Carbonated komatiites and their importance for the Archean deep carbon cycle

Abstract

The deep carbon cycle in the Archean is poorly constrained. Carbonate sedimentation only became an important reservoir for carbon from the late Archean onwards. It has been proposed that the transfer of carbon from the hydrosphere/atmosphere to the lithosphere mainly occurred during alteration of basalts of the oceanic crust [1]. While carbonation of ancient ultramafic rocks including komatiites has been described, it is often assumed that this carbonation occurred much later than the komatiite formation.In this contribution, we investigate the role of carbonation of komatiites for the Archean deep carbon cycle. The Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, provides a well-preserved pre-3 Ga terrestrial record and hosts hydrated komatiites that erupted ~3.48 Ga ago. We present data from samples of the ICDP drill core BARB1, transecting ultramafic lavas of the Komati Formation at depths of 108.77 to 112.73 meters. These komatiites have remained shielded from surface alteration throughout geological history and thus have not been affected by carbonate formation related to recent weathering.The 3 m thick komatiite flow is covered by an andesitic volcaniclastic rock, where carbonate is intergrown with titanite in an albite-biotite-amphibole assemblage. A U-Pb age for titanite of 3266 ± 44 Ma demonstrates carbonate formation prior to this metamorphic overprint. In the uppermost 1.2 m of the komatiite flow only calcite is present, coexisting with chlorite, serpentine, tremolite, talc and magnetite. The volume of calcite decreases from 8-10 vol% in the first 40 cm to 2-6 vol% at 1 m depth where mainly spinifex textured komatiite is present. At 2-3 m depth, cumulate textures predominate, and the volume of calcite is always