332 Metamorphic Petrology (Posters)

Wednesday, 30 October 2013
9:00 AM - 6:30 PM
Hall D

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

Paper number: 332-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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, ~Fe0.83S) 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 KD = XSiDiamond/XSiMelt, is observed to be in the range of ln KD of -11 to -4.6, indicating strong partitioning of Si into the melt. There was no clear correlation of KD or of XSiDiamond with 1/T; however, additional cathodoluminescence imaging is required to identify the main overgrowth areas.