COMPARING A 910 CARAT TYPE IIA DIAMOND WITH SMALLER CLIPPIR DIAMONDS

There are multiple distinct processes and settings in which natural diamonds form. Recognizing and studying these different varieties of diamond genesis often yields valuable insights into mantle geology. A relatively new variety to be recognized stems from large and highly pure (often Type IIa) crystals dubbed CLIPPIR diamonds. The CLIPPIR acronym (Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed) emphasizes the conspicuous physical features that make this group of diamonds stand out, but it is their inclusions that have confirmed CLIPPIR diamonds form in a different way compared to other kinds of diamonds. A 2016 study established that inclusions in CLIPPIR diamonds point to a sublithospheric origin at 360–750 km depths, with diamond growth involving a subduction-related metallic liquid. However, this interpretation was based on inclusions in relatively “small” diamonds, with the largest specimen examined being 32 carats. The extension to +100-carat-sized diamonds like the 3,106 carat Cullinan is an inference based only on qualitative physical properties.

An opportunity to examine inclusions within a 910 carat Type IIa diamond from the Letseng mine, Lesotho, has provided insight into this largest size range. This diamond contains examples of metallic Fe-Ni-C-S melt, previously documented as the most common material trapped as inclusions in smaller CLIPPIR diamonds. It also contains a retrogressed Na- and K-silicate assemblage interpreted as former CF or NAL phases, which are components predicted to be part of basaltic ocean crust subducted to lower mantle depths beyond 660 km. These findings confirm that the sublithospheric genetic model constructed from smaller diamonds also applies to the largest examples within the CLIPPIR suite. They also provide new evidence suggesting CLIPPIR diamond formation can involve oceanic crust in addition to the serpentinized peridotitic mantle portion of subducted slabs indicated by previous Fe isotope analyses.