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

• Published in: Contributions to mineralogy and petrology 2020-10, Vol.175 (11), Article 103
• Main Authors: Blundy, Jon, Melekhova, Elena, Ziberna, Luca, Humphreys, Madeleine C. S., Cerantola, Valerio, Brooker, Richard A., McCammon, Catherine A., Pichavant, Michel, Ulmer, Peter
• Format: Article
• Language: English
• Subjects: Algorithms; Analytical methods; Andesite; Basalt; Composition; Crystallization; Earth and Environmental Science; Earth Sciences; Electron probes; Entrapment; Exchanging; Fayalite; Forsterite; Geology; Inclusions; Iron; Lattice strain; Magnesium; Manganese; Melts; Mineral Resources; Mineralogy; Olivine; Original Paper; Oxidoreductions; Petrology; Sciences of the Universe; Synchrotrons; Temperature effects; Wustite
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-020-01736-7

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 f O 2 (iron-wüstite 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 f O 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– f O 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.