Ferric Iron Partitioning between Pyroxene and Melt: Experiments, Microbeam analysis, and Consequences for Mantle Redox

Abstract

Pyroxene is the chief reservoir of Fe3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe3+ partitioning have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with fO2 buffered and modulated by Ru+RuO2 and Fe-Pt alloy capsules, respectively, between ∆QFM -2.68 and +5.13. Fe3+/FeT of both cpx and melt were determined by Fe K-edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al2O3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe3+ is moderately incompatible in cpx and correlates with cpx Al2O3 content, increasing from 0.05±0.09 to 0.81±0.04. Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe3+/FeT for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al2O3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe3+, leading to greater Fe2O3 contents in high degree partial melts, and this accounts for an inverse relationship between Na2O and Fe2O3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict for partial melting of the mantle, and so they do not accurately determine the relationship between the fO2 and Fe2O3 of peridotite in basalt source regions. To estimate the Fe3+/FeT ratio of the mantle source of MORB, we modeled liquid Fe2O3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe3+/FeT ratios between 0.02–0.06. A source with an Fe3+/FeT ratio of 0.038±0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe3+/FeT but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe3+/FeT ratios. Therefore, elevated Fe3+/FeT ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.