Water transport in continental subduction zones: Constraints from eclogite from the Dabie orogen, east-central China

Water in nominally anhydrous minerals may exist as structural hydroxyl and molecular H2O, and their inter-transformation can occur during subduction zone metamorphism. This issue is examined here by a systematic study on water in garnet and omphacite for ultrahigh-pressure metamorphic eclogites from the Dabie orogen. IR spectra of garnet at different preheating temperatures imply that the broad absorption band at 3400-3450 cm-1 mainly represents molecular H2O. Molecular H2O in garnet can be intrinsic origin, but is mainly originated from external fluid during exhumation. The positive correlation between molecular H2O and structural hydroxyl in garnet suggests their inter-transformation. Molecular H2O that was formerly included in garnet or derived from decomposition of hydrous minerals was transformed gradually to structural hydroxyl in garnet during subduc-tion. OH- modes at higher wavenumber in garnet are more stable and more enriched in D, and D-poor molecular H2O is preferentially lost during dehydration. Hydroxyl in both anhydrous and hydrous minerals would be transformed to molecular H2O during exhumation and redistributed into garnet or retrograde minerals. The addition of molecular H2O into garnet during exhumation causes increase of structural hydroxyl but decrease of delta D value, and affects accurate estimate of water content in peak garnet. Different from the large variation of garnet water contents, omphacite tends to be water-saturated at high-pressure conditions and its water content is mainly controlled by Ca-Eskola. The subducting mafic crust can transport a considerable amount of water to subarc depths to result in the formation of relatively water-rich mantle regions.