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

Abstract

Abstract 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, $${\mathrm{K}}_{\mathrm{D},{\mathrm{Fe}}^{2+}-\mathrm{Mg}}^{\mathrm{ol}/\mathrm{liq}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:mi>Mg</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ol</mml:mi> <mml:mo>/</mml:mo> <mml:mi>liq</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , 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 $${\mathrm{K}}_{\mathrm{D},{\mathrm{Fe}}^{2+}-\mathrm{Mg}}^{\mathrm{ol}/\mathrm{liq}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:mi>Mg</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ol</mml:mi> <mml:mo>/</mml:mo> <mml:mi>liq</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> values from the n = 14 experiments run on pre-saturated Re wire loops were fit to a regular solution model that relates $${\mathrm{K}}_{\mathrm{D},{\mathrm{Fe}}^{2+}-\mathrm{Mg}}^{\mathrm{ol}/\mathrm{liq}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:mi>Mg</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ol</mml:mi> <mml:mo>/</mml:mo> <mml:mi>liq</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> 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 $${\mathrm{K}}_{\mathrm{D},{\mathrm{Fe}}^{2+}-\mathrm{Mg}}^{\mathrm{ol}/\mathrm{liq}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:mi>Mg</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ol</mml:mi> <mml:mo>/</mml:mo> <mml:mi>liq</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> 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 $${\mathrm{K}}_{\mathrm{D},{\mathrm{Fe}}^{2+}-\mathrm{Mg}}^{\mathrm{ol}/\mathrm{liq}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>,</mml:mo> <mml:msup> <mml:mrow> <mml:mi>Fe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>-</mml:mo> <mml:mi>Mg</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ol</mml:mi> <mml:mo>/</mml:mo> <mml:mi>liq</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> 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.

Topics & Concepts