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Determination of Oxygen Fugacity Using Olivine-Melt Equilibrium

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Determination of Oxygen Fugacity using Olivine-Melt Equilibrium: Implications for the Redox States of Mid-Ocean Ridge Basalt and Ocean Island Basalt Mantle Source Regions THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Kenneth James Peterman, B.S. Graduate Program in Earth Sciences The Ohio State University 2017 Master's Examination Committee: Dr. Michael Barton, Advisor Dr. Berry Lyons Dr. Tom Darrah Copyright by Kenneth James Peterman 2017 Abstract In order to connect volcanic rocks to their mantle sources, it is essential to consider redox equilibria and their dependence on temperature, pressure, chemical composition, and oxygen fugacity. Oxygen fugacity (fO2) is an intensive variable that strongly affects the behavior of those elements in magmas that are sensitive to changes in redox state, such as Fe, and therefore Mg-Fe silicates, such as olivine. Since fO2 plays an important role in fractional crystallization, in principle, it is possible to estimate fO2 from analyses of olivine in equilibrium with the melt. This research describes a new method based on this principle called the Olivine-Melt Equilibrium Method. This method first calculates Fe3+ and Fe2+ from a relationship involving the partitioning of Mg and Fe2+ between olivine and melt. The ratio of Fe3+/Fe2+ expresses the change in the valence state of Fe, which is related to the redox state of the magma. The calculated Fe3+ and Fe2+ contents of the melt can then be used to determine the fO2 at which magma crystallized from a model described by Kress and Carmichael (1991). This model expresses a 3+ 2+ relationship between the Fe /Fe ratio of the melt, fO2, temperature, pressure, and melt composition. The Olivine-Melt Equilibrium Method has the advantage that olivine and glass compositions are determined by an Electron Probe Micro Analyzer (EPMA), so analyzed Fe3+ and Fe2+ of the melt is not required. This is useful because glass analyses in literature typically report all Fe as ∑FeO, rather than distinguishing between Fe2O3 and ii FeO. Therefore, there is no need for scarce and specialized analytical methods, such as synchrotron-based techniques, to distinguish between the different oxidation states of Fe. Additionally, this method takes advantage of the fact that olivine is ubiquitous in basaltic lavas, unlike Fe-Ti oxides used to estimate fO2 from geothermometer-oxybarometers. We have calculated oxygen fugacities from published analyses of coexisting glass and olivine pairs in 982 samples from two different tectonic settings. The results (expressed as ΔFMQ) for Mid-Ocean Ridge Basalts (MORB) from the Mid-Atlantic Ridge (−1.55 ± 0.75), the East Pacific Rise (−0.65 ± 0.51), the Juan de Fuca Ridge (−0.77 ± 0.42), and the Galápagos Spreading Center (+0.08 ± 0.48) agree with results obtained using other methods and average −1.09 ± 0.89. Ocean Island Basalts (OIB) from Iceland and the Galápagos Islands (ΔFMQ = −0.43 ± 0.71 and −0.33 ± 0.35 respectively) also yield values consistent with those obtained by other methods and fall in the same range as MORB. However, lavas from the Canary Islands are more oxidized than typical MORB and OIB, with average ΔFMQ = +0.68 ± 0.52. The results for MORB and OIB potentially provide evidence for redox heterogeneity in the mantle, possibly as the result of crustal recycling. However it is necessary to evaluate the possibility that fO2 changes during magma ascent before concluding that the oxygen fugacities of erupted magmas directly reflect those of the mantle source regions. iii Acknowledgments Foremost, I would like to thank Dr. Michael Barton for his guidance during my graduate studies, and for making insightful comments and revisions to this thesis. I would also like to thank Dr. Berry Lyons and Dr. Tom Darrah for their helpful advice and time spent reviewing this thesis. I would like to thank Friends of Orton Hall and its donors for supporting my research. Without their support, I would not have been able to collect samples from Iceland during the summer of 2016. I would like to thank my brother, David Peterman, and Collin Oborn for their assistance with sample collection. These samples will tremendously benefit my research in the future. Additionally, FOH assisted with travel funding to attend the 2016 American Geophysical Union Fall Meeting. iv Vita December 24th, 1991 ......................................Born, Columbus Ohio June 2010 .......................................................Watkins Memorial High School May 2014 – April 2015 ..................................Undergraduate Teaching Associate, School of Earth Sciences, The Ohio State University May 2015 ......................................................B.S. Earth Sciences, with Research Distinction, The Ohio State University August 2015 to present .................................Graduate Teaching Associate, School of Earth Sciences, The Ohio State University Fields of Study Major Field: Earth Sciences v Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. iv Vita ...................................................................................................................................... v List of Figures .................................................................................................................... ix List of Tables ..................................................................................................................... xi Chapters: 1. Introduction ..................................................................................................................... 1 1.1 Oxygen Fugacity ....................................................................................................... 2 1.2 Buffers ....................................................................................................................... 3 2. Previous Methods for Estimation of Oxygen Fugacity................................................... 5 2.1 Determination of fO2 using Geothermometer-Oxybarometers ................................. 5 3+ 2+ 2.2 Determination of fO2 using Fe /Fe Ratios ............................................................ 7 3. Olivine-Melt Equilibrium – Previous Work ................................................................... 9 3.1 Exchange of MgO and FeO between Olivine and Liquid ......................................... 9 vi 4. Methodology ................................................................................................................. 13 4.1 Calculation of Oxygen Fugacity from Olivine-Melt Equilibrium .......................... 13 5. Geological Settings ....................................................................................................... 18 5.1 Mantle Source Regions of MORB and OIB............................................................ 18 5.2 Mid-Atlantic Ridge Geological Setting................................................................... 20 5.3 East Pacific Rise Geological Setting ....................................................................... 21 5.4 Juan de Fuca Ridge Geological Setting................................................................... 22 5.5 Galápagos Spreading Center and Galápagos Islands Geological Setting ............... 22 5.6 Iceland Geological Setting ...................................................................................... 24 5.7 Canary Islands Geological Setting .......................................................................... 25 6. Results ........................................................................................................................... 27 6.1 Application to Natural Samples .............................................................................. 27 6.2 Results for Each Geological Setting ....................................................................... 28 7. Discussion ..................................................................................................................... 30 7.1 Comparison between Different Methods for Determining fO2 ............................... 30 7.2 Redox Heterogeneity of MORB and OIB Mantle Source Regions ........................ 31 7.3 Does the Oxidation State of Basalt Reflect that of its Source? ............................... 33 7.4 Comparison of Olivine-Hosted Melt Inclusions and Pillow Glass ......................... 39 7.5 Post Entrapment Modifications to Olivine-Hosted Melt Inclusions ....................... 41 vii 8. Conclusions ................................................................................................................... 43 9. Future Work .................................................................................................................. 45 9.1 Proposed Research in Iceland ................................................................................. 45 9.2 Analysis of Different Tectonic Environments ........................................................ 46 9.3 Determination of Temporal Variations in fO2 ......................................................... 47 List of References ............................................................................................................. 48 Appendix A: Figures ........................................................................................................
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