Effect of redox on Fe-Mg-Mn exchange between olivine and melt and an oxybarometer for basalts

The Fe-Mg exchange coefficient between olivine (ol) and melt (m), defined as Kd(FeT-Mg) = (Fe-ol/Fe-m)center dot(Mg-m/Mg-ol), with all Fe-T expressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on Kd(FeT-Mg) using experimental, olivine-saturated basaltic glasses with variable H2O (<= 7 wt%) over a wide range of fO(2)(iron-wustite buffer to air), pressure (<= 1.7 GPa), temperature (1025-1425 degrees C) and melt composition. The ratio of Fe3+ to total Fe (Fe3+/n-ary sumation Fe), as determined by Fe K-edge mu XANES and/or Synchrotron Mossbauer Source (SMS) spectroscopy, lies in the range 0-0.84. Measured Fe3+/n-ary sumation Fe is consistent (+/- 0.05) with published algorithms and appears insensitive to dissolved H2O. Combining our new data with published experimental data having measured glass Fe3+/Sigma Fe, we show that for Fo(65-98) olivine in equilibrium with basaltic and basaltic andesite melts, Kd(FeT-Mg) decreases linearly with Fe3+/Sigma Fe with a slope and intercept of 0.3135 +/- 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 +/- 0.0011. This is the value at Fo(50) olivine; at higher and lower F0 the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/Sigma Fe in olivine-bearing experimental melts, from which fO(2) can be calculated. In contrast to Kd(FeT-Mg), the Mn-Mg exchange coefficient, Kd(Mn-Mg), is relatively constant over a wide range of P-T-fO(2) conditions. We present an expression for Kd(Mn-Mg) that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for Kd(FeT-Mg) and Kd(Mn-Mg) to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/Sigma Fe to a precision of <= 0.04.