rhyolitic melt which will be relatively viscous, with low diffusion rates. Removal of Fe^+ from the melt to form oxide minerals will tend to reduce the f0 2 locally as well as saturating the melt in sulphur. The degree of reduction required is not high, as the magma only lies about 0.5 log units above the sulphide-sulphate reaction curve. This process has been suggested for the formation of a number of different accessory phases in magmas (Bacon 1989). Another possibility is that sulphides are derived from the more reducing mafic magma and have been preserved by incorporation into phenocryst phases. Pyrrhotite analyses can be used to estimate the sulphur fugacity in the magma, provided the temperature is known (Toulmin & Barton, 1964). A number of pyrrhotite analyses have been used in this way to calculate the sulphur fugacity of Lascar magmas (Table VII.2). Sulphur fugacities are found to be unusually high relative to other calc-alkaline volcanics, including the sulphur-rich El Chichon eruption (Whitney, 1984). Values vary from -1.2 to +2.8 log units (error approx. ±0.6). High fS 2 is expected to be necessary for sulphur gas phase buffering of fÛ 2 . However, these results may only indicate the heightened sulphur fugacity in the boundary layers adjacent to growing phenocryst phases and may not be indicative of overall conditions. Anhydrite does not always occur in Lascar magmas, in spite of the almost ubiquitous high magmatic f 0 2 . This might indicate large fluctuations in fS 2 which would strongly control the stability of anhydrite. 263 Table Vil.2: Sulphur fugacities calculated for Lascar magmas from pyrrhotite compositions, compared with results from other silicic magmas. Analysis. Temperature °C. log fOg log fSg Lascar. LA1S1 (900) + 2 . 0 LA1S2 (900) +2 . 8 LAI S3 (900) + 1.7 LA103S1 900 - 1 0 . 2 - 1 . 2 LAI 21 81 900 - 1 0 . 0 + 2 . 0 LAI 2182 900 - 1 0 . 0 + 1 . 8 Bishop Tuff 770 -14.4 -2.9 Fish Canyon Tuff 800 - 1 1 . 6 -0 . 2 Julcani 880 -9.8 +0 . 8 St. Helens 950 - 1 0 . 2 -0 . 6 El Chichon 850 - 1 1 . 0 -0 . 2 NOTE: Temperatures in brackets are estimated due to post-eruptive re-equilibration of oxides. Values from other volcanoes are from Whitney (1984). 264 perhaps due to differences in the frequency of mafic magma influxes or the efficiency of the degassing system. Alternatively, anhydrite may have been removed from many of the older rocks by weathering, despite the arid climate (Anhydrite phenocrysts were not found in proximal or distal ash samples from the 1990 eruption of Mount Pinatubo, presumably due to removal either in the eruption cloud or following deposition; Matthews et al, 1992). However, although anhydrite was not present in the 1986 and 1990 Lascar eruption products, which are unlikely to have suffered major weathering before collection, it appeared in the more volatile-rich 1993 magmas. This change in the phase assemblage can be correlated with an increase in calculated f 0 2 in the 1993 magma of about 0.55 log units relative to the 1990 magma (Figure VII.9). It is postulated that the 1986 and 1990 magmas may have bypassed the subvolcanic magma chamber on the evidence of mineral compositions and whole rock geochemistry, whereas the 1993 magmas contain disequilibrium textures indicative of mixing between andesitic and dacitic magmas in a magma chamber. If this is true then the change in f 0 2 can be related to differences in the buffering ability of coexisting sulphur-rich gases. The 1986 and 1990 magmas are postulated to have partially crystallised in a conduit during their transport to the surface. The magmatic sulphur fugacity is therefore likely to have remained low since efficient degassing of this magma has been occurring through the summit crater and the f 0 2 would not have become elevated. Anhydrite could not form because of the low f 0 2 and fS^. In contrast, the 1993 magmas were retained in a magma chamber under high fS 2 conditions which produced an elevated f 0 2 during fractional crystallisation, and anhydrite formed in the sulphur-rich, oxidising conditions. This model demonstrates the importance of petrological monitoring of volcanoes in deducing information about the 265 1986-93 1993 Scoria Q - 1 0 - 1993 Pumice O 1986 Lava 0 SAMPLES 1990 Lava. — — - FMQ - 1 2 - — MH Po/Ah -1 3 - -1 4 - -1 5 -^ - 800 850 900 950 1000 1050 1100 T *C FIGURE VII.9: Plot of calculated log(f 0 2 /bar) against temperature X for 1986, 1990 and 1993 magmas. The 1993 samples clearly have an elevated f 0 2 relative to the earlier magmas. subvolcanic plumbing sytem. VII.5.3.3 Sulphur Isotopes and Implications for Sources of Magmatic Sulphur. In order to study the possible effects of contamination by sulphate from the Salar de Atacama, 9 3 4 g analyses of native sulphur from Lascar and neighbouring volcanoes are plotted against latitude in Figure VII.10, using data provided by Marcos Zentilli. The isotopic values are mostly slightly negative (d^^S = -1.4 to -6.0 permil) except for Lascar which straddles 0 permil. There is no obvious change in the sulphur isotopes between volcanoes alongside the Salar de Atacama and those further north, although Lascar appears to contain slightly heavier sulphur. This is not thought to be the result of assimilation of evaporites since Cerro Tumisa, which is closer to the Salar, shows no such anomaly. The variation in B^^S of native sulphur from different centres is most likely to be the result of shallow hydrothermal processes. The deposition of sulphates such as anhydrite and alunite in supergene zones above the magma chamber can deplete the fluid phase in heavy sulphur due to the preferential fractionation of ^^S into sulphates at low temperatures (Ohmoto & Rye, 1979; Thode, 1991). This is demonstrated in a study of Volcan Copiapo (Stark & Zentilli, 1992; Zentilli et al, 1991 ; Zentilli & Stark, 1992). Supergene sulphate deposits at Marte on the eastern side of the caldera complex are enriched in ^^S ( 4-4 . 1 to +15.3 permil) relative to near-surface sulphur deposits (- 1 . 6 to -15.4 permil). The authors suggest that disproportionation of magma-derived SO 2 at depth could have produced an acid-sulphate fluid with H 2 SO 4 high in ^^S and H 2 S gas depleted in ^^S; 267 TABLE VII.3: Isotopic composition of native sulphur and sulphate minerals from various volcanic centres of the Central Andes. CENTRE LAT. °S SAMPLEMINERAL 3 3 4 9 Apagado 22.03 8 ulphur -3.0 Lascar 23.37 LA-150 8 ulphur +0.9* LA-152 8 ulphur -1.4* Purico 23.00 8 ulphur -5.2 Sasiel 22.30 8 ulphur -6 . 0 Tumisa 23.28 8 ulphur -4.8 Copiapo 27.18 MO-24 8 ulphur - 1 0 . 2 MO-25 8 ulphur -7.3 to -7.4 MZ-4-89 8 ulphur -7.1 MZ-16A-89 8 ulphur -13.5 MZ-17-898 8 ulphur -15.4 MZ-18-898 8 ulphur - 1 2 . 0 MZ-4-89 Gypsum - 1 . 6 MZ-17-89A Minimiite -9.6 MZ-18-89A Alunite - 1 0 . 0 Marte W Z-118-84 Alunite +4.1 WZ-120-84 Alunite +15.3 WZ-120-84 Alunite +14.9 NOTE: Values for Volcan Copiapo and the nearby Marte Au deposit are from Zentilli et al, 1991 and M. Zentilli, pers. comm. * Impure gas caused analytical problems. Accuracy probably ±1 to 2 permil. All analyses carried out by Kreuger Enterprises, Inc., Geochron Labs. Div., 24 Blackstone St., Cambridge, Mass., USA. 268 S. de Atacama Lascar ï Apagado ro -3 -O T> -4 - Tumisa Purico Sasiel -6 - 22.0 22.5 23.0 23.5 Latitude FIGURE VII.10: plotted against latitude for native sulphur from a number of volcanoes in the Atacama area. The position of the Salar de Atacama is shown. Analyses provided by Marcos Zentilli. 2^4 SOg + ZHgO = H 2 SO4 + HgS Near-surface oxidation of this gas could have produced the isotopically light sulphur deposits; 2HgS + Og Sg + 2HgO The sulphur from Lascar, which was found impregnating pumice in a landslip scar on the western side of Centre II along with barite, aluminium sulphate and silica, has a range of values which straddle the “magmatic sulphur” value of 0 permil (Nielsen et al, 1991). This would suggest that depletion of the sulphur gases in was not occurring at depth during the formation of Centre II, indicating either that supergene enrichment did not occur or that the gases were at too high a temperature for isotopic fractionation to have a significant effect. The native sulphur deposits in these volcanoes have probably formed in a number of different environments. The older centres sampled are dissected by erosion to expose their altered cores, which have been mined for their sulphur. These have probably formed by mixing between magmatic gases and meteoric water. The samples from Lascar are more likely to have formed in a shallow environment and are more reminiscent of fumarolic deposits. The sulphur condensation which is thought to colour the active plume yellow results from reactions between gases on cooling and oxidation in the atmosphere. Two reactions are possible candidates for the formation of this sulphur and both may be taking place. The first is simple oxidation of HgS in the plume as 270 described above and the second is a reaction between SO 2 and H 2 S; 4H2S + 2SO2 = 3S2 + 4H2O These results suggest that assimilation of evaporites is not important in supplying sulphur to the system and that the main source of the sulphur is the primary mafic magma. This magma could have collected its sulphur either at source or during its passage through the mantle wedge and the lower crust. The near-magmatic isotopic signature of Lascar sulphur implies that the mantle is the ultimate source of this sulphur. VII.6 VOLATILE TRANSFER TO THE GROUNDWATER SYSTEM. Minerals from cold fumaroles on the southern flank of Lascar, from the springs at Tumbre, and contained in fragments of the hydrothermal alteration zone ejected during the 20 February 1990 eruption, have been analysed by X-ray diffraction. Sulphate minerals dominate the assemblages in all three cases, indicating that sulphate is the dominant anionic phase in hydrothermal fluids and in the groundwater system. The following minerals have been identified by interpreting the XRD traces; SM 93/48 Fragment of hydrothermal alteration zone lying in impact crater. Rozenite FeS 0 4 .4 H2 0 Alunite or Natroalunite (K,Na)Al 3 (S 0 4 )2 (0 H ) 6 Gypsum CaS 0 4 271 L S I Sublimates from cold fumaroles on the southern flank. Variscite AIPO 4 .2 H2 O Tamarugite NaAI(S 0 4 )2 .6 H2 0 S M 9 3 /1 8 Precipitates around cold water springs at Tumbre. Thenardite Na 2 S 0 4 Bloedite Na 2 Mg(S 0 4 )2 .4 H2 0 Halite NaCI These minerals are interpreted as the result of alteration of volcanic rocks by acid sulphate fluids, which leached out Na, Al, Fe, Ca and Mg. The alunite-bearing assemblage from the active crater is typical of intra- volcanic alteration zones in the area and occurs together with sulphur deposits. The cold fumaroles on the lower southern flank occur where erosion has exposed steeply-dipping strata (Saltar Flow). Cold fluids from the crater are thought to migrate down-dip in the flanks of the volcano until they reach such an area where the strata are truncated and evaporate, depositing their dissolved salts on the surface of the rock. Elsewhere on the flanks the ground is covered in similar minerals, indicating that much of the cone is saturated with similar fluids. The occurrence of variscite is unusual. Presumably phosphorus is also leached in large quantities from the rocks by decomposition of apatite. The cold springs at Tumbre are thought to represent a zone of mixing between groundwater and the hydrothermal system, producing 272 bicarbonate-sulphate-chloride water. Analysis of this water has been undertaken (Gardeweg & Medina, 1993) and the dominant dissolved species in November 1989 were Na+ (44 mg/l), Ca^+ (39 mg/l), Mg2+ (18 mg/l), SO 4 2 - (65 mg/l), Ch (43 mg/l) and HCOg* (16.6 mg/l). The pH at 2 2 - 24°C was 8.33. The springs were monitored from 26 February to 9 March 1993 and the temperature was found to be constant at 14-15°G with a pH of 6.04 to 6.28 (at this temperature). These can therefore be classified as “near neutral pH sodium-bicarbonate-sulphate-chloride waters” (Henley & Mahon, 1982). Such waters are produced by the interaction of COg and HgS gases in steam with unmineralised or dilute chloride waters. Clearly such springs are likely to tap some (albeit modified) magmatic volatiles and therefore may provide a sensitive indicator of changes in the hydrothermal system resulting from the influx of more gases from rising magma beneath the volcano. Monitoring the compositional variation of such waters might provide further data which would be useful in predicting eruptions. Unfortunately these springs have suffered contamination due to the emplacement of a pyroclastic flow during the 19-21 April 1993 eruption, so that any such variations are likely to be masked for some time. Other water samples have been analysed in the field. A sample of old snow collected from the southern flank was found to be rather more acid (pH = 3.64 at 24°G), probably as a result of precipitation of snow through the degassing plume of the volcano and fluxing of acidic fluids through the sample on the ground. Laguna Lejia contains extremely saline water due to evaporation of the lake and has a pH of 7.98 at 22-24°G (analysed in November 1989; Gardeweg & Medina, 1993). Laminated 273 sediments exposed on the south side of the lake contain calcite, monohydrocalcite and minor albite (probably detrital). Desert roses of gypsum also occur in these sediments. Due to the extreme salinity of this evaporite lake and its distance from the volcano it is unlikely to be useful in monitoring changes in the groundwater system due to magmatic events. VII.7 DISCUSSION AND CONCLUSIONS. This study of the behaviour of various volatiles in the magmatic and hydrothermal system of Lascar is closely integrated with the models for the evolution of the magmas in the deep supply system, in the magma chamber and in the sub- and intra-volcanic environment. The following conclusions have been drawn: 1 ) The main source of most of the volatiles (H 2 O, CO 2 , SO2 , H2 S and 0 1 ) in the system is the primary magma ascending from the mantle wedge. The ultimate sources of these volatiles are likely to be the subducted oceanic crust, with its upper layers having been altered by reaction with seawater at the spreading ridge crest, and the mantle wedge which overlies the slab. Some contribution from the lower crust is also possible. Sulphur isotopic studies on Lascar and neighbouring volcanoes precludes a significant contribution from the evaporite deposits of the Salar de Atacama, at least in the case of the sulphur. 2) Carbon dioxide, having a very low solubility in basaltic magmas at low pressure, is almost completely degassed from the ascending magma in the lower to middle crust and is thought to accomany the primary magma as a separate gas phase. Fractional crystallisation in lower to middle 274 crustal magma chambers removes anhydrous mineral phases such as olivine, clinopyroxene and spinel and may saturate the melt in H 2 O, causing exsolution of some of the dissolved water. The gas phase accompanying the magma when it enters the magma chamber is thus thought to consist mostly of H 2 O and CO 2 . The flat trend obtained for Cl with increasing Si in the glass inclusions in early phenocrysts of the Soncor mafic inclusion LA124 implies that some chlorine may partition into this gas phase at an early stage. The rapid decrease in S contents with increasing Si in these glass inclusions indicates that a large percentage of the dissolved sulphur is also degassed from the magma at this stage. However, in other samples of the primary magma, such as the mafic inclusions in the Capricorn Lava, fractional crystallisation had not occurred to such a large extent prior to injection of the magma into the magma chamber, so that most of the original chlorine and sulphur was probably still retained in the melt. 3) On its injection into the magma chamber the primary basaltic or basaltic andésite magma is quenched from 1025-1060 °C to 880-950 °C, depending upon the composition of the resident magma. The greatest temperature contrast occurs when the resident magma is dacitic. This drop in temperature is likely to dramatically reduce the solubilities of H 2 O and S in the magma, causing the exsolution of a mixed gas phase containing H 2 O, SO 2 and H 2 S. The rapid drop in Cl contents of glass inclusions and matrix in magma chamber derived rocks indicates that a large proportion of the dissolved chlorine partitions strongly into this vapour phase. Additionally, water and chlorine are incorporated into amphiboles and, occasionally, biotite. Rapid supersaturation of the melt in water during quenching of the mafic magma produces mafic inclusions with a mesh of interlocking acicular 275 amphibole crystals, which become destabilised on mixing into the dacitic host magma. Larger amphiboles form on the margins of the magma chamber, possibly mostly on the roof where water is concentrated by upward migration of bubbles. 