Type II diamonds: Flamboyant Megacrysts?

Abstract

Type II diamonds from Premier are characterised by light carbon isotope-enriched compositions, markedly different from the signatures of the associated Eclogitic (E-type) and Peridotitic (P-type) diamonds at the same locality. This provides strong evidence that these Type II stones represent a third and hitherto unrecognised diamond paragenesis. Their isotopic signature is closely similar to that of rare Orapa diamonds with websterite inclusions, arguing for a common paragenesis. The Orapa websterite garnet inclusions are Ti- and Cr-rich relative to those from other kimberlites, and overlap the compositional field for Orapa garnet megacrysts. These chemical characteristics, coupled with the large size and irregular shape of Type II diamonds, links them to the megacryst suite. This conclusion is supported by evidence which shows that at the low temperature range of megacryst crystallisation, the associated residual liquids, if buffered by mantle wall rocks, at oxygen fugacities between the WM and IW, would be in the diamond P-T-f O2 stability field. The low N concentrations and light cabon isotopic-enriched fingerprint of the Type II stones are consistent with their crystallisation from such evolved magmas. It is demonstrated that in addition to the irregular and often large (megacryst) Type II stones, there are two further suites of N-poor diamonds which can be assigned to either an eclogitic or peridotitic paragenesis on the basis of their inclusions, physical properties and carbon isotopic signatures. Diamonds with complex growth zones and deformation textures can be explained in terms of dynamic mantle processes, linked to formation of the sheared peridotites, within the thermal aureole surrounding the kimberlite magma in which the megacryst suite crystallised. Framesites, fibrous cubic diamonds, and eclogitic diamonds which formed just prior to kimberlite eruption, can also be linked to the processes responsible for crystallisation of the megacryst suite, which is considered to be cognate to the host kimberlite magma.