Contrasting oxygen isotopes in garnet from diamondiferous and barren eclogitic parageneses

Eclogite is a minor mantle lithology, present in subducted slivers in cratonic roots. Mantle eclogites carry O and C isotopic signatures from surface organic and inorganic carbon and also are modified by reaction with fluids in the lithosphere. One third of the diamonds mined worldwide are sourced from mantle eclogites, and individual eclogite xenoliths contain up to 20 vol. % diamond. It is critically important to understand where the diamond carbon comes from, and how the diamonds form, for insights on the carbon cycle, diamond exploration, and processes in the lithospheric mantle. Few samples and methods are available to constrain diamond formation in eclogites; in this work we focus on oxygen isotopes in eclogitic garnets. New analyses of garnet/majorite found as inclusions in the Cullinan diamonds reveal a statistically significant systematic difference between delta O-18 in garnet associated, and unassociated, with diamond. This contrast persists between garnet from diamondiferous and barren eclogite xenoliths and cannot be due to shielding of diamond inclusions from equilibrating with the common mantle values of delta O-18. We propose that diamond-forming metasomatic reactions triggered by carbonatitic fluids may contribute up to 1.5 parts per thousand to the shift of delta O-18 to higher values in eclogitic diamondiferous paragenesis, but cannot fully account for the observed difference of 2.5 parts per thousand.