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

Abstract : The Fe-Mg exchange coefficient between olivine (ol) and melt (m), defined as Kd Fe T −Mg = (Fe ol /Fe m)•(Mg m /Mg ol), with all Fe T expressed as Fe 2+ , is one of the most widely used parameters in petrology. We explore the effect of redox conditions on Kd Fe T −Mg using experimental, olivine-saturated basaltic glasses with variable H 2 O (≤ 7 wt%) over a wide range of fO 2 (ironwüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025-1425 °C) and melt composition. The ratio of Fe 3+ to total Fe (Fe 3+ /∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0-0.84. Measured Fe 3+ /∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H 2 O. Combining our new data with published experimental data having measured glass Fe 3+ /∑Fe, we show that for Fo 65-98 olivine in equilibrium with basaltic and basaltic andesite melts, Kd Fe T −Mg decreases linearly with Fe 3+ /∑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 Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe 3+ /∑Fe in olivine-bearing experimental melts, from which fO 2 can be calculated. In contrast to Kd Fe T −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 Fe T −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 Fe 3+ /∑Fe to a precision of ≤ 0.04.
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Jon Blundy, Helena Melekhova, Luca Ziberna, Madeleine C.S. Humphreys, Valerio Cerantola, et al.. Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts. Contributions to Mineralogy and Petrology, Springer Verlag, 2020, 175 (11), ⟨10.1007/s00410-020-01736-7⟩. ⟨insu-03092886⟩

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