Isovolumetric Silicification of Early Archean Komatiites: Geochemical Mass Balances and Constraints on Origin

Abstract

Stratiform quartz-phyllosilicate-carbonate alteration zones are a common feature of Archean greenstone terranes, and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth. Komatiites and komatiitic basalts of the uppermost Onverwacht Group (ca. 3.4-3.5 Ga), Barberton greenstone belt, South Africa, were regionally metasomatized prior to major tectonic deformation to the assemblage: quartz + dioctahedral mica + chlorite dolomite. Subsequent metamorphism of these rocks has been restricted to greenschist facies, and they thus record processes that may be obscured in analogous sequences from more highly metamorphosed terranes. The isovolumetric nature of much of the alteration and the relative immobility of elements such as Al, Zr, and Y permit a quantitative assessment of elemental gains and losses during metasomatism. For each (one ton) of MgO were removed and added. There were also significant enrichments in K, Ba, and Rb, and the nearly complete removal of Fe, Ca, and Sr. Thermodynamic considerations support the hypothesis that the komatiites were altered by cooling, mildly-acidic fluids. Constraints imposed by the aqueous solubility of mobile components require in excess of the equivalent of of rock. Low-temperature, thermally-driven subseafloor convection of pore fluid provides a credible mechanism for producing this alteration. Local igneous sources of heat are not required, but convection could have been enhanced by the higher regional heat fluxes postulated for the Archean. There is no evidence to support the hypothesis that silicification was the result of subaerial exposure and weathering. The ultimate sources and sinks of the mobile components involved in metasomatism are not yet known but may have included rocks of the underlying Onverwacht Group and/or superjacent Early Archean sea water.