Metamorphism and melting of picritic crust in the early Earth

Abstract

Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium in the bulk composition. Tonalitic melts were generated in MAOC 11 and 13 at pressures ≥12.5kbar along with the residual phases garnet+clinopyroxene+plagioclase±quartz (±orthopyroxene in the presence of quartz and at lower pressures) in the absence of amphibole. Basaltic melts were generated at pressures ≤15kbar predominantly in the presence of granulite facies residues such as amphibole±garnet±plagioclase+orthopyroxene that lack quartz in all MAOC compositions.The tonalitic melts generated in MAOC 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern MORB is a viable source for the generation of early Archean high-Si, low-MgO melts, and therefore TTG-like continental crust in the Archean. The favoured settings for the generation of these melts at pressures up to 15kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones, both without interaction of the melts with the mantle during passage to the surface. Tonalitic melts may have formed in deeper subduction zones at 20kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction. This process may have become more important during the progressive cooling of the Earth.