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

In 19 Siberian eclogitic diamonds, inclusion thermobarometry and redox reconstruction indicate sampling at 180–280 km and oxidized conditions linked to water(carbonate)-rich fluids.

• Why it matters: Eclogite xenoliths can preserve igneous redox signals from subducted oceanic crust, but long metasomatism in the subcontinental mantle has limited accurate reconstructions.
• What they did: The study analyzed garnet and clinopyroxene inclusions in touching and nontouching pairs from diamonds shielded from post-entrapment overprint, deriving temperatures of 1030–1540°C (±60°C) and redox conditions.
• The result: Most samples indicate oxidized source conditions consistent with diamond formation from water(carbonate)-rich fluids, and bulk composition suggests an Archaean convecting mantle region more oxygen-depleted than today.

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