4) Sulphur gases are believed to be particularly important in controlling the evolution of Lascar magmas. The increase in f 0 2 relative to FMQ with decreasing temperature is thought to be the result of buffering by SO 2 -H 2 S equilibrium. Unusually high sulphur fugacities are required in order for this equilibrium reaction to override the normal Fe^+-Fe^+ buffering of f 0 2 , which would be expected to produce a trend parallel to FMQ. Consequently, most Lascar magmas are relatively oxidised, lying well within the stability field of sulphate, and anhydrite is an important phenocryst phase in the Piedras Grandes and Soncor flows and in the 1993 magmas. This effect can be used as a monitor of the efficiency of degassing from the magma, since it appears that the magmas must be very volatile-rich before anhydrite can become stable. The correlation between the presence of anhydrite (and biotite) in the magma and the violence of eruptions through the history of the volcano is thought to confirm this relationship. The prehistoric Soncor Flow and its precursor, the Piedras Grandes Flow, are the only eruptions prior to 1993 which are known to have contained these two minerals and represent the most powerful eruptive phase in the history of Lascar. The 1993 eruption, which is thought to represent a switch in the plumbing system from a fissure system fed from depth to a volatile-rich shallow magma chamber, contains both anhydrite and biotite phenocrysts and may indicate that the severity of future eruptions may be rather higher 276 than in the recent past. Thus, petrological monitoring of eruption products coupled with the above models of the relationship between volatile behaviour and eruption style may be used as a powerful predictive tool in the case of this volcano. 5) As magma rises from the magma chamber towards the surface it continues to degas because of the decrease in pressure. This degassing is vividly demonstrated by the recent activity in the crater, where powerful jet fumaroles emit steam and sulphur gases at 800-900 °C. The eruptive cycles of the present (1984-1993) activity involve growth of lava domes followed by collapse of these domes within a system of concentric fractures. During 1993 it has become clear that the entire crater floor is subsiding in a similar manner and it is likely that the three craters of Centre III have formed in this way. A possible mechanism for this behaviour is degassing of a column of expanded “foam” in the conduit, produced as a result of vésiculation of the rising magma. As gas is removed from this system following the growth of a dome its volume decreases, causing it to collapse plastically and drawing down the crater floor. From the examination of older eroded volcanic centres in the area it is clear that the volcanic edifices above such sulphur-rich magma chambers commonly develop extensively altered cores containing native sulphur, alunite, gypsum and a variety of other minerals including clays. These deposits often represent the upper parts of more extensive mineralised zones immediately below the volcanic edifice which can contain base and precious metal deposits. The breakdown of SO 2 at depth to H 2 S and H 2 SO 4 leads to the formation of acid-sulphate fluids which react with the 277 host rocks to produce sulphate minerals such as gypsum and alunite. The removal of heavy sulphur from the gas in the formation of such fluids is a likely explanation for the slightly light isotopic composition relative to “magmatic sulphur” of the shallow native sulphur deposits in some of the centres sampled. These deposits form by oxidation of the H 2 S gas at shallow levels. The alteration zone within the active crater of Lascar contains native sulphur and alunite with minor rozenite and gypsum. This is probably the result of further reaction of magmatic SO 2 (degassed from the conduit below the crater) with groundwater to form acid sulphate brines near the surface. These fluids become more dilute and oxidised with distance from the crater, depositing mainly water soluble sulphate minerals around cold fumaroles and springs. 6 ) The models presented in this chapter allow some predictions to be made about the effects of shallow magmatic and hydrothermal systems on major volatile cycles, notably of chlorine and sulphur. The primary magma supplying the system which underlies Lascar is not considered to be particularly volatile-rich in comparison with other volcanic centres in the Central Andes. However, the occurrence of anhydrite in some of the magmas indicates that the magma chamber conditions and processes do have a large effect on the fate of some of these volatiles. Firstly, the large temperature contrast between injected and resident magmas, which is a function of the time interval between magma influxes, causes rapid exsolution of most of the dissolved water, sulphur and chlorine from the melt. Secondly, if the magma chamber remains a closed system with respect to these volatiles following such an influx of magma then the sulphur gases can effectively buffer the oxygen fugacity leading to increasingly oxidised 278 conditions and the main sulphur-bearing mineral phase becomes anhydrite rather than pyrrhotite. When a pathway is eventually opened to the surface the eruption is likely to be violent due to the high buildup of volatile pressure and a large quantity of water, sulphur and chlorine gases is released. This has been demonstrated by the 1991 eruption of Mount Pinatubo, Philippines, which erupted 20 million tonnes of SO 2 with its anhydrite- bearing dacitic magma. The climatic effects of such an event, although relatively short-lived, can be significant, since the resultant stratospheric sulphate aerosol causes reflection of solar radiation. Conversely, if the time interval between magma influxes is relatively short, as was the case with Centre I, then the temperature and compositional contrast between the injected and mafic magmas is likely to be less since fractional crystallisation cannot proceed to such a large extent between influxes. Under these conditions the pathway to the surface is also less likely to become closed, so that the magma can degas more efficiently and continuously and the buildup of volatiles in the magma chamber does not develop to such an extent. Sulphur and halogen gases are likely to be emitted in a more passive manner instead of being injected to high atmospheric levels in large, short-lived events. Such a system would not be expected to significantly affect the climate since the eruption would be less likely to inject gases into the stratosphere. The development of supergene mineralised zones beneath such volcanoes is likely to “fix” a significant proportion of the sulphur before it can be emitted. Such zones are more dependent upon factors such as the porosity of the local country rocks and the availability of groundwater to react 279 with the magmatic gases and set up a convecting system above the magma chamber. Despite the extremely arid climate there is ample evidence for a well-developed water table beneath the volcano which is manifested by reliable springs such as are seen in Tumbre. It is considered likely that a significant proportion of the sulphur and chlorine supplied from the subduction system is absorbed into the local drainage systems, eventually to be deposited in large evaporite deposits such as the Salar de Atacama. The surface environment of the Central Andes may therefore be acting as a huge filter, extracting sulphur and halogens which would otherwise be supplied to the atmosphere and oceans. 280 CHAPTER VIII SUMMARY AND CONCLUSIONS VII.1 SUMMARY. This Study has led to the development of a number of new models, particularly concerning the behaviour of sulphur in shallow evolved calc-alkaline magma chambers. This detailed pétrographie study of a single volcanic centre contrasts with the usual techniques which concentrate on whole rock geochemistry and stable and radiogenic isotope analyses. Although such methods have been applied here they are used to complement and extend the information obtained from optical petrography and electron microprobe analyses, from which a detailed picture of the processes taking place within the magma chamber has been obtained. The following points provide a synopsis of the main conclusions. 1 ) Lascar is a typical example of a Central Andean stratovolcano. it has erupted magmas of mostly andesitic composition and occasionally of dacitic composition (55 to 6 8 wt% Si0 2 ). These form lava flows and pumiceous pyroclastic flows which have built up a complex of edifices over three eruptive centres and extend for up to 27 km from the volcano. Lascar became active during the last glaciation and is probably less than 27,000 years old. The present activity, involving cycles of dome building and collapse punctuated by vulcanian to sub-plinian eruptions, appears similar to prehistoric eruptive phases. 281 2) The “primary” magma entering the shallow magmatic system is a basalt or basaltic andésite (55 to 57 wt% Si 0 2 ) which is manifested as mafic inclusions in more evolved lavas and pyroclastic flows. This magma has ascended through the thickened (70 km thick) crust and fractionated on the way, possibly in lower to mid crustal magma chambers, crystallising olivine, clinopyroxene and Cr-spinel. It either enters the shallow magma chamber in this form, as in the case of the mafic inclusions in the Capricorn Lava, or undergoes further fractionation to form orthopyroxene, plagioclase and magnetite phenocrysts, as in the case of the mafic inclusion LA-124 from the Soncor Flow. The calculated temperatures of these early magmas vary from 1025 to 1060 °C. 3) The magma chamber itself, which lies at an unknown depth beneath the volcano, contains andesitic to dacitic magmas. Following the injection of a batch of basaltic andésite the two magmas mix by overturning the magma chamber. Further fractionation occurs during and subsequent to these mixing events leading to complex compositional zoning patterns in pyroxenes and plagioclase. A number of disequilibrium textures result from this process including the formation of reaction coronae on olivine phenocrysts. If the injection of magma into the magma chamber occurs frequently then the compositional and thermal contrast between the two end members is relatively low and the disequilibrium textures are less well developed, as in the case of the Centre I andésites. However, if a long period of fractionation occurs in the magma chamber before such an event then the contrasts are much more marked, so that mixing occurs between basaltic andésite and dacitic magmas, leading to well-developed disequilibrium textures as in the case of the Soncor pyroclastic flow. The 282 andesitic magmas have temperatures from 900-940 °C and the dacitic magmas are rather cooler, at 880-920 °C. The magmas which result from this mixing process are dominantly 2 -pyroxene andésites and dacites with minor olivine. Hornblende and occasionally biotite are seen in more hydrous magmas which may have formed on the margins of the magma chamber. The various magmas chambers which have underlain the volcano in the past have shown a fluctuation between these two types of behaviour. The andesitic lavas and pyroclastic flows of Centre I are the result of frequent influxes of basaltic andésite magma, which continuously stirred and mixed with the magma chamber. The Soncor eruption represents a period of long fractionation with little or no mafic input, followed by an injection event leading eventually to a large, violent eruption. The lavas and pyroclastic flows of Centres II and III show seem to indicate that both processes occurred at different times, and possibly that a complex plumbing system was present, allowing some magma to bypass the chamber and crystallize on its way to the surface. 4) The behaviour of volatile elements in the magmatic system is intimately related to this process of fractional crystallisation punctuated by periodic influxes of hotter, more primitive magma. The source of most of the volatiles (HgO, Cl and S) has been identified as the incoming magma, and these volatiles are exsolved almost completely when this magma suffers quenching. Such a process has been suggested as the source of the 20 million tonnes of SO 2 emitted during the 1991 eruption of Mount Pinatubo, Phillipines (Matthews et al, 1992). It is clear that such influxes have the potential to trigger eruptions since they result in a large release of gases 283 which are likely to increase the internal pressure of the magma chamber and eventually act as a propellant for the erupted magma. However, in most cases it can be demonstrated that a period of fractionation occurs between the injection of primitive magma and the eventual eruption of mixed magmas. This would presumably further increase the amount of gases present in the magma chamber by the growth of anhydrous phenocrysts. Following such mixing events further degassing is likely to result as crystallization of the mixed magmas proceeds. It appears that the volatiles released on injection and quenching of the gas-rich basaltic andésite are not sufficient to trigger an eruption. Rather, the volatile pressure in the chamber is augmented by the gases released during this later crystallization stage, eventually leading to failure of the overlying rocks and causing an eruption. 6) The occurrence of anhydrite in some erupted rocks, notably the Soncor Flow and the 1993 products, indicates that the oxygen fugacity in the magma chamber is elevated relative to the incoming magma. This is confirmed by calculations of f 0 2 from mineral equilibria, which place most Lascar magmas comfortably within the field of sulphate stability. A model has been developed (Matthews et al, in press) in which the mixed sulphur- bearing gas phase can effectively override the normal Fe^+-Fe^+ equilibria and drive the system to higher fOg relative to FMQ with decreasing temperatures. Conversion of SOg into HgS as the temperature decreases produces oxygen which is dissolved in the melt by conversion of ferrous iron to the ferric state. This reaction can override the normal iron buffering of oxygen fugacity simply because of the greater valence change of sulphur 284 relative to iron. A high fS 2 is required for such sulphur gas phase buffering to occur. An important point is that sulphur in such evolved magma chambers is present mainly as a mixed gas phase, with lesser amounts in phenocryst phases such as sulphides or anhydrite. The amount dissolved in the melt is relatively unimportant due to the low solubility of both sulphide and sulphate in rhyolitic melts. This means that calculations of the amount of sulphur present in the magma by comparison of glass matrix and inclusions are likely to result in a serious underestimate and the idea of ‘excess’ sulphur emissions during eruptions will, in many cases, be a misnomer. The same process might operate in other sulphur-rich magma chambers which contain phenocrystic anhydrite and it is likely that other anhydrite-bearing volcanic centres will be found in the future both in the Central Andes and other continental arcs. In fact, anhydrite-bearing calc-alkaline magmas may not be so rare as has previously been thought. The large amounts of sulphur released from “Lascar-type” magma chambers may have implications for the estimation of the global flux of sulphur from volcanoes to the atmosphere. If more centres are found to in which the above processes have operated in the past then the long-term volcanic SO 2 flux is likely to be higher than presently estimated. An additional outcome of this model is that when such magma chambers solidify completely the f 0 2 of the lower-temperature fluids associated with them will increase still further, approaching the magnetite-hematite buffer curve. This not only provides a mechanism for supplying large amounts of sulphur to the late-stage hydrothermal system of a shallow granitoid intrusion but means that this sulphur will be present dominantly as oxidized species (SO 2 , SO3 and H 2 SO 4 ). 285 Liquid immiscibility has been identified in mixed magmas of Lascar and the adjacent Volcan Aguas Calientes, producing coexisting Fe-rich and rhyolitic liquids. This has been attributed to the quenching and oxidation of the mafic magma during mixing events, which is thought to briefly stabilise such immiscible liquids. Hence, magma mixing and unmixing occur simultaneously. Such a process has been suggested as the source of the iron oxide-phosphate lavas of nearby El Laco (Matthews, 1993). 