Primordial neon and the deep mantle origin of kimberlites

The genesis of kimberlites – Earth’s deepest-derived melts – remains an unresolved question despite the economic and scientific interest surrounding these diamond-bearing continental magmas. One critical question is whether they tap ancient, deep mantle or the shallow convecting mantle with partial melting triggered by deep-mantle plumes or plate tectonics. To address this question, we report the compositions of He-Ne-Ar isotopes, formidable tracers of the occurrence of primordial material in the mantle, in magmatic fluids trapped in olivine from kimberlites worldwide. We show that two kimberlites have Ne isotopes less nucleogenic than the upper mantle, which unequivocally requires a deep mantle origin. This is corroborated by previous evidence of negative W isotope anomalies and the location of these kimberlites along age-progressive hot-spot tracks. The lack of strong primordial He isotope signatures indicates overprinting by lithospheric and crustal components, which suggests that Ne isotopes are more robust tracers of deep-mantle contributions in intraplate magmas.