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$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mg = (Feol/Fem)·(Mgm/Mgol), with all FeTexpressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgusing experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range offO2(iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑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+/∑Fe, we show that for Fo65–98olivine in equilibrium with basaltic and basaltic andesite melts,$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgdecreases linearly with Fe3+/∑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 Fo50olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/∑Fe in olivine-bearing experimental melts, from whichfO2can be calculated. In contrast to$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mg, the Mn–Mg exchange coefficient,$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mg, is relatively constant over a wide range of P–T–fO2conditions. We present an expression for$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mgthat incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgand$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mgto olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/∑Fe to a precision of ≤ 0.04.