Abstract

Low concentrations (tens to hundreds of parts per million) of water in nominally anhydrous minerals (NAMs) of the mantle have been shown to significantly affect the rheology, depth of depressurization melting, and many other properties of the mantle. In this contribution, we evaluate the effect of trace concentrations of water in NAMs on melting of lower continental crust. Water locked in structural sites and in fluid inclusions in nominally anhydrous minerals in lower crustal granitoids may act as a flux for partial melting of these source rocks. Water concentrations of 3000 ppm in minerals that make up large volumes of crustal rocks (K-feldspar, plagioclase, quartz) would lower the dry solidus of granite by 273 °C at 1 GPa. Measurements and maps of water concentration in variably deformed samples of the Stevenson Granite from northern Saskatchewan (Canada) show that, during deformation and recrystallization of K-feldspar megacrysts, water migrated from the interior of megacrysts to finer-grained matrix material. Dark, fine-grained, water-richer matrix material consisting of quartz, plagioclase, alkali feldspar, and fine iron oxides are interpreted to be former melt films that resulted, at least in part, from fluxing by NAM-derived water. Deformation may play a role in moving water from NAMs to phase boundaries where generation of partial melt may lead to further rock weakening and further release of water from NAMs.

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