Sodes, Occurring at 2.5 M. Y., 0.9 M. Y., and 0.4 M. Y. Ago

AN ABSTRACT OF THE THESIS OF r

James Gregory Clark for the degree ofMaster of Science (Name) (Degree)

/9,7j- in Oceanography presented on 4' (Major) (Date) Title: AGE, CHEMISTRY, AND TECTONIC SIGNIFICANCE OF

EASTER AND SALA Y GOMEZ ISLANDS Redacted for privacy Abstract approved: Jack Dymond

Easter Island and Sala y Gomez are part of the Sala y Gomez Ridge, a broad band of high topography and scattered seamounts extending ESE from the East Pacific Rise.It has been proposed that the Sala y Gomez Ridge results from the movement of the Nazca Plate over a fixed melting spot in the mantle.To test this hypothesis volcanic rocks from Easter Island and Sala y Gomez were analyzed for their K-Ar ages and major element abundances. Subaerial Easter Island was constructed in three distinct episodes, occurring at 2.5 m. y., 0.9 m. y., and 0.4 m. y. ago.The youngest rocks on the island are the Roiho olivine basalts, and are probably less than 50,000 years old.Eruptive activity on Sala y Gomez was essentially contemporaneous with the early volcanism on Easter Island, No migration of volcanism with time is apparent along the Sala y Gomez Ridge, thus a major criterion of the melting spot hypothesis is not fulfilled. Volcanic rocks from Easter Island constitute a tholeiitic differentiation series; they are chemically similar to those from other islands situated near mid-ocean rise crests.The wide compositional spectrum is most likely the result of fractional crystallization from a basaltic parent liquid, though the data is ambiguous for the highly silicic differentiates.The youngest basalts possess more alkaline affinities which are probably not related to fractional crystallization from the earlier basalts.The alkaline nature of these rocks may be the result of a downward migration of the fusion zone with time, as the island moved eastward over a progressively thickening lithosphere. Volcanic rocks from Sala y Gomez belong to an alkali olivine basalt series.The fundamental chemical differences between the Easter Island and Sala y Gomez suites suggest that the two islands were not derived from a common source, as predicted by the melting spot hypothesis. The evidence does not support a melting spot origin for Easter Island, Sala y Gomez, and the Sala y Gomez Ridge. An alternative model involving diapiric intrusion and decompression melting of asthenosphere material along a major fracture in the Nazca Plate provides a better explanation for the data.Synchronous volcanism along the eastern extension of the Easter Island transform fault has given rise to the islands and seamounts on the Sala y Gomez Ridge. Age, Chemistry, and Tectonic Significance of Easter and Salay Gomez Islands

by James Gregory Clark

A THESIS submitted to Oregon State University

in partial fulfillment of the requirements for the degree of Master of Science June 1975 APPROVED:

Redacted for privacy

Assoc1'e Professol'of Oceanography in charge of major

Redacted for privacy

Dean of Graduate School

Date thesis presented on 4i&.4 /Y4Rc Typed by Margie Woiski for James Gregory Clark ACKNOWLEDGEMENTS

I am indebted to my advisor, Dr. Jack Dymond, for providing patience, encouragement, and especially endurance in the course of this work.I also wish to acknowledge the other members of my committee,Drs.J.B. Corliss,E. J. DaschdJgor their comments and suggestions. Dr. Donald Heinrichs aided in the initial sampling on Easter Island, and Dr. P. E. Baker of the University of Leeds provided additional samples from his Easter Island collection.I am grateful to Dr. David Piper, who supervised the XRF analyses at the Univer. sity of Washington, and to Marilyn Lindstrorn, who led me through the fractional crystallization calculations. Ron Stillinger taught me everything I know about AAS, and Magdalena Catalfomo made me wash my bottles.I have profitted greatly from discussions with Drs. A. R. McBirney and D. K.. Rea, and also Dennis Nelson. A special thanks goes to Dr. Lewis Hogan, who gave me expert assistance and instruction in mass spectrometry, who was always willing to help me solve a problem, who tried to quantify my science,

andwhotaught me to filet a bluegill.Though they had absolutely nothing to do with my project, I wish to express my appreciation to Bill, John, John, Mitch, Cliff, Margaret and Simon,whohelped me to keep science in its proper perspective. The cover is an original linoleum cut done by Boyd Hanna, a prominent artist who is also my father.in.-law. Margie Wolski is responsible for the excellent typing and Kathryn Torvik for the excellent drafting. The work would never have been completed without the love and understanding of my wife, Kris, Funding was provided by the National Science Fo.indation (Contract No. GA-27548A). TABLE OF CONTENTS Page

INTRODUCTION 1

LOCATION AND GEOLOGY 9

Location 9 Geology 10

EXPERIMENTAL METHODS 13

Field Work 13 K-Ar Analyses 14 Chemistry 15 Crystal Fractionation Calculations 16

DATA PRESENTATION 19

K-Ar 19 Chemistry 27 Ba salts 29 Differentiated rocks 33 Comparisons with other provinces 35 SalayGomez 36 Fractional Crystallization Data 38

DISCUSSION 49 Chronology of Volcanism on Easter and SalayGomez Islands 49 Volcanic production rates 55 SalayGomez 55 Major Element Chemistry of Easter Island and SalayGomez 57 Temporal variations on Easter Island 62 Comparison with hot spot volcanism 66 Genetic Relations of the Easter Island Suite 68 Origin of the SalayGomez Ridge 73 Tectonic setting 73 Physiography 74 SalayGomez Ridge 77

CONCLUSIONS 85 Table of Contents, continued

BIBLIOGRAPHY 87

APPENDIX 1:Comparisons of published analyses of 96 U. S. Geological Survey standard rocks, AGV-1, W-1, and BCR-1 with analyses of this study APPENDIX 2:Chemical analyses of rocks from Easter 104 Island and Sala y Gomez

APPENDIX 3a: Analyses of rocks and phenocrysts used 113 in crystal fractionation calculations

APPENDIX 3b: Results of crystal fractionation 115 calculations

APPENDIX 4:Petrography 125 LIST OF FIGURES Figure Page

1 Major island chains in the Pacific Ocean 2

2 Index map of the southeast Pacific Ocean 6

3 Bathymetry of the South Pacific Ocean 7

4 Geology of Easter Island 11

5 Sample locations 20

6 K-Ar ages and stratigraphic positions of lavas from Poike and Rano Kau, Easter Island 24

7 Age histogram for major Easter Island volcanoes and SalayGomez 25

8 Oceanic island rocks plotted in the system Ne-Ol-An-Hy-Qz 30

9 AFM diagrams for rocks from the major Easter Island volcanic centers and an Easter Island dredge haul 32

10 Total alkalis vs. silica diagram for Easter Island series 34

11 AFM diagram for SalayGomez volcanic rocks 37

12 Stratigraphic position of samples used in Rano Kau crystal fractionation model 39

13 Fractional crystallization sequence for rocks from Rano Kau 40

14 Fractional crystallization on Poike and Terevaka 43

15 Possible crystal fractionation origin for the trachytes and rhyolits on Easter Island 46

1 6 Chronology of Easter Island volcanism 50 List of Figures, continued

17 Mid-ocean ridge thermal structure 63

18 Factors governing compositional differences between Easter Island and Sala y Gomez 64

19 Physiographic features of the Sala y Gomez Ridge region (schematic representation) 75

ZO Origin of the Sala y Gomez Ridge 84 LIST OF TABLES Table Page

1 K-Ar age data on volcanic rocks from Easter Island and SalayGomez 21

2 Classification of the Easter Island and Salay Gomez volcanic series 28

3 Rano Kau cry stall fractionation model-weight fraction of components from computer analysis 41

4 Fractional crystallization on Poike and Terevaka 44

5 Possible fractional crystallization origin for silicic differentiates on Easter Island 47

6 Volumes and rates of volcanic production for Easter Island 56 AGE, CHEMISTRY, AND TECTONIC SIGNIFICANCE OF EASTER AND SALA Y GOMEZ ISLANDS

INTRODUCTION

Numerous island and seamount chains stripe the Pacific Basin in a pattern that seems far from irregular (Fig. 1).Knowledge of the history and origin of these features is essential toour understanding of Pacific Ocean tectonics, and various hypotheses have stirredcon- siderable controversy during the past decade. Most studies have focused on the Hawaiian-Emperor chain (Dana, 1849, 1890; Eaton and Murata, 1960; Macdonald, 1949, 1968; Macdonald and Katsura, 1964; Jackson, Silver and Dairymple, 1972).The geomorphology of the Hawaiian volcanoes led Dana (1849, 1890) to suggest that the islands became progressively older to the northwest. Radiometricage determinations later substantiated this hypothesis and also demonstrated an apparent southeastward migration of volcanism for the Emperor Sea- mounts (McDougall, 1964; Funkhouser etal., 1968; Dalrymple, 1971; Clague and Dalryniple, 1972; Ozima etal., 1970).Recent studies indicate that similar migration of volcanism is common to other Pacific island chains- the Austral Islands (Johnson and Malahoff, 1971), the Marquesas Islands (Duncan and McDougall, 1974) and the Society Islands (Dymond, 1975). Betz and Hess (1942) attributed the linearity of the Hawaiian 2

MAJOR ISLAND CHAINS IN THE PACIFIC OCEAN

EAST PACIFfC OCEAN r1y LEGEND

'I1* ,7RIDGE CRESTS - - -- TRENCHES FRACTURE ZONE

Fracture Zone 40 tASfld0CHO o 4000 8000 M

ABOVE - DEPTH - SEA LEVEL

FrZone clipper p.t_ GoIopaq.LFr-Zona . A 4 4 MARQUESAS r4.jSLANDs

/ ii ) cI4lpl

2 L

1600

Figure 1.Major island chains in the Pacific Ocean. 3 Islands to volcanic activity along a major fracture in the oceanic crust,Wilson (1963, 1965) concluded that evidence was lacking for such a major fault zone, and proposed instead that island and seamount chains record oceanic plate motion over fixed magma reser- voirs, or hot spots, in the upper mantle. Morgan (1971, 1972a, b) presented plate motion studies consistent with this interpretation and reviewed supportive geochemical and geophysical evidence. He visualized hot spots as the surface expression of deep mantle convective plumes, which, besides giving rise to linear island and seamount chains, provide the motive force for sea floor spreading. Clague and Jarrard (1973) have shown that nearly all linear island and seamount chains on the Pacific Plate are parallel to small circles generated about a Hawaiian pole at 72°N, 83°W, or an Emperor pole at 17°N, 107°W.They assumed that various Pacific hot spots are fixed relative to each other, and were then able to predict ages of seamounts and islands in other chains by using the proposed rotational motion of the Pacific Plate relative to the Hawaiian hot spot. Geochemical and petrologic data from Pacific island and seamount chains have been largely ignored in the rush by plate tec- tonicists to accommodate the trends of these features to current con- cepts of plume geometry.Implicit in Morgan s model of deep mantle convective plumes (Morgan, 1971, l972a, b) are points of upwelling which display unique petrochemical properties.Vogt and Johnson (1973) describe plume-derived ocean crust as being especially enriched in Fe, Ti, and the large ionic lithopile elements. A point of upwelling might then be expected to impart similar chemical characteristics and volcanic style to all islands and seamounts in the chain which originates from it.While this pattern is well-documented for the Hawaiian Islands (Macdonald and Katsura, 1964; Macdonald, 1968) and possibly the Marquesas (Chubb, 1930; Bishop, Woolley and Din, 1973), data from other Pacific islands cast serious doubts on the validity of the hypothesis (McBirney and Gass, 1967). McBirney and Gass (1967) have demonstrated that a correlation exists between heat flow, distance from mid-ocean rises and the degree of silica saturation of oceanic island volcanic rocks.Strongly over saturated rocks are found on or near mid-ocean rise crests, whereas under saturated, alkaline rocks occur on the flanks and in areas far from such rises. Current petrogenetic theory (O'Hara, 1965, 1968; Green and Ring- wood, 1967; Kushiro, 1968) suggests that this pattern results from shallow, low pressure melting near rise crests, and melting at greater depths away from them. The plume hypothesis has also been criticized for being simplistic and essentially unnecessary to explain the origin of these features and migration of volcanism along them.Jackson and Wright (1970) continue to favor the propagating fracture hypothesis (Betz and Hess, 1942).Because hot spot volcanism might be caused by 5 pressure release associated with such a fracture, Jackson etah (197Z) prefer the term 'melting spot' rather than hot spot.Various models have made use of asthenosphere-lithosphere interaction to explain the melting and subsequent volcanic propagation of Pacific island chains; notable among these are the shear-melting model of Shaw (1973) and the asthenosphere counterfiow hypothesis of McDougall (1971).Stress analysis of lithospheric plates led Turcotte and Oxburgh (1973) to suggest that oceanic island and seamount chains result from crustal extension due to tensional stresses, both thermal stresses in a cooling lithosphere and membrane stresses due to changes in the radius of curvature of a spreading plate.Finally, Runcorn (1974) has presented an eloquent and compelling argument that a deep mantle thermal plume is difficult to reconcile with the principles of mechanics. He argues, from mantle viscosity considerations, that the maintenance of a pipe-like plume configuration in the lower mantle is high unlikely given the time scale on which the supposed plumes must operate (>50 m.y.). The Sala y Gomez Ridge, and the islands and seamounts which form it (Figs. 2 and 3), have been attributed both to a deep mantle plume (Morgan, 1972a, b) and to an elongate fracture transverse to the East Pacific Rise (Herron, 1972a). A detailed petrochemical and K-Ar age study of Easter Island and Sala y Gomez, and their relation to the Sala y Gomez Ridge, is presented here with the aim of 6

uo° \ K0° 900 70°

EADING 'GALAPAG I ' L...... CENTERJ $ I ------!--...... -0.-.-.tl Q,3oO°/:..... _oe / 0° /

i.. . RI GE jj.4\

00 90 70 Figure 2.Index map of the southeast Pacific Ocean. A I I c / 20 * / / b i .

(; 0 / 9 ) 1_ / - ...- - I .-.- --.---

{1 . f---.I 1200 1150 110° 105° 100° 95° 90° 85° 80°

Figure 3.Bathymetry of the South Pacific Ocean.

1 contributing to the understanding of the origin of linear island and seamount chains in the Pacific Basin.The more specific purposes of this study are to: 1) establish a detailed chronology of volcanism for Easter and Sala y Gomez Islands, which can be applied to specific local geologic problems and to the question of migration of volcanism along the Sala y Gomez Ridge. 2) establish the timing of the petrologic and chemical variations within a single island (Easter), and between two islands along the Sala y Gomez Ridge (Easter and Sala y Gomez). 3) establish the genetic relationships of volcanic rocks from Easter and Sala y Gomez Islands through the use of comparative chemistry and a computerized fractional crystallization model. 4) consider the origin of the Sala y Gomez Ridge in view of all available geochemical and geophysical data. LOCATION AND GEOLOGY

Location

Easter Island and Sala y Gomez are the only two islands in a vaguely defined, east-west trending zone of seamounts and higher topography known as the Sala y Gomez Ridge (Fisher and Norris, 1960; Figs, 2 and 3).Easter Island (27°S, 109°W) is situated 530 km east of the crest of the East Pacific Rise; Sala y Gomez (26°S, lO5°W) lies 41 5 km further east-northeast. As originally defined by Fisher and Norris (1960) the Sala y Gornez Ridge extends nearly due east from its intersection with the East Pacific Rise at approximately (27°S, 114°W), along latitudes 25°-27°S, to about 90°W longitude, where it meets the fossil spreading center, the Galapagos Rise (Menard, Chase and Smith, 1964).The relationship of the Sala y Gomez Ridge to the Easter Island Fracture Zone (Sykes, 1967) is poorly understood, but Principal (1974) suggests that the ridge trends east-southeast into the fracture zone and may possibly be coextensive with it. This portion of the Pacific lies close to the equator of the Nazca- Pacific pole-of-rotation axis (Anderson etal., 1974), and thus has the fastest spreading rate in the world mid-ocean ridge system.The half-spreading rate has been estimated at greater than 9.0 cm yr' (Herron, 1972a), Marine magnetic data presented by Herron (1972a) shows Easter Island situated near anom3ly three and Sala y Gomez near anomaly five.From spreading rate data and the magnetic time scale (Heirtzler etal., 1968), the sea floor beneath Easter Island should be about 6, 0 m, y. old, while that beneath Sala y Gomez should be close to 10.5 m.y. old,