7) The 1986 and 1990 lavas are similar to other Centre III lavas and the prehistoric Chaile Flow in that they do not appear to have resided in the magma chamber. This conclusion is based in a lack of well-developed disequilibrium textures, relatively low variability of pyroxene compositions and the evidence of whole rock geochemistry. This group of rocks follows a trend from the most mafic compositions towards a relatively depleted Na^C content indicating plagioclase-dominant fractionation prior to eruption, in contrast to most other rocks which show a slight increase in Na^C with increasing Si 0 2 - These magmas are interpreted as having crystallised on transport to the surface via a separate feeder system which may be a fissure aligned along the Miscanti Line, a fault passing through the volcano. In contrast, the 1993 magmas show good evidence that their origin is a magma chamber similar to that which has supplied most of the prehistoric erupted material, since they contain pyroxenes with a wide compositional range, biotite phenocrysts with reaction coronae and anhydrite. Their calculated oxygen fugacities are displaced upwards relative to those of the 1986 and 1990 magmas and they contain anhydrite phenocrysts, suggesting that sulphur gas phase buffering of f 0 2 has occurred. Whole rock compositions are suggestive of mixing between magmas of the 1986-93 type and a typical 286 more evolved magma chamber. In the absence of a good seismic network this petrological monitoring of the plumbing system is significant in predicting the future eruptive style of the volcano since future activity is likely to be fed directly from this volatile-rich magma chamber. The transition from short-lived vulcanian blasts of shallow origin to sustained sub-plinian eruptions from deeper levels is neatly explained by this model. It is probable that the fissure-fed system was able to degas passively on ascent as is indicated by the powerful jet fumaroles in the active crater, it appears that this degassing is continuing at present so that a double feeder system may now be present beneath the volcano. 8 ) Strontium and neodymium isotopic analyses of Lascar eruption products indicate that assimilation of relatively radiogenic crust by the magmas occurs during crystallization. In other words, between periods of injection the dominant process is assimilation-fractional crystallization (A.F.C.). Since no isotopic analyses of basement rocks are available it is not possible to positively identify the contaminant or quantify the amount of assimilation. However, the large numbers of skarn xenoliths which occur in certain flows (notably the Tumbres Flow and Capricorn Lava) indicate that metasomatic alteration of carbonate wallrocks has occurred during the history of the volcano. 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SAMPLE DESCRIPTIONS Samples are described in terms of location, stratigraphie unit and minerals which have been identified. 311 SAMPLE No. AT-9 DESCRIPTION Tumisa Basic Inclusion MINERALS Hornblende, Biotite, Plagioclase, Orthopyroxene, Magnetite, llmenite. Apatite, Pyrrhotite, Barite (Cavity Infill), Interstital Glass SAMPLE No. LA-001 DESCRIPTION No Sample MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite, Apatite, Pyrrhotite, Iddingsite SAMPLE No. LA-002 DESCRIPTION No Sample MINERALS Olivine, Orthopyroxene, Augite, Plagioclase, Magnetite, llmenite. Apatite SAMPLE No. LA-003 DESCRIPTION No Sample MINERALS Olivine, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Apatite SAMPLE No. LA-004 DESCRIPTION Hydrothermally altered glassy fragment. No sample. MINERALS Quartz SAMPLE No. LA-006 DESCRIPTION Andésite. 1986 Bomb. MINERALS Olivine, Orthopyroxene, Augite, Plagioclase, Magnetite, llmenite. Apatite, Fe-Cu Sulphides SAMPLE No. LA-01 1 DESCRIPTION Basic Inclusion from 5 m.y. dome South of Lascar. No sample. MINERALS Biotite, Hornblende, Plagioclase, Quartz, Magnetite, llmenite. Apatite, Pyrrhotite, Chalcopyrite SAMPLE No. LA-013 DESCRIPTION No Sample MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Opaques SAMPLE No. LA-014 DESCRIPTION No Sample MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite, Interstital Glass SAMPLE No. LA-015 DESCRIPTION No Sample. Glassy lava. Darker brown glass, pale brownish glass. MINERALS Hornblende, Orthopyroxene, Augite, Plagioclase, Quartz, Magnetite, llmenite. Apatite SAMPLE No. LA-016 DESCRIPTION No Sample MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Opaques 3IZ SAMPLE No. LA-017 DESCRIPTION No Sample MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Opaques SAMPLE No. LA-018 DESCRIPTION No Sample Centre 1 lava. MINERALS Hornblende, Olivine, Orthopyroxene, Augite, Plagioclase, Magnetite, llmenite. Apatite SAMPLE No. LA-019 DESCRIPTION No Sample Centre 1 lava M INERALS Olivine, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Apatite SAMPLE No. LA-100 DESCRIPTION 1990 Bomb. 2-pyroxene andésite. S. Flank. MINERALS Olivine, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite, Sulphides, Apatite SAMPLE No. LA-101 DESCRIPTION 1990 Bomb. 2-pyroxene andésite. S. Flank. MINERALS SAMPLE No. LA-102 DESCRIPTION 1990 Bomb. 2-pyroxene andésite. S. Flank. MINERALS SAMPLE No. LA-103 DESCRIPTION Andesitic Pumice containing lava xenolith and metasedimentary xenolith. Soncor Flow. Tumbre MINERALS PUMICE: Orthopyroxene, Clinopyroxene, Biotite, Plagioclase, Magnetite, llmenite. Apatite. XENOLITH: Orthopyroxene, Clinopyroxene, Biotite, Plagioclase, Magnetite, llmenite. Quartz, CuFe Sulphide, Zircon, Monazite, Apatite. SAMPLE No. LA-104 DESCRIPTION Lava xenolith from pumice flow. Tumbre. MINERALS SAMPLE No. LA-105 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS Carbonate (Acid) SAMPLE No. LA-106 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS ZlJ SAMPLE No. LA-107 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS Carbonate (Acid) SAMPLE No. LA-108 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Wollastonite Rock.Tumbre. MINERALS Wollastonite, Dlopside-Hedenbergite, Fassaitic Augite Grossular-Andradite, Ancrthite, Quartz, Apatite, Wilkeite, Calcite, Monazite, Barite SAMPLE No. LA-109 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS Carbonate (Acid) SAMPLE No. LA-110 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow.Tumbre. MINERALS Minor Carbonate (Acid) SAMPLE No. LA-111 DESCRIPTION Calc-silicate xenolith from pumice flow. Buchite. MINERALS Diopside-Hedenbergite, Ancrthite, Quartz, Sphene, Apatite, Glass (Devitrified, Cl-rich), Wollastonite, Anhydrite SAMPLE No. LA-112 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Wollastonite Rock. Tumbre. MINERALS INNER ZQNE Wollastonite, Ancrthite, Garnet, Calcite, Quartz. QUTERZQNE Wollastonite, Ancrthite, Garnet, Quartz, Sphene, "Diopside", Magnetite, Mn-oxide. SAMPLE No. LA-113 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS Wollastonite, Garnet, Ancrthite, Quartz, Apatite, Sphene, Zircon, Calcite Veins SAMPLE No. LA-114 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS SAMPLE No. LA-115 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Diopside Andradite Metaquarzite. Tumbre. MINERALS Diopside, Andradite, Ancrthite, Quartz SAMPLE No. LA-116 DESCRIPTION Calc-silicate xenolith from Tumbres pumice flow. Tumbre. MINERALS Carbonate (Acid) SAMPLE No. LA-117 DESCRIPTION Large acid pumice fragment from Tumbres pumice flow. Tumbre, MINERALS SAMPLE No. LA-118 DESCRIPTION Partially Melted dacite (Xenolith) from Tumbres pumice flow. Tumbre. MINERALS Biotite, Hornblende, Plagioclase, Quartz SAMPLE No. LA-119 D ESCRIPTION Dacite (Xenolith from Tumbres pumice flow) with enclosed homfels xenolith. Tumbre. MINERALS lava Biotite, Hornblende, Orthopyroxene, Plagioclase, Quartz, Opaques HORNFELS Biotite, Orthopyroxene, Plagioclase, Quartz, Magnetite, llmenite. Zircon, Monazite, Fe sulphides. SAMPLE No. LA-120 DESCRIPTIO N 1990 Bomb with partially melted granitic xenolith. N. Flank. MINERALS SAMPLE No. LA-121 DESCRIPTIO N Dacite Pumice. Soncor Flow. Talabre Viejo. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Apatite SAMPLE No. LA-122 DESCRIPTION Acid Pumice. Soncor Flow. Talabre Viejo. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Apatite, Cu-Fe sulphides. Anhydrite SAMPLE No. LA-123 DESCRIPTION Dark Pumice. Tumbres Flow. Ouebrada de Talabre. MINERALS SAMPLE No. LA-124 DESCRIPTION Basaltic Inclusion from pumice flow. Soncor Flow. Talabre Viejo. M INERALS Olivine, Orthopyroxene, Augite, PIgeonlte, Fassaite, Plagioclase, Cr-Splnel, Magnetite, llmenite, Fe-Cu Sulphides. SAMPLE No. LA-125 DESCRIPTION Tumbre young andésite. Talabre Lava. MINERALS SAMPLE No. LA-126 DESCRIPTION Andésite lava (boulder). Centre I andésite. Atx)ve Tumbre. M INERALS Orthopyroxene, Clinopyroxene, Plagioclase, Opaques SAMPLE No. LA-127 DESCRIPTION Blocky andesite lava. Centre I andésite. Above Tumbre. MINERALS 3(r SAMPLE No. LA-128 DESCRIPTION Oxidised pumice fragments. Tumbres Flow. Above Tumbre. MINERALS SAMPLE No. LA-129 DESCRIPTION Blocky dacite lava. Centre II lava. N. Flank. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Apatite, Intersitial Glass SAMPLE No. LA-130 DESCRIPTIO N Blocky lava. Centre II lava. N. Flank. MINERALS SAMPLE No. LA-131 DESCRIPTION Blocky porphyritic lava. Centre I andesite. W side of volcano. MINERALS SAMPLE No. LA-132 DESCRIPTION Large dacite boulder with glassy rim. Piedras Grandes Flow. W side of volcano. MINERALS SAMPLE No. LA-133 DESCRIPTION Blocky porphyiitic lava. Centre I andesite. W side of volcano. MINERALS SAMPLE No. LA-1330 DESCRIPTION Blocky porphyritic lava. Centre I andesite. W side of volcano. MINERALS SAMPLE No. LA-134 DESCRIPTION Blocky porphyritic lava. Centre I andesite. W side of volcano. MINERALS SAMPLE No. LA-135 DESCRIPTION Blocky porphyritic lava. Centre I andesite. W side of volcano. MINERALS SAMPLE No. LA-136 DESCRIPTION Pumice from small flow. Tumbres Flow. SW side of volcano. MINERALS SAMPLE No. LA-137 DESCRIPTION Blocky porphyritic lava. Centre II. SW side of volcano. MINERALS SAMPLE No. LA-138 DESCRIPTION Blocky porphyritic lava. Centre II. SW side of volcano. MINERALS 3 1 ^ SAMPLE No. LA-139 DESCRIPTION Blocky dacite lava. Capricorn Lava. SW side of volcano. MINERALS SAMPLE No. LA-140 DESCRIPTION Blocky, banded dacite lava witti calc-silicate xenolith. Capricorn Lava. SW side of volcano. MINERALS lava Hornblende, Olivine, Orthopyroxene, Clinopyroxene, Plagioclase, Opaques, Fe-Cu Sulphides, Apatite. CALC-SILICATE XENOLITH Biotite, Diopside, Plagioclase, Quartz, hemoilmenite,, magnetite, ilmenite, sphene. SAMPLE No. LA-141 DESCRIPTIO N Mafic Inclusion from lava LA-140. Capricorn t_ava. SW side of volcano. MINERALS Hornblende, Olivine, Orthopyroxene, Augite, Plagioclase, Cr-Spinel, Magnetite, llmenite, Apatite, Fe-sulphides SAMPLE No. LA-142 DESCRIPTION Mafic Inclusion from lava LA-140. Capricorn Lava. SW side of volcano. MINERALS Hornblende, Olivine, Orthopyroxene, Augite, Plagioclase, Cr-Spinel, Magnetite SAMPLE No. LA-143 DESCRIPTION Mafic Inclusion from lava LA-140. Capricorn Lava. SW side of volcano. MINERALS Hornblende, Olivine, Orthopyroxene, Augite, Plagioclase, Cr-Spinel, Magnetite, llmenite. Apatite SAMPLE No. LA-144 DESCRIPTION Metasedimentary Xenolith from lava LA-140. Capricorn Lava. SW side of volcano. MINERALS Augite, Plagioclase, Quartz, Magnetite, Garnet, Apatite, Sphene SAMPLE No. LA-145 DESCRIPTION Pumice fragment with partially melted dacite xenolith. Tumbres Flow. SW side of volcano. MINERALS SAMPLE No. LA-146 DESCRIPTION Mafic Inclusion from lava LA-140. Capricorn Lava. SW side of volcano. MINERALS SAMPLE No. LA-147 DESCRIPTION Acid pumice. Soncor Flow. Camar. MINERALS Olivine, Orthopyroxene, Clinopyroxene, Plagioclase, Quartz, Magnetite, llmenite. Apatite 3 1 ? SAMPLE No. LA-148 DESCRIPTION Calc-silicate bomb. W side of volcano. M IN E R A LS Ancrthite, Diopside, Sphene, Zircon, Apatite, Quartz SAMPLE No. LA-149 DESCRIPTION Sulphate-impregnated pumice. Centre II. W Flank of volcano. MINERALS SAMPLE No. LA-150 DESCRIPTION Sulphate-impregnated pumice. Centre II. W Flank of volcano. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite Sulphur, Aluminium Sulphate, Barite, Silica. Rutile SAMPLE No. LA-151 DESCRIPTION Sulphate-impregnated pumice. Centre II. W Flank of volcano. MINERALS SAMPLE No. LA-152 DESCRIPTION Sulphate-impregnated pumice. Centre II. W Flank of volcano. MINERALS Sulphur SAMPLE No. LA-153 DESCRIPTION Oxidised lava from near Southem rim of crater. Centre III. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite. Quartz SAMPLE No. LA-154 DESCRIPTION Porphyritic lava. Centre II. S Flank. MINERALS SAMPLE No. LA-155 DESCRIPTION Banded Pumice from Tumbre. Soncor Flow. MINERALS Hornblende, Biotite, Orthopyroxene, Clinopyroxene, Plagioclase, Quartz, Magnetite, llmenite, Iddingsite, Anhydrite, Fe-Cu Sulphides. SAMPLE No. LACO 1 DESCRIPTION El Laco. Mass of green radiating acicular orthopyroxene. MINERALS Orthopyroxene Magnetite Quartz SAMPLE No. LACO 2 DESCRIPTION El Laco Magnetite Lava. Mass of large magnetite crystals. MINERALS Magnetite, Vivianite, Variscite, Iron Phosphate Fe4(P04)3, Anhydrite, Quartz, Monazite, Xenotime SAMPLE No. LAS 15 DESCRIPTION Banded Calc-Silicate Xenolith. Alternating "Diopside-Rock" and Wollastonite-Rock". Tumbres Flow, Tumbre. MINERALS ‘ DIOPSIDE ROCK* Orthopyroxene, Diopside, Plagioclase, Quartz, Apatite, llmenite, Zircon. ‘WOLLASTONITE ROCK* Wollastonite, Diopside, Quartz, veins of Calcite, Mg Silicate. ‘ DIOPSIDE ROCK* Diopside, Anorthite, Quartz, Apatite, Zircon, Magnetite, llmenite. SAMPLE No. LAS 23 DESCRIPTION Rhyolite (Tumbres Flow). Actually a buchite xenolith. MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Quartz, Magnetite, Apatite, Zircon, Thorite, Rhyolite Glass matrix SAMPLE No. LAS 25 DESCRIPTION Andesite from bomb agglutinate flow (Saftar Flow). MINERALS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, llmenite, Apatite, Dacitic Glass matrix SAMPLE No. LAS 29-1 DESCRIPTION Pumice. Soncor Flow. MINERALS SAMPLE No. LAS 29-2 DESCRIPTION Andesite Pumice from mixed pumice flow (Soncor Flow). MINERALS Orthopyroxene, Clinopyroxene, Hornblende, Plagioclase, Magnetite, llmenite, Dacitic Glass matrix. Iron oxide melt globules SAMPLE No. LAS 30 DESCRIPTION Homblende-cumulates in pumice from Soncor Pyroclastic Flow. MINERALS Hornblende, Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, Apatite, Pyrrhotite, Anhydrite, Dacitic Matrix Glass, Fe oxide melt globules SAMPLE No. LAS 36-1 DESCRIPTION Pumice from Soncor Pyroclastic Flow containing cumulates. MINERALS Orthopyroxene, Clinopyroxene, Hornblende, Olivine,, Biotite, Plagioclase, Magnetite, llmenite. Apatite, Rhyolitic Glass Matrix SAMPLE No. LAS 36-2 DESCRIPTION Pumice from Soncor Pyroclastic Flow containing cumulates. MINERALS SAMPLE No. LAS 36-3 DESCRIPTION Pumice from Soncor Pyroclastic Flow containing cumulates. MINERALS SAMPLE No. LAS 39 DESCRIPTION Metasomatized Lava Xenolith MINERALS SAMPLE No. LAS 41 DESCRIPTION Hornblende-rich pumice. Soncor Flow. MINERALS SAMPLE No. LAS 47 DESCRIPTION Prismatic jointed block from Soncor Flow. M IN ER A LS Orthopyroxene, Clinopyroxene, Plagioclase, Quartz, Magnetite, llmenite SAMPLE No. LAS 63 DESCRIPTION Breadcrusted bomb from Tumbres Flow. Loc. 67. MINERALS SAMPLE No. LAS 64 DESCRIPTION PUMICE FROM TUMBRES FLOW. LOG. 67. MINERALS SAMPLE No. LAS 68 DESCRIPTION Tufa from Talabre pre-Tumbres alluvium. Same as 02. M IN E R A LS Carbonate (Fizzes with HOI). SAMPLE No. LAS 73 DESCRIPTION Calcrete from top of Tumbres flow. Small gorge below Tumbre. M IN E R A LS Carbonate (Fizzes with HCI). SAMPLE No. LAS 79 DESCRIPTION Mafic scoria of Soncor Flow. Loc. 105. Pampa Lejia. MINERALS SAMPLE No. PWF1 DESCRIPTION 1993 Acid Pumice. Toe of longest pyroclastic flow in Tumbres Valley. M IN E R A LS Orthopyroxene, Augite, Plagioclase, Magnetite, llmenite. Apatite, Chalcopyrite, ANHYDRITE SAMPLE No. PWF2 DESCRIPTION 1993 Pumice. Block from fallout field S of Lascar. Interior of dome? M IN E R A LS Orthopyroxene, Clinopyroxene, Biotite, Plagioclase, Magnetite, llmenite. Apatite, ANHYDRITE SAMPLE No. SM93/01 DESCRIPTION Prismatic jointed block from Soncor Flow. Cerro Opia. MINERALS SAMPLE No. SM93/02 DESCRIPTION Moderately crystal-rich pumice with very large px. phenocryst from Soncor Flow. Cerro OpIa. MINERALS SAMPLE No. SM93/03 DESCRIPTION Crystal-rich pumice with hbl, plag phenocrysts. Soncor Flow. Cerro Opia. MINERALS 1X0 SAMPLE No. SM93/04 DESCRIPTION Calc-silicate?? rock with extensive veining. Soncor Flow. Cerro Opia. MINERALS SAMPLE No. SM93/05 DESCRIPTION Tourmaline-quartz-plagioclase rock. Probably a vein from the Palaeozoic. Cerro Opia. MINERALS SAMPLE No. SM93/06 DESCRIPTION Vesicular crystal-poor pumice. Soncor Flow. Flow front. MINERALS SAMPLE No. SM93/07 DESCRIPTION Small fragment of dense hbl-plag-qtz cumulate. Soncor Flow. Flow front. MINERALS SAMPLE No. SM93/08 DESCRIPTION Alteration zone from top of "pink quartz" flow. Loc. 97. M IN ER A LS Cartxjnate (Fizzes with HCI). SAMPLE No. SM93/09 DESCRIPTION Aphyric lava fragment from loc. 97. Similar to fragments throughout Soncor Flow. MINERALS SAMPLE No. SM93/10 DESCRIPTION Piedras Grandes lava. Probably from Volcan Aguas Calientes. (loc. of MLA101). MINERALS Orthopyroxene, Hornblende, Biotite, Plagioclase, Magnetite, llmenite. Apatite, Anhydrite SAMPLE No. SM93/11 DESCRIPTION Calc-silicate xenolith from Piedras Grandes lava. (loc. of MLA101). MINERALS SAMPLE No. SM93/12 DESCRIPTION Tumbres pumice, dark with inclusion of pale pumice. Loc. 14. MINERALS SAMPLE No. SM93/13 DESCRIPTION White xenolith in Tumbres Pumice. Loc. 14. MINERALS SAMPLE No. SM93/14 DESCRIPTION Calc-silicate xenolith from Tumbres pumice. Loc. 14. MINERALS SAMPLE No. SM93/15 DESCRIPTION Wollastonite rock from Tumbres Flow. Loc. 66. M IN ER A LS Carbonate (Fizzes with HCI). 