Water Content and Deformation of the Lower Crust beneath the Siberian Craton: Evidence from Granulite Xenoliths

Water in the lower crust plays a critical role in rheological layering of the continental lithosphere. Sixteen granulite xenoliths were collected from the Late Devonian Udachnaya and Komsomolskaya kimberlites in the Siberian Craton. Mafic granulite samples experienced pressures of 0.6–1.0 GPa and temperatures of 549°–800°C using the Grt-Cpx (garnet-clinopyroxene) Fe-Mg thermometer, which are consistently lower than equilibrium temperatures of 737°–899°C from the REE-in-Grt-Cpx thermobarometer. Compared with pseudosection calculations, our samples experienced continuous cooling since the last granulite facies metamorphism. Moderate to high water content was measured in clinopyroxene (334–977 ppm H2O), garnet (23–149 ppm H2O), and plagioclase (157–779 ppm H2O), resulting in the bulk water content of 267–707 ppm H2O in granulite samples. Given the very limited later metasomatism and hydrogen loss, water content in granulite xenoliths probably represents in situ water-rich lower crust of the Siberian Craton from 1.8 Ga to the Late Devonian. Clinopyroxene and plagioclase show weak crystallographic preferred orientations, whereas garnet has random orientation. Compared with previous studies, the Precambrian lower crust in stable cratons contains comparable or less water than Phanerozoic lower crust in orogenic belts. Magma underplating in cratons can trigger partial melting of ancient water-rich granulites and produce heterogeneous water distribution in the lower crust.