Geology

Early contributions to the geology of Easter Island were made by Chubb (1933), LaCroix (1936) and Bandy (1937).Recent compre-. hensive studies by Baker (1966), Baker etal, (1974), and Gonzalez- Ferran and Baker (1974) provide further essential detail and illumin- ate some of the problems toward which the present study is directed. Easter Island is roughly triangular in shape, has an area of 160km2, and is entirely volcanic in origin.It comprises three principal volcanoes and over 70 subsidiary eruptive centers (Fig. 4).Each of the main volcanoes has a different structure - Poike, to the east, is a simple strato-volcano; Rano Kau, to the southwest, has a well- developed central caldera; Terevaka, to the north, is a complex fissure volcano.The morphology of Poike and Rano Kau suggests they are relatively old compared to Terevaka and its parasitic centers, Marine erosion has severely reduced both Poike and Rano Kau; wave-cut cliffs surround Poike on all sides and Rano Kau to the west, south, and east, nearly breaching the caldera at its southern Hanqo Oteo

HANGA

TEREVAKA RANO KAU POIKE MUGEARITE B BENMREITE RHYOLITE a aTRACHYTEDOMES

HAWAHTE a ffffllflBENMOREITE BASALT B BASALT FLOWS FLOW HAWAIITE KM llffl1J L-2fIFLOWS 0 I 2 5 PYROCLASTIC PYROCLASTIC CENTRES JCENTRES CONTOUR INTERVAL=100 BASALT FLOWS C Older LAVA FLOW BOUNDARY çYounger EASTER iSLAND CRATER Figure 4.Geology of Easter Island (from Baker, 1966). I- 12 edge. Rano Raraku, a parasitic center associated with Terevaka volcanism, also has a well-developed, marine-eroded cliff at its southeast face. Easter Island volcanic rocks are predominantly plagioclase- phyric and consist of a rather complete tholeiitic differentiation series,They resemble Ascension and Bouvet Islands in the Atlantic (Baker et al,, 1974) and the Galapagos Islands in the Pacific (McBirney and Williams, 1969); all are relatively close to the crests of mid-ocean ridges,Poike and Rano Kau ended eruptive activity with the extrusion of highly silicic differentiates - quartz trachyte for Poike and a peralkali.ne rhyolite for Rano Kau. The geology of Sala y Gomez has been described by Fisher and Norris (1960).It is a small, isolated volcanic islet that marks the subaerial peak of a large seamount.The seamount extends more than 50 km southeast and 30 km southwest of the islet; its extension to the west, north and northeast is not known.The islet itself is saddlebag- shaped, with a maximum length of 700 m east-west and a maximum width of 400 m north-south. No volcanic vents were found on the island, though one may have existed in the embayment on its south side.Three distinct rock units were recognized - two units of mugearitic lava, one red to dark gray and the other dark gray, separated by a calcareous marine sedimentary deposit. 13

EXPERIMENTAL METHODS

r':. i_I Tfl..1_

Two weeks of field work were carried out on Easter Island by Jack Dymond, Donald Heinrichs, and myself during February and March of 1972, We were aided by an excellent geologic map of the island (Fig. 4) prepared by P. E. Baker of the University of Leeds. More than 125 lava flows and pyroclastic centers were sampled for geochemical and paleomagnetic studies.Flows were taken in sequence where possible.Samples were collected for paleomagnetic study in accordance with procedures outlined in Doell and Cox (1965). In addition to hand samples, cores of rock, 10-20 cm long and 2. 54 cm in diameter, were taken using a portable, gasoline-powered core drill with a diamond bit.Use of the drill made it possible to obtain samples from the least-altered portions of lava flows.Additional samples for this study were provided by P. E. Baker. Two dredge hauls from the Easter Island volcanic pile were taken during a 1973 cruise of the Hawaiian Institute of Geophysics R/V Kana Keoki.The samples were dredged from depths of 1100 and 1800 meters. Dredge rocks from Sala y Gomez are from a dredge haul taken during the YALOC-74 cruise of the Oregon State University R/V

Yaquina,The single subaerial sample was obtained from C. E. Engel 14 of the 15.S. Geological Survey and R. L. Fisher of the Scripps Institution of Oceanography.

K-Ar Analyses

Potassium abundances were determined by atomic absorption using techniques described in the next section.The precision of the analyses was less than Z%. Argon measurements were made by isotope dilution mass spectrometry.The rock sample is fused by induction heating in a high vacuum pyrex and metal extraction line.Pre-fusion pressures within the extraction system are commonlyio8tolO9torr.A tracer of Ar38is added to the system from a bulb-type reservoir (Bieri and Koide, 1967) before fusion.Evolved gases are purified over a series of hot titanium getters.The isotopic composition of the argon is determined using a Reynolds-type mass spectrometer (Reynolds, 1956) operated in the static mode, and equipped with an ion multiplier.Digital recording of the mass spectrometer signals on paper tape facilitated data processing, permitting utilization of sophisticated techniques for improving the signal-to-noise ratio and the precision of the analyses (Standley, 1972). Argon determinations were made on whole rock, crushed whole rock and mineral separate samples.The absolute abundances of atmospheric argon were commonly very low; however, the young ages 15 of the rocks and low potassium concentrations led also to low per- centages of radiogenic argon.The precision values assigned to each age determination are estimates of the standard deviation of analytical precision using the procedure outlined by Cox and Dalryniple (1967).

Chemistry

Chemical analyses for major element abundances were obtained by atomic absorption spectrophotometry (AAS) and X-ray flourescence (XRF). Rock powders were split into appropriate sample sizes (generally about 50 milligrams) using an aluminum microsplitter. For AAS analyses the samples were dissolved in Teflon-lined

pressure bombs using HF and aqua regia, and heated to 110°C fori4 hours (Buckley and Cranston, 1970),Excess HF was neutralized with boric acid (Bernas, 1968).CsCl was added to both standards and un- knowns to supressionization effects caused by the presence of Na and K in the samples.Fe, Mn, Na and K were analyzed using an air- acetylene flame; a hotter nitrous oxide-acetylene flame was used for the determination of Si, Al, Mg, Ti, and Ca.Precision for most analyses is 2-5%, except for Ti (5-10%) and Mn (l0%). Analyses of USGS standard rocks, AGV-1, W-1, and BCR-1, suggest that no systematic errors are present (Appendix 1). Ti and Mn abundances were determined by XRF when possible, as the precision of this technique was judged to be appreciably better 16 than that of AAS for these two elements. Sample powders for XRF analysis were fused with a Li2B2O7-Li2CO3 fluxin vitreous carbon crucibles. A 2:1 flux-to- sample ratio was found to be the optimum mixture for decreasing matrix effects, while maintaining proper in- tensity of the X-ray photopeaks (John Kummer, personal communication).Analyses were done at the University of Washington under the direction of David Piper; the non-dispersive X-ray spectrometry (NDX) mode of X-ray flourescence analysis was used.The NDX sys- tern utilizes the semi-conductor radiation detector asa transducer; a multi- channel analyzer provides for rapid multi-element analyses.The precision of the Ti and Mn determinations was generally less than 5%.

Crystal Fractionation Calculations

A computerized least- squares petrologic mixing model was used to test the hypothesis that the Easter Island volcanic series is related by fractional crystallization.Possible mineral additions or subtractions necessary to generate a differentiated suite from a parental basalt were calculated by computer using a program written by David Lindstrom, University of Oregon Center for Volcanology.This program was based on the mixing program developed by Bryan etal. (1969).The model suggests the existence of a slowly crystallizing magma chamber, where separation of various mineral phases produces a wide range of residual liquid compositions. 17 These residual liquids may then be erupted as mugearites, benmoreites, etc. Let M £ mass of parent rock, P m mass of product rock, and m mass of phenocryst (or xenocryst) mineral m. Then M equals m + mm. The mass of any oxide n in the parent rock should equal the sum of the masses of that oxide in the constituent phases (residual minerals and product rock). A series of equations can be written, one for each oxide in a standard chemical analysis (eight in this case- Si02, TiOZ, A1203, FeO, MgO, GaO, NaO and K20).These equations will be of the general form: p=M pn m= 1 m n m m p m p m where C and C= . X and X refer to the mM n n weight percent of oxide n in components p (product rock) and m (either parent rock or constituent minerals), respectively.The values of

C andmare known, and values for Cmust be calculated.If pn n ni n m + 1, this series of equations can be solved in such a manner as to minimize the square of the difference between the observed and calculated composition of the product rock.If

N M m )2 E = (CXv - n=1 m=1 m n we can minimize for thekthcomponent by setting the derivative of with respect to Ck equal to zero:

ack -o and,

N M A_ CXm)Xk=0. N (CX- n aCk n=1 m=1 run This gives us a set of M linear equations whose solutions represent the best estimate of the weight fractions of phenocrysts to be added to or subtracted from a parent rock to produce the product rock. Phenocryst compositions were estimated from optical data (Bandy, 1937; Baker etaL, 1974) and from microprobe data (Baker etal., 1974).The Ca:Na:K ratios of plagioclases, the Ca:Mg:Fe ratios of pyroxenes and the Fe:Mg ratios of olivines were fit into existing mineral analyses taken from Deer, Howie and Zussman (1963). As better phenocryst analyses become available for the rocks studied, the differences between the calculated and observed product compositions should decrease. 19

DATA PRESENTATION

K-Ar

Thirty-four subaerial lavas and two submarine dredge rocks from Easter Island were dated by the potassium-argon method; two submarine dredge rocks and one subaerial lava from Sala y Gomez were also dated.Sample locations for the Easter Island rocks appear in Figure 5, and Table 1 presents the results of the 39 K-Ar age determinations from the two islands.The Easter Island samples are grouped according to their association with the island's three major volcanic centers, The results are internally consistent and agree well with the known stratigraphy (Fig. 6),Poike has a volcanic history spanning more than two million years, whereas Rano Kau was active for some 600,000 years, and the large and structurally unique Terevaka for about 400, 000 years (Fig. 7).The minimal data for Sala y Gomez suggest that it was active at the same time as Poike (Fig. 7). The oldest calculated age was obtained from the base of the north cliff of Poike (Figs. 5 and 6).EC-307, a highly feldspar-phyric basalt, has an age of 2, 5 ± 0.3 m. y.Baker etal. (1974) report an age of 3 m. y. for a sample collected at the foot of this cliff.Ages range up-section to .36 ± 0.2 m.y. for EH-361, the cliff's uppermost flow (Fig. 6).The three conspicuous trachyte domes on the north 17756 Hanga Oteo 17729 EC-241 EC-243 .O0

/ ,.- M. Te Puha roa '---s 55 17731 7 '\ \ 17732 I P Q'/)flM: \ EH-361 EC-3l9 EC-311 I 1/ 1 / EREVAK Ec-322 EC-7 M.Parehe ) / 1 Rqn0 Aroi( ,' EH-122 ... - OMO1ro' 0 H- 0 .. t352.H-35I'\ \._. 0 MotuTautaro £1 o Pl.Ii EH3SbPuAkat1k, "-- H- I EC-201 (3/ - .EH-302 EC206 ROIHQ' ,-- Anamara EC396 --SPOIKE-- _-;___ -- EH-296 ma vaka Kipu 0: Vai tea raku EH-42 0 EH 39 H-88 Ec-224 7PctuuEC-4I3, Kotw EC-3$3

HANGA ROA Retu I EC-190 '. ,---'. \ p --' .J EC- 193 --

1/'-- EC- 176 EC-177 EH-40 M. EH-20 ri KM EC-165 EC-166 EH-27 EC-125 EC-153 0 I 2 3 4 5 EH-29 _--200------/ EIASTER ISLAND CONTOUR INTERVALIOO RANO EH61 KAU OIr4 LAVA FLOW BOUNDARY EH-50 .5 younger EH-54 EH-56 CRATER EH-57 Figure 5. Sample locations. 0 TABLE 1.K-Ar Age Data on Volcanic Rocks From Easter Island and Salay Gomez

40 Ar radiogenic 40 Ar radiogenic Age 100 Sample % K 108ccSTP/g 40Artotal io6yr., Rock Type

POIKE

EC-307 (fids.) 0. 075 0.761 5.6 2.54±.28 Basalt EH-296 0.299 2.294 12,9 1.92.11 Hawaiite EC-383 0.737 5.569 11.6 1.89.11 Hawaiite 17732 0.280 0.993 2.0 0.89 ±.19 Basalt EH-39 0.182 0.546 2.7 0.75±15 Basalt EH-42 0.481 1.141 2.3 0.59 ± 12 Hawaiite EC-311 0.724 1.549 3.6 0.54 ±11 Basalt' EC-319 0.567 0.920 6.,1 0.41 ±.05 Hawaiite EH-361 (fids.) 0.108 0.157 0.7 0.36±.21 Basalt

RANO KAtJ EH-29 (flds.) 0.058 0.220 2.3 0.94±.19 Basalt EH-27 0.535 1.464 3.5 0.68 ±.14 Hawaiite

EH-.Z0 0.283 0. 762 7. 7 0. 67± .06 Hawailte EH-57 0.453 0. 869 5. 7 0. 48 ± 06 Hawaiite N) TABLE 1.Continued

40 40 Ar radiogenic Age Ar radiogenic 100 Sample % K o8 SPIg 40Artotal l0 r; Rock Type

EH-10 3.275 4.277 45. 6 0. 33 ± . 006 Rhyolite

TERE VAKA

EC-370 0.475 0.456 2.9 0. 24 ±.05 Hawaii te EC-125 0. 682 0. 586 2. 7 0. 22 ±. 04 Hawaiite EH-288 1.035 0.874 3.3 0. 21 ± . 04 Hawaiite EC-396 0.596 0.490 2.2 0.21 ± .04 Hawaii te EC-323 0.584 0.449 5.5 0.19 ± .04 Hawaii te EC-384 0.487 0.343 2.5 0.18 ± .04 Hawaiite EC-177 0. 534 0. 358 2.4 0.17 ± 03 Hawàiite EC-190 0.976 0.636 2.7 0. 16 * .03 Hawaiite EH-88 0.551 0.318 0.4 0.14 ± .15 Hawaiite 17756 0.670 0.370 1.0 0. 14 ± .06 Hawaii te EC-241 0. 551 0. 262 0. 9 0.12 ± .05 Hawaiite EC-166 0.728 0.320 1.1 0. 11 ± 04 Hawaiite EC-206 0.537 0.232 4.0 0.11 ±.0Z Ba salt TABLE 1.Continued

40 Ar radiogenic 40 Age Ar rathogenicxlOO -8 40 6 Sample %K 10 ccSP/g Ar total J0yr1. Rock Type

EC-413 0.818 0.311 2.2 0.10±.02 Hawaiite EC-153 0.873 0.322 2.1 0.09±.02 Hawaiite EC-224 0. 722 0. 012 0.04 0. 004 05 Hawaiite EC-ZOl 0. 781 -- -- 0± .05 Hawaiite EH-122 (fids.) 0.140 -- -- 0±.85 Basalt EH-302 0.546 -- -- 0 e.21 Hawaiite

EASTER ISLAND DREDGE HAUL

KK72-35-1 0.704 0.431 1.4 0. 15±05 Hawaiite KK72-35-1 0.704 0.425 1.3 0.15 ±.05 Hawaiite KK7Z-34-43 0.660 0.354 1.7 0.13 ±.03 Hawaiite

SALA Y GOMEZ Y73-4-30-20 1.405 10.926 21.0 1.94 ±.07 Haw3iite PV-302 2.425 13. 010 22.8 1.34.04 Mugearite Y73-4-30-4 2.418 12. 703 15.9 1.31±.06 Mugearite 24 K-AR AGES AND STRATIGRAPHIC POSITIONS OF LAVAS FROM POIKE AND RANO KAU, EASTER ISLAND

POIKE North Trachyte South

0.36 my

0,54 my'E-II - 0.59 my 0.89myfT3 unconformity 0.75 my 2.54rny/EC-307 EC-383

RANO KAU West East 1cY\ Zi?nyORITO Ef\0.48 my ii

67 my

Figure 6.K-Ar ages and stratigraphic positions of lavas from Poike and Rano Kau, Easter Island. 25 AGE HISTOGRAM FOR MAJOR EASTER ISLAND VOLCANOES AND SALA V GOMEZ

4 SALA Y GOMEZ 2 0 (4 POIKE Ui (I) >- -J z 4 RANO KAU 02 In'0LU 2

I I I I I 0 0.5 1,0 1.5 2.0 2.5 3.0 AGE IN MILLIONS OF YEARS Figure 7. Age histogram for major Easter Island volcanoes and Sala y Gomez. 26 slope of Poike (Fig. 4) cut the flows composing the cliff and are thus younger than 0.36 m.y. Anejecta block (EH-296) from Maunga Anamarama (Fig. 4) gave a K-Ar age of 1.90.1 m.y.; because of its age it appears to be associated with the Poike volcano.This result suggests that Poike once extended at least 4 km further west; marine erosion has since obliterated the true western extent of the Poike volcano.The possibility of excess argonin this sample cannot be precluded, however. A nearly identical age of 1. 89 ± 0. 1 m. y. was obtained from EC-383, a sample collected slightly west of the south Poike cliff. Results from Rano Kau are in exceilent;agreement with the stratigraphy (Fig. 6).EH-29, a basalt collected near sea level from the western cliff, gave an age of. 94 ±0. Z m. y.The rhylolitic obsidian at Maunga Onto, suggested by Bandy (1937) as the youngest extrusive in the Rano Kau stage, has an age of. 33 ± .006 m. y.; Gonzalez-Ferran, Cordani and Halpern (1974) obtained an age of .32 ± .08 m.y. from a piece of this rhyolite.Volcanism on Rano Kau apparently began with basalt extrusion about 900,000 years ago, continued for some 600, 000 years, and ended with the eruption of peralkaline rhyolites.Similar rhyolites occur on the three small islets to the southwest of Rano Kau (Fig. 4). K-Ar age determinations were made on 20 samples from Terevaka. Ages range from. 41 * .08 m. y. for an xenolith from 27 Rano Raraku to essentially zero age (no detectable radiogenic argon). An age of, 24 ± . 05 m.y. was obtained on a hawaiite collected from the oldest series associated with the main Terevaka shield. From field evidence (Baker, 1966) the youngest extrusives on the island appear to be the Roiho olivine basalts (Fig. 4).The age data support this conclusion- EC-201, from the base of the cliff at Motu Ko Hepoko, had no detectable radiogenic argon, and error limits set a maximum age of less than 50, 000 years for this sample. Ages obtained from the Sala y Gomez samples range from 1.94 ± .07 m.y. to 1.31 ± .04 m. y.Samples Y73-4-30-4 and PB-302, the former a submarine dredge sample and the latter a subaerial flow, have nearly identical ages of 1. 3 m. y., and are also chemically similar.

