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Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean

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Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean AN ABSTRACT OF THE THESIS OF Frank M. Sprtel for the degree of Master of Science in Oceanography presented on June 23, 1997. Title: Two Styles of Oceanic Near-RidgVo1canism for the South Indian Ocean and the NE Pacific Oean Redacted for privacy Abstract approved: David M. Christie Near-ridge volcanism is an integral component of a mid-ocean ridge system that defines the off-axis extent of the magmatic region sunounding the ridge. Perhaps the two most common expressions of near-ridge volcanism in the ocean basins are near-ridge seamounts and intra-transform volcanic centers. Seafloor near-ridge seamount volcanism was examined for the Southeast Indian Ridge (SEIR), a part of the global spreading system that lies between the Australian and Antarctic continents. During the 1995 WESTWARD 9 & 10 cruises, 104 seamounts were discovered adjacent to a undersurveyed and undersampled 2500km section of the SEW from 88°E to 120°E. SEW seamounts are evenly distributed on the Australian and Antarctic plates. Australian plate seamounts are generally larger in size than Antarctic plate seamounts. Moreover, all seamounts aligned in chains were found on the Australian plate. Such an asymmetric distribution of seainount chains also occurs at the Juan de Fuca ridge in the NE Pacific and may be related to mid-ocean ridge migration. Four of the seven SEW seamount chains are aligned in a direction oblique to absolute plate motion. The presence of oblique trending sealnount chains may be related to along-axis asthenospheric flow. Twelve of the 104 seamounts surveyed were sampled by WESTWARD 10. Seamount basalt glasses were analyzed for major, minor, and trace element content. These data indicate that the seamount lavas are similar in composition to axial lavas and have undergone crystal fractionation. Intra-transform volcanism was examined for the Blanco Fracture Zone, a transform fault that connects the Juan de Fuca and Gorda Ridges in the NE Pacific. A 1994 NOAA cruise utilizing the U.S. Navy's Advanced Tethered Vehicle (ATV) discovered fresh- looking lava flows in the East Blanco Depression (EBD), a small basin within the Blanco Fracture Zone. Glasses from pillow basalts recovered by the ATV were analyzed for major, minor, and trace element content. The glass data span a large range in MgO (-5-9.2 wt.%). Minor and trace elements were modeled by batch and open system fractional melting with 6 % retained melt. These data suggest that both batch melting and open system fractional melting may have been involved in the initial stages of the Blanco lava development while a minor crystal fractionation signature was superimposed on these lavas prior to eruption. Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean by Frank M. Sprtel A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented June 23, 1997 Commencement June 1998 Master of Science thesis of Frank M. Snrtel presented on June 23. 1997 Redacted for privacy Major Pro1ssor, representing Redacted for privacy Dei of College of Oceanic & Sciences Redacted for privacy Dean of I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for privacy Frank M. SrteVauthor Acknowledgments I would like to whole-heartedly thank my two advisers, Dave Christie and Bob Duncan, for their advice and support of my interests during my tenure at Oregon State University. Special credit goes to Dave Christie, who has certainly improved my scientific writing! Andy Ungerer, Roger Nielsen, and Peter Michael were great to have as mentors in the lab because of their broad experience in different analytical techniques. A very special thanks to my family for all of their love, support, and reassurance during my thesis work. I owe as much gratitude to my partner, Kate Metzger, for her constant love, understanding, and encouragement. As a fellow Wisconsinite, Brendan Sylvander, was a good friend and extremely helpful in easing the burden of day-to-day graduate level research. And last but not least...I am forever grateful to my fellow distance running partners, Dave, Jeremy, and Dylan for offering me a daily escape from the rigors of my graduate program. This research was supported by a grant from the National Science Foundation. Contribution of Authors Trace element analyses were performed in the laboratory of Dr. Dave Christie who also provided guidance and advise for the two projects contained in this thesis. Dr. Roger Nielsen helped interpret the data from the East Blanco Depression and ran the electron microprobe lab that produced the major element analyses for both projects. Table of Contents Page Chapter I: Introduction I Chapter