The behaviour of ferric iron during partial melting of peridotite

Abstract

In order to better understand the behaviour of Fe3+ during partial melting of a mid-ocean-ridge-basalt (MORB) mantle source, we performed partial melting experiments of a spinel peridotite at 1.5 GPa, between 1320 and 1450 °C, and over a range of oxygen fugacities (fO2) varying from FMQ-3.5 to FMQ+6 (log-units relative to FMQ = Fayalite-Magnetite-Quartz oxygen buffer). Changes in fO2 were achieved using different capsule materials and by varying the oxidation state of the starting material. Fe3+ contents of spinels and glasses were examined using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy in full-field mode. Our results show that this method allows Fe3+/ƩFe measurements to be made of spinels in our experimental charges but is not suitable for quantifying Fe3+/ƩFe of glasses, particularly at relatively low fO2 where melt Fe2O3 contents are small. We therefore employed several methods (Fe3+/ƩFe ratios of spinels measured by XANES and calculated by stoichiometry, and Fe-alloy sliding redox sensors) to assess the fO2 of the samples. These were then used to calculate the Fe3+/ƩFe ratios of glasses using published models, which were further constrained by Mössbauer spectroscopy measurements on an additional set of partial melting experiments.For a particular initial redox state, the Fe3+/ΣFe ratio in melts remains approximately constant with partial melting degree. This is explained by a slight decrease of the bulk partition coefficient of Fe2O3 (DFe2O3peridotite-melt) as the degree of partial melting increases. The Fe3+/ΣFe ratios of resulting melts are thus restricted to a relatively narrow range over large degrees of melting, as observed for global MORB glass analyses. This observation is also confirmed by pMELTS calculations employing different bulk Fe3+/ΣFe ratios. We conclude that the Fe2O3 content in MORB can be accounted for through partial melting of a mantle source containing 0.1–0.5 wt% Fe2O3 and a bulk DFe2O3peridotite-melt ranging from 0.3 to 0.1.