Origin of framesite revisited: Possible implications for the formation of CLIPPIR diamonds

Framesites are polycrystalline diamonds of enigmatic origin, that differ in numerous respects from monocrystalline diamond varieties. Their textures suggest rapid crystallization, while the low N-aggregation state of some stones and zoning in some garnet inclusions point to a short mantle residence time and formation broadly coeval with the host kimberlite. This differs from the much older (Archean to Proterozoic) ages of peridotitic and most eclogitic diamonds. In contrast to the rarity (similar to 2%) of websteritic inclusions in mono-crystalline diamonds, a high proportion (> 70%) of garnet inclusions in framesites were previously assigned to a websteritic paragenesis. However, websteritic and megacryst garnets show a marked compositional overlap, making a simple chemical distinction between these two parageneses inconclusive. We highlight several lines of evidence linking framesites and the megacryst suite to a common paragenesis. Crystallization of megacrysts, like that of frame-sites, was broadly coeval with eruption of the host kimberlite, and some megacryst nodules are characterized by quench textures. Megacrysts are grouped into relatively Cr-rich and Cr-poor sub-populations that may occur within a single hand specimen. Likewise, relatively Cr-rich and Cr-poor garnet inclusions are reported in framesites, occasionally even within the same diamond host. Framesites from individual localities are characterized by closely similar, highly diagnostic C-isotopic signatures, with a majority enriched in C-12 (modal delta C-13 parts per thousand of -18 to -19) relative to peridotitic and eclogitic diamonds. However, each locality invariably has a sub-population with C-isotopic signatures closer to typical mantle values (-5). Like framesites, large exceptional (CLIPPIR) Type II diamonds from the Cullinan and Letseng kimberlites are dominated by relatively C-12-rich isotopic signatures, but also include a subordinate population closer to typical mantle signatures (-5). Several lines of evidence are inconsistent with a sub-lithospheric provenance model for these valuable stones. The remarkable similarity of their C-isotopic signatures to those of framesite diamonds points to a common paragenesis, thus also linking them to the megacryst suite. The large size (> 10 mm) of many of the CLIPPIR Type II stones is consistent with such a linkage, as is the evidence that crystallization of CLIPPIR Type II diamonds was broadly coeval with emplacement of the host kimberlite. We present a magmatic model of CLIPPIR diamond crystallization at the low temperature extreme of the megacryst suite, which formed in a pegmatitic vein network surrounding a pooled magma body within the lithosphere. Late-stage (evolved) megacryst magmas would be Ca-and carbonate rich, providing a chemical environment favourable for the crystallization of breyite under lithospheric P-T conditions.

Automated summary

Framesites are polycrystalline diamonds that differ from monocrystalline diamond varieties in many aspects. They have rapid crystallization textures and short mantle residence time, suggesting formation around the same time as the host kimberlite. The high proportion of garnet inclusions in framesites were previously believed to be from a websteritic paragenesis, but the compositional overlap between websteritic and megacryst garnets makes it inconclusive.