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Blundy, Jon, Melekhova, Elena, Ziberna, Luca, Humphreys, Madeleine C. S., Cerantola, Valerio, Brooker, Richard A., McCammon, Catherine A., Pichavant, Michel & Ulmer, Peter (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): 103.

Author(s) from Durham

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.