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

Blundy, Jon; Melekhova, Elena; Ziberna, Luca; Humphreys, Madeleine C.S.; Cerantola, Valerio; Brooker, Richard A.; McCammon, Catherine A.; Pichavant, Michel; Ulmer, Peter

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


Authors

Jon Blundy

Elena Melekhova

Luca Ziberna

Valerio Cerantola

Richard A. Brooker

Catherine A. McCammon

Michel Pichavant

Peter Ulmer



Abstract

The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as KdFeT −Mg = (Feol/Fem)·(Mgm/Mgol), with all FeT expressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on KdFeT −Mg using experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range of fO2 (ironwüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+ to total Fe ( Fe3+/ΣFe), as determined by Fe K-edge μXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/ ΣFe 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+/ ΣFe, we show that for Fo65– 98 olivine in equilibrium with basaltic and basaltic andesite melts, KdFeT −Mg decreases linearly with Fe3+/ ΣFe 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 Fo50 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 Fe3+/ ΣFe in olivine-bearing experimental melts, from which fO2 can be calculated. In contrast to KdFeT −Mg , the Mn–Mg exchange coefficient, KdMn−Mg , is relatively constant over a wide range of P–T–fO2 conditions. We present an expression for KdMn−Mg that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for KdFeT −Mg and KdMn−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+/ ΣFe to a precision of ≤ 0.04.

Citation

Blundy, J., Melekhova, E., Ziberna, L., Humphreys, M. C., Cerantola, V., Brooker, R. A., …Ulmer, P. (2020). Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts. Contributions to Mineralogy and Petrology, 175(11), Article 103. https://doi.org/10.1007/s00410-020-01736-7

Journal Article Type Article
Acceptance Date Sep 11, 2020
Online Publication Date Oct 15, 2020
Publication Date 2020-11
Deposit Date Oct 28, 2020
Publicly Available Date Oct 28, 2020
Journal Contributions to Mineralogy and Petrology
Print ISSN 0010-7999
Electronic ISSN 1432-0967
Publisher Springer
Peer Reviewed Peer Reviewed
Volume 175
Issue 11
Article Number 103
DOI https://doi.org/10.1007/s00410-020-01736-7

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