SI INCORPORATION IN DIAMOND OVERGROWTHS IN THE PRESENCE OF ENSTATITE, PYRRHOTITE, AND CARBON

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. The presence of trace elements in diamond is known to occur and if calibrated may provide a useful thermometer. To address this, 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 enstatite (943 ± 7°C, 0.57 ± 0.01 GPa), carbon black, and natural pyrrhotite (Po, ~Fe_0.83S) from Sudbury, Canada. Pyrrhotite was used as a flux and it provided sufficient reducing conditions for diamond growth. The reagent mixture had the bulk composition En₄₇Po₃₂C₂₁ by weight. Experiments were completed in a 1,000-ton Walker module multi-anvil press at a pressure of 7.20 ± 0.13 GPa between 1,650 and 1,880°C. Following treatment, entire samples were mounted in epoxy and planed through with diamond laps and polished for electron microprobe work. Diamond overgrowths were identified from their increased 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) at 15 kV and 200 nA. The silicate-carbonate-sulfide melt quenches to a fine-grained heterogeneous cluster of phases ≤ 1 μm in diameter, indicative of segregation on quenching (e.g. Kiseeva et al, J. Pet., 2013). These melt aggregates were analyzed by electron microprobe using a wide beam method.

The identification of diamond overgrowths was less obvious in this silicate-sulfide flux than with invar, Fe-Ni, flux. The Si concentrations in identified diamond overgrowths range from 20 to 4,100 ppm by weight. The distribution of Si between diamond and melt aggregate, expressed as K_D = X_Si^Diamond/X_Si^Melt, is observed to be in the range of ln K_D of -11 to -4.6, indicating strong partitioning of Si into the melt. There was no clear correlation of K_D or of X_Si^Diamond with 1/T; however, additional cathodoluminescence imaging is required to identify the main overgrowth areas.