THE ROLE OF KOMATIITES FOR FELSIC CRUST FORMATION IN THE EARLY EARTH (Invited Presentation)

Komatiites are ultramafic (MgO > 18 wt. %) volcanic rocks found almost exclusively in Archean greenstone belts. They are extensively serpentinised and therefore hydrated, containing minerals such as serpentine, chlorite, amphibole and magnetite. The hydration and dehydration of komatiites may have played a role in continental crust formation in the early Earth. Greenstone belts only form ~20% of preserved Archean cratons; the rest of the cratons are comprised of felsic TTGs (tonalite-trondhjemite-granodiorite). These vast suites of TTG likely represent the Archean continental crust and appear to have been formed at low temperatures (750–950 °C) from water-present melting. Their source lithology has been suggested to be basaltic, which underwent partial melting, fractional crystallisation, and magma mixing, but the source of free water during melting reactions is unclear, as basalts cannot contain enough water themselves. However, hydrated komatiites can retain their mineral bound water to significant temperatures, releasing up to 6 wt.% H₂O at temperatures between 500–800 °C. These temperatures may be attained during burial, underthrusting or some form of subduction of the komatiites, which can release their water and trigger partial melting of basalts or other lithologies. For example, in the Barberton Greenstone Belt, komatiites comprise 17% by mass of the preserved oceanic plateau, while potential source lithologies for TTGs, basalts, komatiitic basalts and lithic sedimentary rocks, comprise 20%, 11% and 21% respectively. At crustal melting conditions (0.8 GPa, 700–750 °C), the contribution of free water from dehydrating komatiites within the entire plateau increases partial melt volumes by two to ten times. This highlights that despite their low abundance in greenstone belts, they may have played an important role in TTG formation and the emergence of the continents in the Archean.