Chemistry

A representative sampling of 40 subaerial lavas from Easter Island, as well as five submarine rocks dredged from the Easter Island volcanic pile, were analyzed for their major element compositions.These analyses are presented in Appendix 2, along with the Barth- Niggli molecular norms, Thornton- Tuttle Differentiation Index (D. I.) and normative plagioclase compositions.Petrographic data for the rocks appears in Appendix 4. Classification criteria for the rocks are given in Table 2.This TABLE 2.Classification of the Easter Island and Sala y Gomez Volcanic Series

BasaltHawaiite Mugearite Bennioreite Trachyte Rhyolite

An NormativeAn + Ab > 50 50-30 30-15 --

Differentiation Index -- 65-75 75-90 >90 29 system is similar to that used in the Baker etal. (1974) study of Easter Island petrology and geochemistry.The more basic rocks were classified by their normative plagioclase content, while rocks more silicic than mugearite were classified according to the Thornton- Tuttle Differentiation Index.The boundaries are somewhat arbitrary, however, as there exists a complete chemical gradation between the various rock types. Detailed major element chemistry of the Easter Island suite has previously been presented by Baker etal, (1974).The Easter Island lavas are of transitional tholeiltic affinities. A wide compositional spectrum exists, with the rocks ranging from basalt, through a rather complete differentiation series, to trachytes and peralkaline rhyolites.

Ba salts

Normative compositions of the basalts and basic hawaiites are plotted in the system Ne-01-An-Hy-Qz in Figure 8.Basaltic rocks from other oceanic islands and the East Pacific Rise are plotted for comparison. Easter Island basalts are hypersthene normative tholeiites, containing minor amounts of either normative quartz or normative olivine.The lavas appear to straddle the plane of silica saturation, marked by the An-Hy tie line, and thus lie in the fields of quartz or olivine thoieiites.There is a slight tendency for basalts from the Terevaka volcano to be olivine normative rather than quartz Ne An Qz

01 8BOUVET ISLAND Hy PREAST PACIFIC RISE s GALAPAGOS ISLANDS THOLEIITES HHAWAIIAN THOLEIITES St H.SAINT HELENA HAS.HAWAIIAN ALKALI BASALTS

Figure 8.Basaltic rocks from Easter Island plotted in thesystem Ne-Ol-An--Hy.-Qz. All analyses from Sala y Gornez are plotted.Data from other oceanic islands and the East Pacific Rise are included for comparison. 0 31 normative; this, however, is dependent on theFe+3/Fe+Zratio and the pattern may not be accurate,(All norms were calculated with the Fe+3/Fe+Zratio standardized at 0. 33.) The Roiho olivine basalts are the latest Terevaka eruptive s and contain 8-10% normative olivine; these rocks comprise a distinctive unit which plots much closer to the critical plane of silica imdersaturation (An-Ol tie line) than other sub-. aerial basaltic rocks from the island0. The basaltic dredge samples are chemically similar to the Terevaka lavas. High iron and low magnesium are notable features of the Easter Island basalts (Fig. 9; Appendix 2).The Terevaka series generally displays the greatest iron enrichment, with total iron as FeO reaching 13-15%. MgO contents average 4-5% in most of the basalts from the three major centers, but reach 7. 4% in the Roiho olivine basalts. Silica is unusually low for tholeiitic basalts, with values as low as 43% reported by Baker etal. (1974) for two Vaitea basalts associated with Terevaka volcanism.Potassium abundances in the basalts range from

19% K20 for a Poilce basalt (EH-39) to.94% K20 for one from the Roiho vicinity (EC-ZOl).The earlier basalts, from Poike and Rano Kau, are generally lower in K20 than the later basalts from Terevaka. Ti02 contents are quite high for tholeiitic basalts, ranging from 2-4%. The basic hawaiites appear to be the most enriched in Ti02, F F F

A A M Figure 9.AFM diagrams for rocks from the major Easter Island volcanic centers and an Easter Island dredge haul.(SK indicates Skaergaard trend.) A=Na O+K 0, F=FeO+O.9Fe 0 ,M=MgO. 2 2 23 33 Differentiated Rocks

The differentiated rocks are quartz normative and become in- creasingly enriched in normative quartz content with progressive degrees of differentiation (Appendix 2),The most differentiated rocks are the rhyolites from Onto, a parasitic center associated with Rano Kau; they contain about 25% quartz in the norm.The presence of minor acmite in the rhyolite norm indicates that these rocks are mildly peralkaline.The most complete differentiated series occurs on Rano Kau (Fig. 9).Intermediate differentiates are lacking on Poike, which ended eruptive activity with the emplacement of three quartz trachyte domes (Fig. 4); the bulk of the Poike volcano consists of interbedded basalts and basic hawaiites. A large portion of the Terevaka shield is composed of intermediate differentiates, and there appears to be no extreme differentiates as are found associated with the other two centers (Fig. 9). The transitional tholeiitic nature of the Easter Island series is illustrated in Figure 10.The analyses cluster about the line used by Macdonald and Katsura (1964) to separate the Hawaiian tholeiitic and alkaline suites.The alkali-silica trends of Ascension, Bouvet, Galapagos, and Easter Islands exhibit a marked similarity.Total alkalis generally increase with silica content up to the extremely differentiated trachytes and rhyolites, which diverge from the L!J

LJ .1

T I /1

I I GALAPAGOS I IA /Ii ISLANDS I / // /

Na208 / A I I/ /1ho! I // K207I 1/ 1' I 1/ Ii/ / / U 0 / 6I // I I f 1/ / 0 II / 0,,I/ I 0,, / 5 04 / ooj oPOIKE / RANO KAU o TEREVAKA 4 7U DREDGE UI.Do /000 ASALA Y GOMEZ 3

2 45 50 55 60 65 70 75 %S102 Figure 10.Total alkalis vs. silica diagram for the Easter Island series. expected pattern.The Poike trachytes have the highest total alkalis of any rocks on the island (9.7%).This diagram suggests slight differences between the products of the three centers; Terevaka intermediate lavas appear slightly more alkaline than corresponding lavas

from the two older centers0

Comparisons with Other Provinces

Easter Island basalts generally display lower abundances of Si02, MgO and CaO compared to oceanic tholeiites from the East Pacific Rise (Engel, Engel and Havens, 1965) and are enriched in Ti02, total iron and total alkalis, although only slightly in the latter. Two of the Easter Island low potassium tholeiites (EH-29 and 17732) are, however, quite similar to oceanic tholeiites, except for their rather high alumina content (19., 5%).Relative to Hawaiian tholeiites (Macdonald and Katsura, 1964; Macdonald, 1968), the Easter Island basalts have low Si02 and MgO, and are enriched in Ti02, total iron and Na20. McBiriiey and Williams (1969) have noted the similarity between the Easter and Galapagos suites, and the new analyses pre-. sented here and in Baker etal. (1974) arein agreement, although the Easter basalts usually have lower MgO, higher Ti02 and even more extreme iron enrichment. 36

Sala y Gomez

The two dated submarine dredge rocks from Sala y Gomez were also analyzed for their major element compositions; an analysis of a subaerial Sala y Gomez lava was previously published in Engel and Engel (1964).These analyses have been plotted in Figures 8 and 10 for comparison with Easter Island, several other oceanic islands, and the East Pacific Rise,Both diagrams illustrate the alkaline and undersaturated nature of the Sala y Gomez suite.All samples are nepheline normative, plotting on the nepheline side of the critical plane of silica undersaturation (01-An tie line) in the system Ne-Ol- An-Hy-Qz. The rocks are classified according to the criteria in Table 2; Y73-4-30-20 is a hawaiite, while Y73-4-30-4 and PV-301 are mugearite s.Normative nepheline contents increase with progressive differentiation.The inferred parent liquid of such a suite is of alkali olivine bas1t composition.The three samples all plot in the alkaline field of the Macdonald and Katsura (1964) alkali-silica classification (Fig. 10). A standard AFM diagram (Fig. 11) shows that the Sala y Gomez rocks are similar to those from other alkaline islands, such as Gough and St. Helena. AFM plots of Easter and Sala y Gomez lavas show a prominent lack of iron enrichment in the alkaline rocks (Figs. 9 and 11).For a given silica content the Sala y Gomez rocks differ from the Easter Island suite in that they are lower in Ti02, F

/ \ £ SALA YGOMEZ / \ G GOUGH ISLAND /, \ S ST HELENA / / \ SK SKAERGAARD / TREND 1/ \ // /1 /1 /1i/I I,II / F ...*.,. \ s' # %% / ,__ .-G A //_ /

M Figure 11 AFM diagram for Sala y Gomez volcanic rocks, (ANa20 + l(O, F = FeO +0 9FeO3 and M MgO) i:i total iron and GaO, and higher in MgO and total alkalis.

Fractional Crystallization Data

Fractional crystallization has long been proposed as the domin-. ant process relating localized igneous suites (Bowen, 1928),I have considered the applicability of a fractional crystallization model for the Easter Island suite by using a computerized petrologic mixing program (Bryan, Fingers and Chayes,1969)to process the rock and phenocryst data.Standard AFM plots of these rocks (Fig.9) plus gradational chemistry suggest that the Easter Island suite has followed a tholeiitic trend of fractional crystallization (Wager and

Brown,1967). The Rano Kau series was chosen for the model because it contains the broadest spectrum of basalts and differentiates (Fig.9), and because the stratigraphic position of the samples is known (Fig.

12).Compositions of samples and phenocrysts used in the model appear in Appendix 3a.Phenocrysts used in the calculations were only those actually observed in the parent and product rocks (Appendix

4).The parental basalt for the Rano Kau suite was chosen for its low stratigraphic position (Fig. 12), and low Si02 and high MgO relative to the differentiated rocks. The results of all crystal fractionation calculations are given in Appendix 3b; those from the Rano Kau series are illustrated graphically (Fig. 13 and Table 3).These data suggest that it is 39

STRATIGRAPHIC POSITION OF SAMPLES USED IN RANO KAU CRYSTAL FRACTIONATION MODEL

Figure 12. Stratigraphic position of samples used in Rano Kau crystal fractionation model. FRACTIONAL CRYSTALLIZATION SEQUENCE FOR ROCKS FROM RANO KAU

EH-6I EH-57 EH-71 EH-56EH-54 EH-50 40 0L +30 DPX PLAG +20

+10 ADDITION

-I

-2

-40 Figure 13. Additions or subtractions of phenocrysts required to generate each rock by fractional crystallization from a parent liquid.The parent liquid for EH-61 is EH-29 (See Appendices Z and 3a).The 1.....A t_. t.,.4.k t'tJ r, 71 t'YTLi nhi__ nJU(. to.iu.it'rri- :,. is xi- UI iJieparenu liquid for each of the other rocks is the rock uiimediately to its left in the diagram (see Table 3) 0 41

TABLE 3.Rano Kau Crystal Fractionation Model - Weight Fraction of Components from Computer Analysis.

EH-61 EH-57 EH-71

EH-29 1.9993 ElI- 61 0.6119 EH-61 3.2570 F070 -.0.0963 F070 0.0485 F070 -0.2286 CPX-1 -0. 1265 CPX-2 0.0843 CPX-2 -0. 5572 AN7O -0,7532 AN5O 0.1938 AN52 -1.2147 MT 0.0109 MT 0. 0095 MT -0. 0705 1LM -.0. 0099 ILM 0,0228 ILM -0. 1890

(residuals)20.786 (residuals)22. 853 (residuals)20. 198 std. error 0.44 std. error 0. 61 std, error 0. 59 (wt. %) (wt. %) (wt. %)

EH-56 EH-54 EH-50

EH-71 1.0391 EH-56 0.9878 EH-54 1.0679 CPX-2 -0.0213 CPX-2 0.0204 CPX-2 -0.0281 AN5O -0.0140Ab62An340r40.0129Ab62An340r4-0,0339 MT 0.0018 MT -0.0030 MT -0.0015 ILM -0.0011 ILM -0.0006 ILM -0.0028

(residuals)20.099 (residuals)20.008 (residuals)20.747 std. error 1,66 std. error 0.13 std. error 0.40 (wt. %) (wt. %) (wt. %) 42 possible to derive the Rano Kau series from a parental tholeiitic basalt (EH-29) by the subtraction or addition of varying proportions of plagioclase, clinopyroxene, olivine, and Fe-Ti oxides.Volumetri- cally, plagioclase is the most important of the phenocryst phases in determining chemical variations.This conclusion is supported by the presence of plagioclase as the dominant phenocryst phase in these rocks (Appendix 4). As might be expected the plagioclase compositions become more sodic with increasing degree of differentiation, and olivine ceases to be an important factor in the more differentiated rocks.In producing EH-57 from EH-61, the data indicate the addition of rather large amounts of olivine, pyroxene and plagioclase. Thin section analysis of EH-57 (Appendix 4) revealed abundant doleritic xenoliths composed of these three minerals.Baker (1974) suggest a hybrid origin for the xenoliths, with a later acid melt disrupting an earlier basic flow and causing some reaction phenomena. Lack of adequate stratigraphy and the absence of a wide spectrum of differentiated rocks on Poike and Terevaka make a similar comprehensive crystal fractionation model for these two centers impossible. Additional calculations were done, however, for a few rocks of known stratigraphy from Poike and Rano Kau.The results (Fig. 14; Table 4) suggest that fractional crystallization was responsible for the chemical variation on these centers also.The variations between samples are not as extreme as in the Rano Kau FRACTIONAL CRYSTALLIZATION ON POIKE AND TEREVAKA (SEE TABLE 4).

a) POIKE (parent liquid is EH-36l) b) TEREVAKA (parent liquid is EC-243)

EH-350 17756 +15 OL +15 Px

+10 PLAG +10

ADDITION +5 +5

Px 0 94 0 OLfl H REMOVAL

PLAG

PLAG -15 -15

Figure 14.Fractional crystallization on Poike and Terevaka (see Table 4). (J) TABLE 4.Fractional Crystallization on Poike and Terevaka

a) POIKE b) TEREVAKA EH-350 17756

EH-361 l.Z217 EC-Z43 l.Z403 AN55 -0.1158 AN7O -0.1184 FO 63 -0. 0053 FO 70 -0. 0317 CPX-1 0.0046 CPX-3 -0.0663 MT -0. 0166 MT 0. 0056 ILM -0. 0058 ILM -0. 0191

(resicluals)Z 3. 334 (residuals)2 1.047

std. error (wt, %)0. 69 std. error (wt. %)0. 37 rocks and, once again, plagioclase removal is the dominant factor in generating the differences,. Lavas from the Hiva-Hiva Series (Gonza1ez-Ferran, 1974), including the Roiho olivine basalts, are considerably more alkaline than other Easter Island rocks (Fig. 8); olivine is their only phenocryst phase (Appendix 4) and occurs in quite substantial amounts (8-10%),It does not appear that these rocks can be related to other lavas from Terevaka by fractional crystallization.The Roiho olivine basalts presently trend toward the nepheline apex in Figure 8, but removal of olivine, their only phenocryst phase, would have an opposite effectthat of driving the normative composition toward the quartz apex. The data become ambiguous and difficult to interpret at the high silica end of the spectrum (trachytes and rhyolites). As a simplifying factor, I chose samples for the calculations on the basis of increasing Differentiation Index (D. I.), rather than on geologic relations. Although the trachytes occur as three domes parasitic on the north slope of Poike, the lack of intermediate differentiates on Poike and the contemporaneity of the trachytes with the Rano Kau rhyolites leads to speculation as to their true association.The calculations I have made were 1) benmoreite (EH-50, Rano Kau) to trachyte (EH-358, Poike), and Z) trachyte (EH-358, Poike) to rhyolite (EH-10, Rano Kau).The results appear in Figure 15 and Table 5.The sum of the POSSiBLE CRYSTAL FRACTIONATIONORIGIN FOR TRACHYTES AND RHYOLITES ON EASTER ISLAND(SEE TABLE 5).

a) Benmoreite (EH-50, RANO b) Trachyte (EH-358, POIKE) is the I(AU) is the parent liquid, parentliquid.

