SI INCORPORATION IN DIAMOND OVERGROWTHS IN THE PRESENCE OF PYROPE: A POSSIBLE GEOTHERMOMETER

Recent work on discerning the formation conditions of diamonds has concentrated on the study of fluid and mineral inclusions of diamonds; however, few diamonds have a usable inclusion assemblage (Stachel & Harris, Ore Geo. Rev., 2008). The presence of trace elements in diamond is known to occur and if calibrated may provide a useful thermometer.

Natural diamonds, crushed and sieved (-100/+325 mesh) from the Udachnaya Pipe, Russia, were reacted in 2.0 mm Ta crucibles with 0.025 mm Ta foil caps in the presence of synthetic pyrope (1010 ± 10 °C, 2.54 ± 0.06 GPa; half-inch piston-cylinder press), carbon black, and an iron-nickel alloy (as used in Kennedy & Kennedy, J. Geophys. Res., 1976). The Fe-Ni alloy was used as a flux and provided sufficient reducing conditions for the growth and synthesis of diamonds. The reagent mixture was mixed in equal proportions by weight. Experiments were conducted in a 1,000-ton Walker module multianvil apparatus at pressures of 6.00 ± 0.14 to 7.60 ± 0.13 GPa and temperatures of 1117 ± 12 to 1889 ± 28 °C using a 14/8 mm assemblage. An 8.00 ± 0.13 GPa experiment resulted in synthesis of metastable lonsdaleite.

Following treatment, entire crucibles were mounted in epoxy and planed through with diamond laps. Diamond overgrowths were identified from increases in cathodoluminescence (Holsing & Jenkins, GSA Abs., 2011) compared to central portions of the treated diamond seeds, and analyzed using an extended 5-minute peak-search technique (Holsing & Jenkins, GSA Abs., 2012). Reactions could be observed from changes in texture, or evidence of melt percolation as the alloy. Although there are large uncertainties in the regression, a possible thermometer is calibrated with this study indicating an apparent ΔH_f of 67 kJ·mol^-1 and an apparent ΔS of 11 J·K^-1·mol^-1 for the reaction Si(melt) = Si(diamond). No pressure effect on Si incorporation into diamond could be observed.