High P and T Diamond Synthesis From Graphite in the Presence of H2O in Natural Metamorphic Rock Systems

We report here high P and T diamond synthesis from graphite in the presence of H₂O and in combination with different minerals so that bulk composition resembles natural diamond-bearing metasediments from ultra-high pressure terranes. Experiments were performed in a multianvil apparatus using a Pt capsule with an enclosed graphite capsule at P=7-8.8 GPa and T=1200-1500°C for time periods ranging from 1 to 138 hours. We have explored the following systems: (1) graphite-brucite, (2) graphite-calcite-talc and graphite-dolomite-talc, (3) graphite-quartz-muscovite and (4) graphite-quartz-H₂O. Diamonds were found in all run products indicating that they crystallized from a supercritical COH fluid formed from the breakdown of water-bearing minerals and subsequent dissolution of graphite into liberated water or fluid-catalyzed graphite-diamond transformation. One of the notable features of diamond crystallization in most of the COH fluid environments investigated here is the considerable induction time that exists prior to diamond nucleation. This incubation period increases dramatically with decreasing temperature and also depends on the bulk composition of the starting material. No diamond was crystallized from graphite in SiO₂-rich system after 43-hour experiments at 8 GPa and 1500°C, while carbonate and brucite systems yielded diamonds after 10-20 hours at similar P and T conditions. Instead of diamonds in our SiO₂-rich experiment, we find fine-grained spherical graphite polycrystalline aggregates nucleated around newly crystallized coesite. This suggests that SiO₂ promotes nucleation of metastable graphite on its surface in the diamond stability field. It is also expected that SiO₂ has a high solubility in supercritical fluid at such P/T conditions. Therefore, the retardation of diamond nucleation from COH fluid saturated in SiO₂ allows us to conclude that free SiO₂ somehow hampers diamond nucleation. We are pursuing greater understanding of the kinetics of this process and the induction time required for diamond nucleation in SiO₂-bearing systems because this system is closest to the dominant rock type hosting natural UHP metamorphic diamonds.