EH-358 EH-IO FA +20 +20 ADDITION +10 E..:JANoRTH

-20 -20 [:1

-30 -30 Li ANORTH -40 -40 Figure 15. Possible crystal fractionation origin for trachytes and rhyolites on Easter Island (see Table 5). C' TABLE 5.Possible Fractional Crystallization Origin for Silicic Differentiateson Easter Island

a) EH-358 (trach.yte, Poike) b) EH-l0 (rhyolite, Rano Kau) EH-50 1.5838 E}i-358 1.7466 AN 50 -0. 2667 FA -0. 0103 FA -0.2714 ANORTH -0. 6180 CPX.-2 -0.1282 MT -0. 0343 MT 0.1237 I LM -0. 0093 ILM -0.0332

(residuals)2 1.396 (residuals)2 10. 770

std. error (wt.%) 2. 54 std. error (wt. %)8. 30 squares for the first calculation was reasonably low, suggesting the possibility that the benmoreite and trachyte may be related by fractional crystallization.The results of the second calculation were not quite as good and show a particularly poor fit with respect to A1203, NaO and KO. This suggests that either the two rocks are not related by fractional crystallization, or that they may be related by this process, but the anorthoclase phenocryst composition is in con- siderab].e error. In considering the applicability of these fractional crystallization data to the processes that actually took place within the volcanic edifice, it must be appreciated that the results only suggest and do not prove that fractional crystallization was responsible for the observed chemicalvariability.Data of this type need supportive evidence that trace elements can provide.Trace element abundances in the rocks and mineral/liquid distribution coefficients should be used in conjunction with the model I have presented to predict trace element concentrations in residual liquids. Agreement between calculated and observed trace element abundances in the differentiated rocks would be strong evidence for the occurrence of fractional crystallization in the Easter Island rocks. DISCUSSION

Chronology of Volcanism on Easter and Sala y Gomez Islands

The geology, petrology and geochemistry of Easter Island were presented elsewhere (Baker, 1966; Baker etal., 1974; Gonzalez- Ferran and Baker, 1974); however, only 11 age determinations of the island have been published (Gonzalez-Ferran, Cordani and Halpern,

1974).These studies pointed to several problems which could be resolved through extensive application of the K-Ar dating method: 1) absolute age and timing of successive episodes of volcanism on Easter Island. 2) relative ages of the Poike and Rano Kau volcanoes.Field evidence suggests that both are older than Terevaka, but gives no clue as to their relative ages (Gonzalez- Ferran and Baker, 1974). 3) age relations of the compositionally diverse, silicic differentiates of Poike and Rano Kau. 4) age of Terevaka volcanism and subsequent history. K-Ar age data for Easter Islandwere presented in Table 1 and Figures 6 and 7.The data suggest that Easter Island volcanism was more complex than is specified by the range in ages alone.Figure 1 6 brings together available age data into a concise model for Easter Island volcanism. 50

CHRONOLOGY OF EASTER ISLAND VOLCANISM

POIKE 2.5 m.y. basalt

POIKE STAGE I-basalt and hawalite eruptions

1.9 m.y howalite Quiescence and Marine Erosion

P0 IKE RANOKAU RANO KAU VOLCANISM- eruption of .94 my. basalt strongly differentiated suite .89m bosaIt (basalt- hawailte- mugearite- basalt bsnmoreite-Na- rhyolite) hawalite POIKE STAGE U-basalts and hawafites howoHte plus large amounts ol pyroclastic H0W 3 trachyte domes in .m.Y.-y-bas.--? mat,rlol; .33m.y. rhyoIits final stage. trahyte nwrju- w-.ru-u-u-u-Mar,neErosion TEREVAKA TEREVAKA VOLCANISM- basalts, .25 my. hawaUtes, mugearites; youngest eruptives are basic Roiho ollvine <5 my. basa Its.

Figure 1 6.Chronology of Easter Island volcanism. 51 The results imply a two episode constructional history for Poike

volcanism.Subaerial eruptions began 2. 5 to3m. y. ago and continued until approximately 1. 9 m.y. ago.This was followed by a period of quiescence and marine erosion of 1, 0 to 1. 6 m.y. duration. Re- newed volcanism began about 900, 000 years ago and persisted for another 500, 000 years.The 36±0. 2 m, y. age obtained from the uppermost flow on the north Poike cliff probably represents the age of the waning stages of Poike volcanism,This interpretation is supported by BakerT s observation of a marked increase in the abun-. dance of pyroclastic material from the top of the cliff up to the central crater of Pu Akatiki (Baker, 1966). Eruptive activity on Rano Kau was essentially contemporaneous with the second stage of Poike volcanism.Both Eh-29, a basalt from the base of Rano Kau, and 17732, the oldest dated basalt from Poike stage U, yield ages. near 900, 000 years (.94±2 and .89±2 m.y., respectively).The similarity of these rocks with respect to petrography (Appendix 4), chemistry (Appendix 2), and age (Fig. 6, Table 1), suggests that they may have initially tapped the same magma source.Basalt flows on Rano Jau, however, are few and occur only near its base; the bulk of that volcano consists of differentiated hawaiites, mugearites, and benmoreites, with the degree of differentiation increasing progressively up-section.On the other hand, most of Poike is built from interbedded flows of basalt and basic 52 hawaiite; three extremely differentiated trachyte domes occur on its north slope (Fig. 4), but there is a distinct lack of intermediate differentiates in the volcano (Fig. 9).The results suggest that the following events took place during the history of Poike and Rano Kau: 1) Rano Kau and Poike stage II volcanism began about 900, 000 years ago, with both volcanoes tapping the same magma source. 2) Rano Kau was somehow sealed off from the primary magma conduit and continued to fractionate as an essentially closed system for about 600,000 years.(The result of this efficient fractionation was a rather complete differentiation series ranging from basalt to rhyolite.) 3) Poike continued to erupt and be replenished with basaltic magma from the main source for approximately the same lengthof time (500, 000 - 600,000 years). Bandy (1937) suggested on the basis of 'clear field evidence' and petrography that the Poike trachytes (incorrectly referred to as olivine dacite tuff) and the rhyolitic obsidians of Rano Kau were con- teinporaneous. He further implied that Maunga Parehe and Rano Kau tapped essentially the same magma source. Baker (1966) and Baker etal. (1974) rejected this interpretation on grounds that the two rocks are compositionally distinct and there is no field evidence for equating them. However, several points favor Bandy's interpretation. As Baker (1966) has noted, all silicic rocks on the island, both the Poike 53 trachytes and the Rano Kau rhyolites, lie along northeasterly trending fractures,Field evidence also shows both types to be the youngest extrusives of their respective volcanoes.Finally, calculated ages are compatible with this conclusion - the trachytes appear to be younger than .36 m.y.,, while the rhyolite has an age of32 m.y. Terevaka is the largest and most complex structurally of the three Easter Island volcanoes,Over 70 subsidiary eruptive centers are associated with it (Baker, 1966).Two major vent alignments are apparent on Terevaka - one runs almost due north, passing from Otuu, through the Terevaka summit, to Hanga Oteo, and the other trends east-southeast through the summit of Terevaka and includes M. Okoro, M. Pui, M. Anamarama, and Rano Raraku (Fig. 4). Terevaka flows abut against the western cliff of Poike and the northern section of Rano Kau; along with the K-Ar data, this evidence clearly indicates that Terevaka is younger than Poike and Rano Kau. Several lines of evidence indicate that thearliest subaerial volcanism on Terevaka was associated with the east- southeast fissure extending from the Terevaka summit through Rano Raraku. The petrography suggests that the Rano Raraku tuff is the only rock type on the island that is partially submarine in origin (Baker, 1966; J. G. Moore, personal communication; Appendi.x4).Secondly, the wave-eroded cliff on the southeast face of Rano Raraku and a similar cliff on the west side of Poike are the only such inland features on the 54 island; the out-pourings of Terevaka lava forced the retreat of the sea which once surrounded Poike, Rano Raraku, and probably Terevaka. Terevaka lavas can be seen to abut against Rano Raraku and the western Poike cliff.This indicates that Rano Raraku is older than other Terevaka stage features. However, marine erosion may have

obliterated other early features of this stage.The age of. 41 ±01 rn y.; obtained from a Rano Raraku xenolith, suggests that the xenoliths may be related to the onset of Terevaka volcanism; this sample was significantly older than other ages obtained from Terevaka lavas.Stratigraphically, the attitude of the flow from which the xeno].iths were derived would have been beneath coincident rocks of the same age from Poike or Rano Kau.Still, one cannot preclude the possibility that the xenolith has lost racliogenic argon, and thus belongs to one of the earlier volcanic episodes.Finally, lavas associated with these eastern centers yielded the oldest calculated ages for Terevaka, suggesting that the earliest volcanism was related to the ESE fracture.Both the Easter Island transform fault (Sykes,- 1967) and a zone of higher topography extending from it trend east- southeast (Fig. 3).Easter Island sits on this zone of higher topography and it is possible that the east-southeast trending Terevaka fissure reflects the regional substructure of the oceanic crust.There is no conspicuous explanation, however, for either the north trending Terevaka fissure or the northeast alignment of silicic differentiates. 55 Volcanic Production Rates

Oceanic islands are regions which result from anomalously high igneous productivity.I have calculated volcanic production rates for the entire subaerial portion of Easter Island and for each of its main volcanoes (Table 6),If eruptive activity spanned 2. 5 m. y. on Easter Island, then the average production rate is approximately 1x 1043 m yr1 This is somewhat misleading, as the most copious volcanism occurred during the Terevaka stage- 5.4 x 1043-1 m yr, ascompared to 0. 6 - 0.7 x 1O4m3yr for Poike and Rano Kau.These figures are three orders of magnitude less than estimates from Hawaii (40xio6 3 -1 m yr) and Iceland (30 x 106 m3 yr-1), both considered to be products of hot spot volcanism (Decker, 1971; Jackson et al.,1972; Schilling, 1973a, b), but compare favorably to estimates from St. Paul Island in the Indian Ocean (104m3yr) (Watkins etal., 1975).

Sala y Gomez

In spite of our limited sampling, the small size of Salay Gomez and the presence of a singular subaerial lava type (mugearite) suggest that the 1.3 m.y. age (PV-302) represents theaverage age of subaerial volcanism for the island.Because dredge sampling lacks stratigraphic control, I can only consider the older dredged rock (1.9 m.y. old) to represent a minimum age for the inception of volcanism TABLE 6,Volumes and Rates of Volcanic Production for Easter Island

a) Volumes P oike 3.6x1043 m Rano Kau 4.4x1093 m Terevaka 18,8x1093 m

b) Volcanic Production Rates Poike (Stage II, appx.) O.6x104 m3 yr-1 Rano Kau O.7x104 m3 yr-1 Terevaka 5.4x104 m3 yr-1

c) Production Rates from Other Provinces Iceland 30 x 106 m3 yr-1 Hawaii 40 x 106 m3 yr-1 Mid-Ocean Ridges 5. 6 x m3yr (avg.) 100 km section of mid-ocean ridge3.3 x 106 m3 yr-1

Ui C' on Sala y Gomez.The limited age data donot indicate a migration of volcanism through time along the Salay Gomez Ridge.Lavas from Sala y Gomez and Poike (Easter Island) are essentially contemporaneous.Thus, one of the major criteria for hot spot volcanism, that of volcanic migration along a chain through time, is not fulfilled.

Major Element Chemistry of Easter Island and Sala y Gomez

Two chemical features of the Easter Island and Sala y Gomez volcanic series which must be explained are: 1) The chemical distinctiveness of lava types from the two islands.Easter Island constitutes a tholeiitic lava suite, while Sala y Gomez lavas belong to the alkali olivine basalt series (Fig. 8). 2) The apparent trend of the Easter Island tholeiitic basalts toward more alkaline compositions with time (Fig. 8). The composition of a basaltic magma derived from a peridotitic mantle source is dependent on 1) the degree of partial fusion of mantle peridotite, 2) the pressure (depth) of partial fusion and subsequent magma segregation, and 3) crystallization differentiation during movement of the liquid to the surface (Yoder and Tilley, 1962; OtHara, 1965, 1968; Green and Ringwood, 1967).Experimental studies (Kuno, 1959; Kushiro, 1965, 1968; O'Hara, 1965, 1968; Green and Ringwood, 1967) have suggested that tholeiitic and alkaline magma types are genetically unrelated.Kushiro (1965) has demonstrated that the composition of magmatic liquids produced by partial fusion of a pen- dotite mantle is related to the lithostatic pressure at which the fusioi takes place.His experiments indicate that increasing pressure will drive the resulting melt toward progressively more silica-poorcorn- positions.Green and Ringwood (1967) suggest that the character of erupted liquids is determined by the pressure (depth) at which the magma separates from its crystalline residuum, and that magma segregation at increasing pressures will also produce progressively more under saturated liquids.In particular, tholeiitic liquids are thought to be derived by extensive partial melting (20-40%) and subsequent magma segregation at depths shallower than 35 1cm, while alkaline liquids are products of limited partial melting (<15%) and magma segregation at depths of 35-70 km (Green and Ringwood, 1967). Engel and Engel (1964) originally proposed from field associations that the transition from tholeiitic to alkaline liquidswas the result of shallow- level fractional crystallization.Experimental studies have shown, however, that low pressure crystal fractionation froma tholeiitic parent liquid will not lead to undersaturated compositions (Yoder and Tilley, 1962).Green and Ringwood (1967) point to orthopyroxene fractionation from an olivine tholeiitic liquid at depths near 60 km as a means of producingmagmas of alkali olivine basalt corn- position, but O'Hara (1970) has noted that fractionation of more than - 59 30% orthopyroxene will not preserve basic igneous chemistry and con- sequent orthopyroxene cumulates are not in evidence0Trace element abundance studies (Gast, 1968) also suggest that tholeiitic and alkaline magmas are probably not related by fractional crystallization0Most workers conclude that tholeiitic and alkaline basalts represent two distinct magma types, and that alkaline basalts characterize a deeper source area, a lesser degree of partial melting in the mantle, and a greater depth of magma segregation (Kuno, 1959; Dickinson and Hatherton, 1967; Gast, 1968; Kushiro, 1968). Basaltic lavas from Easter Island are tholeiitic.Except for the latest extrusives (Roiho olivine basalts), they are almost exactly saturated with silica (Fig. 8).Analyzed rocks from Sala y Gomez do not fall in the basaltic range (Table Z); they are riepheline normative and appear to have been differentiated from a liquid of alkali olivine basalt composition (Fig. 8).The control exerted on magma composition by lithostatic pressure can probably explain these fundamental differences in magma chemistry.Thus, the tholeiite and quartz tholeiite character of the Easter Island basalts may result from magma segregation at relatively low pressure (shallow depth), less than 5 kb (15 kin) according to Green (1971).The parental basalt of the Sala y Gomez series (alkali olivine basalt), however, represents melting and magma segregation at much greater depths, probably in the range of 35-70 km (11-21 kb) (Green and Ringwood, 1967), McBirney and Gass (1967) have demonstrated that tholeiitic oceanic islands, especially those trending toward heavily quartz normative differentiates (e. g. Easter, I3ouvet and the Galapagos Islands), are commonly situated on or near the crests of mid-ocean rises; alkaline islands (e. g. Sala y Gomez, Gough and St. Helena Islands), however, tend to be located far down the flanks of such rises or in areas far removed from ocean rise influence.The McBirney and Gass (1967) model for island volcanism relates the composition of oceanic islands to their distance from mid-ocean rise crests and the observed oceanic heat flow distribution.Thus, the high thermal. gradient associated with hot, upwelling asthenosphere material at mid-ocean ridge crests suggests shallow melting and the production of tholeiitic magmas in that vicinity.In regions of low thermal gradient (away from mid-ocean rise crests) melting occurs at a deeper level (higher pressure) and resulting liquids would be under saturated and alkaline.While this simple model fits quite well for islands in the Pacific Basin, less well for those in the Atlantic, it is in need of refinement. The heat flow values alluded to in the McBirney and Gass (1967) model have been shown to diminish with increasing age of the ocean floor (Sclater and Francheteau,1970).Oxburgh (1971) has interpreted this to indicate a thickening of the oceanic lithosphere away from the ridge axis, the rate of thickening being greatest in the first 61 few hundred kilometers and thereafter becoming less rapid. From an analysis of Rayleigh wave dispersion data, Leeds etal. (1974) reach essentially the same conclusion. More meaningful from a petrologists point of view is the model presented by Bass (1971). He proposed that the base of the lithosphere represents the maximum effective depth to which open, brittle fractures can extend; this depth also reflects the maximum depth from which significant volumes of magma can be tapped (depth of magma segregation).Bass (1971) and Scheidegger (1972) have both inferred that magma segregation occurs at shallower depths beneath fast spreading rises than beneath slow spreading rises, as extruded basalts from the East Pacific Rise (fast spreading) are quartz and strongly hypersthene normative tholeiites, while Mid- Atlantic Ridge (slow spreading) basalts are more olivine normative. This suggests that the higher thermal gradient beneath fast spreading rises effectively reduces the maximum depth of brittle fracture, and thus the maximum depth of magma segregation.Extending this rea- soning away from ridge crests, a spreading and cooling lithosphere which systematically thickens at the expense of the asthenosphere should propagate open, brittle fractures to progressively greater depths.The most common model for basaltic magma production in- volves the diapiric upwelling and decompression melting of asthenosphere material (Green and Ringwood, 1967; Wyllie, 1971).If the basaltic liquids produced were then segregated at the maximum depth 6Z of brittle fracture, their compositions should become progressively more undersaturated with distance from the rise crest.Oxburgh and Turcotte (1968) have derived a thermal model which defines a zone of partial melting in a convecting mantle (Fig. 17).This zone repre-. sents the difference between their predicted model temperature and the olivine tholeiite fusion temperature.The depth to the fusion zone increases and the maximum excess temperatures (increment above the fusion temperature) decrease with distance from the axis of the ascending column; the result should be a decrease in the degree of partial melting with depth to the fusion zone.Again, liquids produced will be progressively under saturated with distance from a ridge crest. Data from Easter Island and Sala y Gomez are entirely consistent with such a petrogenetic scheme.The Easter Island tholeiites are indicative of an origin on or near the East Pacific Rise, while the alkaline Sala y Gomez series probably formed far down the flanks of the rise. The production of Easter and Sala y Gomez lavas by these processes is illustrated in Figure 18, An important implication of this model is that the volcanic rocks composing the two islands were not generated from a common source.

