High water content of garnet in eclogite with implications for water cycle in subduction zones

Nominally anhydrous minerals in metamorphic rocks serve as potential reservoirs for deep water cycle in subduction zones. The water storage capacity of garnet in eclogite-facies rocks is still controversial. To address this issue， we measured the water contents of garnet from the Rongcheng eclogite in the Sulu orogen using Fourier transform infrared spectroscopy. Four types of absorption bands (3400-3480 cm^-1， 3545-3590 cm^-1， 3610-3625 cm^-1 and 3645-3655 cm^-1) have been identified in garnet from the eclogite sample. The garnet grains contain total water of 460-3630 ppm， dominated by structural hydroxyl （460-3150 ppm）. The mapping results of garnet show good correlations between water and other elements， suggesting the main mechanism of hydrogarnet substitution is accompanied by other point defects. A low proportion of molecular water and the lack of correlation between molecular water and structural hydroxyl indicate that retrograde fluids should not be the main source of high water contents in garnet. The structural hydroxyl contents represent the minimum water storage capacity of garnet under eclogite facies. Therefore， our work demonstrates that garnet is capable of carrying considerable water in subduction zones and serves as an important agent for transporting water to the mantle depth.