Muscovite dehydration and melting in deep crust and subducted oceanic sediments

Abstract

Mixtures of natural quartz and synthetic muscovite and sanidine, with and without free water, were reacted in piston-cylinder apparatus between 10 and 35 kb. Extrapolation of experimentally determined melting curves defines an invariant point for the assemblage muscovite + sanidine + corundum + liquid + vapor at 9.5 kb and 825°C, and one for the same assemblage with quartz at 5.8 kb and 730°C. These two points define the high pressure limit for the subsolidus dehydration reactions of muscovite and of muscovite + quartz, which facilitates selection among the varied muscovite dehydration curves previously published. Comparison of published estimates of temperature distribution of subducted lithosphere slabs with muscovite stability ranges indicates that muscovite in subducted oceanic sediments would probably be dehydrated or melted before reaching a depth of 30 km, with a maximum possible depth of 100 km. This suggests that although muscovite could possibly be involved in magma generation at volcanic fronts, it neither contributes water for magmatic processes much beyond Dickinson's arc-trench gap, nor influences the chemical variation in calc-alkaline lavas across arc complexes and in plutonic rocks across batholiths.