Graphite to diamond transformation during sediment-peridotite interaction at 7.5 and 10.5 GPa

Diamond nucleation and growth were investigated experimentally at 7.5 and 10.5 GPa and temperatures up to 1500 degrees C. Samples consisted of two layers: i) H2O- and CO2-bearing model sediment and ii) graphite-bearing garnet harzburgite comprising natural minerals. Two experimental series were conducted, one under a controlled temperature gradient with the sedimentary layer usually in the cold zone and the other under isothermal conditions. In the latter case, diamond seeds were added to the sedimentary mixture. During the experiments, the sedimentary layer partially or completely melted, with the melt percolating and interacting with the adjacent harzburgite. The graphite-to-diamond transition in the peridotite was observed above 1300 degrees C at 7.5 GPa and 1200 degrees C at 10.5 GPa in the temperature-gradient experiments, and at temperatures similar to 100 degrees C lower in the isothermal experiments with diamond seeds. Newly formed diamond occurs mostly as individual grains up to 10 pm in size and is separate from graphite aggregates. In some cases, an association of diamond with magnesite was observed. Diamond nucleation occurs in hydrous and CO2-bearing silicate melt following graphite dissolution and reaystallization. In the case of the diamond-magnesite association, diamond was probably formed through carbonate reduction coupled with graphite oxidation. The composition of the melts ranged from "carbonatitic" with similar to 10 wt% SiO2 and >50 wt% volatiles to hydrous silicate with similar to 40 wt% SiO2 and <10 wt% volatiles. This variation has no strong effect on diamond nucleation or growth. (C) 2018 Elsevier B.V. All rights reserved.