Variation in Fe2+ / Fe3+ in hydrous silicates in subducted oceanic crust

A fundamental observation regarding oceanic subduction systems is that the starting material of subduction (MORB) is less oxidized than a major end-product (primitive magma in arcs), likely as a result of infiltration of mantle source regions by oxidized components from the subducted slab. Important redox-sensitive elements in slabs are S, C, H, and transition elements such as Fe and Mn. Fe is important because it is volumetrically significant, and release of other redox agents by dehydration reactions in subducted altered oceanic crust is controlled in part by Fe³⁺/∑Fe. Lawsonite (Lws) and epidote-group minerals (EGMs) are hydrous Ca-Al silicates that are key phases in subduction-zone H₂O and element cycling owing to their composition and abundance, and the Lws-EGM transition has been linked to significant changes in fluid composition. Fe is a major component in most EGMs and a more minor but common component in Lws. Fe in Lws and metamorphic EGMs is generally assumed to be entirely Fe³⁺ that substitutes for Al in octahedral sites. However, results of Fe-XANES analyses for Lws and EGMs in blueschist and eclogite were acquired at the European Synchrotron Radiation Facility and show a wide range of Fe³⁺/∑Fe. Results for Lws appear robust because there are no signs of beam damage during analysis and there does not appear to be a strong orientation effect (linear dichroism). EGMs similarly show no beam damage effects but, in contrast to Lws, show significant variation in XANES spectra and resulting calculated Fe³⁺/∑Fe as a function of orientation. Although Fe in some Lws analyzed is entirely Fe³⁺, Lws and EGMs from New Caledonia blueschists contain substantial Fe²⁺. Work is in progress to determine what controls Fe³⁺/∑Fe in Lws and EGMs, but a preliminary conclusion is that Fe²⁺ in these phases in subducted oceanic crust may be greater than currently known, with implications for phase equilibria calculations and understanding of subduction redox conditions and processes.