Synthesis of Staurolite in Melting Experiments of a Natural Metapelite: Consequences for the Phase Relations in Low-Temperature Pelitic Migmatites

We document experiments on a natural metapelite in the range 650---775 C, 6---14 kbar, 10 wt % of added water, and 700---850 C, 4---10 kbar, no added water. Staurolite systematically formed in the fluid-present melting experiments above 675 C, but formed only sporadically in the fluid-absent melting experiments. The analysis of textures, phase assemblages, and variation of phase composition and Fe---Mg partitioning with P and T suggests that supersolidus staurolite formed at (near-) equilibrium during fluid-present melting reactions. The experimental results are used to work out the phase relations in the system K2O---Na2O---FeO---MgO---Al2O3---SiO2---H2O appropriate for initial melting of metapelites at the upper amphibolite facies. The P---T grid developed predicts the existence of a stable P---T field for supersolidus staurolite that should be encountered by aluminous Fe-rich metapelites during fluidpresent melting at relatively low temperature and intermediate pressures (675---700 C, 6---10 kbar for XH2O ˆ 1, in the KNFMASH system), but not during fluid-absent melting. The implications of these findings for the scarcity of staurolite in migmatites are discussed.