Water Recycling in the Deep Earth: Insights From Integrated mu-XRF, THz-TDS Spectroscopy, TG, and DCS of High-Pressure Granulite

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. Here, we use an integrated micro X-ray fluorescence spectrometry, transmission terahertz time-domain, thermogravimetry, and differential scanning calorimetry approach for the first time on high-pressure mafic granulite and amphibolite to investigate the water species and contents, as well as exhumation rehydration reactions. Our study demonstrates that garnet, hornblende, and ilmenite contain considerable amounts, and plagioclase contains minor amounts of water as molecular and structural species, whereas 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's 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.