Fe ³⁺ ∕ΣFe variation in lawsonite and epidote in subducted oceanic crust

Abstract. The hydrous Ca–Al silicates lawsonite and epidote group minerals (EGMs) are key phases in subduction-zone H2O and element cycling. In high-pressure–low-temperature metamorphic rocks, Fe in both minerals is typically assumed to be entirely Fe3+, which substitutes for Al in octahedral sites as a major component in most EGMs and as a minor component in lawsonite and zoisite. New Fe micro-X-ray absorption near-edge spectroscopy (μ-XANES) analyses show substantial Fe2+ in lawsonite in blueschist from New Caledonia and zoisite from an unknown locality. Analysed Fe-rich EGMs (epidote, clinozoisite) contain primarily Fe3+. Lawsonite and some EGMs in subducted oceanic crust may contain more Fe2+ than is currently known, with possible implications for understanding subduction redox processes and conditions and why they vary in different subduction zones.