HR: 17:50h
AN: V44A-07    [Abstracts]
TI: Diamond Nucleation from Amorphous Carbon and Graphite with COH Fluids: an in Situ High Pressure and Temperature Laser-Heated Diamond Anvil Cell Experimental Study
AU: * Zhang, J
EM: junfeng.zhang@ucr.edu
AF: Dept. of Earth Sciences/IGPP, Univ. of California, Riverside, CA 92521, United States
AU: * Zhang, J
EM: junfeng.zhang@ucr.edu
AF: Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China
AU: Prakapenka, V
AF: GSECARS, Argonne National Laboratory, Chicago, IL 60439, United States
AU: Kubo, A
AF: GSECARS, Argonne National Laboratory, Chicago, IL 60439, United States
AU: Kavner, A
AF: Dept. of Earth and Space Sciences/IGPP, Univ. of California, Los Angeles, CA 90095, United States
AU: Green, H W
AF: Dept. of Earth Sciences/IGPP, Univ. of California, Riverside, CA 92521, United States
AU: Dobrzhinetskaya, L F
EM: larissa@ucr.edu
AF: Dept. of Earth Sciences/IGPP, Univ. of California, Riverside, CA 92521, United States
AB: Microdiamonds from orogenic belts contain nanometer size fluid inclusions suggesting diamond formation from supercritical COH fluids. Previous studies have shown that diamonds synthesized from high pressure and temperature experiments with supercritical COH fluids are characterized by skeletal morphology and solid oxide inclusions. However, mechanism and kinetics of graphite/carbon-to-diamond transformation promoted by COH fluids at high pressure and high temperature conditions are not well understood. Here we report in situ observations of diamond nucleation from COH fluids in laser-heated diamond anvil cell. Our experimental starting materials were amorphous carbon (impurity < 2ppm) and graphite (99.9% pure). Oxalic acid dihydrate (COOH)₂·2H₂O) was added to amorphous carbon and glucose (C₆H₁₂O₆) was added to both amorphous carbon and graphite. The organic compounds (3 wt.%) provide CO₂- and CH₄-rich fluid environments respectively during their breakdown at high pressure and temperature. The mixtures were kept at temperature of 1400-1700 °C and pressure of 8-10 GPa for 10-30 minutes. Experiments show that only nanocrystals of diamond were nucleated from amorphous carbon in CO2-rich fluid environment. The fastest rate of diamond nucleation and growth of ~15 micron size crystals was found in the mixture of amorphous carbon with glucose (CH₄-rich environment), whereas only nanocrystalline nuclei were produced in the mixture of graphite with glucose. We have also established that under anhydrous conditions, no diamond nucleation occurred in pure graphite, and only nanocrystals of diamond were observed in the amorphous carbon starting material at temperatures 1700-1900 °C. Our results revealed that the kinetics of diamond nucleation depend on the ˇ°precursorˇ±: 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: (1) diamond nucleation from anhydrous graphite starting material is hampered because of the well-known kinetic barrier that requires a huge amount of energy to convert sp2-bonding to sp3-bonding, whereas there is no difficulty for diamond nucleation in amorphous carbon material; (2) the critical role of supercritical COH fluid for promoting the graphite-to-diamond transformation.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting