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Olivine-Melt Equilibrium and the Redox State of Magmas

Peterman, Kenneth James
http://rave.ohiolink.edu/etdc/view?acc_num=osu1714736827298362

Abstract Details

2024, Doctor of Philosophy, Ohio State University, Earth Sciences.
Knowledge of the distribution of Fe2+ and Mg between olivine and melt (the distribution coefficient, KD) is crucial to understand the origin and evolution of magmas. However, there is disagreement regarding which variables (temperature, melt composition, and oxygen fugacity – fO2) influence the value of KD, as well as the magnitude of their effects. To evaluate the dependence of KD on these variables, data were compiled from literature consisting of equilibrium olivine-melt pairs in experiments at controlled temperature, fO2, and 1 atm pressure. The results confirm that KD is essentially independent of temperature and fO2. However, it is strongly dependent on melt composition (particularly the concentration of silica and alkalis). An evaluation of different published formulations for KD using these data demonstrates that the expression of Gee and Sack (1988) is the most accurate and precise. Furthermore, a new and simpler model based on variation of KD with silica and alkalis has been fit to the olivine-melt database. This reproduces KD with the same accuracy and precision as the Gee and Sack (1988) formulation. The olivine-melt database also illustrates that KD can be used to calculate the proportion of the different valence states of iron in the melt (the Fe3+/ΣFe ratio), which cannot be measured using routine analytical techniques. The melt Fe3+/ΣFe can then be related to fO2 using empirical relationships. This method, referred to as the Olivine-Melt Equilibrium (OME) method, reproduces the fO2 imposed on the experiments within ±0.3 log units. This method was applied to compiled data for natural samples from literature from mid-ocean ridges, ocean islands, back-arc basin spreading centers, and volcanic arcs. Olivine-melt calculated values of fO2 for each location investigated agree with the results of independent techniques. These include compiled measurements of Fe3+/ΣFe ratios using Fe K-edge μ-X-ray Absorption Near Edge Structure (XANES) spectroscopy, as well as compiled results of various oxybarometers based on Fe-Ti oxides. These methods show that there is no meaningful difference in the fO2 of basalts from mid-ocean ridges and most ocean islands (fO2 relative to that of the FMQ buffer: ΔFMQ ≈ -0.2), except for unique locations, such as the Canary Islands. However, back-arc basins (ΔFMQ ≈ 0.2) and volcanic arcs (ΔFMQ ≈ 1.0) are systematically more oxidized. There are inherent uncertainties in acquiring data for equilibrium olivine-melt pairs from literature. Furthermore, independent techniques have mostly been applied to samples from the same location, rather than the same samples. To address this, samples were collected from Iceland, and were analyzed by Electron Probe Microanalysis to precisely constrain the compositions of equilibrium olivine-melt pairs. A subset of these samples was also analyzed via XANES spectroscopy. The results of the OME method are identical to those of XANES spectroscopy. Furthermore, these results demonstrate that only the Zhang et al. (2018) XANES calibration is consistent with measured compositions. However, the application of the OME method and XANES spectroscopy to olivine-hosted melt inclusions shows that they are often not in equilibrium with their hosts. Therefore, this complicates the interpretation of data obtained from olivine-hosted melt inclusions.
Michael Barton (Advisor)
Berry Lyons (Committee Member)
Daniel Kelley (Committee Member)
Derek Sawyer (Committee Member)
David Cole (Committee Member)
411 p.
Earth; Geochemistry; Geological; Geology; Petrology; Plate Tectonics
olivine; distribution coefficient; equilibrium; oxygen fugacity; XANES; melt inclusion

Recommended Citations

Citations

  • Peterman, K. J. (2024). Olivine-Melt Equilibrium and the Redox State of Magmas [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1714736827298362

    APA Style (7th edition)

  • Peterman, Kenneth. Olivine-Melt Equilibrium and the Redox State of Magmas. 2024. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1714736827298362.

    MLA Style (8th edition)

  • Peterman, Kenneth. "Olivine-Melt Equilibrium and the Redox State of Magmas." Doctoral dissertation, Ohio State University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=osu1714736827298362

    Chicago Manual of Style (17th edition)

osu1714736827298362
© 2024, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.