Nanocrystalline diamond from the Earth’s mantle underneath Hawaii

Melt inclusions in a mantle-derived garnet pyroxenite xenolith from Salt Lake Crater (Hawaiian island of Oahu) have been analysed by means of transmission electron microscopy, including energy dispersive X-ray analysis, electron energy-loss spectroscopy and electron diffraction. The melt inclusions represent trapped low-degree melt that metasomatised the Hawaiian mantle lithosphere before the eruption of the host magma. The inclusions consist of a C–Cl–S-rich glass of basaltic composition, an exsolved gas–fluid phase (mainly CO2 with minor amounts of CO, H2O, and some unidentified C–H and H–S species) and/or carbonate. Diamond is the major phase in the glass, ranging in size between a few and several hundred nanometres. Two varieties of nanocrystalline diamonds are identified. Polycrystalline aggregates of nanometre-size diamonds are tentatively explained as pseudomorphs after carbonate, while single crystals are suggested to have crystallised from the melt. Other nanocrystalline phases include native Fe and Cu, FeS, FeS2, ZnS, AgS and several Ti, Nb, Zr, Ir, In, Pd-rich phases of unknown composition. Overall, the observed mineral assemblage suggests that the melt was highly reduced and strongly enriched in volatiles and trace elements of contrasting geochemical affinity. Our findings are the first evidence for a metasomatic impregnation of the Hawaiian mantle by low-degree melts. Here, we report the first occurrence of diamond in the modern oceanic mantle.