Smith Lecture - Renée Tamblyn, Universität Bern

Much of the surviving Archean continental crust is comprised of tonalite-trondhjemite-granodiorite (TTG) suites, leading to the question – how did they form? Geochemical, experimental, field, and phase equilibrium forward modeling studies have identified basalt as a likely source of partial melting to form these ubiquitous TTGs, pointing to the importance of metamorphic processes in the generation of the Earth’s crust. Fractionation of phases such as garnet and plagioclase during partial melting and subsequent fractional crystallization have been shown to shape the major and trace element chemistry of TTGs. Additionally, the importance of free water available to assist in partial melting has been highlighted, and likely is sourced from ultramafic lithologies such as komatiites.

Komatiites are ultramafic (MgO > 18 wt. %) volcanic rocks found almost exclusively in Archean greenstone belts. They are extensively hydrated, as they were serpentinised on the ocean floor after their eruption in oceanic plateaux-type settings. The dehydration of komatiites may play a role in tonalite-trondhjemite-granodiorite (TTG) and therefore continental crust formation. Hydrated komatiites can retain their mineral-bound water to significant temperatures, releasing up to 6 wt.% H2O between 500–800 °C. Burial, underthrusting, or subduction of the komatiites would cause them to release their water and trigger partial melting of basalts to form TTGs at these conditions. As such, it appears that greenstone belts may contain the necessary ingredients to produce TTGs, including alkaline, tholeiitic, and high-Mg basalts, and komatiites. However, they also contain Ca-rich sediments (greywackes), carbonates, varied intrusive rocks, felsic and mafic volcaniclastics, and other clastic and chemical sediments, which may also play a role in continental crust genesis. Modeling of partial melting of these lithologies can explore the types of melts produced during water-fluxed melting, to further understand the processes which formed the earliest continental crust.