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

• Published in: Contributions to mineralogy and petrology 2022-10, Vol.177 (10), Article 94
• Main Authors: Saper, L. M., Baker, M. B., Stolper, E. M.
• Format: Article
• Language: English
• Subjects: Basalt; Composition; Corrections; Earth and Environmental Science; Earth Sciences; Experiments; Ferric-ferrous ratio; Fugacity; Geology; Iron; Iterative methods; Liquids; Low pressure; Magnesium; Mineral Resources; Mineralogy; Olivine; Original Paper; Oxygen; Petrology; Silica; Silicon dioxide; Titanium dioxide; Wire
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-022-01955-0

A set of 1-atm gas-mixing experiments ( n  = 31) was run using Re wire loops at low-oxygen fugacity (ƒO 2 , within one half-log unit of the Fe–FeO buffer) and temperatures of 1175–1400 °C to explore the compositional dependence of the Fe 2+ –Mg olivine–liquid exchange coefficient, K D , Fe 2 + - Mg ol / liq , under conditions where corrections for liquid Fe 3+  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 D , Fe 2 + - Mg ol / liq  values from the n  = 14 experiments run on pre-saturated Re wire loops were fit to a regular solution model that relates K D , Fe 2 + - Mg ol / liq  to temperature, melt composition, and olivine composition. Fe 2+ –Mg exchange in the experiments is well-described using two parameters: the SiO 2  content of the liquid and the coexisting olivine composition. Combining our experiments with a literature compilation of low-pressure, low-ƒO 2 experiments (largely on non-terrestrial bulk compositions) produced a combined data set spanning a broad region of composition space (e.g., liquid TiO 2 and Na 2 O + K 2 O 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 Fe 3+ content of all of the experimental liquids is low, the compositional variation seen in K D , Fe 2 + - Mg ol / liq is effectively independent of the compositional effects on liquid Fe 3+ /Fe 2+ ratios. Given olivine–liquid Mg or Fe 2+ partition coefficient information, it is possible to eliminate the explicit dependence of  K D , Fe 2 + - Mg ol / liq  on olivine composition, enabling a simple iterative approach for calculating the composition of coexisting olivine given only the bulk composition (and Fe 3+ /Fe 2+ ratio) of an olivine-saturated liquid.