Carbonated komatiites and their importance for the Archean deep carbon cycle

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 <2 vol%. Iron-bearing dolomite becomes the dominant carbonate with the volume increasing from 2 vol% to 9-13 vol% at the bottom of the flow. The carbonates display elevated Sr, Ba and B contents. The systematic change of carbonate minerals within the single komatiite flow and enrichment of these fluid-mobile elements indicate carbonation by seawater interaction directly after the emplacement of the flow rather than carbonate introduction during the later metamorphic event.

The intense carbonation of the komatiite lavas led to the incorporation of 1.5 to 6.5 wt% of CO₂, illustrating that secondary carbonate is an important sink for carbon. We performed phase equilibria modelling on a komatiite + 5 vol% calcite composition to determine whether such carbon can be recycled back into the mantle during burial of oceanic crust along expected Archean geotherms. During prograde metamorphism, calcite is replaced by dolomite, which is stable up to temperatures of only 700-750 °C [2]. Therefore, carbonate incorporated into altered komatiites will be entirely released during burial/subduction of oceanic crust in the Archean. This efficient recycling of carbon suggests that ingassing of C into the mantle was likely insignificant and might have helped to keep atmospheric CO₂ levels high in the Archean, an important aspect to explain the “faint young sun” paradox.

[1] Nakamura and Kato (2004): Geochem. Cosmochim. Acta 68 4595-4618.

[2] Tamblyn et al. (2023): Earth. Planet. Sci. Lett. 603, 117982