Metamorphism and melting of picritic crust in the early Earth

Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15 wt.% 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 >= 12.5 kbar 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