Temporal Variations on Easter Island

It is possible that the chemical composition of oceanic island eruptive products will reflect the tectonic history of the island as it - -______

TOP OF THE ASCENDING LIMB; HEAVY LINES INDICATEZONE OF FUSION AND ZONES WITHIN IT WHERE THE PREDICTED TEMPERATUREEXCEEDS THE OLIVINE THOLEIITE FUSION TEMPERATURE BY 100, 2000C, ETC. LIGHTER LINES INDICATE PARTICLE PATHS. FROM OXBURGH & TURCOTTE0968) Figure 17.Mid-ocean ridge thermal structure. 0' ri

FACTORS GOVERNING COMPOSITIONAL DIFFERENCESBETWEEN EASTER ISLAND AND SALA Y GOMEZ

EAST PACIFIC RISE EASTER ISLAND SALA V GOMEZ THOL.EIITIC VOLCANISM ALKALINE VOLCANISM

''-RISIP1GDIAPIRS

b)DECOMPRESSION MELTING (ADAPTED FROM GREEN AND R1NGWOOD, 1967)

GEOTHERM

f SOURCE EASTER ISLAND T SOURCE

PARTIAL MELTING BEGINS AT F MAGMA IS SEGREGATED AT M Figure 18.Factors governing compositional differences between Easter Island and Sala y Gomez. 65 moves away from a spreading center or melting spot0For example, the trend of Hawaiian basalts from tholeiitic to alkaline with time has been explained as the result of movement of the islands off a melting spot, and subsequent downward migration of the zone of fusion (Wilson, 1963; McBirney, 1967; Green, 1971).Varying degrees of Hawaiian-S type volcanic development have been observed on oceanic islands such as Reunion (Upton and Wadsworth, 1972) and seamounts such as Cobb Seamount (Scheidegger, 1972). The span of subaerial eruptive activity on Easter Island was about 2. 5 million years. Assuming a constant spreading rate of 9 cm yr (Herron, l972a), the island should have moved a distance of 225 km during this time.This movement may be reflected in the changing magma chemistry of the islands s eruptive centers.The alkalis vs. silica plot (Fig, 10) suggests slight differences between the eruptive products of Terevaka and those of the two older centers (Poike and Rano Kau).The intermediate members of the Rano Kau series appear slightly less alkalic than their Terevaka equivalents. Any differences at the basaltic end are vague, but McBirney and Williams (1969) point to the fortunate tendency for igneous magrnas to differentiate toward end products that magnify chemical characteristics only subtly revealed in their parent basalts,The youngest effusive on the island is the Roiho olivine basalt (EC-207), which also has the most alkaline affinities of any rocks on the island,The Roiho basalts are far higher in MgO and marginally higher in K20 than other Easter Island basalts (Appendix 2); these rocks plot much closer to the critical plane of silica undersaturation than older basalts from the island (Fig. 8).The trend of the Terevaka differentiates (Fig. 10) and the composition of the Roiho basalts demonstrate that the Terevaka stage volcanics posses more alkaline affinities than those from the other centers.It is unlikely that such differences would be generated by fractional crystallization (see above). Removal of observed phenocrysts (olivine) would drive resulting compositions toward increased silica saturation (next section).In view of the previous discussion concerning the pressure control of basalt magma chemistry it seems possible that the eruption of more under saturated magmas in the final stage of activity on Easter Island is related to movement away from the rise crest and subsequent deepening of either the depth of magma segregation or the zone of melting. Such an interpretation is consistent with models of island and ridge volcanism presented by Aumento (1967), McBirney (1967), McBirney and Gass (1967), Harris (1969) and Bass (1971).

Comparison with Hot Spot Volcanism

In these days of "plume consciousness" it seems appropriate to compare the compositions of basalts from Easter Island and Sala y Gomez to supposed plume-derived basalts.The concept of a plume 67 or melting spot source for an island or seamount chain implicitly requires that all products of that source will have a similar chemical character and style of volcanism imparted to them. Well-documented examples of this are the Hawaiian chain (Macdonald, 1968; Jackson, Silver and Dalrymple, 1972), the Marquesas Islands (Duncan and McDougall, 1974; Bishop, Woolley and Din, 1973), and the Iceland- Faeroes basalts (Schilling and Noe-Nygaard, 1975).Vogt and Johnson (1973) suggest that high iron and titanium abundances are characteristic of plume-derived ocean crust.Schilling (1973a, b, c) and Hart, Schulling and Powell (1973) have listed a large group of chemical and petrographic parameters which characterize plume-derived basalts from abyssal tholeiites; among these are:1) high pyroxene to plagioclase phenocryst ratios, 2) highs FeO/MgO ratios, 3) high concentrations of Ti02, K20, P205 and total iron, 4) high values ofSr87/ Sr, and 5) high La/Sm enrichment factors. Neither Easter Island nor Sala y Gomez lavas possess abundant pyroxene phenocrysts; the Easter basalts are heavily plagioclase phyric, with few if any pyroxene phenocrysts (Appendix 4), and the Sala y Gomez lavas are nearly aphyric.While the Easter Island series does possess extreme iron enrichment in some of the basalts and basic hawaiites (Fig. 9), early basalts associated with Rano Kau and Poike stage II (EH-29 and 17732) have total iron abundances of 8-9%, which are similar to typical abyssal tholeiites.It should be emphasized that the trend to iron enrichment displayed on Figure 9 is a normal consequence of fractional crystallization from a tholeiltic parent liquid (Wager and Brown, 1967). FeO/MgO ratios of the Sala y Gomez lavas suggest little iron enrichment relative to MgO in their initial liquid (Fig. 11).Basalts from the relatively young Terevaka stage of Easter Island volcanism do have high Ti02 and KO. I have shown, however, that Terevaka lavas have more alkaline affinities than other Easter Island rocks, suggesting a slightly deeper source and depth of magma segregation. MacGregor (1965) has shown that Ti02 abundances in magmatic liquids are sensitive to pressure; thus the higher Ti02 in the Terevaka lavas probably reflects their greater depths of origin and equilibration.Y73-4-30-2O, from Sala y Gomez, also has high TiC2, again reflecting its greater depth of origin.While the Easter Island and Sala y Gomez lavas possess some of the characteristics which Schilling (1973a, b, c) attributes to a plume source (e. g. high total iron and TIC2), such a source seems unnecessary to explain their petrologic and chemical character. More importantly, the data strongly suggestseparate sources for the Easter Island and Sala y Gomez volcanic rocks, thus precluding a plume relationship for the two islands.

Genetic Relations of the Easter Island Suite

Rock-type abundances and major element chemical variations of oceanic island rocks have often been interpreted as the result of simple crystal fractionation.The chemical variability of lava suites from Ascension (Daly,1925),Trinidade (Almeida,1961),St. Helena

(Baker,1969),and Gough (Le Maitre,1962;Zielinski and Frey,

1970) Islands in the Atlantic and Tahiti (McBirney and Aoki,1968), Socorro (Bryan, 1970) and the Galapagos Islands (McBirney and Williams, 1969) in the Pacific has been ascribed to fractional crystallization,Trace element data (Frey etaL,1968)and isotope data

(GastetaL,1964;Oversby and Gast, 1970), however, suggest serious problems in attributing all of the chemical diversity in oceanic island suites to this process, particularly the more silicic members. Cumulate ejecta blocks, usually considered strong evidence for crystal fractionation, have not been observed on Easter Island; the dolerj.tjc xenoliths found in some of the Rano Kau differentiated rocks do not possess a cumulate texture.Baker etal, (1974) have noted that phenocryst compositions, gradational chemical variations, and volume relations make fractional crystallization the most likely pro-. cess relating the basalts and differentiated rocks. Additional evidence lies in my own observation that the percentage of phenocrysts in the rocks declines with progressive degrees of differentiation. While aphyric basalts occur (north Poike cliff), most of the basalts are heavily plagioclase phyric (Appendix 4); the differentiated rocks, especially the mugearites and benmoreites, tend to be nearly aphyric (Appendix 4).The petrologic mixing calculations lend semi-quantita- tive support to the hypothesis that fractional crystallization is responsible for the observed chemical variability in most of the Easter Island rocks (the exceptions are the Roiho olivine basalts and possibly the trachytes and rhyolites).The Easter Island basalts and differentiates are probably related via fractional crystallization involving the removal of substantial plagioclase and lesser amounts of olivine, clinopyroxene and Fe-Ti oxide minerals.The basaltic parent liquid used in the calculations (EH-Z9) does not possess the high iron and titanium abundances characteristic of plume-derived ocean crust (Vogt and Johnson, l973),The calculations strongly suggest that iron and titanium enrichment in the Easter Island suite is due to the process of fractional crystallization and not to deep- seated differences in source rock chemistry or melting conditions as would be predicted by the plume hypothesis (Morgan, 1972a, b; Schilling, 1973a, b, c). The only real ambiguity in the data lies in the origin of the trachytes and rhyolites.The age data (Table 1; Fig. 16) and the apparent alignment of all silicic rocks on the island along en echelon northeast trending fractures suggest that the trachytes and peralkaline rhyolites are genetically related. My data point to the possibility that the trachytes (Poike) were derived from a liquid of benmoreite composition by fractional crystallization involving removal of plagioclase, fayalitic olivine, clinopyroxene and magnetite (Table 5; Fig. 71 15).Although the trachytes (Poike) and benmoreites (Rano Kau) are associated with different centers, the age and structural relations provide support for this interpretation. More difficulty is encounter- ed in interpreting the trachyte to rhyolite data; the fit of the data is particularly poor with respect to A1203, Na20 and K20.It is possible that the anorthoclase phenocryst compositions are inerror, but it is equally possible that the two rocks may not be related in a simple manner. Sr-isotopic data for basalts and rhyolites from Easter Island was presented by Baker etal. (1974) and Gonzalez-Ferran, Cordani and Halpern (1974).Sr87!Sr86ratios of .7024 to .7035 in the basaltic rocks and 7050 in the rhyolites suggest that the two may not have been derived from a common source and seem to preclude crystal fractionation as a geneticlink,This problem is not unique to Easter Island among oceanic island suites.Gastetal. (1964) and Oversby and Gast (1970) have reported Sr and Pb isotope data from Tristan da Cunha, St. Helena, and Gough Islands which support similar conclu-. sions for the basalts and silicic differentiates on those islands, again suggesting different sources for the two rock types.Trachytes also appear to have higher Sr87/Sr86values than basalts in lavas from Ascension Island (Baker, 1973). Bailey and Macdonald (1970) have sugge sted that the Easter Island commendites (mildly peralkaline obsidian) cannot be products 72 of alkali feldspar fractionation from a trachytic liquid, but might instead be derived from partial melting of metaluminous trachyte,This is difficult to envision in an oceanic environment, but the fact that Easter Island experienced another basaltic stage subsequent to trachyte and rhyolite extrusion may provide a clue as to the mechanism for such partial melting. Holmes (1931) proposed a scheme whereby basaltic magma might invade a body of granitic composition. The basaltic magma, through convective heat transfer, melts a portion of the granitic region and forms a cupola at the upper region of contact.Fractures may then tap both bas3ltic and granitic liquids. Age data (Table 1; Fig. 16) suggests that the trachytes, rhyolites and at least some of the Terevaka lavas were essentially contemporaneous.Perhaps Terevaka magma invaded a trachytic body at depth, giving rise to partial melting and extrusion of the rhyolite.This is an admittedly ad hoc hypothesis, and still does not explain the significant differences in Sr-isotope ratios. However, dependent on how long the trachytic body was segregated at depth, a higherSr87ISr86 ratio would be expected because of the higher Rb/Sr ratio in the trachytes as opposed to the basalt.Rb/Sr ratios in Easter Island trachytes are about 2, while those in the basalts are near 0. 01 (Baker etaL,, 1974); given the youthful age of the rocks, the difference in Rb/Sr ratios is not judged sufficient to generate the observed difference in Sr87/Sr86ratjosAdditional data is needed before 73 this problem can be resolved.

Origin of the Sala y Gomez Ridge

I shall consider the origin of the Sala y Gomez Ridge in view of all available geochemical and geophysical data.

Tectonic Settin

The past 10-30 million years has seen the Pacific Basin under- go major changes in plate configuration (Sciater etal., 1971; Herron, l972a).Spreading along the present East Pacific Rise began approximately 50-65 m. y. ago at 55Os,just north of the Eltanin Fracture Zone.The spreading grew northward, reaching 35°S about 20 m.y. ago (Herron, l972a).Prior to 10 m.y. ago the north-northwest trending Galapagos Rise (Menardetal., 1964) was active east of the present East Pacific Rise; Herron (1972a) has identified the crest of this fossil rise between 10°S and 30°S.In the area of interest ('24°S to 30°S) anomaly 5 sits atop t1ie Galapagos Rise crest; to the west of the fossil rise anomaly 6 appears twice, on crust generated by both the East Pacific and Galapagos Rises.The relationships of this region led Herron (1972a) to suggest that spreading was initiated on the East Pacific Rise about 20 in.y. ago, and that both spreading centers were active untIl 9 nl.y. ago, when the Galapagos Rise became extinct.The present configuration of the Nazca and Pacific 74 Plates is shown in Figure 2.

