Oxygen fugacity, temperature, and pressure of Siberian eclogitic diamonds: Implications for the Archaean convecting mantle redox state

Kimberlite-borne eclogite xenoliths ultimately derive from ancient subducted oceanic crust. Some specimens still record the chemical and redox conditions of their igneous protoliths, allowing a view into contemporaneous convecting mantle sources. However, accurate reconstructions are limited because eclogites have been exposed to metasomatism in the subcontinental mantle for billions of years. Here, we investigated garnet and clinopyroxene inclusions, both touching and nontouching pairs, in 19 eclogitic diamonds from the Siberian craton, hence shielded from post-entrapment overprint. Inclusion compositions indicate average sampling depths of 180 to 280 km, temperatures of 1030° to 1540°C (±60°C), and redox conditions, for most of the samples, oxidized enough for diamonds to have originated from water(carbonate)-rich fluids. The reconstructed bulk composition points to basaltic oceanic crust formed from an Archaean convecting mantle region that was more oxygen-depleted than today, providing compelling evidence of redox heterogeneities in the source rocks of ancient oceanic crust.