3Z\ SAMPLE No. SM93/16 DESCRIPTION Vitrophyre from lascar washed out on alluvial fan. MINERALS SAMPLE No. SM93/17 DESCRIPTION Hydrothermally altered breccia. Alluvial fan. MINERALS SAMPLE No. SM93/18 DESCRIPTION Sulphate sublimates from around springs at Tumbre. M IN E R A LS Thenardite, Bloedite, Halite SAMPLE No. SM93/19 DESCRIPTION Xenolith from andesitic scoria block from top of Soncor Flow (Loc. = LA122). MINERALS SAMPLE No. SM93/20 DESCRIPTION Glassy rim of bomb from Chaile flow. Quebrada Chaile. M IN E R A LS Orthopyroxene, Clinopyroxene, Plagioclase, Magnetite, Apatite SAMPLE No. SM93/21 DESCRIPTION Dense but vesicular andésite from Chaile Flow. Quebrada Chaile. MINERALS SAMPLE No. SM93/22 DESCRIPTION Piedras Grandes lava from Soncor lag breccia. Loc. 100. M IN E R A LS Orthopyroxene, Clinopyroxene, Amphibole, Olivine, Plagioclase, Magnetite, llmenite. Apatite, Anhydrite, Fe (Cu) sulphide SAMPLE No. SM93/23 DESCRIPTION Vitrophyre from breadcrusted bomb. Soncor lag breccia. Loc. 100. MINERALS SAMPLE No. SM93/24 DESCRIPTION FRAGMENT OF ALTERATION ZONE FROM SONCOR LAG BRECCIA. LOC.84. MINERALS SAMPLE No. SM93/25. DESCRIPTION Dense but vesicular andesitic breadcrust bomb. Soncor lag breccia. Loc. 84. MINERALS SAMPLE No. SM93/26 DESCRIPTION Small fragment of bleached pumice from Tumbres plinian airfall. Tumbre. MINERALS SAMPLE No. SM93/27 DESCRIPTION Wollastonite rock from Tumbres Flow. Tumbre. M IN E R A LS Carbonate (Fizzes with NCI). IZZ SAMPLE No. SM93/28 DESCRIPTION Small breadcrusted calc-silicate xenolith. Washed out of Tumbres Flow. Tumbre. MINERALS SAMPLE No. SM93/29 DESCRIPTION Wollastonite rock from Tumbres flow. Tumbre. MINERALS Carbonate (Fizzes with HOI). SAMPLE No. SM93/30 DESCRIPTION Green wollastonite-diopside rock from Tumbres flow. Tumbre. MINERALS Carbonate (Fizzes with HCI). SAMPLE No. SM93/31 DESCRIPTION Wollastonite rock from Tumbres flow. Tumbre. MINERALS Carbonate (Fizzes with HCI). SAMPLE No. SM93/32 DESCRIPTION Wollastonite rock from Tumbres flow. Tumbre. MINERALS Carbonate (Fizzes with HCI). SAMPLE No. SM93/33 DESCRIPTION Hornblende dacite. Block and ash flow of V. Aguas Calientes. Pampa Lejia. Loc. 106. MINERALS SAMPLE No. SM93/34 DESCRIPTION Small hornblende rich mafic inclusion. Block and ash flow of V. Aguas Calientes. Pampa Lejia. Loc. 106. MINERALS SAMPLE No. SM93/35 DESCRIPTION Soncor dacite pumice overlying V. Aguas Calientes block and ash flow. Pampa Lejia. Loc. 107. MINERALS SAMPLE No. SM93/36 DESCRIPTION Hornblende dacite with mafic incl. V. Aguas Calientes. Loc. 108. MINERALS MAFIC INCLUSION: Hornblende, Plagioclase. HOST LAVA: Hornblende, Orthopyroxene, Augite, Plagioclase, Magnetite, llmenite. Apatite, Fe oxide melt incl. SAMPLE No. SM93/37 DESCRIPTION Hornblende dacite with clot of hornblende cumulate. V. Aguas Calientes. Loc. 108. MINERALS Hornblende, Biotite, Magnetite, llmenite. Apatite, Plagioclase, Pyrrhotite, Pyrite, Fe oxide melt incl. SAMPLE No. SM93/38 DESCRIPTION Mafic Inclusion. May not be associated with SM93/36,37. V. Aguas Calientes. Loc. 108. MINERALS Orthopyroxene,Clinopyroxene, Plagioclase, Magnetite, llmenite, Apatite, Chalcopyrite, ANHYDRITE 5 2 3 SAMPLE No. SM93/39 DESCRIPTION Calcite crystals. S. side of Laguna Lejia. M IN E R A LS Calcite SAMPLE No. SM93/40 D ESCRIPTION Desert rose. S. side of Laguna Lejia. MINERALS SAMPLE No. SM93/41 D ESCRIPTION Cartx)nate coat from fossil featfier. S. side of Laguna Lejia. MINERALS Calcite SAMPLE No. SM93/42 DESCRIPTION Cauliflower bomb from Saltar Flow. Zeolitised. Loc. 112. MINERALS SAMPLE No. SM93/43 DESCRIPTION Porpfiyritic lava with cpx megactysts. Saltar flow. Loc. 112. MINERALS MEGACRYST., Augite, Orthopyroxene, Olivine, Cr-Spinel, Magnetite SAMPLE No. SM93/44 DESCRIPTION Vitrophyre from Soncor Flow. S. side of Lascar. MINERALS Orthopyroxene, Augite, Hornblende, Plagioclase, Magnetite, llmenite. Apatite, Anhydrite SAMPLE No. SM93/45 DESCRIPTION Sulphate sublimates from cold fumaroles. S. side of Lascar. MINERALS Variscite, Tamarugite SAMPLE No. SM93/46 DESCRIPTION Old hornblende dacite lava from NE flank. Underlying Saltar flow. Probably V. Aguas Calientes. MINERALS SAMPLE No. SM93/47 DESCRIPTION Fragment of pyroxene cumulate. NE flank. MINERALS Augite, Orthopyroxene,Flagioclase Magnetite, llmenite. Apatite SAMPLE No. SM93/48 DESCRIPTION Fragment of active crater alteration zone. 1990 bomb. MINERALS Rozenite Alunite or Natroalunite Gypsum SAMPLE No. SM93/49 DESCRIPTION Sample of altered pumice with sulphur smell. Tumbres flow. E. of Tumbres. MINERALS SAMPLE No. SM93/50 DESCRIPTION Ash from basal plinian of Soncor Flow. Tumbres. MINERALS 32.^ SAMPLE No. SM93/51 DESCRIPTION Banded pumice of Soncor Flow. Tumbres. MINERALS SAMPLE No. SM93/52 DESCRIPTION Banded pumice of Soncor Flow. Tumbres. MINERALS SAMPLE No. SM93/53 DESCRIPTION Calc-silicate xenolith from homblende dacite lava. Cerro Lejia. MINERALS SAMPLE No. SM93/54 DESCRIPTION Calc-silicate xenolith from homblende dacite lava. Cerro Lejia. MINERALS SAMPLE No. SM93/55 DESCRIPTION Calc-silicate xenolith from homblende dacite lava. Cerro Lejia. MAGNETITE SKARN. M IN E R A LS Augite, Plagioclase, Quartz, Sphene, Magnetite, llmenite SAMPLE No. SM93/56 DESCRIPTION Calc-silicate xenolith from homblende dacite lava. Cerro Lejia. M IN E R A LS ZONE A: Plagioclase, Quartz, Sphene, Magnetite, Dbpside. ZONE B: Diopside, Sphene, Zircon, Plagioclase. ZONE C: Plagioclase, Quartz, Magnetite, Sphene, Monazite ?, Interstitial Glass. SAMPLE No. SM93/57 DESCRIPTION Calc-silicate xenolith from homblende dacite lava. Cerro Lejia. MINERALS SAMPLE No. SM93/59 DESCRIPTION Plutonic xenolith from hornblende dacite lava. Cerro Lejia. MINERALS SAMPLE No. SM93/60 DESCRIPTION Mafic inclusion from hornblende dacite lava. Cerro Lejia. MINERALS SAMPLE No. SM93/61 DESCRIPTION Alteration zone of Cerro Lejia including sulphur. M IN E R A LS Sulphur SAMPLE No. SM93/62 DESCRIPTION Cartx)nate coat from fossil feather. S. side of Laguna Lejia. M IN E R A LS Calcite SAMPLE No. SM93/63 DESCRIPTION Cartxjnate coat from fossil feather. S. side of Laguna Lejia. M IN E R A LS Calcite SAMPLE No. SM93/64 DESCRIPTION Cartx>nate coat from fossil feather. S. side of Laguna Lejia. M IN E R A LS Calcite SAMPLE No. SM93/65 DESCRIPTION Cartjonate coat from fossil feather. S. side of Laguna Lejia. M IN E R A LS Calcite SAMPLE No. SM93/66 DESCRIPTION Basalt from Cerro Overo (Maar Crater). M IN E R A LS LAVA: Olivine, Clinopyroxene, Gr-spinel, Magnetite, Plagioclase XCENOLITH: Plagioclase, Quartz, Biotite, Magnetite, llmenite. SAMPLE No. SM93/67 DESCRIPTION Quartzite from small hill N. of Quebrada Talabre. M IN E R A LS Carbonate (Fizzes with HCI). SAMPLE No. SM93/68 DESCRIPTION Pumice from (Purico) ignimbrite at Toconao, with mafic inclusion. MINERALS SAMPLE No. SM93/69 DESCRIPTION Silica from boiling pool. El Tatio. M IN E R A LS Opal SAMPLE No. SM93/70 DESCRIPTION Silica from warm stream. El Tatio. M IN E R A LS Opal SAMPLE No. SM93/71 DESCRIPTION Silica from margin of travertine terrace (cold). El Tatio. M IN E R A LS Opal SAMPLE No. SM93/72 DESCRIPTION Silica from extinct travertine structure. El Tatio. M IN E R A LS Opal SAMPLE No. SM93/73 DESCRIPTION Silica "pebbles" from area close to active geyser. El Tatio. M IN E R A LS Opal SAMPLE No. TU-46 DESCRIPTION Tumisa Lava M IN E R A LS Orthopyroxene, Hornblende, Plagioclase, Magnetite, llmenite. Apatite, Intersitial Glass SAMPLE No. TU-74 DESCRIPTION No Sample: Thin section only. M IN E R A LS Orthopyroxene SAMPLE No. TU-78 DESCRIPTION No Sample: Thin section only. M IN E R A LS Hornblende, Biotite, Plagioclase, Magnetite, llmenite. Apatite, Zircon SAMPLE No. TU-81 DESCRIPTION Tumisa Lava MINERALS SAMPLE No. TU-95 DESCRIPTION No Sample: Thin section only. M IN E R A LS Olivine, Hornblende, Plagioclase, Gr-spinel, Magnetite, llmenite SAMPLE No. TU-99-3 DESCRIPTION Tumisa Lava M IN E R A LS Magnetite llmenite Pyrrhotite SAMPLE No. VL-1 DESCRIPTION Halite sample from salt dome, Valle de la Luna, San Pedro de Atacama. M IN E R A LS Halite Kutnohorite APPENDIX 2 ANALYTICAL TECHNIQUES. Electron Microprobe. Electron microprobe analyses, including sulphur and chlorine were carried out using a JEOL-733 Superprobe equipped with Link Systems EDS, at Birkbeck College, University of London. An accelerating voltage of 15 kv, and a count time of 100 secs was used. When analysing glasses, the beam was defocussed to lOp and the count time reduced to 30 secs, to limit the volatilisation of sodium. Whole Rock Analyses. Whole rock major and trace elements (Appendix 3) were determined using the Philips PW1400 X-ray fluorescence spectrometer at Royal Holloway and Bedford New College. Major elements were determined on fusion discs and trace elements on pressed powder pellets using the major element composition to calculate matrix corrections. Further details are provided in Thirlwall and Marriner (1986). Ferrous iron was determined by titration against potassium bichromate solution, following digestion of the sample with a mixture of 50% HgSO^ solution and 48% HF solution. HgO- was determined as the weight loss after heating the sample in an oven for 1 hour at 105 °C. Whole rock carbon and sulphur were analysed on powdered samples using a LECO CS-125 carbon-sulphur determinator, by fusion at >1000 °C in a stream of pure oxygen, catalysed by pure iron. Water was absorbed in a tube of magnesium perchlorate, and H 2 O+ measured as the weight gain of the tube minus HgO-. Other samples were analysed by atomic 328 absorption spectroscopy by Tony Osborn at RHBNC, following dissolution of the sample in HCIO/HF solutions. SiOg was determined by the method of Shapiro & Brannock (1956) except that the sample was fused with lithium metaborate. Some whole rock analyses of major and some trace elements have been carried out by atomic absorption spectroscopy at the laboratories of the SNGM, Chile. Details are given in Gardeweg (1991a). These analyses are not listed in Appendix 3. Isotopes. Sr and Nd isotopes were separated using conventional ion- exchange techniques and analysed on the VG354 5-collector mass spectrometer at RHBNC (Thirlwall, 1991). Carbon and oxygen isotopes were analysed at the Stable Isotope Laboratory at RHBNC. Carbon was liberated from a buchite xenolith (LA-111) and a mafic inclusion (LA-141) by step heating in vacuo. At low temperatures oxygen was used to combust organic carbon and at higher temperatures COg gas was liberated by simple pyrolysis. Carbonate in skarn xenoliths was attacked by phosphoric acid at 90°C to liberate COg. The gas was analysed using a mass spectrometer using the calcite standard RHBNC-1. 329 TABLE A3.1: Step heating run carried out on mafic inclusion LA-141. Sample weight = 96.374 mg. Temperature °C Combustion/ ng C ppm C Pyrolysis (C/P) 400 0 1632 -30.14 16.9 600 0 890 -29.41 9.2 800 P 163 -28.24 1.7 900 P 93 1.0 1000 P 42 0.4 1100 P 192 -34.83 Dirty Gas: Low 0. 1200 0 336 -26.39 3.5 TOTAL 32.7 TABLE A3.2: Step heating run carried out on buchite xenolith LA-111. Sample weight = 96.374 mg. Temperature ° C Combustion/ ng C a‘*3c ppm C wt% COg Pyrolysis (C/P) 300 C 768 9 0.003 400 C 1152 -28.85 14 0.005 500 P 640 -19.88 8 0.003 600 P 16 0 0.000 700 P 1024 -17.04 13 0.005 800 P 192 2 0.001 TOTAL 46 330 X-Ray Diffraction. Analyses were carried out by Steve Mirons by the following method. A small portion of each sample was crushed in an agate pestle and mortar until fragments passed a 72-mesh seive. Fragments were mounted in a standard sample holder using the back-packing method. To determine mineralogy, samples were analysed on a Philips PW 1710 diffractometer with a graphite monochromator using Cu K alpha radiation, 40kV, 30mA, scanning through 2 - 60° 2 theta at 0.5° 2 theta per minute, slits 1° 2 theta divergence, 0.2 mm receiving, and 0.5° scatter. Chart speed 1° 2 theta = 10mm on paper. Ratemeter 5X10^ c.p.s. at time constant 5. Minerals were identified by comparing peaks with JCPDS files. Infrared spectroscopy. Samples were prepared as thin sections 50-1 OOp thick and polished both sides. Mid-infrared analyses were carried out using a Bruker IFS45 infrared spectrometer with infrared microscope attachment. Aperture sizes were selected according to the size of the area which was to be analysed. First-Order Carbon Dating. A tufa sample from Talabre Viejo was dated using a modification of the first-order technique developed by Vita-Finzi (1983). The sample was cleaned manually and then in dilute HCI. Approximately 20g of sample was then reacted with dilute HCI to release CO 2 gas. 5 ml of pentan- 1-01 was added to the sample in order to suppress the foaming problem which was apparent in earlier experiments. Long-chain alcohols are well 331 known as foam suppressors (Osipov, 1962). The gas was bubbled through silver nitrate solution to remove the HCI, passed through magnesium perchlorate powder to dry it and bubbled through an equal mixture of Carbo- Sorb and Permafluor V until saturation of the solution was achieved (no further weight gain). The resulting solution was analysed using a scintillation counter, counting for 200 seconds since longer counting times are affected by degradation of the sample. The count rate was 0.9 ± 0.1 cpm above background (8.8 ± 0.1 cpm), equivalent to an age of 11,700 ± 700. This result must be considered provisional in view of the low count rate and the notorious unreliability of ^^0 ages obtained from tufa in the absence of control and measurements on the groundwater in question (0. Vita-Finzi, pers. comm.). 332 APPENDIX 3 WHOLE ROCK ANALYSES, 333 CENTRE I LAVAS- CENTRE II LAVAS- LA127 LA133 LA135 LA130 LA137 LA138 LA154 SiOg 58.22 60.47 60.63 60.28 59.47 57.76 60.03 TiOg 0 . 8 6 0.72 0.72 0.71 0.69 0.85 0.71 AI2 O 3 16.89 18.15 17.86 16.76 15.82 17.15 16.83 2.77 3.11 2.40 2 . 1 2 1.94 3.34 3.56 FeO 3.75 2.35 2.99 3.70 4.28 3.69 2.71 MnO 0 .1 1 0 .1 1 0 . 1 0 0 .1 1 0 . 1 2 0 0.13 0 . 1 2 MgO 4.61 2.55 2.44 3.56 5.04 4.24 3.38 CaO 7.01 5.87 5.79 5.69 6.25 7.30 5.92 Na 2 0 3.74 4.06 4.00 3.77 3.18 3.66 3.64 K2 O 1.50 1.85 1.94 2 .1 1 2 . 0 2 1.62 1.95 ^ 2 ^ 5 0.25 0.26 0.25 0.23 0.18 0.28 0.23 H2 O + 0.73 0.71 0.85 0.95 0.87 0.76 1.24 LOI TOTAL 100.45 1 0 0 . 2 1 9 9 .9 7 9 9.9 8 99.84 100.77 100.32 H 2 0 - 0.14 0.13 0.13 0.18 0.11 0.10 0.22 C02 0.033 0.038 0.026 0.183 0.039 0.029 0.040 S 0.0635 0.0094 0.0048 0.0523 0.0069 0.0032 0.0323 Trace Elements (ppm). Co Ni 52 15 1 2 36 6 8 52 26 Cr 103 24 19 84 199 45 51 V 163 108 105 129 146 165 135 Sc 2 0 14 14 16 2 0 2 0 17 Cu 49 2 2 15 21 39 42 26 Zn 82 76 74 75 71 77 71 Cl 351 130 235 416 250 573 169 Ga 2 0 2 0 19 19 17 2 1 2 0 Pb 9 13 13 13 13 11 1 2 Sr 579 521 512 480 401 701 583 Rb 43 53 70 85 79 46 65 Ba 438 459 439 441 398 383 435 Zr 159 175 177 165 153 145 164 Nb 8 11 11 11 9 8 9 Th 6 8 8 1 0 9 6 8 Y 18 25 24 23 2 2 2 2 2 2 La 2 1 24 24 23 2 1 2 0 24 Ce 48 54 53 50 44 47 50 Nd 23 26 25 24 21 24 23 ®^Sr/®®Sr 0.706693 0.706686 0.706792 ERROR 0.000009 0.000010 0.000011 143Nd/144Nd ERROR 334 PIEDRAS SONCOR FLOW- GRANDES FLOW. Dacite Pumice. Mafic Vitrophyre Andésite Dacite Incl. Pumice Lava LA121 LA122 LA124 SM93/44 LAS29 SM93/22 SiOg 64.94 63.88 56.19 67.66 61.79 63.21 TiOg 0.57 0.64 0.99 0.54 0.72 0.59 AI2 O3 15.32 15.71 16.52 14.72 15.96 15.56 2 . 0 0 2.31 3.34 4.53 5.89 5.18 FeO 2.35 2.34 4.06 MnO 0.08 0.08 0 .1 1 0.09 0 .1 1 0.09 MgO 2.39 2.59 5.81 2.39 3.35 2.92 CaO 3.76 4.49 6.92 3.77 5.44 4.67 N320 3.64 3.45 3.64 3.64 3.76 3.64 K2 O 2.64 2.40 1.39 2 . 6 8 2.06 2.43 ^2^5 0.17 0.19 0.25 0.17 0 . 2 2 0.19 H2 O + 2.44 1.97 0.90 LOI 1.30 0.58 0.82 TOTAL 100.31 100.05 1 0 0 . 1 2 101.49 99.58 1 0 1 . 0 1 H2 O- 0.15 0.24 0.13 0.28 0 .1 1 0 .2 1 CO 2 0.093 0.118 0.073 .0037 .0267 .0030 S 0.1810 0.1430 0.0075 .0141 .0146 .0046 Trace Elements (ppm). Co 17 23 430 Ni 2 2 23 98 32 47 36 Cr 36 33 230 V 95 1 0 0 191 95 129 108 Sc 1 0 1 2 19 9 13 11 Cu 19 33 19 2 2 42 25 Zn 39 63 8 8 53 65 61 Cl 1042 2190 469 Ga 18 18 2 0 Pb 1 0 8 6 Sr 381 416 57 348 446 436 Rb 97 90 34 Ba 481 467 387 440 425 430 Zr 132 144 150 Nb 11 1 0 9 1 2 1 2 11 Th 1 2 1 2 4 Y 19 18 17 17 16 17 La 24 26 19 Ce 50 54 45 Nd 2 2 23 23 ®^Sr/®®Sr 0.706412 0.706455 0.706242 ERROR 0.000008 0.000008 0.000007 143Nd/144Nd 0.512427 0.512468 ERROR 0.000004 0.000008 335 CHAILE TUMBRES FLOW CAPRICORN LAVA- — flow - Host Mafic Mafic Andésite Andésite Dacite Lava Incl. Incl. Pumice Pumice Pumice SM93/20 LA140 LA141 LA143 LA123 LA136 LA117 SiOg 61.84 63.41 57.17 55.52 57.32 58.00 67.79 TiOg 0.79 0.61 0.81 0.93 0.87 0.82 0.48 Alg0 3 16.15 15.81 16.92 16.97 17.03 17.06 15.58 FegOg 5.79 2.52 3.28 3.84 3.96 3.00 2.64 FeO 2.41 3.10 3.32 2.77 3.67 1.32 MnO 0 . 