Melting relations of muscovite-granite to 35 kbar as a model for fusion of metamorphosed subducted oceanic sediments

Abstract

Muscovite-granite was reacted in cold-seal pressure vessels at 2 kbar and in pistoncylinder apparatus between 10 and 35 kbar, with just 0.6 weight per cent water structurally bound in 14 modal per cent muscovite, and with additional water contents varying to 50 weight per cent. Phase relationships are presented through the melting interval with excess water, and with no free water added. Selected reactions above 10 kbars have been successfully reversed. An isobar at 15 kbar shows the effect of varying water contents on the mineral phase boundaries for vapor-present and vapor-absent conditions. For the dry rock, temperatures for the solidus and liquidas (quartz-out) curves, respectively, are 10 kbar-760° C, 1160° C; 15 kbar-810° C, 1220° C; 25 kbar-880° C; 1340° C; 35 kbar-1040° C, 1460° C. The solidus curve corresponds to the melting of muscovite + quartz. With water vapor present, the solidus is considerably lower, 15 kbar-610° C, 25 kbar-665° C. Water solubility in the liquid at 15 kbar is 24±3 weight per cent. Maximum temperatures for quartz and feldspars in the vapor-absent region decrease considerably with increasing water content. Temperatures for the quartz-out curve at 15 kbars are 0.6 % H_2O-1230° C; 24 % H_2O-760° C. At 15 kbars for low water contents, water-undersaturated liquid coexists with quartz and feldspars through hundreds of degrees. Subducted pelagic sediments which metamorphosed to muscovitebearing quartzo-feldspathic rocks would undergo two episodes of melting, beginning at different depths: (1) the first liquid dissolves all pore fluid, and transports it away when it escapes from the crystalline host, (2) reaction of muscovite yields a second liquid, with less dissolved water. According to two published thermal models for a lithosphere slab dipping at 45°, the depths would be (a) 60 km and 92 km, or (b) 17 km and 21 km. Magmas generated by partial fusion in subducted oceanic crust are cooler than the overlying crustal layers and the mantle above the slab by as much as 200° C to 300° C. This must lead to intrusion of relatively cool magma into hot rock. Consequent heating of the magma increases its prospects of reaching high levels in the upper mantle or crust before it solidifies by crossing the solidus curve.