Komatiites are ultrabasic volcanic rocks characteristic of the Archean and Proterozoic eras. In this study, we investigate the petrogenesis of the komatiites of the Barberton greenstone belt in South Africa. We present an original melting model developed on the experimental observation that, at great depths (P>13GPa), komatiitic liquids are denser than solid peridotite, and therefore cannot escape their source. We propose that the fusion of the sources of Barberton komatiites began at pressures greater than 13GPa in the mantle but that the liquids produced were retained in the sources until they crossed, during their ascent, the density limit at which the liquid becomes less dense than the residue (at ~13GPa). Then, the liquids would have started to escape their source, slowly at first, and faster and faster as the density contrast between liquid and solid residue increased. Al-depleted komatiites, which are the most common type in the Barberton belt, were produced as the first liquids that escaped the source soon after they crossed the density limit. The liquids that later escaped the same source at shallower depth, formed Al-enriched komatiites. Al-undepleted komatiites were produced by the same mechanism but from hotter sources. From complementary studies of the three isotopic systems Sm-Nd, Lu-Hf and Re-Os, we constrained the chemical history of the sources of Barberton komatiites. The Sm-Nd and Lu-Hf systems indicate depleted sources whereas the Re-Os system indicates a chondritic source. We propose that the sources of the komatiites had become depleted by the extraction of early crust, and that their compositions in Re and Os were then reset to their chondritic value by contamination by material from the last massive accretion stage of the Earth: “the late veneer”.
