Hydroxyl in clinopyroxene and titanite in a UHP diamond-free garnet-clinopyroxene rock from the Kokchetav Massif, northern Kazakhstan

摘要

Hydroxyls in clinopyroxene and supersilicic titantite in diamond-free garnet-clinopyroxene rock from the Kumdy-Kol area of the Kokchetav Massif were characterized using micro-FT-IR spectroscopy. The presence of supersilicic titanite is evidence of ultra-high pressure (UHP) metamorphic conditions of at least 6 GPa. Significant amounts of hydroxyls were detected suggesting that supersilicic titanite crystallized in an H_2O-rich environment at a depth of >200 km. Clinopyroxenes coexisting with supersilicic titanite contain exsolved K-feldspar and phengite lamellae and were classified into three types on the basis of exsolved lamellae texture. Hydroxyl was identified in types A and B, but was not present in type C. The variety of reintegrated hydroxyl contents in clinopyroxenes reflected the differences of H_2O activities when they crystallized: type C formed at extremely low H_2O activities;;type B crystallized at high H_2O activities;;and type A formed under a range of conditions from high (pyroxene cores) to low (pyroxene mantles) H_2O activities. Clinopyroxenes recorded local heterogeneity of H_2O activity on the scale of individual grains during UHP metamorphism and represent the best indicator of H_2O activity. The fluid environment is an important factor attending petrogenesis during UHP metamorphism. Variations in abundance of hydroxyl in UHP clinopyroxene can be used to clarify fluid environments at great depths.