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 KD) and vice versa (high KD), which produced indistinguishable results. The distribution coefficient, KD, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and KD 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 WG 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.