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

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 H₂O-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 H₂O activities when they crystallized: type C formed at extremely low H₂O activities; type B crystallized at high H₂O activities; and type A formed under a range of conditions from high (pyroxene cores) to low (pyroxene mantles) H₂O activities. Clinopyroxenes recorded local heterogeneity of H₂O activity on the scale of individual grains during UHP metamorphism and represent the best indicator of H₂O 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.