1 0 0.09 0 . 1 2 0 . 1 2 0 . 1 2 0.13 0.08 MgO 2.41 2.79 4.77 5.49 3.76 3.81 1.41 CaO 4.67 4.82 6.76 7.32 7.08 6.56 3.46 NagO 3.22 3.68 3.75 3.68 3.63 3.40 4.09 KgO 2.26 2.38 1.57 1.35 1.54 1.69 2.63 ^2^5 0.25 0.18 0.24 0.26 0.33 0.30 0.17 H20+ 1.28 1.41 1.17 1.19 1.06 1.72 LOI 1.82 TOTAL 99.00 99.98 99.89 99.97 99.60 99.49 101.38 HgO- 0.41 0.18 0.23 0.29 0.09 0.04 0.16 COg 0.0385 0.084 0.244 0.038 0.117 0.039 0.025 8 0.0279 0.0052 0.0093 0.0058 0.0277 0.0782 0.0067 Trace Elements (ppm). Co 2 0 Ni 50 29 64 76 25 24 7 Cr 47 105 129 28 29 9 V 134 104 161 189 148 148 58 Sc 14 13 2 0 23 16 17 9 Cu 36 36 60 37 44 39 8 Zn 67 62 74 81 79 79 57 Cl 127 365 298 700 425 244 Ga 18 2 0 2 0 2 0 2 0 18 Pb 16 15 9 7 11 14 Sr 399 453 625 643 711 616 400 Rb 90 49 38 48 57 109 Ba 445 477 402 334 385 426 577 Zr 150 138 135 151 150 189 Nb 13 1 0 9 9 8 9 1 2 Th 11 7 6 6 7 1 2 Y 18 2 0 2 0 2 0 2 0 2 1 23 La 24 21 19 2 2 2 1 25 Ce 50 47 45 50 49 50 Nd 2 2 23 23 25 24 2 2 ®^Sr/®®Sr 0.706340 0.705947 0.705981 0.705765 0.706148 ERROR 0.000009 0.000007 0.000009 0.000009 0.000009 143Nd/144Nd 0.512425 0.512436 0.512470 ERROR 0.000005 0.000005 0.000005 336 CENTRE LAVAS -1986 TO 1993 PRODUCTS- Talabre 1990 1993 1993 Lava Lava Pumice Scoria LA125 LA153 LA102 PWF1 PWF2 SiOg 58.01 56.25 58.30 59.20 57.82 TiOg 0.85 0.81 0.75 0.72 0.75 AI2 O3 17.68 16.99 16.24 17.37 16.21 F®2®3 4.49 4.69 2.67 6.54 7.29 FeO 2.17 2.44 4.14 MnO 0 .1 1 0 . 1 2 0.13 0 .1 1 0 . 1 2 MgO 3.71 5.18 4.78 3.63 5.04 CaO 7.11 7.70 7.17 6.25 6.98 N8 2 0 3.80 3.61 3.46 3.55 3.35 K2 O 1.54 1.32 1.63 1.97 1.93 ^2^5 0.32 0.23 0.19 0 .2 1 0 . 2 2 H2 O + 0.52 0.47 LOI 0 .0 1 0 .0 1 TOTAL 100.31 99.34 99.93 99.56 99.72 H20- 0 . 1 2 0 .2 1 0.07 0 . 1 2 0.04 C02 0.047 0 . 0 1 0 0.0086 0 . 0 0 2 1 S 0.0015 0.0026 0.0136 0.0103 Trace Elements (ppm). Co 24 30 Ni 24 52 32 32 40 Cr 27 92 90 V 142 168 159 143 173 Sc 17 2 2 21 16 21 Cu 45 51 58 38 50 Zn 76 83 6 8 6 6 65 Cl 276 2 2 1 276 Ga 21 19 18 Pb 1 0 7 1 0 Sr 728 566 462 453 454 Rb 50 39 61 Ba 389 385 382 395 320 Zr 154 135 138 Nb 9 7 8 11 1 0 Th 6 4 7 Y 2 0 19 2 2 19 18 La 23 16 19 Ce 50 39 42 Nd 25 2 0 21 8 7 cSr/°°Sr r/8 6 0.706352 ERROR 0.000009 143 Nd/144 Nd ERROR 337 SKARN XENOLITHS WOLL. DIOPSIDE SKARN SKARN LA108 LA144 SiOg 60.81 70.59 TiOg 0 . 1 0 0.50 AI2 O 3 2.29 12.94 F e jO g 0.67 3.60 FeO MnO 0.06 0.15 MgO 0.30 0.95 CaO 33.09 6.98 Na 2 0 0 . 1 0 2.49 K2 O 0.54 0.26 ^ 2 ^ 5 0.17 0.16 H2 O + LOI 2 .1 1 0.49 TOTAL 9 9 .9 4 99.11 H 2 O- 0.17 0.34 CO 2 0.553 0.0073 S 0 . 0 1 0 1 0.0800 Trace Elements (ppm). Co 9 9 Ni 2 2 11 Cr V 54 69 Sc 2 7 Cu 7 9 Zn 16 42 Cl Ga Pb Sr 393 456 Rb Ba 521 108 Zr Nb 1 2 1 0 Th Y 1 2 19 La Ce Nd 87cr/86S r /° ° S r ERROR 143 Nd/ 144 Nd ERROR 338 APPENDIX 4. SELECTED ELECTRON MICROPROBE ANALYSES. Analyses of minerals and glasses are listed by sample number. For each analyses, weight percent oxides and calculated mineral formulae are presented. Bold figures denote analyses above the detction levels of the electron microprobe and plain text figures denote those below the detection levels (< 2 sigma). Such low concentrations of elements, such as Cr2 0 3 in hornblende, are used in calculations of mineral formulae. This means that on scatter plots of calculated formula units of various elements the points which lie below the detection levels of the instrument do not all reduce to zero, but maintain the scatter which is present above the detection levels. The same method has been used in plotting weight percent oxides in glasses. 339 OLIVINE. ANALYSIS DESCRIPTION LA124 Hoiocrystaiiine Mafic inclusion. Tumbres Flow. 0 2 core Core of olivine phenocryst. 0 2 rim Rim of olivine phenocryst. 04 core Core of olivine phenocryst. 04 rim Rim of olivine phenocryst. 05 core Core of olivine phenocryst. 05 rim Rim of olivine phenocryst. 06 core Core of olivine phenocryst. 06 rim Rim of olivine phenocryst. LA141 Mafic inclusion. Capricorn Lava. 0 4 Olivine Phenocryst. 05 Olivine Phenocryst. 06 Olivine Phenocryst. 07 Olivine Phenocryst. 0 8 Olivine Phenocryst. 09 Olivine Phenocryst. 013 Olivine Phenocryst. 023 Olivine Phenocryst. LA147 Dacitic Pumice. Soncor Flow. 0 1 core Core of olivine phenocryst with opx/magnetite corona. 0 1 rim Rim of olivine phenocryst with opx/magnetite corona. 04 Olivine phenocryst with opx/magnetite corona. LAS36-1 Hornbiende-Rich Pumice. Soncor Flow. 0 1 Olivine phenocryst with opx/magnetite corona. 03 Olivine phenocryst with opx/magnetite corona. SM93/43 Augite Megacryst. Saltar Flow. 0 1 Olivine inclusion in core of megacryst. 0 1 Olivine inclusion in core of megacryst. LA18 Centre i Andésite Lava. 0 1 Olivine phenocryst with orthopyroxene overgrowth. 340 OLIVINE. LA124 LA124 LA124 LA124 LA124 LA124 LA124 LA124 ANALYSIS. 0 2 core 0 2 rim 04 core 04 rim 05 core 05 rim 0 6 core 0 6 rim SlOg 38.91 38.68 40.12 38.54 39.44 39.25 39.93 39.28 Cr2 0 3 0.05 0.09 0.02 0.00 0.07 0.00 0.00 0.07 FeO 14.53 18.63 12.95 19.38 13.18 17.45 15.96 17.94 MnO 0.17 0.26 0.12 0.45 0.15 0.11 0.23 0.16 MgO 43.50 40.81 45.30 40.82 44.95 42.32 43.70 42.06 NIO 0.30 0.10 0.29 0.04 0.12 0.35 0.23 0.19 CaO 0.14 0.15 0.06 0.16 0.13 0.11 0.07 0.10 CAJ N8 2 0 0.18 0.34 0.24 0.39 0.18 0.39 0.38 0.16 • r TOTAL 97.78 99.06 99.10 99.78 98.22 99.98 100.50 99.96 FORMULA (4 OXYGENS) SI 1.000 1.000 1.008 0.994 1.002 1.000 1.004 1.002 Cr 0.001 0.002 0.000 0.000 0.001 0.000 0.000 0.001 Fe 0.312 0.403 0.272 0.418 0.280 0.372 0.335 0.383 Mn 0.004 0.006 0.003 0.010 0.003 0.002 0.005 0.003 Mg 1.667 1.573 1.696 1.569 1.701 1.607 1.637 1.598 NI 0.006 0.002 0.006 0.001 0.002 0.007 0.005 0.004 Ca 0.004 0.004 0.002 0.004 0.004 0.003 0.002 0.003 Na 0.009 0.017 0.012 0.020 0.009 0.019 0.019 0.008 TOTAL 3.003 3.007 2.999 3.016 3.002 3.010 3.007 1 . 0 0 2 % Fo 84.1 79.4 8 6 . 0 78.6 82.0 82.5 82.8 80.5 OLIVINE. LA141 LA141 LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS 04 05 06 07 08 09 013 023 SIO2 39.67 39.57 39.72 39.66 39.74 39.83 38.93 40.00 Cr2 0 3 0.07 0.08 0.05 0.01 0.10 0.04 0.08 0.07 FeO 13.62 15.47 14.13 1 2 . 6 6 13.33 13.95 13.97 12.79 MnO 0.10 0.25 0.10 0.21 0.23 0.33 0.26 0.20 MgO 46.30 44.18 45.77 46.47 45.59 46.09 44.70 46.89 NiO 0.21 0.14 0.45 0.49 0.00 0.33 0.24 0.18 CaO 0.14 0.19 0.11 0.09 0.08 0.05 0.07 0.10 Na2 0 0.13 0.16 0.00 0.00 0.08 0.16 0.30 N.A. v/J f TOTAL 100.24 100.04 100.33 99.59 99.15 100.78 98.55 100,23 r FORMULA (4 OXYGENS) SI 0.990 0.997 0.992 0.992 0.999 0.991 0.992 0.993 Cr 0.001 0.002 0.001 0.000 0.002 0.001 0.002 0.001 Fe 0.284 0.326 0.295 0.265 0.280 0.290 0.298 0.266 Mn 0.002 0.005 0.002 0.004 0.005 0.007 0.006 0.004 Mg 1.721 1.660 1.704 1.733 1.709 1.709 1.697 1.735 Ni 0.004 0.003 0.009 0.010 0.000 0.007 0.005 0.004 Ca 0.004 0.005 0.003 0.002 0.002 0.001 0.002 0.003 Na 0.006 0.008 0.000 0.000 0.004 0.008 0.015 TOTAL 3.012 3.006 3.006 3.006 3.001 3.014 3.017 3.006 % Fo 85.7 83.4 85.2 86.7 85.7 85.2 84.8 86.5 OLIVINE. LA147 LA147 LA147 LAS36-1 LAS36-1 SM93/43 SM93/43 LA18 ANALYSIS 0 1 core 0 1 rim 04 0 1 03 0 1 0 2 0 1 SiOg 39.56 39.29 38.80 38.86 38.47 40.36 39.98 39.88 Cr2 0 3 0.01 0.00 0.00 0.00 0.00 0.00 0.00 N.A. FeO 16.76 19.29 19.93 22.42 22.13 16.97 17.66 15.26 MnO 0 . 2 1 0.23 0.48 0.42 0.46 0 . 2 1 0.16 0.23 MgO 42.41 41.34 40.84 38.78 38.39 43.17 42.42 44.80 NiO 0.22 0.02 0.16 0.09 0.09 0.00 0.09 N.A. CaO 0.05 0.03 0.04 0.00 0.02 0.10 0.18 0.11 NagO 0.21 0.34 0.16 0.39 0.19 0.05 0.02 N.A. TOTAL 99.43 100.54 100.41 100.96 99.75 1 0 0 . 8 6 100.51 100.28 VJ f UJ FORMULA (4 OXYGENS) SI 1.008 1.002 0.996 1.002 1.004 1.012 1.010 0.999 Or 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe 0.357 0.411 0.428 0.484 0.483 0.356 0.373 0.320 Mn 0.005 0.005 0.010 0.009 0.010 0.004 0.003 0.005 Mg 1.611 1.571 1.562 1.491 1.493 1.613 1.597 1.673 NI 0.005 0.000 0.003 0.002 0.002 0.000 0.002 Ca 0.001 0.001 0.001 0.000 0.001 0.003 0.005 0.003 Na 0.010 0.017 0.008 0.020 0.010 0.002 0.001 TOTAL 2.997 3.007 3.008 3.008 3.003 2.990 2.991 3.000 % Fo 81.7 79.1 78.1 75.2 75.2 81.8 80.9 83.7 ORTHOPYROXENE. ANALYSISDESCRIPTION. LA124 Hoiocrystaiiine Mafic Inciusion. Soncor Flow. P 8 Orthopyroxene overgrowth on olivine phenocryst. P9 Orthopyroxene overgrowth on olivine phenocryst. P19 Orthopyroxene microphenocryst in interstitial glass in olivine pyroxene crystal clot. P45 Orthopyroxene in groundmass. P6 6 core Core of orthopyroxene phenocryst. P6 6 rim Rim of orthopyroxene phenocryst. LA121 Dacitic Pumice. Soncor Flow. P20 core Core of orthopyroxene phenocryst. P20 Int Intermediate growth zone of orthopyroxene phenocryst. P20 rim Rim of orthopyroxene phenocryst. P60 core Core of large subhedral orthopyroxene phenocryst. P60 rim Rim of large subhedral orthopyroxene phenocryst. P62 Subhedral orthopyroxene microphenocryst. LA122 Dacitic Pumice. Soncor Flow. P1 Small euhedral orthopyroxene. P20 core Core of orthopyroxene phenocryst. P20 rim Rim of orthopyroxene phenocryst. P25 core Core of orthopyroxene phenocryst. P25 rim Rim of orthopyroxene phenocryst. P27 Orthopyroxene microphenocryst. LA141 Mafic Inclusion. Capricorn Lava. P19 core Core of rounded orthopyroxene phenocryst. P19 rim Rim of rounded orthopyroxene phenocryst. P20 core Core of rounded orthopyroxene phenocryst. P20 rim Rim of rounded orthopyroxene phenocryst. P50 Orthopyroxene intergrown with magnetite. P58 Small euhedral orthopyroxene phenocryst. 344 ANALYSIS DESCRIPTION. LA14Q Dacitic Capricorn Lava. P16 core Core of euhedral orthopyroxene phenocryst. P16 rim Rim of euhedral orthopyroxene phenocryst. P18 core Core of small euhedral orthopyroxene phenocryst. P18 rim Rim of smalleuhedral orthopyroxene phenocryst. P19 Very small orthopyroxene microphenocryst. P20 Very small orthopyroxene microphenocryst. SM93/43 Clinopyroxene megacryst. Saltar Flow. P1 Orthopyroxene inclusion in core of megacryst. P3 Orthopyroxene inclusion in core of megacryst. P7 core Core of orthopyroxene inclusion in rim of megacryst. P7 rim Rim of orthopyroxene inclusion in rim of megacryst. SM93/20 Andésite Lava. Chaile Flow. P1 core Core of orthopyroxene phenocryst. P1 rim Rim of orthopyroxene phenocryst. LAS15 Banded Diopside-Woliastonite Skarn Xenolith. Tumbres Flow. P4 Ferro-hypersthene in pyroxene-rich band. P5 Ferro-hypersthene in pyroxene-rich band. P7 Ferro-hypersthene in pyroxene-rich band. LAS23 Buchite Xenolith. Tumbres Flow. P7 Endiopside. Secondary “phenocryst” in matrix glass. P8 Endiopside. Secondary “phenocryst” in matrix glass. P10 Endiopside. Secondary “phenocryst” in matrix glass. 345 ORTHOPYROXENE. LA124 LA124 LA124 LA124 LA124 LA124 ANALYSIS PB P9 P19 P45 P 6 6 core P 6 6 rim SlOg 53.55 53.99 53.51 55.10 54.51 54.11 TlOg 0.65 0.41 0.35 0.27 0.19 0.28 AI2 O 3 1 . 6 8 1 . 8 8 0.74 0.87 1.59 1.07 Cr2 0 3 0.01 0.00 0.02 0.02 0.08 0.08 FeO 13.23 15.53 12.93 12.32 15.18 16.63 MnO 0.45 0.43 0.46 0.28 0.49 0.48 MgO 27.52 27.13 25.01 28.64 27.11 25.25 CaO 1.90 1.87 4.35 2.05 1.16 1.43 NB2 0 0.09 0.38 0.11 0.11 0.39 0.19 TOTAL 99.08 101.62 97.48 99.66 100.70 99.52 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.935 1.939 1.981 1.968 1.945 1.977 Ti 0.018 0.011 0.010 0.007 0.005 0.008 Al 0.071 0.079 0.032 0.037 0.067 0.046 Cr 0.000 0.000 0.001 0.001 0.002 0.002 Fe^+ 0.029 0.050 0.000 0.019 0.058 0.000 Fe 2 + 0.371 0.356 0.400 0.349 0.395 0.508 Mn 0.014 0.013 0.014 0.009 0.015 0.015 Mg 1.482 1.452 1.380 1.525 1.442 1.375 Ca 0.074 0.072 0.172 0.078 0.044 0.056 Na 0.006 0.027 0.008 0.007 0.027 0.014 TOTAL 4.000 3.999 3.998 4.000 4.000 4.001 En 0.768 0.771 0.706 0.780 0.766 0.709 Fs 0.192 0.189 0.205 0.179 0.210 0.262 Wo 0.040 0.040 0.089 0.041 0.025 0.029 346 ORTHOPYROXENE. LA121 LA121 LA121 LA121 LA121 LA121 ANALYSIS P20 core P20 int P20 rim P60 core P60 rim P62 SlOg 52.57 54.50 52.53 53.18 54.08 53.24 TiOg 0.26 0.15 0.24 0.29 0.07 0 . 2 1 AI2 O 3 2.65 2 . 2 0 2.70 3.64 1 . 0 2 1.17 CfgOg 0.00 0.45 0.49 0.13 0.00 0.03 FeO 19.38 10.87 1 2 . 6 8 13.68 18.65 16.48 MnO 0.43 0.22 0.15 0.29 0.72 0.40 MgO 23.47 29.24 26.83 25.44 24.39 25.02 CaO 1.32 1 . 2 0 1.40 3.05 1.07 1.34 NagO 0.00 0.33 0.69 0.16 0.02 0.16 TOTAL 100.08 99.16 97.71 99.86 1 0 0 . 0 2 98.05 FORMULA (4 CATIONS,, 6 OXYGENS) Si 1.931 1.940 1.913 1.917 2.002 1.973 Ti 0.007 0.004 0.007 0.008 0.002 0.006 A! 0.115 0.092 0.116 0.155 0.045 0.051 Cr 0.000 0.013 0.014 0.004 0.000 0.001 Fe3+ 0.012 0.028 0.077 0.000 0.000 0.000 Fe 2+ 0.583 0.296 0.309 0.412 0.578 0.511 Mn 0.013 0.007 0.005 0.009 0.023 0.013 Mg 1.285 1.552 1.456 1.367 1.345 1.382 Ca 0.052 0.046 0.055 0.118 0.005 0.053 Na 0.001 0.023 0.049 0.011 0.001 0.011 TOTAL 3.999 4.001 4.001 4.001 4.001 4.001 En 0.668 0.818 0.798 0.717 0.698 0.710 Fs 0.303 0.156 0.169 0.216 0.300 0.262 Wo 0.029 0.026 0.033 0.067 0.003 0.028 347 ORTHOPYROXENE. LA122 LA122 LA122 LA122 LA122 LA122 ANALYSIS P1 P20 core P20 rim P25 core P25 rim P27 SiOg 50.72 53.18 53.04 52.91 52.98 53.60 TiOg 0.20 0.19 0.18 0.17 0.18 0.18 Alg0 3 2 . 2 0 0.58 0 . 8 6 1 . 1 1 0 . 8 6 0.76 CrgOg 0.00 0.01 0.00 0.06 0.11 0.01 FeO 19.88 21.05 20.84 20.28 20.30 20.33 MnO 0.76 0.62 0.81 0.62 0.76 0.69 MgO 21.80 23.72 22.91 22.58 22.29 22.78 CaO 1.13 1 . 2 0 1 . 2 2 1.18 1 . 2 0 1.31 Nag 0 0.33 0.37 0.20 0.27 0.05 0.23 TOTAL 97.02 100.92 100.06 99.18 98.73 99.89 FORMULA (4 CATIONS, 6 OXYGENS) SI 1.931 1.943 1.962 1.972 1.991 1.985 Ti 0.006 0.005 0.005 0.005 0.005 0.005 A! 0.099 0.025 0.037 0.049 0.053 0.033 Or 0.000 0.000 0.000 0.002 0.003 0.000 FeS+ 0.051 0.105 0.045 0.015 0.000 0.005 Fe2+ 0.582 0.538 0.600 0.617 0.653 0.624 Mn 0.024 0.019 0.025 0.019 0.020 0.022 Mg 1.237 0.292 1.263 1.254 1.229 1.257 Ca 0.046 0.047 0.048 0.047 0.048 0.052 Na 0.024 0.026 0.014 0.020 0.000 0.016 TOTAL 4.000 4.000 3.999 4.000 3.999 3.999 En 0.662 0.687 0.660 0.653 0.645 0.650 Fs 0.312 0.286 0.314 0.321 0.330 0.323 Wo 0.026 0.026 0.026 0.025 0.025 0.027 348 ORTHOPYROXENE. LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS P19 core PI 9 rim P20 core P20 rim P50 P58 SlOg 52.54 53.18 52.24 52.01 52.72 53.71 TiOg 0.16 0.09 0.16 0 . 2 1 0.32 0.24 AI2 O 3 1.27 0.62 1.29 2.55 1.29 1.63 Cr2 0 3 0.00 0.01 0.00 0.07 0.00 0.04 FeO 19.78 18.87 20.92 17.77 21.47 15.29 MnO 0.72 0.64 0.46 0.35 0.75 0.40 MgO 23.24 24.09 22.53 23.93 21.92 26.92 CaO 1 . 2 1 1.08 1.35 1.26 1.14 1.30 NagO 0.40 0.18 0.17 0.19 0.15 0.16 TOTAL 99.32 98.76 99.12 98.34 99.76 99.53 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.945 1.975 1.951 1.930 1.966 1.939 TI 0.004 0.002 0.004 0.006 0.009 0.006 AI 0.055 0.027 0.057 0.112 0.057 0.069 Cr 0.000 0.000 0.000 0.002 0.000 0.001 Fe3+ 0.076 0.034 0.045 0.026 0.000 0.053 Fe 2 + 0.537 0.552 0.608 0.526 0.670 0.409 Mn 0.023 0.020 0.015 0.011 0.024 0.012 Mg 1.283 1.333 1.254 1.324 1.219 1.449 Ca 0.048 0.043 0.054 0.050 0.046 0.050 Na 0.029 0.013 0.012 0.014 0.011 0.011 TOTAL 4.000 3.999 4.000 4.001 4.002 3.999 En 0.686 0.691 0.654 0.696 0.630 0.758 Fs 0.287 0.286 0.317 0.276 0.346 0.214 Wo 0.027 0.023 0.030 0.028 0.024 0.028 349 ORTHOPYROXENE. LA140 LA140 LA140 LA140 LA140 LA140 ANALYSIS P16 core P16 rim P18 core P18 rim P19 P20 SlOg 53.41 53.53 54.51 52.77 53.01 53.78 TiOg 0.19 0.18 0 . 2 2 0.34 0.26 0.18 AI2 O 3 0.46 0 . 6 6 0.85 1.94 1.43 0 . 8 8 CrgO] 0.00 0.00 0.00 0.10 0.10 0.11 FeO 18.72 19.21 14.68 16.16 14.66 14.64 MnO 0.60 0.60 0.31 0.26 0.26 0.30 MgO 24.33 24.29 27.65 25.44 26.50 26.01 CaO 1 . 2 0 0.94 1.71 2.27 1.77 2.53 NagO 0.20 0.18 0.37 0.09 0.42 0.21 TOTAL 99.11 99.59 100.30 99.37 98.41 98.64 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.975 1.972 1.947 1.925 1.934 1.968 TI 0.005 0.005 0.006 0.009 0.007 0.005 AI 0.020 0.029 0.036 0.084 0.061 0.038 Cr 0.000 0.000 0.000 0.003 0.003 0.003 Fe3+ 0.032 0.030 0.086 0.051 0.084 0.028 Fe 2 + 0.547 0.562 0.352 0.442 0.363 0.420 Mn 0.019 0.019 0.009 0.008 0.008 0.009 Mg 1.341 1.333 1.472 1.383 1.441 1.418 Ca 0.048 0.037 0.065 0.089 0.069 0.096 Na 0.014 0.013 0.026 0.006 0.030 0.015 TOTAL 4.001 4.001 3.999 4.000 4.000 4.000 En 0.692 0.690 0.778 0.720 0.767 0.732 Fs 0.282 0.291 0.186 0.230 0.193 0.217 Wo 0.025 0.020 0.036 0.050 0.039 0.051 350 ORTHOPYROXENE. SM93/43 SM93/43 SM93/43 SM93/43 SM93/20 SM93/2 ANALYSIS P1 P3 P7 core P7 rim PI core PI rim SlOg 56.41 54.72 53.50 54.10 53.04 53.75 TiOg 0.19 0.18 0 . 2 0 0.08 0.26 0.30 AI2 O3 1.53 2.60 2.17 2.05 3.47 1 . 