EXPLORATION IMPLICATIONS OF ISOTOPICALLY HEAVY IRON IN LARGE GEM TYPE IIA DIAMONDS

Large, high-quality type IIa diamonds such as the Cullinan and the Koh-i-Noor are among the most elusive of mined gem diamonds. They belong to a variety of diamonds called CLIPPIR diamonds, an acronym reflecting their distinguishing physical characteristics (Cullinan-like, Large, Inclusion Poor, Pure, Irregular, Resorbed). There is currently no reliable method to predict the occurrence of CLIPPIR diamonds within a deposit, which remains a hurdle for exploration and mining. Mineral inclusions reveal that they originate in the sublithospheric mantle at a depth of 360-750 km, explaining why their occurrence is effectively independent from more common eclogitic and peridotitic lithospheric diamonds and their associated indicator minerals. A recent study of iron isotopes in the metallic inclusions sometimes found in CLIPPIR diamonds has provided additional insight into how they form, giving clues that may aid in exploration.

The metallic inclusions possess a remarkably heavy iron isotopic composition, which is attributed to subducted iron sourced from magnetite and/or Fe-Ni alloys precipitated during seafloor serpentinization of oceanic peridotite. This finding is a big step toward a genetic model for CLIPPIR diamonds because it reveals a key lithological input for their formation, namely subducted serpentinized peridotite. This input may come specifically from cold subducting slabs, whose serpentinized mantle portions can bypass the shallow sub-arc dehydration activity and instead transport serpentinite-derived components such as hydrous minerals to the transition zone/uppermost lower mantle. The results suggest that geochemical signatures related to deeply subducted serpentinites may eventually provide a basis for targeting CLIPPIR diamonds in volcanic deposits at surface.