Physiography

The bathymetry (Fig. 3) suggests five physiographic features of the Sala y Gomez Ridge region which may be important in a discussion of the origin of this feature.These are listed below and are schemat- ically illustrated in Figure 19: 1) There is an east. southeast trending offset in the East Pacific Rise crest near 27°S.Sykes (1967) andAndersonetal, (1975) report first motion directions of 096° to 105° for earthquakes registered along this transform fault. 2) A broad band of seamounts and high topography extends east.- southeast from the above-n-ientjoned offset of the rise crest to about ioiow longitude.Both Easter Island and Sala y Gornez appear to be associated with this feature. 3) A relict fracture zone apparently offsets the fossil rise, stretching from near 25°S, 090°W to approximately 26°S, 099°W. This fracture zone has been termed the Easter Island Fracture Zone by Mammerickxetal (1975).The fossil rise has not been located between this fracture and the Quiros Fracture Zone ('22°S to 25°S), 4) A large chain of seamounts appears to be associated with the relict fracture zone, and extends along it to the north, from 090 ow to

101 °W.It is this seamount chain which has often been referred to as FIG. 19: PHYSIOGRAPHIC FEATURES OF THE SALA Y GOMEZ RIDGE REGION (SCHEMATIC REPRESENTATION)

EPR

1MPSALAYGOMEZ D______F

6\6 //

EPR // EAST PACIFIC RISE CREST GR

GR GALAPAGOS RISE CREST FZ FRACTURE ZONE s SEAMOUNT CHAIN 616 BOUNDARY BETWEEN CRUST GENERATED BY EAST PACIFIC AND GALAPAGOS RISES

MP'if'MICRO-PLATE, CALLED EASTER PLATE BY ANDERSON ET AL (1974) Figure 19.Physiographic features of the Sala y Gomez ridge region (schematic representation). the Sala y Gon-iez Ridge (Fisher and Norris, 1960; Mammerickx .:J:'' 1975), 5) The crest of the fossil rise trends south-southeast from its junction with the relict fracture zone at 094°-095°W longitude,, Of great importance, though not a physiographic feature, is the boundary between crusts generated by the East Pacific and Galapagos Rises.This boundary has been identified from magnetic anomaly patterns below the relict fracture zone at about 099 ow and trends south-southeast from there (Herron, 1972a).One more feature of this region must be noted.Magnetic, seismic and bathymetric data led Forsyth (1973), Herron (1972b) andAndersonetal. (1974) to postulate the presence of two small "micro-plates" on the East Pacific Rise crest.The northern "micro-plate"is proposed to lie on the rise crest just northwest of Easter Island, between 22°S and 26°S; it has been called the "Easter Plate" (Anderson etal., 1974), and is bounded on the north by transform faults and on the south by spreading centers.The southern "micro-plate" is located between 32°S and 34°S, just north of the Chile Fracture Zone,The significance of these features is difficult to evaluate. Anderson etal, (1974) suggest that the northern "micro-plate" may be related to the proposed "Easter Island hot spot" (Morgan, 197Za), but data presented in this paper cast serious doubts on the existence of a hot spot near Easter island. 77 The terminology applied to the Easter Island Fracture Zone and the Sala y Gomez Ridge by MammerickxetaL (1975) is unnecessarily confusing,Most, if not all, of the seamounts north of the relict fracture zone and the relict fracture zone itself lie on crust generated by the fossil Galapagos Rise (Figs. 2, 3 and 19).Easter Island and Sala y Gomez, however, are associated with the zone of higher topography and scattered seamounts sitting on East Pacific Rise-generated crust,Both calculated ages (Fig, 7) and bathymetry (Fig. 3) support this interpretation.The term Sala y Gomez Ridge should more properly be applied to the broad band of high topography and seai-nounts which extends from the East Pacific Rise crest offset at 27°S, 114°W to about 28°S, 101 °W.Similarly, the Easter Island Fracture Zone of Mammerickxetal. (1975) is also a misnomer, as it is probably not related to Easter Island; Sykes (1967) and Herron (1972a) had previously referred to the East Pacific Rise crest offset at 27°S, 114°W as the Easter Island transform fault,In this paper the term TSa1a y Gomez Ridgewill refer to the aforementioned zone of high topography associated with the East Pacific Rise, and the rise crest offset at 27°S, 114°W will be called the Easter Island transform fault.

jGomez Ridge

There have been two trends of thought concerning the origin of r1 the Sala y Gomez Ridge.The currently popular plume hypothesis of Morgan (1971, 1972a, b) attributes tnis feature to the movement of the Nazca Plate over a stationary hot spot,or deep mantle plume, located on the East Pacific Rise at the intersection of the Rise with the Sala y Gomez and Tuamotu Ridges (27°S, 114°W; Figs. 1 and Z). Morgan (1971, l97Za, b) considered the Salay Gomez Ridge to extend from the East Pacific Rise crest to the Nazca Ridge. He placed anomaly 13 near this 'Sala y Gomez Ridge"- Nazca Ridge junction, and suggested that the 'elbow" represented a change in spreading direction on the Nazca Plate, some 38 million years ago. According to this interpretation, analagous "elbows" in the Tuamotu-Line and Hawaiian-Emperor chains (Fig. 1) represent like changes in spreading direction on the Pacific Plate,The possible origin, discussed most recently by Herr on (1972a), ascribes the Salay Gomez Ridge to volcanism along a major fracture in the Nazca Plate. Herron (1972a) has found anomalously low spreading rates five to ten miLlionyears ago between the Easter Island Fracture Zone (presumably the eastward extension of the Easter Island transform fault) and the Chile

Fracture Zone. She interprets this as evidence that the Salay Gomez Ridge may be the result of shearing duringa temporary breakup of the Nazca (and Pacific) Plate five to ten million yearsago, while the Tuamotu Ridge represents a similar feature on the Pacific Plate. Ideas implicit in the hot spot, or plume, hypothesis (Wilson, 79 1963, 1965; Morgan, 1971, 197Za, b) require a hot spot-derived oceanic island chain to possess several characteristics: 1) approximate linearity, and parallelism to the direction of plate motion. 2) progressively increasing ages of shield-building volcanism with distance from the hot spot.

3)similarity of basalt composition and style of volcanism, A close inspection of all relevant data suggests that the Sala y Gomez Ridge fulfills none of these criteria. The general trend of the Sala y Gomez Ridge is east-southeast, essentially the same as the Easter Island transform fault (09 6° to 105°).While the Ridge itself is approximately parallel to the direction of plate motion, the pattern of islands and seamounts upon it is

scattered and irregular (Fig0 3).The zone of island and seamount production appears much wider than expected from current plume and hot spot hypotheses (Harrison etal,,1974), Calculated ages from Easter Island and Sala y Gomez (Fig. 7; Table 1) suggest that the earliest subaerial volcanism on the two islands was essentially contemporaneous. Sea floor ages in the vicinities of Easter Island and Sala y Gomez are about 6 m. y. and 10.5 rn.y., respectively (Herron, l97Za).Given an estimated half- spreading rate of 9 cm yr (Herron, l972a), the youth of the islands (2-3 rn.y,) precludes any possibility that either island (at least the subaerial portion) was formed on the rise crest, the site of the proposed hot spot,Neither ithere any consistent age vs. distance progression, as has been documented for the Hawaiian Islands (McDougall, 1964, 1971), the Marquesas Islands (Duncan and McDougall, 1974), and the Society Islands (Dymond, 1975).If such a pattern exists, evidence must lie deep within the volcanic pile. Finally the chemistry and petrology of rocks from Easter Island and Sala y Gomez do not support a hot spot origin for the Sala y Gomez Ridge.I have shown that there are fundamental differences in flowrock chemistry between the Easter and Sala y Gomez suites.The Easter Island basalts are mostly quartz normative tholeiites, suggesting shallow magma segregation, probably on or near the Rise crest.The preferred parent liquid for the Sala y Gomez rocks, however, is of alkali olivine basalt composition, indicative of a greater depth of magma segregation and a smaller degree of partial melting in the mantle.These lavas possess a chemical character which is inconsistent with an origin on or near a rise crest.It is highly unlikely that volcanic rocks from Easter Island and Sala y Gomez were derived from a common source.I conclude that Easter Island, Sala y Gomez, and probably the entire Sala y Gomez Ridge are not adequately explained by the movement of the Nazca Plate over a hot spot or plume source in the mantle. An hypothesis of origin for the Sala y Gomez Ridge must explain 81 1) the broad zone of high topography and scattered arrangement of islands and searnounts, the apparent contemporaneity of volcanism, and 3) the trend toward silica undersaturation in the volcanic rocks with distance from the East Pacific Rise crest,Available data do not disagree with the suggestion by Herron (1972a) that the Salay Gomez Ridge is a result of volcanism along a major fracture in the Nazca Plate,I now propose a model for the origin of the Sala y Gomez Ridge which expands upon this concept. The close association of the Sala y Gomez Ridge with the Easter Island transform fault suggests a genetic relationship between them. The Sala y Gomez Ridge region is located quite close to the equator of the Pacific-Nazca pole-of-rotation axis; as such it has the fastest spreading rate in the world mid-ocean ridge system (>9,0 cm yr) (Herron, 1972a; Anderson etal., 1974).It is possible that the Easter Island transform fault, which offsets the East Pacific Rise crest by approximately 400 km (the largest fracture zone offset of the rise crest between 0° and 30°S), represents the surface expression of a major zone of lithospheric weakness. Increased stress application due to the high spreading rates in this region may perpetuate this zone along both the eastern and western extensions of the transform outside the offset.Differential spreading rates north and south of the Easter Island transform and its eastern extension between five and ten million years ago may have contributed to, or caused, this tear in the Nazca (and Pacific) Plate (Herron,, 1972a),Such a deep-seated major fracture in the lithospheric piate might initiate, or at least allow, diapiric upwelling of asthenosphere material along its length.This would result in decompression melting similar to that envisioned in Figure 18.The fracture zone would provide rapid egress of the basaltic liquids to the surface, and the magma composition would be governed by the maximum depth of brittle fracture (essentially the thickness of the lithosphere), where magma segregation is proposed to occur, Since the lithosphere thickens with distance from the rise crest (Leeds et al., 1974; Oxburgh, 1971), magmatic liquids should become more under saturated in silica as they erupt farther from the rise crest,This is the pattern observed on Easter Island and Sala y Gomez. Broad upwelling of asthenosphere material may have a

Mdominghl effect on the lithosphere.This could be due either to in- jection of asthenosphere material and subsequent upward displacement of the overlying lithosphere, or to an increased thermal gradient caused by magma production, An increased thermal gradient should depress the main phase boundaries in the mantle, resulting in a volume expansion as low temperature, high pressure minerals invert- ed to their less dense, high temperature equivalents (Gass, 1970). An increase in volume would most easily be relieved by vertical uplift,Perhaps this explains the east-southeast trending zone of high topography along the eastern extension of the Easter Island transform.Tensional cracks associated with this uplift would provide magma conduits foreamount and island volcanism, and would not require a linear alignment of the volcanoes.Finally diapiric uprise may occur contemporaneously along the fracture, thus allow- ing for synchronous volcanism in the islands and seamounts which compose the Sala y Gomez Ridge.The model is illustrated dia- gramatically in Figure ZO; it explains the high topography, synchronous volcanism and compositional relations observed along the Sala y Gomez Ridge.It is worth noting that, of the islands lying along the far western extension of the Easter Island transform fault, Pitcairn, Tahiti and Moorea all were active at nearly the same time as Easter and Sala y Gomez Islands (McDougall, 1974; Dyrnond, 1975).These islands also display progressive silica undersaturation with distance from the rise crest (McBirney and Gass, 1967),It is possible that processes similar to those which generated the Sala y Gomez Ridge were also responsible for the genesis of the islands and seamounts to the west of the East Pacific Rise. ORIGIN OF THE SALAY GOMEZ RIDGE EASTER IS. SALA V GOMEZ EAST PACIFIC RISE Sea Level

- __ 1-Doming of oceanic crust - OCEANIC CRUST

I""N RTIAL MELTING LITHOSPHERE

t I/ ALONGMAJOR FRACTURE

ASTHENOSPHERE

Figure 20. Origin of the Sala y Gozrtez Ridge. 00 CONCLUSIONS

1) Subaerial Easter Island was constructed in three distinct episodes, occurring at 2. 5 m. y., 0. 9 m. y., and 0. 4 m. y. ago.The youngest rocks on the island are the Roiho olivine basalts, which are probably less than 50, 000 years old.

2) Volcanism on Sala y Gomez was essentially contemporaneous with that on Easter Island, and there appears to have been no migration of volcanism with time along the Sala y Gomez Ridge.

3) The Easter Island volcanic suite comprises a tholeiitic differentiation series.The wide compositional spectrum can probably be explained as the result of crystal fractionation from a basaltic

parent liquid, but the data are ambiguous at the high silica end9 and conflict with strontium isotopic evidence.The Easter Island rocks appear similar to volcanic rocks from other islands situated on or near mid-ocean rise crests (e.g. Ascension, Bouvet, and the Galapagos Islands).

4) Early basaltic products on Easter Island were quartz tholeiites, while the most recent extrusive was an olivine basalt with alkaline affinities.This different probably cannot be ascribed to fractional cyrstallization, More likely, it was the result of a downward migration of the zone of fusion with time, as the island was 86 carried some 2Z5 km eastward over a progressively thickening lithosphere.,

5)Volcanic rocks from Sala y Gomez have an alkaline and silica-under saturated character.They were probably derived from

an alkali olivine basalt parent0

6) The fundamental differences in flowrock chemistry between Easter Island and Sala y Gomez suggest that the two islands were not derived from a common source.,The chemical character of the Easter Island basalts indicates that they were generated by partial melting and magma segregation at much shallower depths than were those of Sala y Gomez.

7) Evidence does not support a hot spot origin for Easter Island, Sala y Gomez, or the Sala y Gomez Ridge. Age, chemical, seismic, and bathymetric considerations support a model for the origin of the Sala y Gomez Ridge involving diapiric intrusion and subsequent melting of asthenosphere material along a major fracture in the Nazca Plate.This leads to similar ages of volcanism along the fracture, while magma compositions are governed by magma segregation at the base of a lithosphere which progressively thickens away from the spreading center., BIBLIOGRAPHY

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Comparisons of analyses of U. S. Geological Survey standard rocks, AGV-1, W-1, and BCR-1(s), reported by F'anagan (1969) and Fleischer (1969), with those analyses of this study(C). 97

57.8 58.0 58.5 59.0 59.5 60.0 60.4

I I I I I I

15.7 $6.0 $6.5 $7.0 17.5 17.9

I I I I A1203

6.5 70 7.5

I I I I Fe203

I I I I

0.9 1.5 2.0

I I I MgO

AGV-I 98

4.4 5.0 5.5 6.0 6.4

I I I I I

CoO

Na20 L.

2.7 2.8 2.9 3.0 3.1 3.2

I I I I I I K20

AGV-I 51.0 51.5 52.0 52.5 53.0 53.5

I I

14.0 14.5 15.0 15.5 I I I - J

I'll

10.0 10.5 11.0 11.5 120

I I I I I

w-I 1 00

5.5 6.0 6.5 7.0

I I I I

$0.5 11.0 $1.5 $2.0

I I I

2.0 2.1 2.2 2.3 2.4

I I I I I

NaO Jj

.50 .60 .70 .80 .90 I I I I I

w-I 101

1.0 1.5 2.0

I I I

w-ui 1 OZ

52.7 53.0 53.5 54.0 545 55.0 55.5 55.9

I I I I I I I I

Si02

(2.5 (3.0 (3.5 14.0

I I I 4I0

13.2 13.5

I I Fe203

2.0 2.5 3.0 3.5 3.9

I I I I I MgO

BCR-I 103 6.0 6.5 7.0 7.5 8.0

I I I I I

CaO I

3.0 3.5 4.0

Na20

L4 1.5 1.6 1.7 1.8 1.9

I I I I I I K20

1.9 2.0 2.1 2.2 2.3 2.4 2.5

I p i i I I 1102

BCR-I 104

APPENDIX 2 Chemical Analyses of Rocks From Easter Island and Sala y Gomez

POIKE

EH-296 EC-383 EH-361 EN- 350 EH-39 EH-352

Si02 45.84 47.66 47.89 48. 22 48. 64 48. 68 Tb2 3.90* 3.79* 2.94* 3.29 2. 92* 2. 84 A1203 15.73 14.30 17.87 17.43 15.25 17.54 Fe203 3.96 3.75 2.95 3.02 3.45 2.82 FeO 10.69 10.11 7.97 8.15 9.31 7.62 MnO 0. 23* 0.21* 0.18* 0.29 0.18* 0.27 MgO 4.61 4.19 3.28 3.89 5.19 4.69 CaO 8.44 8.37 8.64 9. 00 9.66 9.86 Na20 3.15 3.81 3.42 3.42 3.04 3.40 K20 0.37 0.91 0.69 0.82 0.19 0.72 Total 96.92 97.10 95.83 97. 53 97.83 98.44

Molecular Norms IL 5.76 5.56 4.34 4.77 4.24 4.06 MT 4.39 4.13 3.27 3.29 3.76 3.02 OR 2.32 5.66 4.32 5.04 1.17 4.36 AB 19.96 36.01 32.55 31.98 28.46 31.31 AN 29.34 20.24 33.26 31.02 28.57 31.26 QTZ -- -- 0.92 1.72 -- NE - - DI 12.02 18.78 9.75 12.38 17.12 15.14 NY 14.74 4.60 11.59 11.13 14.96 6.89 OL 1.48 5.04 -- 0. 39 -- 3.97

