Water incorporation in garnets from ultrahigh pressure eclogites at Shuanghe, Dabieshan

摘要

The hydrogen concentration and composition of garnets in the ultrahigh pressure eclogites at Shuanghe, eastern Dabieshan, were investigated using Fourier transform infrared spectroscopy and electron microprobe analysis. The OH absorption bands can be divided into four groups: (1) 3635-3655 cm(-1); (2) 3600-3630 cm(-1); (3) 3540-3580 cm(-1); and (4) 3400-3450 cm(-1) and the water content ranges from 45 to 2529 ppm. Based on the behaviour of the OH absorption band and the relationship between water content and the composition of garnets, the samples can be divided into two classes: samples with >400 ppm H2O and samples with <= 400 ppm H2O. The water content of the former shows an obvious positive correlation with Ca atoms and a negative correlation with the Si, Mg and Fe2+ atoms per 12 anions, whereas the water content of the latter shows no obvious linear correlation with cations. It is concluded that the major mechanism of hydroxyl incorporation in garnets with >400 ppm H2O is by the coupled substitution 4H + (Z)square ->square + Si-Z in the tetrahedral site, and that several mechanisms are responsible for OH incorporation in garnets with <= 400 ppm H2O.