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 K2 O–Na2 O–FeO–MgO–Al2 O3 –SiO2 –H2 O 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 fluid-present melting at relatively low temperature and intermediate pressures (675–700°C, 6–10 kbar for XH2O = 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.