Fe3+/ΣFe variation in lawsonite and epidote in subducted oceanic crust

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.