Hydrous species in eclogitic omphacite: Implication for metamorphic dehydration during exhumation

Abstract

Water contents were analyzed by infrared spectroscopy for structural hydroxyl in randomly oriented cuboids and doubly polished wafers of single crystal omphacite from an ultrahigh-pressure eclogite in the Dabie orogen. The results are quantitatively compared with bulk water contents measured by the TC/EA-MS continuous flow technique. This enables recognition of various hydrous species in omphacite and their bearings on metamorphic dehydration during continental subduction. The two cuboids yield constant maximal structural water contents of ∼378ppm (H2O wt.) using the omphacite-specific infrared absorption coefficient, which are much lower than the bulk water contents of ∼1496ppm. The excessive water is contributed by the presence of other hydrous species in the form of hydrous mineral inclusions, hydrous symplectite and submicron fluid inclusions. Since the hydrous mineral and/or fluid inclusions may be captured at different metamorphic stages during subduction and exhumation of continental crust and the hydrous symplectite is the secondary rather than the primary, it is suggested that the bulk water contents represent a mixing of various hydrous species with distinct origins. In this regard, it is critical to examine whether water in these secondary hydrous species is of internal origin; if not, it has nothing to do with the transport capacity of water in deeply subducted crustal rocks. Upon stepwise heating up to 700°C, omphacite exhibits a gradual decrease in bulk water contents but an increase in δD values, indicating the preferential loss of D-depleted hydrous species as a whole due to the decompressional dehydration of UHP metamorphic rocks during exhumation. Therefore, the D-depleted hydrous species may be dominated in the hydrous symplectite and inclusion water.