Petrological Evidence from Komatiites for an Early Earth Carbon and Water Cycle

Komatiites from Alexo and Pyke Hill in the Archean Abitibi greenstone belt provide petrological evidence for an early Earth carbon and water cycle, ingassing in the cool Hadean and outgassing in the hot Archean. The komatiites have SiO2 contents that are lower than those expected of advanced volatile-free melting of mantle peridotite. The SiO2 misfit cannot be plausibly accounted for by variations in model Bulk-Earth peridotite composition, perovskite fractionation in a magma ocean, addition of chondrites, a source that had recycled crust added to it, or by chemical alteration during serpentinization. One possible resolution to the silica misfit problem is obtained if the komatiites from Alexo and Pyke Hill were partial melts of carbonated peridotite, a conclusion based on reasonable agreement between the major element compositions of komatiites (i.e. SiO2, Al2O3, FeO, MgO, and CaO) and experimental melt compositions of carbonated peridotite. High-degree melts with olivine as the sole residual phase can have low SiO2 contents owing to carbonate addition. Furthermore, a role for significant H2O is indicated from recent olivine-hosted melt inclusion studies. More work is needed to constrain how much CO2 and H2O is required to resolve the SiO2 misfit, and the T–P conditions of melting. Failure to do so imposes significant uncertainty in Archean mantle potential temperature and geodynamic interpretations. These uncertainties notwithstanding, the komatiites appear to be recording important degassing events in the Archean. Depending on the extent of volatile degassing, hydrous and CO2-rich komatiites could have formed either in mantle plumes or in ambient mantle. Ingassing may have occurred in the Hadean when atmospheric CO2 and H2O were sequestered by reaction with impact ejecta, oceanic crust and mantle peridotite to produce carbonates and hydrous minerals. Parts of the Earth may have been sufficiently cool at some point in the Hadean to ingas the deep mantle, consistent with a variety of constraints from zircon and isotopic studies. Hydrous and CO2-rich komatiites formed from ‘carbonated wetspots’ in mantle plumes or ambient mantle later in the Archean. The drop in komatiite production at the end of the Archean may be a record of significant purging of the mantle in volatiles, affecting biogenic methane production and the evolution of oxygen in the atmosphere.