2 2 Cr2 0 3 0.04 0.38 0.00 0.13 0.04 0.04 FeO 11.35 10.26 15.26 12.33 16.35 19.96 MnO 0.24 0.16 0.39 0.27 0.33 0.56 MgO 29.75 29.75 26.05 27.70 25.22 23.19 CaO 1.30 1.46 1.33 1.33 1.40 1.34 NagO 0.13 0.24 0.23 0.10 0.21 0.21 TOTAL 100.94 99.75 99.13 98.09 100.32 100.57 FORMULA (4 CATIONS, 6 OXYGENS) SI 1.978 1.932 1.945 1.965 1.991 1.990 TI 0.005 0.005 0.005 0.002 0.007 0.006 Ai 0.063 0.108 0.093 0.088 0.031 0.014 Or 0.001 0.011 0.000 0.004 0.000 0.000 FeS+ 0.000 0.022 0.024 0.000 0.000 0.007 Fe2+ 0.333 0.281 0.444 0.374 0.651 0.661 Mn 0.007 0.005 0.012 0.008 0.021 0.015 Mg 1.555 1.565 1.412 1.500 1.212 1.232 Ca 0.049 0.055 0.052 0.052 0.063 0.062 Na 0.009 0.017 0.017 0.007 0.023 0.013 TOTAL 4.000 4.001 4.000 4.000 3.999 4.000 En 0.802 0.822 0.740 0.778 0.718 0.633 Fs 0.172 0.148 0.231 0.194 0.251 0.333 Wo 0.026 0.031 0.029 0.028 0.031 0.035 351 ORTHOPYROXENE. LAS15 LAS15 LAS15 LAS23 LAS23 LAS23 ANALYSIS P4 P5 P7 P7 P 8 P10 SiOg 49.26 49.23 49.24 55.78 56.35 56.58 TIOg 0.13 0.10 0 . 2 0 0 . 2 2 0.04 0.13 Alg0 3 0.73 0.46 0.59 1.57 0 . 6 8 0.74 Crg0 3 0.00 0.02 0.00 0.00 0.12 0.00 FeO 30.73 30.97 31.18 6.98 7.95 7.95 MnO 1.41 1.44 1.55 1.34 1.59 1.47 MgO 13.94 13.53 13.57 32.15 31.90 31.99 CaO 0.85 0.92 0.99 1.08 0.62 0.75 NagO 0.22 0.04 0.39 0.04 0.05 0.02 TOTAL 97.27 96.71 97.71 99.16 99.28 99.63 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.986 2.005 1.979 1.959 1.985 1.986 TI 0.004 0.003 0.006 0.006 0.001 0.004 AI 0.035 0.022 0.028 0.065 0.028 0.031 Cr 0.000 0.001 0.000 0.000 0.003 0.000 Fe3+ 0.002 0.000 0.032 0.004 0.002 0.000 Fe%+ 1.034 1.055 1.016 0.201 0.232 0.253 Mn 0.048 0.050 0.053 0.040 0.047 0.044 Mg 0.837 0.821 0.813 1.683 1.675 1.673 Ca 0.037 0.040 0.043 0.041 0.023 0.028 Na 0.017 0.003 0.030 0.003 0.003 0.001 TOTAL 4.000 4.000 4.000 4.002 3.999 4.000 En 0.439 0.428 0.434 0.874 0.868 0.865 Fs 0.542 0.550 0.543 0.104 0.120 0.120 Wo 0.020 0.021 0.023 0.022 0.012 0.015 352 CLINOPYROXENES. ANALYSIS.DESCRIPTION. LA124 Holocrystalline mafic inclusion. Soncor Flow. P1 Fassaitic augite inclusion with andesitic glass in olivine phenocryst. P4 core Core of euhedral augite phenocryst. P4 rim Rim of euhedral augite phenocryst. P 6 Augite overgrowth on olivine phenocryst. P17 Subcalcic augite inclusion in glass in olivine-augite clot. P23 Augite in olivine-pyroxene clot. LA121 Dacitic Pumice. Soncor Flow. P58 core Large anhedral augite phenocryst. Core. P58 rim Large anhedral augite phenocryst. Rim. P69 core Large ragged augite phenocryst. Core. P69 rim Large ragged augite phenocryst. Rim. LA122 P32 Rounded augite in crystal clot. P33 Rounded augite in crystal clot. LA141 Mafic inclusion. Capricorn Lava. P11 core Augite phenocryst. Core. P11 rim Augite phenocryst. Rim. P13 core Augite phenocryst. Core. P13 rim Augite phenocryst. Rim. P59 Small anhedral augite attached to hornblende. P63 core Rounded augite phenocryst. Core. P63 rim Rounded augite phenocryst. Rim. LA140 Dacitic Capricorn Lava. P 11 Clinopyroxene in small skarn xenolith. P14 Clinopyroxene in small skarn xenolith. PI 7 core Euhedral augite phenocryst. Core. P17 rim Euhedral augite phenocryst. Rim. P34 Small euhedral augite microphenocryst. 353 ANALYSIS. DESCRIPTION. SM93/43 Clinopyroxene Megacryst. Saltar Flow. P2 Core of augite megacryst. P8 Rim of augite megacryst. SM93/20 Andesitic Lava. Chaile Flow. P7 core Augite phenocryst. Core. P7 rim Augite phenocryst. Rim. P9 core Augite phenocryst. Core. P9 rim Augite phenocryst. Rim. LA108 Wollastonite skarn xenolith. Tumbres Flow. A1 Fassaitic augite. A3 Fassaitic augite. AS Fassaitic augite. P1 Diopside. P2 Diopside. P3 Salite. LA111 Buchite Xenolith. Tumbres Flow. P1 Salite. P10 Salite. LA112 Wollastonite Skarn Xenolith. Tumbres Flow. P1 Salite. LA115 Diopside Skarn Xenolith. Tumbres Flow. P1 Salite. P2 Salite. P3 Salite. LA144 Magnetite Skarn Xenolith. Capricorn Lava. P2 Augite. P4 Diopside. LA148 Diopside Skarn Xenolith. Bomb. P2 Salite. P3 Salite. 354 ANALYSIS. DESCRIPTION. LAS15 Banded Wollastonite-Diopside Skarn Xenolith. Tumbres Flow. P8 Ferrosalite. P10 Ferrosalite. LAS23 Buchite Xenolith. Tumbres Fiow. PI Endiopside. P2 Endiopside. P3 Endiopside. 355 CLINOPYROXENE. LA124 LA124 LA124 LA124 LA124 LA124 ANALYSIS P1 P4 core P4 rim P6 P17 P23 SIO 2 45.89 51.41 49.90 50.93 52.16 50.63 TIO2 1.42 0.48 0 . 8 6 0.89 0.91 3.14 AI2 O3 8.46 2.51 3.08 2.48 1.97 0.46 Cr2 0 3 0 . 0 0 0.38 0 . 0 0 0 . 0 1 0 . 0 0 0.19 FeO 7.60 7.63 8.16 11.36 11.08 7.30 MnO 0 . 1 1 0.15 0.35 0.37 0.36 0.17 MgO 13.67 14.65 15.82 15.77 20.37 16.29 CaO 2 0 . 1 0 2 2 . 0 2 20.31 16.42 11.34 19.91 N320 0.62 0.59 0.58 0.70 0.42 0.26 TOTAL 97.87 99.89 99.06 98.93 98.61 98.35 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.724 1.902 1.852 1.908 1.931 1.889 Ti 0.040 0.013 0.024 0.025 0.025 0.013 Al 0.375 0.109 0.135 0.110 0.086 0.138 Cr 0.000 0.011 0.000 0.000 0.000 0.006 Fe3 + 0.142 0.094 0.156 0.073 0.032 0.071 Fe2+ 0.097 0.142 0.097 0.283 0.311 0.157 Mn 0.003 0.005 0.011 0.012 0.011 0.005 Mg 0.765 0.808 0.875 0.880 1.124 0.906 Ca 0.809 0.873 0.808 0.659 0.450 0.796 Na 0.045 0.042 0.041 0.051 0.030 0.019 TOTAL 4.000 3.999 3.999 4.001 4.000 4.000 En 0.595 0.493 0.570 0.503 0.557 0.541 Fs 0.076 0.087 0.063 0.162 0.154 0.094 Wo 0.329 0.420 0.367 0.335 0.288 0.365 356 CLINOPYROXENE. LA121 LA121 LA121 LA121 LA122 LA122 ANALYSIS P58 core P58 rim P69 core P69 rim P32 P33 SlOg 52.79 51.32 50.61 52.76 53.63 52.79 TlOg 0.07 0.73 0.73 0.45 0.32 0.23 AI2 O 3 1.15 3.02 2.85 1.96 1.87 1.72 Cr2 0 3 0.04 0.14 0.14 0.00 0.16 0 . 0 0 FeO 8 . 1 2 7.59 10.07 9.04 8.24 9.23 MnO 0.37 0.29 0.36 0.35 0.31 0.50 MgO 14.74 14.58 13.09 14.53 17.84 14.29 CaO 21.29 20.81 20.63 21.18 18.07 21.17 N320 0.31 0.44 0.47 0.32 0.19 0.53 TOTAL 98.88 98.92 98.95 100.59 1 00.63 100.46 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.978 1.919 1.912 1.950 1.956 1.953 TI 0.002 0.020 0.021 0.012 0.009 0.006 Al 0.051 0.133 0.127 0.085 0.080 0.075 Cr 0.001 0.004 0.004 0.000 0.005 0.000 Fe3+ 0.011 0.017 0.039 0.016 0.000 0.043 Fe 2 + 0.244 0.220 0.279 0.263 0.251 0.243 Mn 0.012 0.009 0.011 0.011 0.010 0.016 Mg 0.823 0.812 0.737 0.800 0.970 0.788 Ca 0.855 0.834 0.835 0.839 0.706 0.839 Na 0.023 0.032 0.034 0.023 0.014 0.038 TOTAL 4.000 4.000 3.999 3.999 4.001 4.001 En 0.444 0.468 0.428 0.441 0.525 0.442 Fs 0.132 0.127 0.162 0.145 0.136 0.136 Wo 0.424 0.405 0.411 0.414 0.339 0.422 357 CLINOPYROXENE. LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS P11 core P11 rim P13 core P1 3 rim P59 P63 core SIO 2 51.81 51.60 51.34 51.38 50.43 52.72 TlOg 0 . 2 2 0.32 0.43 0.24 0.79 0.41 AI2 O3 2 . 2 1 1.19 2.48 0.87 2.53 1.48 0.60 0.13 0.37 0.08 0.05 0.00 FeO 4.55 8.56 5.97 8.31 12.44 8.96 MnO 0.10 0.56 0.28 0.39 0.81 0.39 MgO 16.35 14.98 17.23 13.93 15.45 14.41 CaO 22.38 2 1 . 0 1 20.05 22.24 16.16 20.82 Na20 0.43 0.45 0.44 0.43 0.58 0.56 TOTAL 98.65 98.80 98.59 97.87 99.24 99.75 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.916 1.935 1.899 1.951 1.892 1.962 TI 0.006 0.009 0.012 0.007 0.022 0.011 Al 0.097 0.052 0.108 0.039 0.112 0.065 Cr 0.017 0.004 0.011 0.002 0.001 0.000 Fe3+ 0.075 0.089 0.090 0.076 0.101 0.029 Fe 2 + 0.066 0.179 0.095 0.188 0.290 0.250 Mn 0.003 0.018 0.009 0.012 0.026 0.012 Mg 0.901 0.837 0.950 0.788 0.864 0.800 Ca 0.887 0.844 0.795 0.905 0.650 0.831 Na 0.031 0.033 0.031 0.031 0.042 0.040 TOTAL 3.999 4.000 4.000 3.999 4.000 4.000 En 0.539 0.487 0.569 0.448 0.514 0.440 Fs 0.039 0.104 0.057 0.107 0.172 0.138 Wo 0.422 0.409 0.375 0.445 0.314 0.422 358 CLINOPYROXENE. LA141 LA140 LA140 LA140 LA140 LA140 ANALYSIS P63 rim P11 P14 P1 7 core PI 7 rim P34 SIO2 51.89 51.68 51.36 52.46 48.76 50.68 TIO2 0.53 0.30 0.03 0.37 0.92 0.74 AI2 O3 1.78 2.23 1.43 1.31 4.21 3.35 Cr203 0.15 0.00 0.01 0.11 0.20 0.18 FeO 9.26 9.43 7.75 9.01 9.58 8.88 MnO 0.33 0.66 0.60 0.35 0.23 0.28 MgO 13.89 12.82 13.14 14.68 14.58 14.28 CaO 20.96 22.24 21.86 20.55 19.46 21.40 N 8 2 O 0.79 1.54 1.67 0.58 0.44 0.74 TOTAL 99.58 100.90 97.85 99.42 98.38 100.53 FORMULA (4 CATIONS,, 6 OXYGENS) SI 1.936 1.898 1.932 1.957 1.836 1.866 TI 0.015 0.008 0.001 0.010 0.026 0.020 Al 0.078 0.097 0.064 0.058 0.187 0.145 Cr 0.005 0.000 0.000 0.003 0.006 0.005 Fe3+ 0.072 0.201 0.192 0.049 0.117 0.133 Fe2+ 0.217 0.089 0.052 0.232 0.185 0.140 Mn 0.010 0.020 0.019 0.011 0.007 0.009 Mg 0.772 0.702 0.737 0.816 0.818 0.784 Ca 0.838 0.875 0.881 0.821 0.785 0.844 Na 0.057 0.110 0.122 0.042 0.032 0.053 TOTAL 4.000 4.000 4.000 3.999 3.999 3.999 En 0.445 0.491 0.498 0.452 0.529 0.509 Fs 0.125 0.062 0.035 0.129 0.120 0.091 Wo 0.430 0.447 0.466 0.419 0.352 0.401 359 CLINOPYROXENE. SM93/43 SM93/43 SM93/20 SM93/20 SM93/20 SM93/20 ANALYSIS P2 P8 P7 core P7 rim P9 core P9 rim SlOg 52.36 49.44 51.96 50.16 52.36 51.99 TlOg 0.38 1.03 0.40 0.62 0.72 0.65 AI2 O 3 3.59 4.21 2.25 3.81 1.87 1.74 Cr2 0 g 0.45 0.12 0.00 0.24 0.17 0.15 FeO 6.19 8.76 9.90 10.28 9.05 9.44 MnO 0.00 0.15 0.31 0 . 2 2 0.25 0.29 MgO 15.55 13.77 13.80 1 2 . 8 8 14.67 14.74 CaO 2 1 . 6 8 19.67 2 0 . 1 0 20.37 20.90 20.24 Na2 0 0.37 0.52 0.72 0.42 0.61 0.57 TOTAL 100.57 97.67 99.44 99.00 100.60 99.81 FORMULA (4 CATIONS,, 6 OXYGENS) Si 1.912 1.878 1.943 1.894 1.931 1.934 TI 0.010 0.029 0.011 0.018 0.020 0.018 Al 0.155 0.188 0.099 0.170 0.081 0.076 Cr 0.013 0.004 0.000 0.007 0.005 0.004 Fe3 + 0.016 0.033 0.047 0.028 0.056 0.059 Fe 2 + 0.173 0.245 0.263 0.297 0.223 0.235 Mn 0.000 0.005 0.010 0.007 0.008 0.009 Mg 0.846 0.779 0.769 0.725 0.806 0.817 Ca 0.848 0.800 0.805 0.824 0.826 0.806 Na 0.026 0.039 0.052 0.031 0.044 0.041 TOTAL 3.999 4.000 3.999 4.001 4.000 3.999 En 0.501 0.468 0.442 0.431 0.454 0.460 Fs 0.102 0.147 0.151 0.177 0.126 0.132 Wo 0.397 0.385 0.407 0.392 0.420 0.407 360 CLINOPYROXENE. LA108 LA108 LA108 LA108 LA108 LA108 ANALYSIS A1 A3 A5 PI P2 P3 SIO 2 43.02 46.23 42.93 52.42 53.58 43.98 TiOg 0.89 0.72 1.16 0.00 0.05 1.32 AI2 O 3 9.30 5.98 11.19 0.39 0.64 9.11 Cr2 0 3 0.00 0.07 0.12 0.03 0 . 0 2 0.10 FeO 11.31 9.68 8.64 6.46 2.99 10.75 MnO 0.45 0.55 0.35 0.71 0.69 0.55 MgO 8.83 10.73 8.99 13.57 15.82 9.17 CaO 25.08 24.96 24.42 26.12 26.43 24.85 Na2 0 0.33 0.15 0.07 0.34 0.09 0.22 TOTAL 99.21 99.07 97.87 100.04 1 0 0 . 2 2 100.05 FORMULA (4 CATIONS,, 6 OXYGENS) Si 1.637 1.752 1.646 1.945 1.957 1.660 TI 0.026 0.021 0.034 0.000 0.001 0.037 AI 0.417 0.267 0.506 0.017 0.027 0.405 Cr 0.000 0.002 0.004 0.001 0.001 0.003 Fe3 + 0.283 0.198 0.136 0.117 0.064 0.216 Fe%+ 0.077 0.109 0.141 0.083 0.027 0.123 Mn 0.015 0.018 0.011 0.022 0.021 0.018 Mg 0.500 0.606 0.514 0.751 0.861 0.516 Ca 1.022 1.014 1.003 1.039 1.034 1.005 Na 0.024 0.011 0.005 0.025 0.006 0.016 TOTAL 4.001 3.998 4.000 4.000 3.999 3.999 En 0.549 0.501 0.502 0.463 0.494 0.503 Fs 0.085 0.090 0.138 0.051 0.015 0.120 Wo 0.367 0.409 0.360 0.485 0.490 0.377 361 CLINOPYROXENE. LA111 LA111 LA112 LA115 LA115 LA115 ANALYSIS P1 RIO PI PI P2 P3 SlOg 48.96 52.80 52.67 48.55 50.18 50.24 TlOg 0.10 0.03 0.00 0.23 0.06 0.14 AI2O3 2.39 0.35 0.35 1.97 1.07 0.84 Cr203 0.05 0.14 0.02 0.00 0.00 0.00 FeO 13.75 8 23 8.71 14.68 12.09 9.95 MnO 0.50 0.43 0.49 0.77 0.89 0.77 MgO 9.03 12.54 1 3.02 8.24 10.02 11.44 CaO 24.69 25.42 24.82 24.03 24.27 25.05 Na2 O 0.25 0.09 0.28 0.47 0.14 0.39 TOTAL 99.72 100.03 100.36 98.94 98.72 98.82 FORMULA (4 CATIONS,, 6 OXYGENS) Si 1.879 1.979 1.963 1.887 1.938 1.915 Ti 0.003 0.001 0.000 0.007 0.002 0.004 Ai 0.108 0.015 0.015 0.090 0.049 0.038 Cr 0.002 0.004 0.001 0.000 0.000 0.000 FeS+ 0.147 0.028 0.080 0.159 0.080 0.151 Fe2+ 0.441 0.230 0.191 0.318 0.311 0.166 Mn 0.016 0.014 0.015 0.025 0.029 0.025 Mg 0.516 0.701 0.723 0.477 0.577 0.650 Ca 1.015 1.021 0.991 1.001 1.004 1.023 Na 0.019 0.007 0.020 0.035 0.011 0.029 TOTAL 3.999 4.000 3.999 3.999 4.001 4.001 En 0.295 0.375 0.415 0.326 0.342 0.426 Fs 0.253 0.123 0.110 0.218 0.185 0.109 Wo 0.452 0.501 0.475 0.456 0.473 0.465 362 CLINOPYROXENE. ANALYSIS LA144 LA144 LA148 LA148 LAS15 LAS15 P2 P4 P2 P3 P 8 P10 SiOg 51.95 52.60 51.51 51.78 48.54 49.09 TIOg 0.33 0.08 0.02 0.11 0.06 0.01 AI2O3 2 . 1 2 1.16 1.35 1.23 0.52 0.65 Cr20g 0.08 0.00 0.10 0.07 0.00 0.04 FeO 7.60 5.79 11.06 10.87 19.51 19.09 MnO 1.05 1.04 0.85 0.77 1.64 1.34 MgO 14.05 15.16 1 0 . 2 2 10.76 4.58 4.64 CaO 22.24 22.67 24.68 24.77 22.69 23.04 N 8 2 O 0.59 0.75 0.38 0.36 0.21 0.17 TOTAL 1 0 0 . 0 1 99.25 100.17 100.72 97.75 98.07 FORMULA (4 CATIONS,, 6 OXYGENS) Si 1.925 1.947 1.952 1.947 1.972 1.965 Ti 0.009 0.002 0.000 0.003 0.002 0.001 Ai 0.093 0.050 0.060 0.054 0.025 0.026 Cr 0.002 0.000 0.003 0.002 0.000 0.002 Fe3+ 0.080 0.106 0.061 0.070 0.043 0.053 Fe2+ 0.156 0.073 0.290 0.272 0.620 0.570 Mn 0.033 0.033 0.027 0.025 0.057 0.046 Mg 0.776 0.836 0.577 0.603 0.277 0.302 Ca 0.883 0.899 1.002 0.998 0.988 1.020 Na 0.043 0.054 0.028 0.026 0.016 0.015 TOTAL 4.000 4.000 4.000 4.000 4.000 4.001 En 0.471 0.505 0.339 0.352 0.158 0.174 Fs 0.095 0.044 0.170 0.159 0.354 0.328 Wo 0.435 0.451 0.491 0.489 0.489 0.497 363 CLINOPYROXENE. LAS23 LAS23 LAS23 ANALYSIS P1 P2 P3 SiOg 51.25 52.50 52.98 TIOg 0.38 0.32 0.12 Alg0 3 2.85 1.76 0.94 CrgOg 0.14 0.10 0.00 FeO 5.50 5.62 4.47 MnO 0.50 0.92 0.99 MgO 15.45 15.88 16.65 CaO 22.04 21.67 21.46 Nag 0 0.28 0.56 0.53 TOTAL 98.11 99.33 98.14 FORMULA (4 CATIONS, 6 OXYGENS) SI 1.912 1.938 1.969 TI 0.011 0.009 0.003 AI 0.125 0.077 0.041 Or 0.004 0.003 0.000 FeS + 0.045 0.066 0.055 Fe2+ 0.127 0.107 0.084 Mn 0.016 0.029 0.031 Mg 0.859 0.874 0.923 Ca 0.881 0.857 0.855 Na 0.020 0.040 0.038 TOTAL 4.000 4.000 3.999 En 0.507 0.511 0.520 Fs 0.075 0.063 0.047 Wo 0.417 0.427 0.433 364 AMPHIBOLE. ANALYSIS. DESCRIPTION. LA141 Mafic Inclusion. Capricorn Lava. U1 Amphibole Inclusion in augite phenocryst. U2 Amphibole Inclusion in augite phenocryst. A10 Acicular amphibole. A11 Amphibole overgrowth on olivine phenocryst. A12 core Amphibole phenocryst. Core. A12 rim Amphibole phenocryst. Rim. LA14Q Dacitic Capricorn Lava. A20 Amphibole phenocryst with opx/magnetite corona. A21 Amphibole phenocryst with opx/magnetite corona. A22 Amphibole phenocryst with opx/magnetite corona. LA18 Centre i Andésite. A1 Amphibole phenocryst. A2 Amphibole phenocryst. A3 Amphibole phenocryst. SM93/10 Piedras Grandes Flow. A1 Amphibole Phenocryst. A2 Amphibole Phenocryst. SM93/44 Vitrophyre. Soncor Flow. A1 core Amphibole Phenocryst. A1 rim Amphibole Phenocryst. LAS29-2 Andesitic Pumice. Soncor Fiow. A1 core Amphibole Phenocryst. A1 rim Amphibole Phenocryst. 365 AMPHIBOLE. LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS. U1 U2 A10 A 11 A12 core A12 rin SIO 2 45.48 43.77 42.89 41 .62 42.05 42.28 TiOg 1.24 1.35 2.79 2.81 2.62 2.69 AI2 O 3 9.97 11.46 11.34 12.44 12.47 11.57 FeO 7.79 8.54 11.49 10.34 10.42 10.28 MnO 0.07 0.07 0.13 0.02 0 . 2 1 0 . 2 2 MgO 16.49 16.02 14.71 14.72 14.67 15.08 CaO 1 2 . 1 2 1 2 . 2 1 11.55 1 1 .75 11.55 11.71 NagO 2.13 2.44 2 32 2.57 2 . 6 6 2.37 KgO 0.45 0.40 0.44 0.43 0.53 0.47 Cl 0.04 0.05 0.05 0.00 0.09 0.05 - 0 = Cl - 0 . 0 1 - 0 . 0 1 - 0 . 0 1 0 . 0 0 - 0 . 0 2 - 0 . 0 1 TOTAL 95.77 96.30 97.70 96.70 97.25 96.71 FORMULA (Leake, 1978) Si 6.204 6.435 6.370 6.396 6.277 6.346 Ti 0.191 0.190 0.371 0.351 0.365 0.338 AI 2.040 1.746 1.747 1.729 1.911 1.926 Fe3+ 1.011 1.001 0.517 0.512 0.505 0.398 Fe 2 + 0.312 0.214 0.896 0.936 0.898 0.939 Mn 0.035 0.002 0.016 0.018 0.015 0.014 Mg 3.206 3.412 3.083 3.059 3.029 3.039 Ca 1.711 1.661 1.753 1.767 1.785 1.761 Na 0.631 0.602 0.640 0.635 0.633 0.682 K 0.108 0.079 0.108 0.096 0.097 0.104 TOTAL 15.449 15.342 15.501 15.499 15.515 15.547 Cl 0.005 0.007 0.012 0.005 0.008 0.007 366 AMPHIBOLE. LA140 LA140 LA140 LA18 LA18 LA18 ANALYSIS. A20 A21 A22 Al A2 A3 SIO2 44.49 44.23 43.24 42.33 42.80 42.12 TlOg 3.56 3.19 3.26 3.10 3.27 3.06 AI2 O 3 10.57 10.69 10.13 1 1 . 0 0 10.94 10.64 FeO 11.40 11.56 1 1 . 1 1 11.79 11.51 11.63 MnO 0.25 0.19 0.17 0.19 0.16 0.16 MgO 14.50 14.62 14.31 13.59 13.72 13.83 CaO 11.63 11.64 11.32 11.42 1 1 . 2 1 11.37 Na2 0 2.46 2.67 2.54 2.44 2.46 2.45 K2 O 0.54 0.52 0.42 0.67 0.62 0.60 Cl 0.00 0.07 0.00 N.A. N.A. N.A. - 0 = Cl 0 . 0 0 - 0 . 0 2 0 . 0 0 TOTAL 99.42 99.36 96.50 96.53 96.69 95.86 FORMULA (Leake, 1978) Si 6.619 6.366 6.196 6.079 6.110 6.162 TI 0.136 0.148 0.303 0.309 0.286 0.295 Al 1.710 1.966 1.931 2.142 2.136 1.987 Fe3+ 0.316 0.437 0.764 0.597 0.626 0.683 Fe 2 + 0.633 0.602 0.624 0.666 0.640 0.570 Mn 0.009 0.009 0.016 0.003 0.026 0.028 Mg 3.577 3.472 3.167 3.204 3.177 3.275 Ca 1.890 1.903 1.787 1.839 1.798 1.829 Na 0.601 0.689 0.650 0.727 0.749 0.670 K 0.083 0.074 0.081 0.080 0.099 0.088 TOTAL 15.574 1 5.666 15.519 15.646 15.647 15.499 Cl 0.010 0.012 0.012 0.000 0.022 0.013 367 AMPHIBOLE. SM93/10 SM93/10 SM93/44 SM93/44 LAS29-2 LAS29 ANALYSIS. Al A2 Al core A1 rim Al core Al rim SIO2 43.49 45.45 43.85 43.48 42.13 43.44 TlOg 1.78 1.78 3.39 3.17 3.26 3.08 AI2 O 3 12.13 10.46 10.20 9.97 10.88 11.18 FeO 11.09 10.26 11.63 11.77 11.26 10.94 MnO 0.29 0.02 0.13 0.14 0.12 0.11 MgO 15.08 16.17 14.24 13.95 13.64 13.96 CaO 11.19 10.95 11.26 11.21 11.18 11.25 Na2 0 2.28 2.19 2.27 2.22 2.19 2.41 K2 O 0.59 0.44 0.58 0.51 0.51 0.56 Cl 0.02 0.03 0.05 0.02 0.03 0.03 - 0 = Cl 0.00 - 0.01 - 0.01 0.00 - 0.01 - 0.01 TOTAL 97.94 97.74 97.59 96.44 95.19 96.95 FORMULA (Leake, 1978) SI 6.357 6.330 6.360 6.260 6.296 6.263 TI 0.383 0.344 0.361 0.345 0.362 0.342 Al 1.781 1.803 1.756 1.918 1.897 1.865 Fe3+ 0.390 0.441 0.431 0.426 0.434 0.480 Fe2+ 0.972 0.942 0.935 1.032 0.983 0.966 Mn 0.031 0.023 0.021 0.024 0.020 0.020 Mg 3.087 3.118 3.136 2.995 3.009 3.064 Ca 1.781 1.785 1.784 1.810 1.767 1.812 Na 0.687 0.741 0.724 0.699 0.701 0.707 K 0.098 0.095 0.079 0.127 0.116 0.114 TOTAL 15.567 15.622 15.587 15.636 1 5.585 15.633 Cl 0.000 0.017 0.000 368 BIOTITE. ANALYSIS. DESCRIPTION. LAS36-1 Hornblende-rich pumice. Soncor Flow. A2 Biotite phenocryst. A3 Biotite phenocryst. A4 Biotite phenocryst. SM93/10 Piedras Grandes Fiow. A5 core Biotite phenocryst. Core. A5 rim Biotite phenocryst. Rim. SM93/37 Volcan Aguas Calientes. Mafic inclusion in dacitic lava. A5 Biotite phenocryst. B1 Biotite phenocryst. PWF1 1993 Andesitic Scoria. A1 Biotite phenocryst with thick reaction corona. B1 Biotite phenocryst with thick reaction corona. 