Partitioning of Fe₂O₃ in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record f_O2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and f_O2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe₂O₃ between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe₂O₃ (DFe2O3spl/melt) increases as spinel Fe₂O₃ concentrations increase, independent of increases in f_O2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find DFe2O3opx/melt = 0.63 ± 0.10 and DFe2O3cpx/melt = 0.78 ± 0.30. MORB Fe₂O₃ and Na₂O concentrations are consistent with a modeled MORB source with Fe₂O₃ = 0.48 ± 0.03 wt% (Fe³⁺/ΣFe = 0.053 ± 0.003) at potential temperatures (T_P) from 1320 to 1440 °C. The temperature-dependence of the DFe2O3spl/melt function alone allows ~ 40% of the variation in MORB compositions. If we allow DFe2O3opx/melt and DFe2O3opx/melt to also vary with temperature by tying them to spinel Fe₂O₃ through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe₂O₃ concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe³⁺/ΣFe = 0.03 that predict f_O2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with f_O2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.