Effect of pressure on Fe3+/ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients

Abstract

Experiments establishing the effect of pressure on the Fe 3+ /ΣFe ratio of andesitic silicate melts buffered by coexisting Ru and RuO 2 were performed from 100 kPa to 7 GPa and 1400-1750°C. Fe 3+ /ΣFe ratios were determined by room temperature Mössbauer spectroscopy, but corrected for the effects of recoilless fraction. Fe 3+ /ΣFe ratios in quenched glasses decrease with increasing pressure consistent with previous results between 100 kPa and 3 GPa (O'Neill et al., 2006), but show only small pressure effects above 5 GPa. Ratios also decrease with increasing temperature. Mössbauer hyperfine parameters indicate mean coordination of Fe 3+ ions of ~5 in glasses, with no dependence on the pressure from which the glasses were quenched, but show an increase with pressure in mean coordination of Fe 2+ ions, from ~5 to ~6. XANES spectra on these glasses show variations in pre-edge intensities and centroid positions that are systematic with Fe 3+ /ΣFe, but are displaced from those established from otherwise identical andesitic glasses quenched at 100 kPa (Zhang et al. 2016). These systematics permit construction of a new XANES calibration curve relating pre-edge sub-peak intensities to Fe 3+ /ΣFe applicable to high pressure glasses. Consistent with

Core formation and magma ocean outgassing set planetary N-S-C ratios

“…The success of core formation and an oxidized atmosphere setting (S/N) OE , (C/N) OE , and (C/S) OE ratios given a gas-poor bulk Earth ( Figure 6) is consistent with the recent predictions of magma ocean ƒO 2 profiles with depth 40,41 , where deep magma oceans are expected to co-exist with oxidized atmospheres.…”

“…If it is, then this model is identified as successful and conditions are logged (Figure 4d-f). the pressure-dependence of Fe +3 -Fe +2 equilibria 40,41 . Satisfying (C/S) OE ratios, on the other hand, requires oxidizing atmospheric conditions.…”

“…We then calculate bulk Earth nitrogen and carbon contents as a function of sulfur content using the S/N and C/N ratios from different groups of chondrites7 .For each core formation pressure evaluated, we also explore the effect of atmospheric ƒO 2 (IW+4 to IW-3) as an independent variable(Figure 4a-c). Variations in atmospheric state may be driven by changes in ferric iron stability with pressure40,41 and can lead to more reducing or more oxidizing shallow conditions compared to that associated with metal-magma reactions at depth. Atmosphere ƒO 2is important for N and C because their volatility also depends on their dominant valance state.…”

What Factors Affect the Duration and Outgassing of the Terrestrial Magma Ocean?

“…The assumption of greenhouse gases H 2 O and CO 2 as major species is in accordance with an oxidized MO surface (Hirschmann 2012;Zhang et al 2017) and bulk silicate Earth .…”

A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres

“…Such an MO involved mechanism has yet to be fully established because relevant redox controlling reactions are still poorly constrained in realistic magma ocean scenarios. Several studies have inferred the oxygen fugacity (f O 2 ) profile of silicate melts at high pressures using the experimental data at zero or relatively low pressures and is applicable only for shallow magma oceans 13,14 . The oxygen fugacity is a function of pressure, temperature, and composition, thus likely varying greatly within MOs that could have extended very deep, even covering the entire mantle regime.…”