369 BIOTITE. LAS36-1 LAS36-1 LAS36- 1 SM93/10 SM93/10 SM93/C ANALYSIS. A2 A3 A4 A5 core A5 rim A5 SlOg 38.44 38.39 39.09 37.36 38.28 38.66 TIOg 5.64 5.30 5.07 5.18 6 . 0 1 4.27 AI2O3 12.51 12.78 12.56 13.55 13.96 14.57 FeO 11.72 11.71 10.93 13.83 14.21 12.72 MnO 0.00 0.03 0.18 0.02 0.00 0.03 MgO 16.42 16.50 17.18 15.00 15.07 16.74 CaO 0.16 0.19 0.01 0.18 0.03 0.15 NagO 0.84 0.87 0 . 8 6 0.48 0.93 1.14 KgO 9.14 8 . 8 8 8.92 7.57 8 . 8 8 8.04 Cl 0.18 0.16 0.15 0.18 0.08 0.15 - 0 = Cl - 0.04 - 0.04 - 0.03 - 0.04 - 0 . 0 2 - 0.03 TOTAL 95.01 94.77 94.92 93.31 97.43 96.44 FORMULA (22 OXYGENS) SI 5.686 5.683 5.749 5.628 5.566 5.609 Ti 0.627 0.590 0.561 0.587 0.657 0.466 AI 2.182 2.230 2.178 2.406 2.393 2.492 Fe 1.450 1.450 1.344 1.742 1.728 1.543 Mn 0.000 0.004 0.022 0.003 0.000 0.004 Mg 3.620 3.640 3.766 3.367 3.266 3.619 Ca 0.025 0.030 0.002 0.029 0.005 0.023 Na 0.241 0.250 0.245 0.140 0.262 0.321 K 1.725 1.677 1.674 1.455 1.647 1.488 TOTAL 15.556 15.554 15.541 15.357 15.524 15.565 Cl 0.045 0.040 0.037 0.046 0.020 0.037 370 BIOTITE. SM93/37 PWF1 PWF1 ANALYSIS. B1 A1 B1 SIO 2 38.20 37.77 38.42 TIO2 4.64 5.97 5.96 AI2 O 3 13.51 14.07 14.33 FeO 13.45 10.25 10.05 MnO 0.13 0.03 0.00 MgO 15.42 16.83 16.76 CaO 0.12 0.04 0.15 Na2 0 1 . 1 2 0 . 6 8 0.94 K2 O 8.42 8.37 8.51 01 0.08 0 . 1 1 0.19 - 0 = 0 ! - 0 . 0 2 - 0 . 0 2 - 0.04 TOTAL 95.07 94.10 95.27 FORMULA (22 OXYGENS) SI 5.668 5.569 5.591 Ti 0.518 0.662 0.652 AI 2.363 2.446 2.458 Fe 1.669 1.264 1.223 Mn 0.016 0.004 0.000 Mg 3.410 3.698 3.635 Ca 0.019 0.006 0.023 Na 0.322 0.194 0.265 K 1.594 1.574 1.580 TOTAL 15.579 15.417 15.427 Cl 0.020 0.027 0.047 371 PLAGIOCLASE. ANALYSIS DESCRIPTION LA124 Holocrystaliine Mafic Inciusion. Soncor Flow. F1 Plagioclase intergrown with magnetite. Inclusion in augite. F4 Plagioclase phenocryst in clot with olivine, augite. F30 core Core of plagioclase phenocryst. F30 rim Rim of plagioclase phenocryst. F31 core Core of plagioclase phenocryst. F31 rim Rim of plagioclase phenocryst. LA122 Dacitic Pumice. Soncor Flow. F4 core Core of plagioclase phenocryst. F4 rim Rim of plagioclase phenocryst. LA121 Dacitic Pumice. Soncor Flow. F10 Plagioclase phenocryst. F20 core Core of large euhedral plagioclase phenocryst. F20 rim Rim of large euhedral plagioclase phenocryst. F28 Plagioclase microphenocryst in crystal clot. LAS29-2 Andesitic Pumice. Soncor Flow. F1 core Core of euhedral plagioclase phenocryst. F1 rim Rim of euhedral plagioclase phenocryst. F2 core Core of euhedral plagioclase phenocryst. F2 rim Rim of euhedral plagioclase phenocryst. F3 core Core of euhedral plagioclase phenocryst. F3 rim Rim of euhedral plagioclase phenocryst. LA141 Mafic inclusion. Capricorn Lava. F31 core Core of large plagioclase phenocryst. F31 rim Rim of large plagioclase phenocryst. F32 Plagioclase intergown with orthopyroxene, magnetite. F33 Plagioclase intergrown with orthopyroxene, magnetite. F38 core Core of large plagioclase phenocryst. F38 rim Rim of large plagioclase phenocryst. 372 LA100 1990 Andésite Lava. F20 Small plagioclase. F21 Small plagioclase. F50 core Core of large plagioclase phenocryst. F50 rim Rim of large plagioclase phenocryst. F52 core Core of anhedral plagioclase phenocryst. F52 rim Rim of anhedral plagioclase phenocryst. LA108 Woliastonite Skarn Xenoiith. Tumbres Flow. FI Anorthite in diopside-rich outer crust. LA111 Buchite Xenoiith. Tumbres Flow. F2 Anorthite. LA112 Woliastonite Skarn Xenoiith. Tumbres Flow. F2 Anorthite. F4 Anorthite. LA115 Diopside Skarn Xenoiith. Tumbres Flow. F4 Anorthite. F5 Anorthite. 373 PLAGIOCLASE. LA124 LA124 LA124 LA124 LA124 LA124 ANALYSIS FI F4 F30 core F30 rim F31 core F31 rim SlOg 59.49 47.10 49.35 49.53 48.78 52.00 AI2 O 3 24.88 32.44 31.24 31.49 31.69 28.72 FeO 1.28 0.64 0.87 0.73 0 . 6 6 1 . 0 2 CaO 8.65 16.68 15.70 15.63 15.76 13.40 Na2 Ü 5.92 2.26 2.43 2.75 2.59 3.96 K2 O 0.47 0.01 0.41 0.05 0.08 0.13 TOTAL 100.69 97.13 1 0 0 . 0 0 100.18 99.56 99.23 FORMULA (32 OXYGENS) SI 10.614 8.754 9.070 9.067 8.992 9.568 AI 5.233 7.108 6.769 6.796 6.887 6.230 Fe 0.191 0.099 0.134 0.112 0.102 0.157 Ca 1.654 3.322 3.092 3.066 3.113 2.642 Na 2.048 0.814 0.866 0.976 0.926 1.413 K 0.107 0.002 0.096 0.012 0.019 0.031 TOTAL 19.847 20.099 20.027 20.029 20.039 20.041 An 0.461 0.807 0.770 0.763 0.773 0.660 Ab 0.512 0.192 0.207 0.234 0.223 0.333 Or 0.027 0.000 0.023 0.003 0.005 0.007 374 PLAGIOCLASE. LA122 LA122 LA121 LA121 LA121 LA121 ANALYSIS F4 core F4 rim F10 F20 core F20 rim F28 SiOg 58.25 58.61 50.02 51.58 49.78 52.30 AlgOs 25.84 26.16 31.06 30.31 31.64 29.85 FeO 0.45 0.50 0.58 0.59 0.53 0.43 CaO 8.33 8.28 15.03 13.93 15.48 13.05 NagO 6.26 6.41 2.95 3.36 2.71 3.85 KgO 0.64 0.62 0.06 0 . 2 0 0.09 0.13 TOTAL 99.77 100.58 99.70 99.97 100.23 99.51 FORMULA (32 OXYGENS) Si 10.473 10.455 9.175 9.401 9.091 9.534 AI 5.477 5.502 6.717 6.513 6.812 6.415 Fe 0.068 0.075 0.089 0.090 0.081 0.066 Ca 1.605 1.583 2.954 2.721 3.029 2.549 Na 2.182 2.217 1.049 1.187 0.960 1.361 K 0.147 0.141 0.014 0.047 0.021 0.030 TOTAL 19.952 19.973 19.998 19.959 19.994 19.955 An 0.418 0.412 0.741 0.695 0.760 0.653 Ab 0.545 0.552 0.255 0.293 0.235 0.340 Or 0.037 0.035 0.003 0.012 0.005 0.007 375 PLAGIOCLASE. LAS29-2 LAS29-2 LAS29-2 LAS29-2 LAS29-2 LAS29-2 ANALYSIS F 1 core F1 rim F2 core F2 rim F3 core F3 rim SiOg 51.69 49.53 54.06 54.19 48.75 50.29 AI2O3 30.69 31.60 28.40 28.76 31.64 31.50 FeO 0.48 0.53 0.44 0.42 0.70 0.55 CaO 13.61 14.86 11.03 11.54 15.50 14.89 Na20 3.86 3.05 4.82 5.00 2.76 3.36 K2O 0.16 0.17 0 26 0.18 0.14 0.06 TOTAL 100.49 99.74 99.01 100.09 99.49 100.65 FORMULA (32 OXYGENS) Si 9.373 9.090 9.865 9.802 8.996 9.144 AI 6.560 6.837 6.110 6.133 6.883 6.752 Fe 0.073 0.081 0.067 0.064 0.108 0.084 Ca 2.644 2.922 2.157 2.237 3.065 2.901 Na 1.357 1.085 1.705 1.754 0.988 1.185 K 0.037 0.040 0.061 0.042 0.033 0.014 TOTAL 20.044 20.055 19.965 20.032 20.073 20.080 An 0.661 0.727 0.557 0.562 0.757 0.713 Ab 0.330 0.263 0.427 0.428 0.236 0.283 Or 0.009 0.010 0.015 0.010 0.008 0.003 376 PLAGIOCLASE. LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS F31 core F31 rim F32 F33 F38 core F38 rim SiOg 55.04 55.65 50.46 51.45 56.98 53.65 AI2 O 3 27.88 27.19 30.65 29.80 26.65 28.43 FeO 0.44 0.60 1.25 1.13 0.32 0.37 CaO 10.71 9.98 14.48 13.47 9.33 11.25 NagO 4.97 5.14 3.23 3.75 6.19 4.97 KgO 0.30 0.31 0.17 0.16 0.36 0.25 TOTAL 99.34 98.87 1 00.24 99.76 99.83 98.92 FORMULA (32 OXYGENS) SI 9.994 10.132 9.231 9.423 10.268 9.815 AI 5.968 5.836 6.610 6.435 5.662 6.132 Fe 0.067 0.091 0.191 0.173 0.048 0.057 Ca 2.084 1.947 2 838 2.644 1.801 2.205 Na 1.750 1.815 1.146 1.332 2.163 1.763 K 0.069 0.072 0.040 0.037 0.037 0.083 TOTAL 1 9.932 1 9.893 20.056 20.044 19.979 20.055 An 0.542 0.519 0.719 0.673 0.457 0.551 Ab 0.441 0.462 0.272 0.318 0.534 0.429 Or 0.017 0.018 0.009 0.009 0.009 0.020 377 PLAGIOCLASE. LA140 LA140 LA100 LA100 LA100 LA100 ANALYSIS F20 F21 F50 core F50 rim F52 core F52 rim SlOg 56.36 52.53 53.98 54.06 51.29 54.49 AI2 O 3 26.48 26.26 29.26 28.54 30.06 28.04 FeO 0.85 1.79 0.54 0.74 0.67 0.60 CaO 9.40 11.87 12.18 11.80 13.88 1 1 . 2 1 N320 5.51 3.92 4.26 4.29 3.46 4.65 K2 O 0.40 0.19 0 . 2 1 0.29 0.18 0.29 TOTAL 99.00 96.56 100.43 99.72 99.54 99.28 FORMULA (32 OXYGENS) SI 10.252 9.909 9.733 9.819 9.398 9.920 AI 5.678 5.840 6.220 6.111 6.494 6.018 Fe 0.129 0.282 0.081 0.112 0.103 0.091 Ca 1.832 2.399 2.353 2.297 2.725 2.187 Na 1.943 1.434 1.489 1.511 1.229 1.641 K 0.093 0.046 0.048 0.060 0.042 0.067 TOTAL 19.927 19.910 19.924 19.910 19.991 20.015 An 0.491 0.644 0.613 0.605 0.690 0.572 Ab 0.486 0.345 0.375 0.380 0.300 0.412 Or 0.023 0.011 0.012 0.015 0.010 0.017 378 PLAGIOCLASE. LA108 LA111 LA112 LA112 LA115 LA115 ANALYSIS FI F2 F2 F4 F4 F5 SlOg 43.25 43.73 44.36 43.14 43.06 43.97 AI2 O3 35.71 36.69 35.69 35.39 35.24 33.82 FeO 0 . 2 2 0.42 0.55 0.33 0.85 0.89 CaO 19.74 19.88 2 0 . 0 2 19.62 20.32 20.04 Na2 0 0.41 0.15 0.34 0.22 0.51 0.73 K2 O 0.04 0.07 0.04 0.03 0.00 0 . 0 0 TOTAL 99.37 100.94 1 0 1 . 0 0 98.73 99.98 99.45 FORMULA (32 OXYGENS) SI 8.074 8.034 8.153 8.102 8.038 8.244 A! 7.860 7.946 7.734 7.835 7.756 7.476 Fe 0.034 0.065 0.085 0.052 0.133 0.140 Ca 3.949 3.913 3.943 3.948 4.065 4.026 Na 0.148 0.053 0.121 0.080 0.185 0.265 K 0.010 0.016 0.009 0.007 0.000 0.000 TOTAL 20.075 20.027 20.045 20.024 20.177 20.151 An 0.962 0.983 0.969 0.979 0.958 0.940 Ab 0.036 0.013 0.029 0.020 0.042 0.060 Or 0.002 0.004 0.002 0.002 0.000 0.000 379 GARNET. ANALYSIS DESCRIPTION LA1Q8 Woliastonite Skarn Xenoiith. Tumbres Flow. XI Garnet inclusion in woliastonite in central zone of xenoiith. X2 Garnet inclusion in woliastonite in central zone of xenoiith. X4 Garnet in diopside-rich outer crust of xenoiith. X5 Garnet intergrown with clinopyroxene in outer crust of xenoiith. X7 Garnet inclusion in woliastonite in central zone of xenoiith. LA112 Concentrically Zoned Woliastonite Skarn Xenoiith. Tumbres Flow. X5 Garnet in outer zone of xenoiith. PS Garnet in outer zone of xenoiith. LA113 Woliastonite Skarn Xenoiith. Tumbres Flow. X2 Garnet in unaltered wollastonite-rich core. X3 Garnet in unaltered wollastonite-rich core. LA115 Diopside Skarn Xenoiith. Tumbres Flow. 0 7 Roikiloblastic Garnet. 0 8 Poikiloblastic Garnet. LA144 Magnetite Skarn Xenoiith. Capricorn Lava. X3 Garnet. 380 GARNET. LA108 LA108 LA108 LA108 LA108 LA112 ANALYSIS XI X2 X4 X5 X7 X5 SiOg 37.91 38.05 35.28 34.79 34.31 36.07 AI2 O 3 15.17 15.68 8 . 2 0 7.74 1 0 . 1 1 7.99 CrgOg 0 .1 1 0 . 1 0 0.19 0 . 0 0 0.17 0.32 TIOg 0.42 0.67 3.13 3.65 5.11 1.09 FeO 6.79 8.49 16.77 16.27 13.21 16.86 MnO 0.41 0.74 0.35 0.56 0.50 0.26 MgO 0 . 2 1 0.28 0.57 0.67 0.91 0.15 CaO 36.65 36.40 33.86 33.89 34.78 34.22 TOTAL 97.67 100.41 98.35 97.57 99.10 96.96 FORMULA (16 CATIONS,, 24 OXYGENS) SI 5.862 5.689 5.708 5.677 5.474 5.903 AI(IV) 0.138 0.131 0.292 0.323 0.526 0.097 AI(VI) 2.627 2.721 1.272 1.166 1.375 1.445 Or 0.014 0.013 0.024 0 . 0 0 0 0 .0 2 1 0.041 Tl 0.049 0.077 0.381 0.448 0.613 0.135 Pe3+ 1.137 1.095 2.235 2 . 2 2 0 1.763 2.307 Pe 2 + 0 . 0 0 0 0 . 0 0 0 0.034 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 Mn 0.054 0.014 0.048 0.078 0.068 0.035 Mg 0.048 0.064 0.137 0.164 0.215 0.037 Ca 6.072 6.017 5.870 5.924 5.945 6 . 0 0 0 TOTAL 16.000 16.000 16.000 16.000 16.000 16.000 381 GARNET. LA112 LA113 LA113 LA115 LA115 LA144 ANALYSIS P5 X2 X3 G7 G8 X3 SIO 2 35.70 36.91 36.33 35.24 34.10 35 .6 5 AI2O 3 6.45 6.96 5.31 5.11 3.70 1.91 Cr203 0 . 1 0 0.29 0.09 0 . 2 2 0 . 0 0 0 . 2 0 TIO 2 0.99 0.88 0.97 0.94 2.18 0 .64 FeO 18.82 17.40 20.04 21.10 2 2.45 25.23 MnO 0 . 0 0 0.24 0.18 0.47 0.58 0.88 MgO 0.28 0.13 0.14 0.16 0.24 0 .2 1 CaO 33.74 33.07 33.22 34.20 3 3.62 32.44 TOTAL 96.08 95.88 96.28 67.44 96 .8 7 97.16 FORMULA (16 CATIONS,, 24 OXYGENS) SI 5.923 6.127 6.049 5.809 5.698 5.975 AI(IV) 0.077 0 . 0 0 0 0 . 0 0 0 0.191 0.302 0.025 AI(VI) 1.184 1.361 1.041 0.802 0.427 0.353 Or 0.014 0.039 0 . 0 1 2 0.029 0 . 0 0 0 0.026 Tl 0.123 0 . 1 1 0 0 . 1 2 2 0.117 0.274 0.081 Fe3+ 2.611 2.126 2.603 2.908 3.138 3.486 Fe 2+ 0 . 0 0 0 0.289 0.187 0 . 0 0 0 0 . 0 0 0 0.050 Mn 0 . 0 0 0 0.034 0.026 0.066 0.082 0.125 Mg 0.069 0.032 0.034 0.038 0.059 0.053 Ca 5.998 5.882 5.927 6.039 6.019 5.825 TOTAL 16.000 16.000 16.000 16.000 16.000 16.000 382 WOLLASTONITE. ANALYSIS DESCRIPTION LA108 Woliastonite Skarn Xenoiith. Tumbres Flow. W1 Woliastonite in central zone of xenoiith. W2 Woliastonite in central zone of xenoiith. X50 Woliastonite in central zone of xenoiith. LA112 Woliastonite Skarn Xenoiith. Tumbres Flow. X2 Woliastonite in central zone of xenoiith. LAS15 Banded Diopside-Wollastonite Skarn Xenoiith. Tumbres Flow. X4 Early woliastonite with high Mg, Fe, Mn content. X6 Late woliastonite with low Mg, Fe, Mn content. X7 Late woliastonite with low Mg, Fe, Mn content. X6 Early woliastonite with high Mg, Fe, Mn content. X9 Late woliastonite with low Mg, Fe, Mn content. LA111 Buchite Xenoiith. Tumbres Flow. X50 Relict woliastonite inclusion in quartz. 383 WOLLASTONITE. LA108 LA108 LA108 LA112 LAS15 LAS15 LAS15 LAS15 LAS15 LA111 ANALYSIS W1 W2 X50 X2 X4 X6 X7 X8 X9 X50 SIO2 50.32 50.75 51.45 51.52 47.03 52.27 51.13 50.16 50.35 52.79 FeO 0.11 0.34 0.25 0.76 7.27 1.47 1.61 7.74 1.76 0.88 MnO 0.52 0.28 0.37 0.22 1.91 0.85 0.87 2.07 0.96 0.23 MgO 0.19 0.21 0.24 0.21 0.95 0.17 0.09 0.58 0.08 0.29 CaO 48.49 49.13 47.51 47.65 37.25 45.01 45.21 37.10 45.83 47.13 TOTAL 99.63 100.71 99.82 100.36 94.41 99.77 98.91 97.65 98.98 101.32 oo oso -r FORMULA (6 OXYGENS) SI 1.968 1.965 1.996 1.991 1.972 2.024 2.006 2.018 1.985 2.012 Fe 0.004 0.011 0.008 0.025 0.255 0.048 0.053 0.260 0.058 0.028 Mn 0.017 0.009 0.012 0.007 0.068 0.028 0.029 0.071 0.032 0.007 Mg 0.011 0.012 0.014 0.012 0.059 0.010 0.005 0.035 0.005 0.016 Ca 2.032 2.038 1.975 1.973 1.674 1.867 1.901 1.599 1.936 1.925 TOTAL 4.032 4.035 4.005 4.008 4.028 3.977 3.994 3.983 4.016 3.988 SPINELS. ANALYSIS DESCRIPTION LA124 Holocrystaliine Mafic Inclusion. Soncor Flow. M1 Cr-Spinel inclusion in olivine phenocryst. M3 Cr-Spinel inclusion in olivine phenocryst. MG Cr-Spinel in clump with olivine phenocrysts. M9 Magnetite intergrown with plagioclase. Included in augite phenocryst. M13 Large magnetite in clump with plagioclase and pyroxenes. M30 Groundmass magnetite. LA122 Dacitic Pumice Soncor Flow. M2 Magnetite intergrown with orthopyroxene phenocryst. M18 Magnetite phenocryst. LA121 Dacitic Pumice. Soncor Flow. M59 Magnetite inclusion in orthopyroxene phenocryst. M72 Magnetite phenocryst. LAS29-2 Andesitic Pumice. Soncor Flow. M7 Large euhedral magnetite. M8 Small euhedral magnetite. LA141 Mafic Inclusion. Capricorn Lava. M1 Cr-Spinel inclusion in olivine phenocryst. M10 Cr-Spinel inclusion in orthopyroxene phenocryst. M11 Cr-Spinel phenocryst. M9 Magnetite phenocryst M13 Magnetite phenocryst. M41 Magnetite intergrown with ilmenite. Inclusion in hornblende. SM93/43 Augite Megacryst. Saltar Flow. M1 Cr-Spinel inclusion in core of megacryst. M2 Cr-Spinel inclusion in core of megacryst. M4 Magnetite inclusion in rim of megacryst. 385 ANALYSIS DESCRIPTION LA100 1990 Andésite Lava. Ml Magnetite phenocryst. M il Magnetite phenocryst. M l 3 Magnetite inclusion in orthopyroxene phenocryst. 386 SPINEL. LA124 LA124 LA124 LA124 LA124 LA124 ANALYSIS Ml M3 M6 M9 M13 M30 SIO 2 0.16 0.31 0.13 0.59 0.26 0.48 TiOg 0.99 0.80 3.07 10.40 5.49 1 2 . 6 6 AI2 O 3 15.36 17.08 7.14 2 . 8 6 4.04 1.96 Cl' 2 0 3 38.40 39.52 25.36 2.39 10.84 0 . 2 2 FeO 29.52 27.00 51.03 68.92 68.34 74.02 MnO 0.41 0.07 0.36 0.23 0.29 0 . 6 8 MgO 10.30 11.13 5.76 4.98 3.79 2.91 NiO 0.17 0.38 0.00 0.00 0.00 0.03 CaO 0.05 0.01 0.00 0 . 2 0 0.03 0.14 Na^O 0.16 0.06 0.24 0.37 0.40 0 . 2 2 TOTAL 95.52 96.36 93.09 90.94 93.48 93.32 FORMULA (3 CATIONS, 4 OXYGENS). SI 0.005 0.010 0.005 0.022 0.010 0.018 Tl 0.025 0.020 0.084 0.294 0.152 0.357 AI 0.602 0.657 0.304 0.127 0.175 0.087 Or 1.009 1.019 0.725 0.071 0.315 0.007 Fe3+ 0.337 0.268 0.808 1.197 1.213 1.170 Fe%+ 0.484 0.469 0.736 0.968 0.889 1.154 Mn 0.012 0.002 0.011 0.007 0.009 0 . 0 2 2 Mg 0.510 0.541 0.311 0.279 0.208 0.163 Ml 0.005 0.010 0.000 0.000 0.000 0.001 Ca 0.002 0.000 0.000 0.008 0.001 0.006 Na 0.010 0.004 0.017 0.027 0.029 0.016 TOTAL 3.001 3.000 3.001 3.000 3.001 3.001 387 SPINEL. LA122 LA122 LA121 LA121 LAS29-2 LAS29 ANALYSIS M2 M18 M59 M72 M7 M 8 SIO 2 0.06 0 . 2 0 0.26 0.05 0.23 1.49 TIO2 7.08 7.81 7.12 0.00 9.71 0.19 AI2 O 3 1.96 2 . 2 0 1.51 0.00 3.27 0.28 CT2 0 3 0.29 0.16 0.26 0.00 0.30 0.00 FeO 74.25 79.77 77.25 87.17 79.25 88.26 MnO 0.11 0.09 0.47 0.00 0.16 0.00 MgO 1.74 0.39 1.65 0.11 2 . 8 8 0.59 NIO N.A. 0.00 0.00 0.00 0.00 0.00 CaO 0.06 0.02 0.04 0.03 0.08 0.06 NB2 0 0.36 0.54 0.32 0.43 0.35 0.26 TOTAL 85.91 91.18 8 8 . 8 8 87.79 96.23 90.94 FORMULA (3 CATIONS, 4 OXYGENS). Si 0.002 0.008 0,010 0.002 0.008 0.058 Tl 0.217 0.228 0.212 0.000 0.264 0.006 AI 0.094 0.101 0.071 0.000 0.139 0.013 Or 0.009 0.005 0.008 0.000 0.009 0.000 Fe3+ 1.488 1.461 1.502 2.030 1.331 1.877 Fe 2 + 1.048 1.130 1.057 0.906 1.062 0.991 Mn 0.004 0.003 0.016 0.000 0.005 0.000 Mg 0.106 0.023 0.097 0.007 0.155 0.034 NI 0.000 0.000 0.000 0.000 0.000 Ca 0.003 0.001 0.002 0.001 0.003 0.002 Na 0.029 0.041 0.025 0.034 0.025 0.020 TOTAL 3.000 3.001 3.000 3.000 3.001 3.001 388 SPINEL. LA141 LA141 LA141 LA141 LA141 LA141 ANALYSIS M1 M10 M il M9 M13 M41 SIO2 0 . 2 2 0.25 0.28 0.33 0.28 0.31 TIO2 1 . 1 0 1.71 1.97 7.59 6.50 3.52 AI2 O 3 12.92 10.34 11.84 2.60 2.76 2.62 Cr2 0 3 35.52 28.69 30.74 0.23 0.26 1.07 FeO 39.82 46.06 46.87 78.52 75.83 79.64 MnO 0.44 0.20 0.29 0.54 0.38 0.17 MgO 6 . 