D.I. 32.3 41.7 37.8 37. 0 31.3 35.7

An Ab + An 36.0 50.5 49. 2 50.1 50.0

* Analysis done by XRF 105

Appendix 2, Continued:

POIKE,continued

EC-307 EH-351 EH-42 17732 EH-358

Si02 49.70 49.81 49.86 49.88 66.31 Ti02 2.80* 3.25 3.06* 1.97* Ø34* A1203 16.46 15.25 14.52 19.54 15.81 Fe203 2. 87 3. 18 3.21 2.43 3. 59 FeO 7.74 8.58 8.67 6.55 1.19 MnO 0.19* 0.31 0.21* 0.15* 0.13* MgO 4. 81 4.46 5. 25 4. 01 0. 03 CaO 10.12 9.32 9.65 11.39 0.38 Na20 3. 12 3. 50 3.20 2. 85 5.97 K20 0.69 0.85 0.55 0.35 3.75 Total 98.50 98.51 98.18 99.12 97.50

Molecular Norms IL 4.01 4.68 4.42 2.80 0.48 MT 3. 09 3.43 3. 48 2. 59 2. 41 OR 4.19 5.19 3.37 2.11 22.60 28. 82 32.45 29.78 26. 07 54. 69 AN 29.71 24.16 24.49 40.24 1.92 QTZ 1.16 0.69 1.88 1.93 15.48

NE------DI 17.56 18.88 20.10 13.87 -- HY 11. 45 10. 54 12. 50 10.40 0. 09

OL------

D.I. 34.2 38.3 35.0 30.1 92.8

Ab+An 50.8 42.7 45.1 60.7 3.4 Appendix 2,Continued

RANO KAU

EH-20 EH-57 EH-29 EH-27 EH-61 EH-71

Si02 47.71 49.27 49.71 49.87 50.00 59.66 Ti02 3.24* 3.46* 2.13* 3.11* 3.62 1.59* A1203 14.73 14.86 19.37 14.10 15.25 14.11 Fe203 3. 42 3.46 2. 29 3. 25 3.46 2. 23 FeO 9.22 9.34 6.18 8.78 9.33 6.15 MnO 0.20* 0.22* 0.14* 0.19* 0.26 0.21* MgO 5. 27 5.25 4. 63 5. 19 4. 52 1.48 CaO 9.88 8.87 10.99 9.35 8.69 4.21 Na20 3. 28 3. 17 2.98 3. 75 3.40 4. 57 K20 0. 34 0. 50 0. 45 0. 63 0. 74 2. 16 Total 97. 29 98.40 98. 87 98. 22 99. 27 96. 42

Molecular Norms

IL 4.72 5.00 3.02 4.47 5. 19 2.33 MT 3.74 3.75 2.43 3.51 3. 72 2. 50 OR 2. 10 3. 06 2.70 3. 84 4. 50 13. 39 AB 30.81 29.52 27.21 34.75 31.42 43.07 AN 25. 60 25.77 38. 80 20.42 24. 87 12. 19 QTZ -- 2. 05 0. 86 -- 2. 03 ii. 98

NE------DI 20.55 15.90 13.32 21.97 15.60 7.79 HY 10.87 14.95 11.66 10.12 12.67 6.75 OL 1.60 -- -- 0.92 -- -- AC------

0.1. 32. 9 34. 5 30. 8 38. 6 37.9 68.4

Ab+An 45. 4 46. 6 58. 8 37. 0 44. 2 22. 1

* Analysis done by XRF 107

Appendix 2, Continued

RANO KAU, Continued

EH-56 EH-50 EH-54 EH-.10

Si02 60.25 61.11 61.22 73.82 Ti02 1.58 1.48 1.54 0.08* A1203 14.21 14.36 14.38 12.80 Fe203 2. 30 2. 22 2. 28 0. 82 FeO 6.20 5.99 6.15 2.00 M.nO 0.27 0,26 0.26 0. 10* MgO 1.36 1.37 1.53 0.00 CaO 3.74 3.70 4.20 0. 58 Na20 4. 92 5. 06 4. 96 5. 56 K20 2. 15 2. 27 2. 14 3. 77 Total 96.98 97. 82 98. 66 99. 53

Molecular Norms IL 2.29 2.13 2.20 0.11 MT 2.51 2.39 2.44 0.86 OR 13.23 13.82 12.93 22.36 AB 46. 02 46. 83 45. 56 47. 78 AN 10.77 10.07 10.90 -- QTZ 11.36 11.07 11.14 24.69

NE------Dl 6.85 7.08 8.34 2.31 HY 6. 97 6. 62 6. 50 1. 43

OL------AC ------0.13 D.I. 70.6 71.7 69.6 94.8

Ab+An 19.0 17.7 19.3 0.0 108

Appendix 2, Continued

TEREVAKA

EC-243 EH-302 EC-207 EH-288 EC-201 17756

Si02 47.21 48.22 48.25 48.56 48.86 49.17 Ti02 374* 3. 00* 2. 73* 2. 98* 2. 99* 3. 60* A1203 14.26 15.99 15.97 15.34 16.45 13.91 Fe203 3. 83 3.82 2.98 3.62 3.09 4.10 FeO 10.34 10.31 8.05 9.76 8.34 11.08 MnO 0.23* 0.25* 0.19* 0.27* 0.18* 0.25* MgO 4.71 3.35 7.35 3.37 6.42 4.14 GaO 9.28 7.26 9.75 5.04 9.02 8.40 Na20 3.49 3.90 3.03 4. 17 3.39 3. 89 K20 0. 63 0. 59 0. 63 1. 17 0. 94 0. 75 Total 97. 72 96.69 98.93 94.28 99.68 99.08

Molecular Norms IL 5.46 4.42 3.86 4.48 4.21 5.19 MT 4.19 4.22 3.16 4.09 3.26 4.44 OR 3.90 3.69 3.78 7.46 5.61 4.59 AB 32. 82 37. 03 27. 62 40.43 30. 73 36. 16 AN 22.40 25.78 28.55 21.26 27.15 18.93 QTZ -- 0.10 -- 0.37 -- -- NE - - DI 20. 66 9.84 16.46 4.60 14.42 19.38 HY 5.71 14.93 8.17 17.32 5.62 8.78 OL 4.86 -- 8.40 -- 9.01 2.54

DI. 36.7 40. 8 31.4 48. 3 36. 3 40.7 An Ab+An 406 41. 1 50. 8 34. 5 46. 9 34. 4

*Analysis done by XRF 109

Appendix 2, Continued

TEREVAKA, Continued

EC-385 17731 EH.-88 EC-322 EC-369

S102 49.17 49.18 49.20 49.86 50.49 TiC2 3. 22* 3.96* 3.13* 3.46* 2.83* A1203 15.48 13.30 15.81 14.69 14.78 Fe203 3. 63 4.04 3.03 3.63 3.33 FeO 9. 81 10.92 8.17 9.79 8.99 MnO 0.19* 0.24* 0.20* 0. 22* 0. 20* MgO 4. 62 4.38 6.75 4.52 6.34 CaO 10.30 8.51 8.74 9.15 9.83 Na20 3.35 3.57 3.09 3.10 3.37 K20 0.63 0.75 0.82 0.75 0.55 Total 100.40 98.85 98.94 99.17 100.71

Molecular Norms IL 4.56 5.74 4.44 4.99 3.96 MT 3.86 4.40 3.23 3.93 3.50 OR 3.79 4.61 4.94 4.58 3.27 AB 30. 59 33.36 28.28 28. 79 30.41 AN 25.78 18.79 27.37 24.78 23.70 QTZ -- 1.12 -- 2.72 -- NE - - DI 20.96 20.13 13.47 17.74 20.25 HY 8.48 11.85 16.46 12.48 13.26 OL 1.98 1.82 -- 1.67 D.I. 34.4 39.1 33.2 36.1 33.7 An Ab+An 36.0 49.2 46.3 43.8 110

Appendix 2,Continued

TEREVAKA

EC-125 EC-153 EC-165 EC-413 17729 EC-193 EH-122 EC-176

Si02 50.58 51.19 51.24 51.34 51.35 51.88 51.95 52.34 Ti02 3.81* 3.36 3.19* 3.25* 2.96* 3.07* 2.91* 3.01* A1203 14. 16 15. 20 14. 87 15. 04 16.49 15. 21 17. 53 15. 02 Fe203 3. 68 3q44 3. 59 3. 56 3.29 3. 69 2. 86 3. 52 FeO 9.92 9.29 9. 69 9. 61 8.88 9.97 7. 73 9. 51 MnO 0. 23* 0. 36 0. 25* 0. 23* 0. 20* 0. 26* 0. 18* 0. 24* MgO 4. 82 3. 80 3. 64 3. 76 4. 03 3. 52 3. 36 3.49 GaO 8.36 7. 57 7.44 6. 61 9.78 7. 08 10. 26 7.25 Na20 3.62 4.13 4.15 3.86 3.47 4.03 3.41 4.23 K20 0.82 1. 02 0.97 0.93 0.72 0.96 0. 60 1. 13 Total 100.00 99.36 99.03 98. 19 101.17 99. 67 100.79 99.74

Molecular Norms IL 5.43 4.80 4.58 4.71 4.15 4.38 4.09 4.28 MT 3.93 3.69 3.86 3.87 3.46 3.96 3.02 3.76 OR 4. 95 6. 17 5. 90 5.72 4. 28 5. 81 3. 58 6. 82 AB 33.23 38.00 38.37 36.06 31.38 37. 10 30.91 38.79 AN 20.42 20.42 19.65 21.82 27.49 21.10 31.05 19.06 QTZ 1.60 0.89 1.25 3.53 1.77 2.50 3.58 1.74 NE ------DI 15.90 14.45 14.69 9.85 17.10 11.93 20.25 14.14 HY 14.95 11.60 11.71 14.45 10.36 13.23 13.26 11.41 OL ------1.67 --

D. I. 39. 9 45. 1 45. 5 45. 3 37. 4 45.4 38. 1 47. 3

Ab+An38.1 35.0 33.9 37.7 46.7 36.3 50.1 32.9

*Analysis done by XRF 111

Appendix 2,Continued

EASTER ISLAND DREDGE

KK72- 34-7 KK7Z-34-4 KK7Z.-35.. 1 KK7Z-34-34 KK7Z.-34-43

Si02 48.77 49.00 49.69 49.96 50.99 Ti02 2.73* 2.96 339* 2.80* 2.85* A1203 16.53 16.02 15.81 15.79 15.94 Fe203 2.92 2. 96 3.24 3. 06 2. 86 FeO 7. 88 7.98 8.74 8.27 7.71 MnO 0.19* 0.29 0.20* 0.19* 0.19* MgO 6.16 7.15 4.36 5.90 5.92 CaO 9.82 9.79 9.66 9.73 9.71 Na20 3.28 3.29 3.28 3. 34 3.27 K20 0.74 0.74 0. 85 0.79 0.74 Total 99.02 100.18 99.22 99.83 100.18

Molecular Norms IL 3.86 4.13 4.85 3.95 4.00 MT 3.10 3.10 3.48 3.24 3.01 OR 4.44 4.38 5.16 4.72 4.40 AB 29.92 29. 61 30.26 30.33 29. 57 AN 28. 65 26.82 26.62 26.06 26. 83 QTZ - - -- 1.15 -- 0.72 NE - - DI 16.68 17.49 18.10 18.22 17.36 HY 6.18 4.79 10.38 10.43 14.11 OL 7.15 9.67 -- 3.07

D.I. 34.4 34.0 36.6 35.0 34. 7 An Ab+An 48.9 47.5 46.8 46.2 47. 6

*Analysis done by XRF 112

Appendix 2, Continued

SALA Y GOMEZ

Y73-4-30-Z0 PV-301 Y73-4-30-4

Si02 47.90 52.48 53.59 Ti02 3.63 1.77 1.86 A1203 17.12 16.86 18.00 Fe203 2.63 2.26 1.85 FeO 7.11 6.11 5.00 MnO 0.25 0.16 0.27 MgO 6.03 4.10 3.85 GaO 8.06 5.47 5.25 Na20 3.96 5.43 5.44 1(O 1.63 2.82 2.81 Total 98. 32 97.46 97. 92

Molecular Norms IL 5.14 2.49 2.60 MT 2.79 2.39 1.94 OR 9.78 16.85 16.67 AB 29.89 38.29 41.99 AN 24. 50 13. 46 1 6. 47 QTZ ------NE 3.73 6.62 4.24 DI 12.89 11.20 7.75 HY ------OL 11.29 8.70 8.33 D.L 43.4 61.8 62.9

Ab+An 45. 0 26. 0 28.2 113

APPENDIX 3a Analyses of Rocks and Phenocrysts Used In Crystal FracUonatiori Calculations

RANO KAU

EH-29 EH-61 EH-57 EH-71 EH-56 EH-54 EH-50 EH-10

Si02 49.71 50.00 49.27 59.66 60.25 61.22 61.11 73.82 Ti02 2.13 3. 62 3.46 1. 59 1.58 1. 54 1.48 0.08 A1203 19.37 15.25 14.86 14.11 14.21 14.38 14.36 12.80 FeO 8. 24 2. 44 12.45 8. 20 8. 27 8. 20 7.99 2. 74 MgO 4.63 4.52 5.25 1.48 1.36 1.53 1.37 0.01 GaO 10.99 8. 69 8.87 4.21 3.74 4.20 3.70 0. 58 Na20 2.98 3.40 3. 17 4. 57 4.92 4. 96 5. 06 5. 56 K20 0.45 0.74 0. 50 2. 16 2. 15 2. 14 2, 27 3.77 Total 98. 50 98. 66 97. 83 95.98 96.48 98. 17 97.34 99. 36

POIKE TEREVKA

EH-361 EH-350 EH-358 C-243 17756 Si02 47.89 48.22 66.31 47. 21 48. 96 Ti02 2. 94 3. 29 0. 34 3.74 3. 60 A1203 17.87 17.43 15.81 14. 26 13.91 FeO 10.62 10.87 4.42 13.79 14.77 MgO 3. 28 3. 89 0. 03 4.71 4.14 GaO 8. 64 9.00 0. 38 9.28 8.40 Na20 3.42 3.42 5.97 3.49 3.89 K20 0,69 0.82 3.75 0.63 0.75 Total 95.35 96.94 97.01 97.11 96.42 114

PHENOCR YS TS

An70 An55 An52 An50 Ab62An340r4 Fo70 Fo63 FA

S102 51.01 54.48 54.78 56.00 58.00 37.70 38. 10 30. 56 Ti02 0. 06 0. 03 0. 03 0. 03 0. 02 0.01 0. 01 0. 72 A1203 30. 99 28. 86 28. 69 28. 00 26.44 0. 01 0. 01 0. 09 FeO 0.49 0.60 0. 55 0.36 0.20 26.80 31. 50 60. 81 MgO 0. 10 0.05 0.05 0. 05 0.03 35. 10 30. 50 3.47 GaO 13.83 10.92 10.72 9.90 7.09 0.25 0.02 1.13 Na20 3.29 4.94 5. 04 5.47 7. 15 0. 01 0. 01 0. 01 K20 0.20 0. 17 0. 17 0.20 0.70 0.01 0.01 0.01

CPX-1 CPX-2 GPX-3 MT ILM S102 51.00 51.00 51.00 0.27 0.01 Ti02 0. 60 0. 60 0. 60 0. 01 52. 00 A1203 1.50 1.50 1.50 0.21 0.01 FeO 13.28 17.09 15.99 98.20 48.00 MgO 10. 16 9. 25 8. 64 0.01 0.01 GaO 22. 61 19.77 21.72 0. 01 0.01 Na20 0. 50 0. 50 0. 50 0. 01 0. 01 K20 0.10 0. 10 0. 10 0.01 0.01

Source: Deer, Howie and Zussman, 1967 (see text, page 115

APPENDIX 3b Results of Crystal Fractionation Calculations

(i) EH-29 to Eh-61 (Rano Kau)

EH- 61 obs EH- 61est Diff. %Diff. E. Diff. Si 50.00 50.884 0.884 1.77 0.59 Ti 3. 62 3. 62 0. 00 0. 00 0. 00 Al 15.25 15.196 -0.054 -0.36 -0.18 Fe 12.44 12.44 0.00 0.00 0.00 Mg 4. 52 4. 517 -0. 003 -0.07 -0.04 Ca 8.69 8.67 -0. 0 -0.22 -0.11 Na 3. 40 3. 416 0. 016 0. 46 0. 23 K 0.74 0.735 -0. 005 -0.61 -0.41

Average 0. 123 0. 44 0. 18

Sumof Squares 0.786 3.90 0. 54

Vector Coeff. Std. Dev.

