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 vol- canic 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 epi- sodes, 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 differ- entiation 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 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 vol- canoes 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 differ- ences 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 deter- minations 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 0 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 sea- mount 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 con- vective plumes, which, besides giving rise to linear island and sea- mount chains, provide the motive force for sea floor spreading.