Abstract
The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as {text{Kd}}_{{{text{Fe}}^{T} {-} {text{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 {text{Kd}}_{{{text{Fe}}^{T} {-} {text{Mg}}}} using experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range of fO2 (iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+ to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑Fe 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+/∑Fe, we show that for Fo65–98 olivine in equilibrium with basaltic and basaltic andesite melts, {text{Kd}}_{{{text{Fe}}^{T} {-} {text{Mg}}}} decreases linearly with Fe3+/∑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 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+/∑Fe in olivine-bearing experimental melts, from which fO2 can be calculated. In contrast to {text{Kd}}_{{{text{Fe}}^{T} {-} {text{Mg}}}}, the Mn–Mg exchange coefficient, {text{Kd}}_{{{text{Mn}} {-} {text{Mg}}}}, is relatively constant over a wide range of P–T–fO2 conditions. We present an expression for {text{Kd}}_{{{text{Mn}} {-} {text{Mg}}}} that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for {text{Kd}}_{{{text{Fe}}^{T} {-} {text{Mg}}}} and {text{Kd}}_{{{text{Mn}} {-} {text{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+/∑Fe to a precision of ≤ 0.04.
Highlights
The exchange of iron and magnesium between olivine and coexisting melt bears directly on the generation and chemical evolution of basaltic magmas
We suggest that the apparent pressure increase in KdFeT−Mg observed by Ulmer (1989) is a combination of two effects: the decrease in Fe3+∕ΣFe that occurs with increasing pressure at fixed temperature along an fO2 buffer; and the more reduced nature of his higher pressure experiments compared to those at one atmosphere
The experimental data presented above show the strong dependence of Fe–Mg exchange on fO2, but the relative insensitivity of Mn–Mg exchange to the same parameter
Summary
The exchange of iron and magnesium between olivine and coexisting melt bears directly on the generation and chemical evolution of basaltic magmas. At equilibrium KdFe2+−Mg is expected to vary with the free energy of exchange reaction (1) for the pure end-members, as well as any non-ideal interactions of F e2+ and Mg dissolved in melts and in olivine. Despite these expectations, the seminal work of Roeder and Emslie (1970) showed that KdFe2+−Mg is remarkably constant over a wide range of pressure, temperature and composition (P–T–X), such that a value of 0.30 ± 0.03 can be used with some confidence to describe melting and crystallisation in a wide variety of olivine-bearing systems We refer to this as the ‘canonical’ value of KdFe2+−Mg
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