Preferential mobilisation of oxidised iron by slab-derived hydrous silicate melts

Abstract

The role of slab-derived hydrous silicate melts in the transfer of the oxidised signature in subduction zones remains poorly constrained. We have investigated the mobility and redox state of iron in hydrous silicate melts by carrying out solubility measurements of hematite–magnetite assemblages in a piston-cylinder apparatus combined with electron microprobe and Mössbauer analysis of the recovered glasses. The experiments were performed at subcritical conditions, i.e. two-fluid phases coexisting with the solid assemblage. We observe concentrations of total FeO as high as 1.85 ± 0.18 wt. % (2.07 ± 0.41 wt. % in saline systems) at 2 GPa and 900 °C, with Fe3þ/Fetot ratios of 0.79 ± 0.04 (0.45 ± 0.07) that indicate the dominance of oxidised iron in the melt phase. Combined with thermodynamic modelling to reconstruct the composition and speciation of the coexisting fluid phase, we demonstrate that hydrous silicate melts can transport 20 times more dissolved iron, preferentially as oxidised iron, than aqueous fluids at sub-arc conditions. Our results support the efficient dissolution of ‘fluid-insoluble’ iron oxides in slab-melts, which are thus efficient agents for the transfer of oxidised iron to the mantle wedge, ultimately contributing to the oxidation of the arc magma source.