Constraints from experimental melting of amphibolite on the depth of formation of garnet-rich restites, and implications for models of Early Archean crustal growth

Abstract

The felsic continental crust formed in the early Earth most likely resulted from melting of basaltic protoliths, but the geodynamic processes leading to partial melting are still debated. Attempts to reconcile geochronological data, thermal modeling and experimental results have led to two major alternative scenarios: (1) partial melting of subducted oceanic slabs and (2) dehydration melting at the base of thick (or thickened) oceanic/simatic protocrusts. Existing experimental data on melting of metabasalt suggest that garnet only becomes an important residual phase (>10wt%) at depths >50–60km, which has been seen as difficulty for model 2. We present results of amphibolite dehydration-melting experiments at pressures of 5–15kbar and provide constraints on melting reactions of a hydrated metabasalt with SiO2 of 47.5wt% and Al2O3 of 16.4wt%. Our experiments demonstrate that felsic melts and complementary restites with ∼20wt% garnet can form at ca. 900°C and 12kbar, conditions corresponding to the base of a 40-km thick basaltic protocrust that might be prevalent in the Early Archean. Based on phase proportions determined experimentally and trace element partitioning data, our modeling suggests that such partial melts resemble the Early Archean tonalite–trondhjemite–granodiorite (TTG) suites, including high Al2O3 and low MgO contents, and modestly high La/Yb and Sr/Y ratios. The garnet-rich restite is calculated to be denser than the underlying Early Archean lherzolitic upper mantle and would have the potential to delaminate. Our experimental results and combined geochemical modeling are consistent with models where the initial growth of continental crust on the Archean Earth occurred in non-subduction settings by anatexis of the base of basaltic plateaus.