The largest diamonds are hosted in iron-rich substrate accreted at the base of the lithosphere

Diamonds larger than 100 carats are some of the most valuable gemstones ever unearthed. They are hosted in mantle-derived magmas called kimberlites, but occur at few locations globally. Beyond their large size and rarity, these diamonds exhibit distinctive attributes such as exceptional clarity and irregular shape, leading to the CLIPPIR acronym1. The carbon isotopes of these diamonds indicate their origin from subducted slab material 2, 3, 4. While the formation of CLIPPIR diamonds in the mantle transition zone (MTZ) appears robustly constrained by the occurrence of majorite inclusions1, the nature of the CLIPPIR diamond substrate remains obscure. Here we show that CLIPPIR diamonds are associated with kimberlites tapping vertically extensive, Fe-rich and deformed domains at the base of the lithosphere. Beyond enrichment in Fe, these domains exhibit light oxygen and heavy Fe isotopes, which indicate a major role of subducted basaltic material that experienced hydrothermal alteration at the Earths surface. The association of CLIPPIR and other sub-lithospheric diamonds with these anomalous Fe-rich domains that are rarely sampled by kimberlites and their similar isotopic anomalies point to a genetic relation. Considerations on kimberlite genesis in the upper convective mantle and partial retrogression of majorite inclusions suggest that the CLIPPIR substrate originally stalled in the MTZ, where the diamonds grew, before being accreted at the base of the lithosphere. The geographic overlap between CLIPPIR diamond locations and the loci of large igneous provinces points to accretion of subducted slab material including dense eclogitic crust via buoyant mantle upwellings. Beyond providing the largest diamonds, these Fe-rich, isotopically anomalous domains contribute to the isotopic heterogeneity of intraplate magmas globally.