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UHP-03 Ultra-high pressure metamorphism: Minerals, microstructures and nanoscale observations - Part 1
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Kinetics of diamond nucleation: laser-heated diamond anvil cell experiments assisted with synchrotron in-situ X-ray measurements
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Junfeng Zhang, University of California (United States)
Vitaly Prakapenka, Argonne National Laboratory (United States)
Atsushi Kubo, Argonne National Laboratory (United States)
Abby Kavner, University of California (United States)
Harry Green, University of California (United States)
Larissa Dobrzhinetskaya, University of California (United States)
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Microdiamond from orogenic belts containing nanometeric size fluid inclusions is a pristine geological material for understanding processes responsible for diamond nucleation and growth at high pressures and high temperatures in subduction zone environments. Previous studies have demonstrated that diamonds may be synthesized in laboratory from a supercritical COH fluid at wide range of T = 1200 °C 1500 °C, and P = 7 -10 GPa. However, mechanism and kinetics of diamond nucleation from graphite and amorphous carbon in presence of such a fluid are not well constrained. We have conducted several ultrahigh-pressure experiments in laser-heated diamond anvil cell assisted with synchrotron in-situ microX-ray measurements. With these techniques we were able to establish time during which the diamonds nucleation took place. Powdered amorphous carbon (impurity < 2 ppm) and graphite (99.9 % pure) were used for starting materials as a carbon source. Oxalic acid dihydrate (COOH)2•2H2O) and glucose (C6H12O6) in amount of 3 wt % were added to the starting material as a source of fluid. The prepared samples heated by laser beam at temperature of 1400-1700 °C were kept at pressures 8-10 GPa for 10-50 minutes. The run products show that nanocrystals of diamond were nucleated from amorphous carbon in CO2-rich fluid environment. The fastest rate of diamond nucleation and growth (15 micron size crystals) was observed in the mixture of amorphous carbon with glucose which provides CH4-rich environment. Only nanometric diamond nuclei were observed in the run product started with graphite and glucose. We have also established that under anhydrous conditions, no diamond nucleation took place in the pure graphite, but a few nanometric size diamond crystals were observed in the amorphous carbon at T = 1700-1900 °C, P = 10 GPa. Our results revealed that the kinetics of diamond nucleation depend on the starting materials: diamond nucleates and grows faster from amorphous carbon than from graphite in the presence of COH fluid; in our anhydrous experiments diamond nucleates only from amorphous carbon. These results demonstrate that diamonds nucleation from anhydrous graphite is hampered because of the well-known kinetic barrier, and emphasize the promoting role of a supercritical COH fluid for graphite transformation to diamond.
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