A New Oxybarometer for Basalts Based on Olivine-Melt Mn-Fe2+ Exchange Coefficient and FeT/Mn Ratios in Olivine and Melt

Melt Fe3+/FeT ratios are widely used as a proxy for the oxygen fugacity (fO2) in magma systems. A particular challenge is a difficulty in quantifying the Fe2+ and Fe3+ contents, which are not easily measurable by conventional techniques. Using the Mn-Fe 2+ exchange coefficient (KD) between olivine (ol) and melt (m), this study develops a novel approach to this problem using the relations....hip of Fe3+ m.... / FeT m....= 1 - KDMn-Fe2+ ol/m *.... ( FeT ol/M....nol)/( FeT m /Mnm). The key for applying this equation is to combine well-de....fined KDMn-Fe2+ ol/m with accurate Mn and FeT analyses of olivine and coexisting melt. The Mn and FeT analyses are readily measured by electron microprobe. To d....efine KDMn-Fe2+ ol/m, we performed experiments at 1.0-1.75 GPa and 1250-1450 degrees C under graphite-buffered conditions, where Fe T can be used as Fe 2+ to cal....culate KDMn-Fe2+ ol/m. The results show that.... KDMn-Fe2+ ol/m = 0.730 +/- 0.025 is nearly constant over a realistic range of experimental conditions (temperature, pressure, and melt composition) relevant to basaltic magmas, allowing calculation of melt Fe 3+/Fe T ratios without the need for any further knowledge of these parameters. A comparison with published data, where melt Fe 3+/FeT ratios have been directly measured, demonstrates that our calculation method is as accurate as Mossbauer spectroscopy and XANES techniques, thus providing a very simple and accessible approach to melt Fe3+/FeT ratio estimation. However, it should be noted that the accuracy of this method is critically reliant on: (a) accurate Mn and Fe analyses and (b) olivine-melt chemical equilibrium.