II: A Morphologic, Kinematic, and Petrologic Study of Near-Axis Southeast Indian Ridge Seamounts 4 Abstract 5 Introduction 5 Methods 9 Observations 11 Distribution of seamounts along the SEIR 11 Seamount distribution with individual segments 13 SEW seamount chain orientation 21 SEW seamount chemistry 23 Discussion/Interpretation 31 Distribution of seamounts along the SEW 31 Seamount distribution within individual segments 35 SEW searnount chain orientation 39 SEW seamount chemisny 42 Conclusion 45 References 46 Chapter ifi: The Petrogenesis of Intrabasin Volcanics from the East Blanco Depression 49 Abstract 50 Introduction 50 Geologic History 51 Methods 54 Results 54 Petrography and mineral chernistiy 54 Chemistry of East Blanco Depression lavas 56 Table of Contents, Continued Page Discussion 59 Polybaric fractional crystallization model 59 Batch and fractional melting models 63 Open system fractional melting model 67 Crystal fractionation correction for the open system fractional melting model 69 Two scenarios for the petrogenesis of the Blanco lava suite 69 Conclusion 71 References 73 Chapter IV: Summary 75 Bibliography 78 Appendices 83 List of Figures Figure Page 11.1 a Map of the Southeast Indian Ridge from 88°-120°E 6 II.lb-e Boxed sections of the SEW 7 11.2 Map of the East Pacific Rise from 5°-15°N 10 11.3a-b Histograms of seamounts on the Australian Plate 12 11.4a-b Histograms of seamounts on the Antarctic Plate 12 11.5 The ll1°ESeamountField,Box4 14 11.6 SEW seamount height versus longitude 15 11.7 SEW seamount basal diameter versus longitude 16 11.8a-b Histograms of SEW and EPR seamount heights 17 11.9 a-b Histograms of SEW and EPR seamount basal diameters 18 11. lOa-b SEW seamount chains and individual seamounts position in segment 19 11.1 la-b SEW and EPR seamount chain position in segment 20 11.12 Bathymetry map of 89°E seamount chain 22 IL 13 Bathymetry map of 99°E seamount chain 24 11.14 Bathymetiy map of 90°E seamount chain 25 11.15 Bathymetry map of 101°E seamount chain 26 11.16 Bathymetry map of 101 °50'E seamount chain 27 IL 17 Bathymetry map of 107°E seamount chain 28 11.18 SEW seamount major and minor element variation diagrams 29 11.19 SEW seamount trace element spider diagram 30 11.20a-b Davis and Karsten (1986) seamount chain evolution model 33 11.21 Along-axis mantle flow model of West and Christie (1996) 36 11.22 SEW seamount chain apparent flow velocity versus longitude 40 List of Figures, Continued Figure Page 11.23a-f Oblique seamount chain formation model 41 11.24 SEIR seamount La/Sm versus MgO variation diagram 43 111.1 East Blanco Depression location map 52 111.2 Bathymetry map of the East Blanco Depression 53 111.3 Anorthite composition of plagioclase phenocrysts in glass versus Na20/CaO of host glass 55 111.4 Fosterite composition of olivine phenocrysts versus Mg# compared with calculated olivine composition for host glass versus Mg# 55 ffl.5a-b K20 versus MgO and Ti02 versus MgO variation diagrams for Juan de Fuca, Gorda, and Blanco lava suites 57 ffl.6a-b CaO versus MgO and K .OITiO2 versus MgO variation diagrams for Juan de Fuca, Gorda, and Blanco lava suites 58 111.7 Blanco lava suite spider diagram 60 ffl.8a-b CaO versus MgO and Ti02 versus MgO variation diagrams for polybaric crystal fractionation 61 HJ.9a-b K20 versus MgO and (La/Sm)n versus MgO variation diagrams for polybaric crystal fractionation 62 IIJ.lOa-b Ti versus Zr and Ti/Zr versus (La/Sm)n variation diagrams for batch and open system fractional melting 64 111.1 la-b (La/Sm)n versus Ba and (La/Sm)n versus K20 variation diagrams for batch and open system fractional melting 65 111.12 (CeIYb)n versus Sm variation diagram for batch and open system fractional melting 66 List of Appendices Page Appendix I: Electron Microprobe and ICP-MS Analytical Techniques 84 Electron Microprobe Analytical Technique 84 ICP-MS Analytical Technique 84 Sample preparation 84 Sample digestion and dilution 85 Appendix II: Basalt Glass Analyses for SEIR Seamount Lavas 86 Table All.1 87 Appendix III: Basalt Glass Analyses for East Blanco Depression Lavas 106 Table All. 1 107 Appendix IV: Batch and Open System Fractional Melting Model Parameters 110 Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean Chapter I Introduction The global mid-ocean ridge system, traced for 80,000 km throughout the world's oceans (Mammerickx, 1989), is the largest volcanic construct on earth. Mid-ocean ridges mark divergent plate boundaries where two oceanic plates spread apart from each other as new oceanic crust forms. Although mid-ocean ridges are loci of most oceanic volcanism, their volcanic activity is supplemented by eruptions at other locations on the seafloor such as near-ridge seaxnounts, intra-transform extensional basins, and hotspots.
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