GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 189-5
Presentation Time: 9:00 AM-6:30 PM


PETERMAN, Kenneth, MCCANN, Veronica and BARTON, Michael, School of Earth Sciences, The Ohio State University, 125 S Oval Mall, Columbus, OH 43210

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 Fe-Mg 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 requires a model that describes the relationship between Fe3+/Fe2+, T, P, melt composition, and fO2. To determine the Fe3+/Fe2+ of melt in equilibrium with olivine, an olivine-melt distribution coefficient (KD) is utilized. Therefore, this method can be used even if the Fe3+/Fe2+ ratio of the melt is unknown.

This research includes application of the Olivine-Melt Equilibrium Method to datasets from various published experimental runs under anhydrous conditions at 1 bar. Since there are several different KD equations and empirical relationships between Fe3+/Fe2+ and fO2, this research involves using these experimental datasets in order to constrain which equation best reproduces the fO2 measured during these experiments. First, published experimental data with analyzed Fe2O3 and FeO contents were used to calculate fO2 using different Fe3+/Fe2+ models. The model that best reproduces the fO2 imposed on the experiments was selected after filtering the runs to the range of fO2 expected for natural samples (FMQ ± 3). Next, to select the appropriate KD equation, the Olivine-Melt Equilibrium Method was applied to experimental data in which the Fe2O3 and FeO contents are unknown, with total Fe reported as FeOT. These datasets were filtered to ensure the olivine-melt pairs represent equilibrium conditions. The Fe3+/Fe2+ model of Kress and Carmichael (1991), and the KD of Gee and Sack (1988) were found to best reproduce experimental results (± 0.3 log units fO2).