Olivine and Fe-isotopes in kimberlites indicate an iron-rich substrate for CLIPPIR and other sub-lithospheric diamonds.

CLIPPIRs (Cullinan-like, large, inclusion-poor, pure, irregular, resorbed) are the rarest and most exceptional gem-quality diamonds, but their occurrence is reported in few kimberlites. Despite their light carbon isotopic composition indicating a genetic link with subducted slabs, the nature of their growth substrate is poorly constrained. Using olivine geochemistry as a proxy of the composition and depth of entrained mantle wall-rock, we show that CLIPPIRs are predominantly associated with Fe-rich kimberlites sampling vertically extensive, Fe-rich domains at the base of the lithosphere. These domains have light oxygen and heavy iron isotopic signatures, consistent with hydrothermally-altered oceanic crust. The geographic association and Fe-C-O isotopic affinity between CLIPPIR and other sub-lithospheric diamonds with these Fe-rich domains suggests a common origin. Partial retrogression of majorite inclusions is consistent with a model in which subducted CLIPPIR substrate was accreted to the lithosphere by buoyant mantle upwelling, following temporary stalling within the mantle transition zone. Subsequent interaction between early kimberlitic melts and these domains produced megacrysts at the base of the lithosphere. These megacrysts were then sampled along with CLIPPIR and other sub-lithospheric diamonds by ascending kimberlite melts. Beyond providing the largest diamonds, these Fe-rich, isotopically anomalous domains contribute to the isotopic heterogeneity of intraplate magmas globally.