An unexpected assemblage: Moissanite and carbonaceous material from SSZ mantle rocks of the Dun Mountain Ophiolite, New Zealand

Abstract Moissanite (SiC) and other super-reducing minerals have been reported from supra-subduction zone (SSZ) ophiolites, yet their origin remains controversial given the oxidized conditions expected in forearc mantle. In this study, hundreds of moissanite grains have been recovered from mantle rocks of the Early Permian Dun Mountain Ophiolite, New Zealand. We report the first occurrence of moissanite found in direct association with disordered carbonaceous material (CM), which records low peak formation temperatures of < 334 °C. Carbon isotope analyses reveal characteristic light compositions for both moissanite (δ¹³C = − 33.7 to − 19.2‰; mean − 25.0‰) and associated CM (δ¹³C = -39.2 to -29.0‰; mean − 35.3‰). The light C isotope signatures of both phases are consistent with moissanite formation via fractionation of ultra-reduced, carbon-rich fluids derived from organic-rich sediments carried by a subducting lithospheric slab. Preservation of moissanite in an oxidized forearc mantle could be aided by low temperatures and relatively short mantle residence times. Whilst the total mass of moissanite formed under these settings is low, these findings highlight the occurrence of localised ultra-reducing conditions at shallow depths in SSZ environments and identify an under-recognised pathway for carbon cycling in subduction zones.