Recycling Subducted Organic Carbon as Diamonds: An Example From the New Caledonia Forearc Ophiolite

Abstract

Micro-diamonds and moissanite (SiC) have been identified in ophiolitic mantle harzburgites and chromitites of the New Caledonian Peridotite Nappe. The pale yellow (100–250 μm) micro-diamonds and light blue moissanite (mean −26.5‰, range −33.5‰ to −23.8‰ and mean −26.9‰, range −31.8‰ to −25.6‰ respectively) exhibit consistently strong negative δ¹³C values consistent with vegetal (C3) photosynthesis. Preservation of U-Pb ages amongst co-occurring rutile xenocrysts, with a closure temperature of 620 ± 20°C, constrains the maximum thermal conditions experienced by these rocks. These temperatures indicate that the New Caledonian diamonds did not form under the deep mantle conditions typical of conventional diamond genesis but instead within a distinct supra-subduction zone (SSZ) forearc setting. The association with moissanite suggests formation within anoxic, organic carbon-rich sediments at the top of the subducting slab or within the subduction channel under localized super-reducing conditions. In light of mantle heterogeneity, extension of the known distribution of ophiolitic diamonds to the Southern Hemisphere supports interpretation of their formation in relation to an SSZ process rather than a deep mantle source. It also highlights a previously unrecognized aspect of the global carbon cycle, underscoring the significance of SSZ forearc ophiolites in deep carbon transport and transformation.