THE RELATIONSHIP BETWEEN GEM-QUALITY TYPE IIA AND TYPE IIB DIAMONDS

Geographically, a link has long been recognized between large, gemmy type IIa diamonds and blue, boron-bearing type IIb diamonds. Mining localities noted for their type IIb diamonds, such as the historic Golconda region of India or the Cullinan mine in South Africa, also happen to be rich in large, gemmy type IIa diamonds (or more precisely, CLIPPIR diamonds: Cullinan-like, Large, Inclusion-poor, Irregular, Resorbed). However, the geological origin of these diamonds has only come into focus within the last four years, from a systematic initiative to characterize their rare inclusions. Based on the inclusions, it is now apparent that both CLIPPIR and type IIb diamonds originate from the sublithospheric mantle, at depths several times greater than most other kinds of gem diamonds from the lithospheric mantle. The relationship between these two groups in the mantle during diamond formation remains to be clarified.

The finding of relatively abundant metallic Fe-Ni-C-S inclusions in CLIPPIR diamond is not mirrored by a similar abundance of metallic phases in type IIb diamonds, suggesting an important difference in their formation conditions. Beyond this contrast, however, both diamond varieties have a range of carbon isotopic compositions extending to light values and both have at least some silicate inclusions linked to metamorphosed oceanic crust, indicating subduction plays a key role. Ongoing research suggests deeply subducted serpentinized peridotite in the mantle portion of slabs may be a common thread in the genetic relationship between CLIPPIR and type IIb diamonds.