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University of Florida Thesis Or Dissertation GEOLOGY AND PETROGENESIS OF LAVAS FROM AN OVERLAPPING SPREADING CENTER: 9°N EAST PACIFIC RISE By V. DORSEY WANLESS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010 1 © 2010 V. Dorsey Wanless 2 To my family and friends 3 ACKNOWLEDGMENTS I would like to thank my advisor (Michael Perfit), collaborators (W. Ian Ridley, Emily Klein, Scott White, Paul Wallace, John Valley, and Craig Grimes) and supervisory committee (Paul Mueller, Ray Russo, George Kamenov, and David Richardson) for their mentoring and encouragement throughout this study. Additionally, I would like to thank the Captain, officers and crew of the R/V Atlantis for all their help during cruise AT15-17, the MEDUSA2007 Science party (including S. White, K. Von Damm, D. Fornari, A. Soule, S. Carmichael, K. Sims, A. Fundis, A. Zaino, J. Mason, J. O’Brien, C. Waters, F. Mansfield, K. Neely, J. Laliberte, E. Goehring, and L. Preston) for their diligence in collecting data and samples for this study. I thank the ROV Jason II shipboard and shore-based operations group for their assistance in collecting these data and HMRG for processing all DSL-120 side scan and bathymetry data collected during this cruise. Discussions with S. White and A. Goss are gratefully acknowledged. Thanks to G. Kamenov and the UF Center for Isotope Geosciences for laboratory assistance and to the Department of Geological Sciences staff for all of their help. Finally, I thank my friends and family for all their support over the years. This research was supported by the National Science Foundation (grants OCE-0527075 to MRP and OCE-0526120 to EMK). 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...................................................................................................... 4 LIST OF TABLES ................................................................................................................ 8 LIST OF FIGURES .............................................................................................................. 9 ABSTRACT........................................................................................................................ 12 CHAPTER 1 INTRODUCTION ........................................................................................................ 14 2 DACITE PETROGENESIS ON MID-OCEAN RIDGES: EVIDENCE FOR OCEANIC CRUSTAL MELTING AND ASSIMILATION ............................................ 17 Abstract ....................................................................................................................... 17 Introduction ................................................................................................................. 18 Geologic and Tectonic Setting ................................................................................... 22 9°N East Pacific Rise – Overlapping Spreading Center ..................................... 22 Juan de Fuca Ridge Propagating Ridge Tip and Axial Seamount ..................... 24 Galápagos Spreading Center- Extinct OSC or Propagating Ridge Tip? ........... 25 Petrography ................................................................................................................ 26 Geochemical Methods ................................................................................................ 26 Geochemical Results .................................................................................................. 27 Major Element Results ......................................................................................... 27 Trace Element Results......................................................................................... 28 Isotopic Data ........................................................................................................ 29 Petrogenetic Models For High-Silica Lavas ............................................................... 29 Crystal Fractionation ............................................................................................ 30 Rayleigh fractional crystallization .................................................................. 30 Crystal-melt segregation model .................................................................... 33 In situ crystallization calculations .................................................................. 34 Partial Melting (Anatexis) ..................................................................................... 35 Assimilation Fractional Crystallization ................................................................. 37 Discussion ................................................................................................................... 41 Petrogenesis of High Silica Lavas ....................................................................... 41 Geochemical evidence of partial melting ...................................................... 41 The need for crystallization, assimilation and altered crust in dacite petrogenesis ............................................................................................... 44 Isotopic signature of assimilation .................................................................. 45 AFC Processes and Tectonic Setting ................................................................. 48 Model for Formation of MOR Dacites .................................................................. 49 Relationship of Melt Lens to Dacites at 9°N........................................................ 50 5 Effects of Assimilation on Typical MORB Compositions .................................... 51 Conclusions ................................................................................................................ 52 Acknowledgements..................................................................................................... 53 3 ROLE OF ASSIMILATION IN THE PETROGENESIS OF LAVAS ON MID- OCEAN RIDGES INFERRED FROM Cl, H2O, CO2 AND OXYGEN ISOTOPE VARIATIONS .............................................................................................................. 79 Introduction ................................................................................................................. 79 Geologic Setting ......................................................................................................... 81 Analytical Methods and Results ................................................................................. 83 Major and Trace Elements................................................................................... 83 Volatile Elements ................................................................................................. 84 Oxygen Isotope Analyses and Results ............................................................... 85 Discussion ................................................................................................................... 85 Magma Crystallization versus Assimilation-Fractionation-Crystallization Processes ......................................................................................................... 85 Evidence for Assimilation from Oxygen Isotopes ............................................... 87 CO2 and H2O Degassing, Magma Ascent Rates and Depth of Assimilant ........ 89 Source of Assimilant ............................................................................................ 91 Oceanic Plagiogranites ........................................................................................ 92 Conclusions ................................................................................................................ 93 4 CRUSTAL DIFFERENTIAION AND SOURCE VARIATIONS AT THE 9°N OVERLAPPING SPREADING CENTER; EAST PACIFIC RISE ............................ 102 Introduction ............................................................................................................... 102 Background, Tectonic Setting and Geology of the 9oN OSC .................................. 104 Overlapping Spreading Centers ........................................................................ 104 Tectonic Setting and Previous Studies of 9°N OSC ......................................... 104 9°N OSC Geology .............................................................................................. 106 8°37’ N EPR Deval ............................................................................................. 108 Geochemical Methods .............................................................................................. 109 Geochemical Results ................................................................................................ 110 Geochemistry of East Limb Lavas ..................................................................... 110 Basalts ......................................................................................................... 111 Basaltic andesites/low-P2O5 andesites ....................................................... 111 High-P2O5 andesites.................................................................................... 112 Dacites ......................................................................................................... 112 Isotopic compositions of East Limb lavas ................................................... 113 West Limb Lavas................................................................................................ 113 8°37’ EPR Lavas ...............................................................................................
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