2 2 6.14 3.83 2 . 2 1 2.37 2 . 1 1 NIO 0.29 0.71 0.42 0.34 0.00 N.A. CaO 0.12 0.06 0.03 0.00 0.03 0.09 N320 0.35 0.45 0.35 0.43 0.42 0.13 TOTAL 97.00 94.61 96.62 92.79 88.99 89.66 FORMULA (3 CATIONS, 4 OXYGENS). SI 0.007 0.009 0.010 0.012 0.011 0.012 Tl 0.028 0.045 0.052 0.215 0.191 0.103 AI 0.517 0.426 0.486 0.115 0.127 0.120 Or 0.954 0.793 0.846 0.007 0.008 0.033 Fe3+ 0.482 0.704 0.562 1.457 1.493 1.627 Fe%+ 0.649 0.644 0.802 1.011 0.986 0.963 Mn 0.013 0.006 0.009 0.017 0.013 0.006 Mg 0.315 0.320 0.199 0.124 0.138 0.122 Ni 0.008 0.020 0.012 0.010 0.000 Ca 0.004 0.002 0.001 0.000 0.001 0.004 Na 0.023 0.031 0.024 0.031 0.032 0.010 TOTAL 3.000 3.000 3.003 2.999 3.000 3.000 389 SPINEL. SM93/43 SM93/43 SM93/43 LA100 LA100 LA100 ANALYSIS Ml M2 M4 Ml M il M13 SlOg 0.30 0.41 0.27 0.38 0.25 0.36 TiOg 1.23 3.13 7.51 14.79 1 0 . 2 1 4.92 AlgOg 10.71 9.86 4.96 1.35 2 . 1 0 4.00 Cr2 0 3 1 0 . 2 2 10.42 0.45 0.09 0.16 0.06 FeO 63.90 62.01 73.25 68.29 72.28 72.83 MnO 0.07 0.06 0.19 0.45 0.32 0.13 MgO 6 . 1 2 5.43 3.71 2.15 1.82 2.36 NiO 0.00 0.00 0.05 0.00 0.00 0.00 CaO 0.39 0.37 0 . 2 0 0.17 0.00 0.11 Nag 0 0.20 0.31 0.21 0.60 0.29 0.21 TOTAL 93.14 92.00 90.80 88.27 87.43 84.98 FORMULA (3 CATIONS, 4 OXYGENS). SI 0.011 0.015 0.010 0.015 0.010 0.015 Tl 0.033 0.084 0.213 0.444 0.309 0.150 AI 0.444 0.417 0.220 0.063 0.100 0.191 Or 0.284 0.296 0.013 0.003 0.005 0.002 Fe3+ 1.194 1.111 1.336 1.064 1.280 1.494 Fe^+ 0.684 0.749 0.969 1.214 1.153 0.979 Mn 0.002 0.002 0.006 0.015 0.011 0.004 Mg 0.321 0.290 0.208 0.128 0.109 0.143 NI 0.000 0.000 0.002 0.000 0.000 0.000 Ca 0.015 0.014 0.008 0.007 0.000 0.005 Na 0.014 0.022 0.015 0.046 0.023 0.017 TOTAL 3.002 3.000 3.000 2.999 3.000 3.000 390 ILMENITE. ANALYSIS DESCRIPTION LA121 Dacitic Pumice. Soncor Fiow. M54 Ilmenite inclusion in orthopyroxene phenocryst. M71 Ilmenite inclusion in orthopyroxene phenocryst. LA122 Dacitic Pumice. Soncor Fiow. M1 Ilmenite inclusion in orthopyroxene phenocryst. M6 Small rounded ilmenite phenocryst. LAS47 Prismatic Jointed Green Dacite Biock. Soncor Fiow. M3 Ilmenite grain. M4 Ilmenite grain. LAS29-2 Andesitic Pumice. Soncor Fiow. M1 Ilmenite in crystal clot with orthopyroxene phenocrysts. LA141 Mafic inciusion. Capricorn Lava. M10 Ilmenite phenocryst. M39 Small anhedral ilmenite. M40 Ilmenite intergrown with magnetite, included in hornblende crystal. LA140 Dacitic Capricorn Lava. M23 Ilmenite phenocryst. M27 Ilmenite phenocryst. M20 Hemoilmenite xenocryst. M50 Hemoilmenite in magnetite skarn xenoiith. M51 Hemoilmenite in magnetite skarn xenoiith. LA100 1990 Andésite Lava. M3 Ilmenite inclusion in orthopyroxene phenocryst. M3 RPT Ilmenite inclusion in orthopyroxene phenocryst. M5 Ilmenite inclusion in orthopyroxene phenocryst. 391 ILMENITE. LA121 LA121 LA122 LA122 LAS47 LAS47 ANALYSIS M54 M71 Ml M 6 M3 M4 SIO 2 0.15 0 . 2 2 0.32 0.25 0.25 0.30 TIO2 32.78 37.24 27.56 27.22 41.29 43.51 AI2 O 3 0.27 0.31 0.43 0.52 0.24 0.14 Cr2 0 3 0.10 0.04 0.11 0.30 0.00 0.16 FeO 54.99 55.36 57.03 51.15 51.91 46.99 MnO 0.23 0.27 0.10 0.00 0.27 0.64 MgO 2 . 0 1 2 . 2 1 1.74 1.56 2.45 3.84 NIO 0.00 0.00 N.A. N.A. 0.00 0.00 CaO 0.46 0.04 0.04 0.09 0.10 0.00 Na2 0 0.07 0.16 0.19 0.35 0.24 0.27 TOTAL 91.08 95.85 87.52 81.44 96.75 95.85 FORMULA (2 CATIONS, 3 OXYGENS). Si 0.004 0.006 0.009 0.007 0.006 0.008 Tl 0.657 0.711 0.572 0.607 0.782 0.824 AI 0.008 0.009 0.014 0.018 0.007 0.004 Or 0.002 0.001 0.002 0.007 0.000 0.003 FeS + 0.672 0.562 0.834 0.769 0.427 0.342 Fe^+ 0.554 0.613 0.483 0.499 0.666 0.648 Mn 0.005 0.006 0.002 0.000 0.006 0.014 Mg 0.080 0.084 0.072 0.069 0.092 0.144 NI 0.000 0.000 0.000 0.000 Ca 0.014 0.001 0.001 0.003 0.003 0.000 Na 0.004 0.008 0.010 0.020 0.012 0.013 TOTAL 2 . 0 0 0 2 . 0 0 1 1.999 1.999 2 . 0 0 1 2 . 0 0 0 Ilmenite 0.642 0.701 0.558 0.589 0.772 0.810 392 ILMENITE. LAS29 -2 LAS29 -2 LA141 LA141 LA140 LA140 ANALYSIS M1 M10 M39 M40 M23 M27 SIO 2 0.14 0 . 2 2 0.31 0 . 2 0 0.23 0 . 2 0 TIO2 35.42 27.03 33.60 23.82 31.28 30.70 AI2 O 3 0 . 6 6 0.69 0.43 0.53 0.35 0.46 Cr2 0 3 0.11 0.26 0.02 0.63 0.09 0.13 FeO 53.85 63.42 56.45 64.86 62.22 61.70 MnO 0.14 0.00 0.28 0.03 0.16 0.10 MgO 2.36 1.62 2 . 0 2 1 .70 1.75 1.91 NIO 0.00 0.00 N.A. N.A. N.A. N.A. CaO 0.05 0.44 0.18 0.03 0.08 0.06 N320 0.17 0.30 0.43 0.18 0.23 0.32 TOTAL 92.92 93.98 93.72 91.98 96.39 95.58 FORMULA (2 CATIONS, 3 OXYGENS). SI 0.004 0.006 0.008 0.005 0.006 0.005 Tl 0.694 0.521 0.651 0.469 0.591 0.583 AI 0.020 0.021 0.013 0.016 0.010 0.014 Or 0.002 0.005 0.000 0.013 0.002 0.003 Fe3+ 0.593 0.933 0.692 1.032 0.807 0.821 Fe%+ 0.581 0.426 0.525 0.388 0.501 0.483 Mn 0.003 0.000 0.006 0.001 0.003 0.002 Mg 0.092 0.062 0.078 0.066 0.066 0.072 NI 0.000 0.000 Ca 0.001 0.012 0.005 0.001 0.002 0.002 Na 0.009 0.015 0.021 0.009 0.011 0.016 TOTAL 1.999 2 . 0 0 1 1.999 2 . 0 0 0 1.999 2 . 0 0 1 Ilmenite 0.682 0.502 0.628 0.453 0.574 0.564 393 ILMENITE. LA140 LA140 LA140 LA100 LA100 LA100 ANALYSIS M20 M50 MSI M3 M3RPT MS SiOg 1.16 0.12 0.34 0.08 0.31 0.32 TIO2 14.46 16.10 16.55 33.38 33.45 40.88 AI2 O3 0.74 0.58 0.49 0.64 0.56 0.45 Cr2 0 3 0.09 0.07 0.27 0.03 0.10 0.05 FeO 72.17 72.86 71.55 54.67 54.41 48.61 MnO 0.00 0.09 0.02 0.19 0.09 0.28 MgO 0.89 0.89 0.64 1.87 1 . 8 8 3.31 NIO N.A. 0.00 0.00 0.00 0.00 0.00 CaO 0.05 0.14 0.01 0.10 0.03 0 . 1 2 N 83 0 0.20 0.34 0.16 0.24 0.22 0.23 TOTAL 89.76 91.19 90.03 91.20 91.05 94.25 FORMULA (2 CATIONS, 3 OXYGENS). SI 0.031 0.003 0.009 0.002 0.008 0.008 Tl 0.290 0.318 0.333 0.668 0.670 0.788 AI 0.023 0.018 0.015 0.020 0.018 0.014 Or 0.002 0.001 0.006 0.001 0.002 0.001 Fe3+ 1.345 1.358 1.305 0.651 0.635 0.406 Fe2+ 0.262 0.243 0.297 0.565 0.578 0.636 Mn 0.000 0.002 0.000 0.004 0.002 0.006 Mg 0.035 0.035 0.026 0.074 0.075 0.126 NI 0.000 0.000 0.000 0.000 0.000 Ca 0.001 0.004 0.000 0.003 0.001 0.003 Na 0.010 0.017 0.008 0.012 0.011 0.011 TOTAL 1 .999 1.999 1.999 2 . 0 0 0 2 . 0 0 0 1.999 Ilmenite 0.291 0.290 0.325 0.654 0.662 0.777 394 APATITE GROUP MINERALS. ANALYSIS. DESCRIPTION. LA100 1990 Andésite Lava. XI Apatite Inclusion in orthopyroxene phenocryst. LA141 Mafic Inclusion. Capricorn Lava. X6 Apatite inclusion in magnetite phenocryst. XII Apatite inclusion in magnetite phenocryst. XI 2 Apatite inclusion in magnetite phenocryst. XI 3 Apatite inclusion in magnetite phenocryst. XI 6 Apatite inclusion in magnetite phenocryst. LAI 43 Mafic Inclusion. Capricorn Lava. XI Apatite inclusion in orthopyroxene phenocryst. X4 Apatite inclusion in orthopyroxene phenocryst. L A I47 Dacite Pumice. Soncor Flow. X2 Apatite inclusion in magnetite phenocryst. X7 Apatite inclusion in magnetite phenocryst. L A I08 Woliastonite Skarn Xenoiith. Tumbres Flow. X6 Apatite in outer zone of xenoiith. X3 Wilkeite in outer zone of xenoiith. X8 Wilkeite inclusion in woliastonite. X51 Wilkeite in thin veinlet. X52 Wilkeite in thin veinlet. X53 Wilkeite in thin veinlet. 395 APATITE GROUP MINERALS. LA100 LA141 LA141 LA141 LA141 LA141 LA143 LA143 LA147 LA147 ANALYSIS XIX6 XII X12 X13 X16 XI X4 X2 X7 CaO 52.33 53.69 52.12 53.05 52.71 52.98 53.71 54.05 52.48 53.23 FeO 0.51 2.55 2.07 2.18 2 . 0 0 2.18 1.03 0.92 2.99 2.58 MgO 0.38 0.27 0 . 2 2 0.16 0.31 0 . 2 1 0.33 0.24 0.27 0.36 Na2 0 0.24 0.23 0.00 0.46 0.30 0.38 0.30 0.23 0.30 0.46 P2O5 43.25 42.35 42.09 42.43 42.38 42.66 42.53 42.50 41.57 41.50 SIO2 0.28 0 33 0.32 0.36 0.43 0.29 0.63 0 . 6 6 0.38 1 . 2 0 SO 3 0.02 0.34 1 , 1 2 0.71 0.80 0.65 0.73 0.62 0 . 6 8 0.60 Cl 0.50 0.91 0.95 0.77 0.87 0.56 0.75 0.71 0.89 0.74 CAJ - 0 = Cl - 0 . 1 1 - 0 . 2 1 - 0 . 2 1 - 0.17 - 0 . 2 0 - 0.13 - 0.17 - 0.16 - 0 . 2 0 - 0.17 -0 TOTAL 97.40 100.46 98.68 99.95 99.60 99.78 99.84 99.54 99.36 100.50 FORMULA (6 PO4 + SO4 + SIO4) Ca 9.099 9.474 9.105 9.264 9.182 9.230 9.285 9.363 9.350 9.304 Fe 0.069 0.352 0.282 0.297 0.272 0.297 0.139 0.124 0.415 0.352 Mg 0.092 0.066 0.052 0.038 0.075 0.050 0.078 0.057 0.067 0.088 Na 0.075 0.073 0.000 0.146 0.093 0.119 0.092 0.071 0.096 0.147 PO 4 5.943 5.905 5.811 5.854 5.834 5.873 5.811 5.818 5.851 5.731 SiO, 0.045 0.054 0.052 0.059 0.069 0.047 0.101 0.107 0.063 0.196 SO 4 0.012 0.041 0.137 0.087 0.097 0.079 0.088 0.075 0.085 0.073 TOTAL 15.335 15.965 15.439 15.745 15.622 15.695 15.594 15.615 15.927 15.891 Cl 0.138 0.372 0.261 0.213 0.240 0.155 0.205 0.195 0.251 0.203 APATITE GROUP MINERALS. LA108 LA108 LA108 LA108 LA108 LA108 ANALYSIS X6 X3 X8 X51 X52 X53 CaO 56.95 56.55 56.76 55.71 55.34 55.80 FeO 0.11 0.09 0.11 0.17 0.01 0.00 MgO 0.08 0.12 0.00 0.08 0.14 0.07 Na2 0 0.12 0.22 0.01 0.09 0.19 0.11 P2O5 43.29 29.57 30.07 22.94 24.30 32.45 SiOg 0.61 5.76 5.79 9.08 8.73 5.52 SO 3 0.76 7.65 6.64 10.04 9.47 5.69 Cl 0 . 2 0 0.32 0 . 2 2 0.33 0.35 0.26 - 0 = 01 - 0.05 - 0.07 - 0.05 - 0.07 - 0.08 - 0.06 TOTAL 102.07 1 0 0 . 2 1 99.55 98.37 98.45 99.84 FORMULA (6 PO4 + SO4 + S1O4 ) Ca 9.678 9.950 10.071 9.939 9.772 9.626 Fe 0.015 0.013 0.015 0.024 0.001 0.000 Mg 0.018 0.029 0.000 0.020 0.035 0.017 Na 0.037 0.068 0.002 0.029 0.060 0.034 P O 4 5.813 4.111 4.216 3.234 3.390 4.424 S IO 4 0.097 0.945 0.958 1.511 1.438 0.888 S O 4 0.090 0.943 0.826 1.254 1.171 0.688 TOTAL 15.748 16.059 16.088 16.011 15.867 15.677 Cl 0.053 0.088 0.061 0.094 0.098 0.072 SULPHIDES ANALYSIS DESCRIPTION LA1 Centre II Lava. 51 Rounded pyrrhotite inclusion in ilmenite phenocryst. 52 Rounded pyrrhotite inclusion in ilmenite phenocryst. 53 Rounded pyrrhotite inclusion in ilmenite phenocryst. LA121 Dacitic Pumice. Soncor Flow. 81 Pyrrhotite inclusion in orthopyroxene phenocryst. 82 Pyrrhotite inclusion in orthopyroxene phenocryst. LA155 Banded Pumice. Soncor Flow. 8 MG1 Pyrite. Crystallised sulphide melt droplet included in ilmenite. LASSO Hornblende-rich pumice. Soncor Flow. 81 Pyrrhotite microphenocryst. 81 Pyrrhotite microphenocryst. PWF1 1993 White Pumice. XI Chalcopyrite inclusion in ilmenite phenocryst. 398 SULPHIDES. LA1 LA1 LA1 LA121 LA121 LA155 LAS30 LAS30 PWF1 ANALYSIS S1 S2 S3 S1 S2 SMG 1 S 1 S 2 X 1 Fe 55.86 54.68 55.88 52.89 54.51 45.73 55.99 56.15 27.62 Mg N.A. N.A. N.A. 0.06 0.01 0.09 0.00 0.03 0.71 Ml 0.75 0.65 0.76 1.73 1.77 0.00 0.98 0.97 0.48 Cu 0.00 0.00 0.00 0.00 0.26 N.A. N.A. N.A. 37.28 Ca N.A. N.A. N.A. N.A. N.A. N.A. 0.25 0.25 N.A. Ti 1 . 0 0 0.87 1.31 0.02 0.00 0.35 0.13 0.11 N.A. S 39.81 40.54 39.19 37.64 38.38 52.69 38.90 38.91 30.65 TOTAL 97.42 96.74 97.14 92.34 94.92 98.86 96.25 96.39 96.74 -0 FORMULA (2 SULPHIDES) Fe 1.611 1.549 1.637 1.614 1.631 0.997 1.653 1.657 1.035 Mg 0.004 0.001 0.005 0.000 0.002 0.061 Ni 0.028 0.024 0.029 0.069 0.069 0.000 0.038 0.037 0.017 Cu 0.000 0.000 0.000 0.000 0.007 1.228 Ca 0.010 0.010 Ti 0.034 0.029 0.045 0.001 0.000 0.009 0.005 0.004 S 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 TOTAL 3.673 3.602 3.711 3.688 3.708 3.011 3.706 3.710 4.341 GLASSES ANALYSIS. DESCRIPTION. LA124 Holocrystalline Mafic Inclusion. Soncor Flow. G1 Andesitic glass inclusion in olivine phenocryst. 02 Andesitic glass inclusion in olivine phenocryst. 04 Andesitic glass inclusion in olivine phenocryst. 014 Andesitic intersitial glass in olivine-augite crystal clot. 023 Dacitic glass inclusion in olivine phenocryst. 021 Dacitic glass inclusion in magnetite phenocryst. LAS-30 Hornblende-Rlch Andesitic Pumice. 01 Rhyodacitic matrix glass. 04 Rhyodacitic matrix glass. 08 Rhyodacitic matrix glass. Z1 Iron-rich immiscible melt globule. Z2 Iron-rich immiscible melt globule. Z3 Iron-rich immiscible melt globule. LAS29-2 Andesitic Pumice. Soncor Flow. 03 Rhyolitic glass inclusion in augite phenocryst. 04 Dacitic matrix glass. 05 Dacitic matrix glass. LA122 Dacitic Pumice. Soncor Flow. 01 Rhyolitic glass inclusion in orthopyroxene phenocryst. 02 Rhyolitic glass inclusion in apatite phenocryst. 03 Rhyolitic glass inclusion in augite phenocryst. LA141 Mafic Inclusion. Capricorn Lava. 030 Rhyolitic glass matrix. 032 Rhyolitic glass matrix. 035 Rhyolitic glass matrix. LA140 Dacitic Capricorn Lava. 010 Rhyolitic glass matrix. 011 Rhyolitic glass matrix. 013 Rhyolitic glass matrix. 400 ANALYSIS. DESCRIPTION. LAG 1986 Andésite Lava. G30 Rhyolitic interstitial glass. G31 Rhyolitic interstitial glass. G32 Rhyolitic interstitial glass. LA100 1990 Andésite Lava. G10 Rhyolitic glass inclusion in augite phenocryst. G12 Rhyolitic glass inclusion in augite phenocryst. G15 Rhyolitic glass inclusion in orthopyroxene phenocryst. PWF1 1990 White Pumice. G1 Rhyolitic glass matrix. G2 Rhyolitic glass matrix. G3 Rhyolitic glass matrix. PWF2 1990 Andesitic Scoria. G 1 Dacitic glass matrix. G2 Dacitic glass matrix. G3 Dacitic glass matrix. LA111 Buchite Xenolith from Tumbres Flow. G13 Interstitial devitrified brown glass. G16 Interstitial devitrified brown glass. G31 Colourless glass inclusion in quartz. LAS23 Buchite Xenolith from Tumbres Flow. G5 Matrix glass. G6 Matrix glass. G8 Matrix glass. 401 GLASSES. LA124 LA124 LA124 LA124 LA124 LA124 ANALYSISG1 G2 G4 G14 G23 G21 SIO 2 55.28 55.28 56.94 63.64 66.84 68.34 TIO2 0.98 1.05 1.24 1.94 0.56 0.89 AI2 O3 22.79 2 2 . 6 6 22.07 13.92 17.17 14.61 FeO 4.48 5.18 4.59 5.40 1.05 3.97 MnO 0.00 0.00 0.00 0.00 0.23 0.00 MgO 1.78 1 . 6 6 1.55 0.62 0.52 0.53 CaO 2.44 2.40 1.97 1.04 0.82 1 . 1 1 Na2 0 3.46 3.28 3.49 3.05 3.79 3.64 K2 O 2.93 3.18 3.29 3.39 3.61 3.81 P 2 O 5 0.17 0.39 0.38 1.09 0.77 0.85 Cl 0.19 0.12 0.13 0.19 0.23 0.09 S 0.50 0.36 0.36 0.00 0.25 0.01 TOTAL 95.00 95.56 96.01 94.28 95.84 97.85 H2 O (diff) 5.00 4.44 3.99 5.72 4.16 2.15 402 GLASSES. LAS30 LAS30 LAS30 LAS30 LAS30 LAS30 ANALYSISG1 G4 G8 Z1 Z2 Z3 SIO 2 69.34 70.15 70.78 28.26 29.72 35.48 TIO2 0.45 0.71 0.38 0.59 0.67 0.31 AI2 O 3 15.60 16.12 15.60 6 . 6 8 6.87 7.09 FeO 3.02 1.05 2.44 52.17 51.98 44.08 MnO 0.00 0.03 0.04 0.26 0 . 0 2 0 . 0 0 MgO 0.69 0.92 0.85 2.84 2.82 0.69 CaO 2.16 2.45 2.61 0.25 0 . 2 2 0.67 N8 2 0 4.46 4.62 4.22 0.79 0.90 1.05 K2 O 3.04 3.14 3.20 0.38 0.41 0.90 P2 O 5 0.00 0.39 0.07 0.00 0.02 0.00 Cl 0.14 0.14 0.15 0.07 0.13 0.03 s 0.07 0.00 0.06 0.08 0.04 0.02 TOTAL 98.97 99.72 100.40 92.37 93.80 90.32 mmmm H2 O (diff) 1.03 0.27 0 . 0 0 m m mm mmmm 403 GLASSES LAS29-2 LAS29-2 LAS29-2 LA122 LA122 LA122 ANALYSIS G3 G4 G5 G 1 G2 G3 SlOg 75.10 66.95 68.45 76.22 74.97 74.49 TIOg 0.22 0.75 0.59 0.61 0.46 0.28 AI2 O 3 13.79 15.29 15.30 12.18 12.43 11.69 FeO 0.36 3.31 2.71 2.28 1 . 2 1 1.67 MnO 0.19 0.18 0.00 0.16 0.00 0.20 MgO 0.00 1.13 0.99 0.26 0.23 0.31 CaO 1.58 2.92 2.07 0.89 1.32 0.83 NagO 6.47 4.80 4.47 1 . 8 8 3.51 1.99 KgO 0.91 2.63 3.56 4.51 4.51 4.14 PgOs 0.24 0.27 0.44 N.A. 0.12 0.00 Cl 0.00 0.03 0.08 N.A. 0.07 0.14 S 0.00 0.00 0.10 N.A. 0.01 0.07 TOTAL 98.86 98.26 98.76 98.99 98.84 95.81 HgO (diff) 1.14 1.74 1.24 1 . 0 1 1.16 4.19 404 GLASSES. LA141 LA141 LA141 LA140 LA140 LA140 ANALYSIS 030 032 035 010 G 1 1 G13 SIO 2 73.14 72.38 73.62 75.91 76.01 75.23 TIO 2 0.59 0.62 0.65 0.65 0.33 0.60 AI2 O 3 11.62 12.79 12.07 1 1 . 1 0 11.16 1 1 . 0 0 FeO 1.29 1 . 0 1 0.85 1 . 0 2 1 . 0 0 1.04 MnO 0.00 0.00 0.06 0.04 0.12 0.00 MgO 0.03 0.06 0.04 0.00 0.34 0.09 CaO 0.44 0.67 0.64 0.33 0.58 0.44 N 8 2 0 2.19 2.61 2.22 3.04 2.99 2.78 K2 O 4.86 4.94 4.81 5.11 5.02 4.94 P2O5 0.00 0.30 0.20 0.00 0.00 0.23 Cl 0.03 0.13 0.11 0.09 0.02 0.16 s 0.03 0.00 0.04 0.04 0.02 0.05 TOTAL 94.22 95.51 95.31 97.33 97.60 96.56 H2 O (diff) 5.78 4.49 4.69 2.67 2.40 3.44 405 GLASSES LA6 LA6 LA6 LA100 LA100 LA100 ANALYSIS G30 G31 G32 G10 G12 G15 SIO 2 76.31 74.97 75.87 71.18 71.21 72.80 TIOg 0.56 0.67 0.60 1 . 2 0 1.35 0.51 AI2 O3 10.75 12.48 13.05 11.71 11.84 12.44 FeO 1.45 1.53 0.96 2.30 2.51 2 . 2 0 MnO 0.00 0.05 0.13 0.00 0 . 0 0 0.06 MgO 0.24 0.14 0.16 0.63 0.50 0.33 CaO 0.58 0 . 8 8 1.35 0.44 0.49 1.41 NagO 3.41 4.07 3.67 4.38 4.47 3.71 KgO 4.59 4.28 4.01 5.30 5.23 4.14 P 2O 5 0.26 0.22 0.12 0.39 0.28 0.16 Cl 0.01 0.09 0 . 1 1 0.33 0.24 0.10 s 0.04 0.00 0.00 0.08 0.00 0.00 TOTAL 98.20 99.38 100.03 97.94 98.12 97.86 HgO (diff) 1.80 0.62 0 . 0 0 2.06 1 . 8 8 2.14 406 GLASSES. PWF 1 PWF 1 PWF1 PWF2 PWF2 PWF2 ANALYSISG 1 0 2 03 0 1 0 2 03 SiOg 72.53 73.02 73.27 62.25 62.50 65.05 TIOg 0.10 0.26 0.00 0.41 0 . 8 6 0.99 AI2 O 3 12.81 13.32 12.70 17.13 17.16 15.52 FeO 1.90 1.47 1.36 4.11 4.19 5.06 MnO 0.01 0.15 0.00 0.00 0.00 0.06 MgO 0.07 0.41 0.33 1.54 1.45 1.83 CaO 1.37 1.34 1.39 5.19 5.23 3.94 NagO 3.93 3.89 3.82 4.05 3.94 4.26 KgO 4.33 4.42 4.27 2.24 2 . 2 2 2.78 P2O5 0.00 0.00 0.39 0.06 0.40 0.14 01 0.05 0.08 0.02 0.09 0.02 0.13 S 0.01 0.00 0.00 0.02 0.00 0.10 TOTAL 97.1 1 98.36 97.55 97.09 97.97 99.86 HgO (diff) 2.89 1.64 2.45 2.91 2.03 0.14 407 GLASSES LA111 LA111 LA111 LAS23 LAS23 LAS23 ANALYSIS G13 G16 G31 G5 G6 G8 S 1 0 2 6 6 . 2 0 64.04 51.75 74.59 74.60 74.12 TiOg 0.65 0.56 0.50 0.31 0.39 0.25 AI2 O 3 10.84 10.40 11.09 1 1 . 1 2 11.08 1 1 . 1 1 FeO 3.04 3.04 4.15 0.97 1 . 1 1 1.16 MnO 0.08 0.10 0.00 0.00 0.18 0.00 MgO 0.27 0 . 2 0 1.55 0.19 0.25 0.24 CaO 10.18 9.99 24.09 0.90 0.81 1.18 Nag 0 0.66 0.50 1.24 2.26 2.74 2.75 KgO 4.38 4.24 0.50 4.00 3.98 3.82 P2O5 0.17 0.11 0.00 0.20 0.41 0.00 Cl 1.25 1.31 0.51 0.11 0.00 0.07 s 0.09 0.08 0.02 0.05 0.02 0.00 TOTAL 97.81 94.57 95.38 94.70 95.57 94.70 HgO (diff) 2.19 5.43 4.62 5.30 4.43 5.30 408