Compositional layering in a highly diamondiferous eclogite xenolith and its implications for diamond genesis

Abstract

We examined a compositionally layered diamondiferous eclogite, which yields a diamond grade of approximately 8400 carats per metric ton, discovered in a new collection from the Roberts Victor Mine, South Africa, with the aim of further constraining eclogitic diamond genesis. This high concentration of diamond, the relatively large size of the xenolith host (15 x 12 x 8 cm) together with its compositional layering shown by modal variations of garnet, clinopyroxene and diamond, provide a rare opportunity to test whether the diamond formation was contemporaneous with or post-dated the formation of the host eclogite. We will address this issue based on the results of (1) 3D mapping of the rock using the HRXCT technique, (2) major element analyses of garnet and clinopyroxene across the layering, (3) FTIR analysis of the nitrogen content and aggregation state of the diamonds, and (4) Carbon isotope analyses of the diamonds.