EH-29 1.9993± 0.0215 An70 -0.7532± 0.0156 Fo70 -0.0963± 0.0031 CPX-1 -0.1265± 0.0054 MT 0. 0109± 0.0017 ILM -0. 0099± 0. 0020

Total 1.0243± 0.0274 116

(ii)EH-61 to EH-57(Rano Kau)

EH-57 obs EH-57est Diff. %Diff. E. Diff. Si 49.27 47.582 -1.688 -3.43 -1.14 Ti 3. 46 3. 46 0. 000 0. 00 0. 00 Al 14.86 14. 887 0.027 0. 18 0.09 Fe 12.45 12.45 0.000 0.00 0.00 Mg 5. 25 5. 259 0. 009 0. 17 0. 09 Ca 8. 87 8. 916 0. 046 0. 52 0. 26 Na 3.17 3.184 0.014 0.43 0.21 K 0. 50 0. 501 0. 001 0. 16 0. 08 Average 0 223 0 61 0 23 Sum of Squares 2.853 12.28 1.44

Vector Coeff. Std. Dev.

EH-61 0.6119± 0.0140 An50 0.1938± 0.0111 Fo70 0.0485± 0. 0036 CPX-2 0.0843± 0. 0089 MT 0.0095± 0.0028 ILM 0.0228± 0. 0030

Total 0. 9709± 0. 0207 117

(iii)ELI-61 to EH-71 (Rano Kau)

EH-71 obs EH-71est Diff. %Diff. E. Diff.

Si 59. 66 59. 249 -0.411 -0.69 -0.23 Ti 1.59 1.59 0.00 0.00 0.00 Al 14.11 13.969 -0.141 -1.00 -0.50 Fe 8.20 8.20 0.00 0.00 0.00 Mg 1.48 1.48 0.00 0.00 0.00 Ca 4.21 4.211 0. 001 0. 03 0.01 Na 4. 57 4. 666 0.096 2.10 1.05 K 2.16 2.141 -0.019 -0.90 -0.45

Average 0. 084 0. 59 0.28 Sum of Squares 0.198 6.70 1.61

Vector Coeff. Std. Dev. EH-61 3.2570± 0.0536 An52 -1.2147± 0.0313 Fo7 0 -0. 2286± 0.0052 CPX-2 -0.5572± 0.0093 MT -0.0705± 0.0028 ILM -0.1890± 0.0039

Total 0. 9970 ± 0. 0632 118

(iv) EH-71 to EH-56 (Rano Kau)

H-56 obs EH-56est Diff. %Diff. E. Diff. Si 60.25 60. 118 -0.132 0.22 -0.07 Ti 1. 58 1. 58 0. 00 0. 00 0. 00 Al 14.21 14.237 0.027 0.19 0.09 Fe 8. 27 8. 27 0. 00 0. 00 0. 00 Mg 1.36 1.34 -0.02 -1.48 -0.74 Ca 3.74 3.814 0.074 1.98 0.99 Na 4.92 4. 661 -0. 259 -5.26 -2. 63 K 2. 15 2. 239 0. 089 4. 16 2. 08

Average 0.075 1.66 0.83 Sumof Squares 0.099 51.18 12.79

Vector Coeff. Std.Dev.

EH-71 1.0391± 0.0319 An50 -0.0140± 0.0183 CPX-2 -0.0213± 0.0065 MT 0.0018± 0.0042 ILM -0.0011± 0.0033

Total 1.0043± 0.0377 119

(v) EH-56 to EH-54 (Rano Kau)

EH-54 obs EH-54est Diff. %Diff. E. Diff.

Si 61.22 61.305 0.085 0.14 0.05 Ti 1.54 1.54 0.00 0.00 0.00 Al 14.38 14.408 0.028 0.19 0.10 Fe 8. 20 8. 20 0. 00 0. 00 0. 00 Mg 1. 53 1. 533 0,003 0. 18 0. 09 Ca 4.20 4.190 -0.010 -0.24 -0.12 Na 4. 96 4. 962 0. 002 0. 05 0. 02 K 2.14 2.135 -0.005 -0.24 -0.12 Average 0.017 0.13 0.06 Sumof Squares 0.008 0.21 0.05

Vector Coeff. Std. Dev.

EH-56 0.9878± 0.0025 Ab62An340r4 0.0129± 0. 0016

CPX-2 0.0204± 0.0004 MT 0.0030± 0. 0003 ILM 0. 0006± 0. 0002

Total 1.0175± 0. 0030 120

(vi) EH-54 to EH-50 (Rano Kau)

ER-SO obs ER- 50est Diff. %Diff. E. Diff. Si 61.11 61.972 0.862 1.41 0.47 Ti 1.48 1.48 0.00 0.00 0.00 Al 14.36 14. 417 0.057 0.39 0.00 Fe 7. 99 7. 99 0. 002 0. 00 0. 00 Mg 1. 37 1. 372 0.002 0. 18 0.09 Ca 3.70 3.688 -0. 012 -0.32 -0.16 Na 5. 06 5. 04 -0. 02 -.0. 40 -0. 20 K 2.27 2.259 -0. 011 -0.50 -0.25 Average 0.121 0.40 0.17

Sum ofSquares 0.747 2. 69 0.40

Vector Coeff. Std. Dev.

EH-54 1.0679± 0.0070 Ab62An340r4 -0.0339± 0. 0044 CPX-2 -0. 0281± 0. 0012 MT -0. 0015± 0. 0007 ILM -0. 0028± 0. 0006

Total 1.0015± 0.0084 121

(vii) EH-361 to EH-350 (Police)

EH-350 obs EH-350est Dill. %Diff. E. Dill. Si 48.22 50.043 1.823 3.78 1.26 Ti 3. 29 3. 29 0.00 0.00 0.00 Al 17.43 17. 338 -0. 092 -0. 53 -0.26 Fe 10. 87 10. 87 0. 00 0. 00 0. 00 Mg 3.89 3. 883 -0. 007 -0. 17 -0.08 Ca 9. 00 8. 956 -0. 044 -0.48 -0. 24 Na 3.42 3.411 -0.009 -0.26 -0.13 K 0. 82 0. 817 -0. 003 -0.31 -0. 16

Average 0. 247 0. 69 0. 27 SumofSquares 3.334 15.00 1.76

Vector Coeff. Std. Dev.

EH-361 1. 2217± 0.0266 An55 -0.1558± 0.0199 Fo63 -0.0053± 0. 0044 CPX-1 0.0046± 0.0091 MT -0. 0166± 0.0029 ILM -0.0058± 0. 0033 122

(viii)EC-243 to 17756(Terevaka)

17756 17756 ob8 est Diff. %Diff. E. Diff.

Si 48.96 47.937 -1.023 -2.09 -0.70 Ti 3. 60 3. 60 0. 00 0.00 0.00 Aj 13.91 13.919 0.009 0.07 0.03 Fe 14.77 14.77 0.00 0.00 0.00 Mg 4. 14 4. 143 0. 003 0. 08 0.04 Ca 8. 40 8. 424 0. 024 0. 28 0.14 Na 3.89 3.905 0.015 0.40 0.20 K 0. 75 0. 751 0. 001 0. 08 0. 04

Average 0. 134 0. 37 0. 14

Sumof Squares 1. 047 4. 62 0, 55

Vector Coeff. Std. Dev. EC-243 1.2403± 0.0125 An70 -0.1184* 0.0083 Fo70 -0.0317± 0.0022 CFX-3 -0. 0663± 0. 0053 MT 0.0056± 0. 0020 ILM -0. 0191 0. 0020

Total 1.0104± 0.0163 123

(ix) EH-50 (Rano Kau) to EH-358 (Poike)

iH-35 obs EH-3 58est Diff. %Diff. E. Diff.

Si 66.31 67. 052 0.742 1.12 0.37 Ti 0. 34 0. 340 0. 00 0. 00 0. 00 Al 15.81 15.085 -0.725 _4.59 .2.29 Fe 4.42 4.420 0.00 0.00 0.00 Mg 0. 03 0. 030 0. 00 0. 00 0. 00 Ca 0. 38 0. 380 0.00 0. 00 0. 00 5. 97 6.489 0. 519 8. 70 4. 35 K 3.75 3. 5Z7 -0.223 -5.94 -2.97 Average 0 276 2 54 1 25 Sumof Squares 1.396 133.24 33.14

Vector Coeff. Std. Dev.

EH-50 1.5838± 0.1206 An50 -0.2667± 0. 0859 PA -0,2714± 0.1211 CPX-2 -0.1282± 0.0318 MT 0.1237± 0. 0625 ILM -0.0332± 0. 0026

Total 1. 0081± 0. 2037 124

(x) EH-358 (Poike) to EU-b(Rano Kau)

EU-i 0 obs EU- 10est Diff. %Diff. E. Diff.

S 73.82 74.108 0.288 0.39 0.13 Ti 0. 08 0. 080 0. 00 0. 00 0. 00 Al 12.80 16.019 3.219 25.15 12.57 2.74 2.74 0. 00 0. 00 0. 00 Mg 0.01 0.010 0.00 0. 00 0.00 Ca 0.58 0.429 -0. 151 -26.01 -13.00 Na 5. 56 5. 557 -0. 003 0. 05 0. 03 K 3.77 3.218 -0. 552 -14.63 -7.32

Average 0. 527 8. 30 4. 14

Sum of Squares 10.770 1522.97 380.72

Vector Coeff. Std. Dev.

EH-358 1.7466± 0.8937 FA -0.0103± 0.0056 ANORTH -0. 6180± 0.7139 MT -0.0343± 0.0286 ILM -0.0093± 0.0053

Total 1.0748± 1.1442 APPENDIX 4 Petrography

Phenocrysts and Micro henocrysts

'v'5-20% s..' <5% Sample No. Rock Type Location PL OL CPX ORE Matrix Remarks

POIKE

EH-361 basalt top of north '.6/b'/ PL, CPX, ORE, PL An60; OL cliff Glass, OL altered to iddingsite EC-307 basalt base of northXf/ PL, OL, CPX, PLAn64; some PL cliff ORE, Glass phenocrysts are strongly resorbed; OL altered to iddingsite EH-296 hawailte inclusion at v / ./ './ PL, CPX, ORE PL'An60; OL altered Anamarama to iddingsite; trachytic texture EH-42 hawaiite base of south / PL, CPX, OL, nearly aphyric; few cliff ORE PL microphenocrysts are clustered EH-39 basalt base of south './ PL, CPX, OL, nearly aphyric; few cliff ORE, OPX(?) PL micropheno.- crysts are clustered; " microve sicular Appendix 4, Continued

Phenocrysts and Sample No. Rock Type Location Microphenocrysts Matrix Remarks PLOLCPXORE

17732 basalt base of north 'v'// PL,CPX, OL, OL altered to cliff ORE iddingsite EH-352 basalt north slope ./// / FL,OL, CPX, PLAn48; OL ORE altered to iddingsite; inter granular texture EC-350 hawaiite Pu Akatiki. '<4' V FL,CPX, OL, PLAn68; olivine ORE altered to iddingsite; vesicular EC-351 hawaiite north slope Vi / FL,OL, CPX PLiAn60 ORE

EC-383 hawaiite west of south .1 FL,CPX, OL, PLAn62; pilotaxitic cliff ORE texture EH-358 trachyte dome on north V ./ FL,AMPH, QZ, trachytic texture; slope ORE also anorthoclase phenocrysts; OL is fayalite RANO KAU

EH-29 basalt base of west 4/ / / FL,CFX, OL, cliff ORE EH-20 hawaiite above base of4' J FL,CPX, ORE west cliff Appendix 4,Continued

Phenocrystsand Microphenocrysts Sample No. Rock Type Location PLOLCPX ORE Matrix Remarks

EH-57 hawaiite inside caldera, // PL, OL, CPX, nearly aphyric; below Orongo ORE doleritic xenoliths of PL, CFX, ORE; highly oxidized appearance EH-61 hawaiite inside caldera V1' PL, CPX, OL, PL'An49; OL ORE, Glass altered to iddingsite; highly oxidized appearance EH-27 hawaiite feeder dike in PL, CPX, ORE, microvesicular and west cliff Glass aphyric EH-71 mugearite east slope J FL, CPX, ORE, doleritic xenoliths of Glass PL, CPX, OL, ORE EH-56 benmoreite inside caldera 1 Glass, PL, ORE doleritic xenoliths as above EH-54 benmoreite inside caldera III Glass, FL, ORE doleritic xenoliths, plus few OL xenocry sts EH-50 benn-xoreite inside caldera 1 Glass, FL, CFX, doleritic xenoliths ORE

-J Appendix 4, Continued

Pheriocrysts and Microphenocrysts Sample No. Rock Type Location PLOL CPX ORE Matrix Remarks EH-lO rhyolite M. Onto Glassy Obsidian microlites of alkali flds., few zircon, px, some fayalitic OL TERE VAKA

EH-302 hawaiite between west PL,CPX, ORE aphynic Poike and M. Glass Anamar ama EC-413 hawailte Vai tea // / / PL,CPX, OL, intergranular tex- ORE tune; PL"..An4550 EC-243 hawaiite km W of PL,OL, CPX nearly aphyric Anakena Bay ORE 177Z9 hawaiite NNW coast 1/ PL,CPX, OL, zoned PLvAn60_50 ORE EC-322 hawaiite north coast, 1/ PL,OL, CPX, PL phenocrysts are near Poike ORE strongly zoned EC-369 hawaiite E. of Hanga 1.1 PL,CPX, ORE nearly aphynic; Tetenga inter granular texture

EC-193 hawaiite .B km E. of PL,CPX, OL, nearly aphyric; sub- Ahu Akahanga ORE ophitic texture Appendix 4, Continued

Phenocrysts and Microphenocrysts Sample No. Rock Type Location FL OL CPX ORE Matrix Remarks

EC-176 hawaiite 1 km NE of I I FL, CPX,OL FL microphenocrysts Koe Koe ORE are zoned and altered; OL altered to ORE EC-153 hawaiite E. of Hana I / / FL, CPX,OL, microglomeropor- Tee ORE phyritic; intergranular texture; OL altered to iddingsite EC-125 hawaiite S. coast- FL, CFX,OL, sub-ophitic texture MotuOoki ORE

17731 hawaiite NW Coast FL, CPX,ORE aphyric; intergranu- OL lar texture; slightly vesicular 17756 hawaiite Hanga Otteo FL, CPX,ORE, aphyric; intergranu- OL lar texture slightly vesicular EH-Z88 hawaiite E. slope, I J Glass, PL, hyalopilitic texture; M. Pui CPX, ORE small xenolith of brown glass with FL xstls. EC-166 hawaiite E. of Hana I FL, CPX,OL intergranular tex- Tee ORE ture; PL microphenocrysts are strongly resorbed Appendix 4, Continued

Phenocrysts and Microphenocrysts Sample No. Rock Type Location PL OL CPXORE Matrix Remarks

EC-1 65 hawaiite E. of Hana PL,OL, CPX, nearly aphyric; inter- Tee ORE granular texture EH-35 hawaiite R. Raraker PL,CPX,OL xenolith in R. Raraku ORE tuff; nearly aphyric; subophitic texture

EH-37 tuff R. Raraker Brown glass presence of mid- coated glass usphe_ rules" suggests partial submarine origin

EH-122 basalt W. of M. O'Koro PL,OL, CPX PL-An50 oscillatory zoning60'

EC-385 hawaiite Hanga Nui PL,CPX, OL, oscillatory zoning of ORE PL phenocrysts

EH-88 hawailte E. of Vaka Kipu PL,OL, CPX, nearly aphyric ORE

EC-207 basalt Roiho PL,OL, CPX, ORE EC-201 hawante Moto Ko FL,OL, CPX, partial alignment of Hepoko ORE PLiaths

EASTER ISLAND DREDGE HAUL 0 KK7Z-35-1 basic 1800 m PL,CPX, ORE hawaute Glass w

Appendix 4, Continued Phenocrysts and Microphenocrysts Sample No. Rock Type Location PL OL CPX ORE Matrix Remarks

KK72-34-43 basic iJOOm PL, OL, CPX, hawaiite ORE

KK7Z.-34-7 basic ilOOm PL, OL, CPX, hawalite ORE KK72-34-34 basic 1100m Glass, PL, CPX hawaiite KK7Z-34-4 basic ilOOm Glass, PL, CPX hawaiite

SALA Y GOMEZ

PV- 302 mugearite subaerial PL, OL, CPX, trachytic texture; ORE 60-80% of rock is PL; OL altered to iddingsite Y74-4-30-4 mugearite dredge PL, Glass, CPX, trachytic texture; ORE ore and CPX are rare Y74-4-30-20 hawaiite dredge PL, Glass, OL, PL,An; vesi- ORE, minor CPX cular;Otaltered to iddingsite

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