Water content and species in high-pressure granulite: An evaluation based on μ-XRF, THz-TDS spectroscopy, TG and DCS

Subduction-related dehydration and exhumation-related rehydration play an important role in water recycling on Earth. Water can be transported to the deep mantle through crustal subduction, whereas the behavior of water in the subducted crust during exhumation remains enigmatic. In subduction and exhumation processes, the oceanic crust is transformed into high-pressure mafic granulites and amphibolite through high and low temperature and pressure metamorphism, respectively. During the metamorphism, water can be transported into the deep mantle via subduction-related dehydration and returned to the surface through exhumation-related rehydration processes. As a result, the anhydrous and hydrous minerals are formed within the high-pressure mafic granulites and amphibolites.

In this contribution, we use an integrated micro X-ray fluorescence spectrometry (μ-XRF), transmission terahertz time-domain (THz-TDS), thermogravimetry (TG) and differential scanning calorimetry (DSC) approach on high-pressure mafic granulite and amphibolite to investigate the water species and contents, as well as exhumation rehydration reactions. Our study demonstrates that the high-pressure mafic granulite recorded peak/post peak metamorphism with the formation of Grt + Hbl + Pl + Qtz + Ilm + Ru. Amphibolite experienced retrograde metamorphism with an assemblage of Grt + Hbl + Pl + Qtz. Garnet, hornblende, and ilmenite contain considerable amounts of water as molecular and structural species. Plagioclase contains minor amounts of molecular and structural water. Quartz contains only a minor amount of structural water. Water released from garnet and external fluids from the grain boundary will either migrate into hornblende, plagioclase, ilmenite and quartz, or stored in the mantle wedge, or further subducted into the deep mantle. We suggest that water recycling between the Earth surface and deep mantle is an unequilibrium process, and the lower crust and mantle may store a significant amount of water in deep Earth and can function as a container to feed and maintain the water recycling balance.