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

Abstract

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.

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