HYDROXYL COMPONENT OF ULTRAHIGH-PRESSURE ECLOGITES FROM THE KOKCHETAV MASSIF, KAZAKHSTAN

Water is introduced into the mantle by hydrated oceanic lithosphere at subduction zone, but mostly released at relatively shallow depth (~50 km) by the amphibolite-eclogite transformation. However, nominally anhydrous minerals can dissolve substantial amounts of hydroxyl as reported in mantle xenoliths (e.g., Bell and Rossman, 1992). In this study, we measured hydroxyl components of major constitute minerals of eclogites from the Kokchetav ultrahigh-pressure metamorphic terrane, by using infrared spectroscopy and secondary ion mass spectrometry. The diamond-grade eclogite is mostly composed of omphacite and garnet with minor amounts of coesite and rutile, and yields P-T conditions of ca. 60 kbar and 1000°C. Infrared spectra reveals three hydroxyl absorption bands for the omphacite in the regions of 3440-3460 cm-1, 3500-3530 cm-1 and 3600-3625 cm-1, and ion micro-probe analysis yields 1680 ppm OH by weight. The garnet has a single peak at 3580-3630 cm-1, and contain up to 240 ppm OH. The hydrogen concentrations of these minerals increase systematically with recrystallized pressure from quartz-eclogite through coesite-eclogite to diamond-grade eclogite. Ca-Eskola component in the omphacite have a positive correlation with the OH content, therefore vacancy is most likely mechanism to incorporate hydroxyl in the clinopyroxene. According to modal proportion of the rocks, the eclogites contain approximately 430 ppm H2O at depths greater than 150 km. This suggests that subducted oceanic crust can carry significant amounts of water into the upper mantle although no hydrous minerals are stable at such depth. Our results have important implications for the distribution of water at the mantle.