An FTIR Study of Kyanite in the Maobei Kyanite-Bearing Eclogites from the Sulu Orogenic Belt, Eastern China

Abstract

As a minor phase, kyanite has been repeatedly shown to have experienced ultrahigh pressure (UHP) metamorphism together with its host eclogites. Thus, it could play some role in transporting water into the deep earth. Here we present a detailed investigation of water concentrations of kyanite, and for reference, of garnet and omphacite from four Maobei eclogites in the Sulu orogenic belt, eastern China. Fourier transform infrared (FTIR) measurements show that kyanites, garnets, and omphacites all have distinct hydroxyl absorption bands due to OH groups bound in their crystal structure. The FTIR profile analyses on ten grains from different samples reveal a homogeneous distribution of water across kyanite, suggesting insignificant water loss during exhumation. The calculated water concentrations in kyanite (21 wt ppm–41 wt ppm) are comparable to those reported previously for kyanite from various geological occurrences when using the most recent calibration. They are however much lower compared with those in garnet (46 wt ppm–83 wt ppm) and omphacite (302 wt ppm–548 wt ppm) from the Maobei eclogites. This implies that kyanite is not a major water carrier in eclogites considering its low volume fraction and contributes negligibly to transport water into the deep mantle accompanying subducted oceanic crust until its possible transformation to AlSiO3OH.