An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa

Abstract

The partitioning of Mg and Fe²⁺ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO₂ has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low K_D) and vice versa (high K_D), which produced indistinguishable results. The distribution coefficient, K_D, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and K_D is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with $W_{G_{Mg - Fe}^{ol} }$ between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with $W_{G_{Mg - Fe}^{ol} } = 5625 \pm 574 J/mol$ (implying 5450 J/mol at 1 bar) and $W_{G_{Mg - Fe}^{opx} } = 2145 \pm 614 J/mol$, but the two W _G parameters are so highly correlated with each other that our data are almost equally well fit with $W_{G_{Mg - Fe}^{ol} } = 3700 \pm 800 J/mol$, as obtained by Wiser and Wood. This value implies $W_{G_{Mg - Fe}^{opx} } = 280 \pm 900 J/mol$, apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.