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Published October 2022 | public
Journal Article

Fe²⁺-Mg partitioning between olivine and liquid at low oxygen fugacity: an experimental and thermodynamic framework


A set of 1-atm gas-mixing experiments (n = 31) was run using Re wire loops at low-oxygen fugacity (ƒO₂, within one half-log unit of the Fe–FeO buffer) and temperatures of 1175–1400 °C to explore the compositional dependence of the Fe²⁺–Mg olivine–liquid exchange coefficient, K^(ol/liq)_(D,Fe²⁺-Mg), under conditions where corrections for liquid Fe³⁺ are small. The bulk compositions used for these experiments include a picrite, a high-alumina basalt, and a suite of three MORB compositions with variable Fe/Mg. The K^(ol/liq)_(D,Fe²⁺-Mg) values from the n = 14 experiments run on pre-saturated Re wire loops were fit to a regular solution model that relates K^(ol/liq)_(D,Fe²⁺-Mg) to temperature, melt composition, and olivine composition. Fe²⁺–Mg exchange in the experiments is well-described using two parameters: the SiO₂ content of the liquid and the coexisting olivine composition. Combining our experiments with a literature compilation of low-pressure, low-ƒO₂ experiments (largely on non-terrestrial bulk compositions) produced a combined data set spanning a broad region of composition space (e.g., liquid TiO₂ and Na₂O + K₂O contents up to 18.4 wt% and 8.4 wt%, respectively). Fitting this expanded experimental database required two additional liquid compositional terms: Ti, and a Si-(Na + K) cross-term. Because the Fe³⁺ content of all of the experimental liquids is low, the compositional variation seen in K^(ol/liq)_(D,Fe²⁺-Mg) is effectively independent of the compositional effects on liquid Fe³⁺/Fe²⁺ ratios. Given olivine–liquid Mg or Fe²⁺ partition coefficient information, it is possible to eliminate the explicit dependence of K^(ol/liq)_(D,Fe²⁺-Mg) on olivine composition, enabling a simple iterative approach for calculating the composition of oexisting olivine given only the bulk composition (and Fe³⁺/Fe²⁺ ratio) of an olivine-saturated liquid.

Additional Information

We would like to acknowledge the thoughtful suggestions of Paul Asimow, John Eiler, and Claire Bucholz on an earlier version of this work. The comments of two anonymous reviewers lead to improvements in the manuscript.

Additional details

August 22, 2023
October 24, 2023