minerals Article Water Content in Garnet from Eclogites: Implications for Water Cycle in Subduction Channels Yiren Gou 1, Qin Wang 1,* , Yan Li 1,2 and Richard Wirth 2 1 State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China;
[email protected] (Y.G.);
[email protected] (Y.L.) 2 GeoForschungsZentrum Potsdam, Section 3.5 Surface Geochemistry, D-14473 Potsdam, Germany;
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[email protected]; Tel.: +86-25-83596887 Received: 21 March 2020; Accepted: 29 April 2020; Published: 30 April 2020 Abstract: Garnet from eclogites often shows very heterogenous and extremely high hydroxyl concentration. Eight eclogite samples were selected from the Sulu ultrahigh-pressure terrane and the Sumdo high-pressure metamorphic belt (Lhasa). The mean hydroxyl concentration in pyrope-rich and almandine-rich garnet varies from 54 to 427 ppm H2O and increases with the retrogression degree of eclogites. TEM observations reveal nanometer-sized anthophyllite exsolutions and clinochlore inclusions in water-rich domains in garnet, where anthophyllite is partly replaced by clinochlore. Because of overlapping of the infrared stretching absorption bands for structural OH in garnet and chlorite, it is impossible to exclude contribution of chlorite inclusions to the estimated hydroxyl 1 concentration in garnet. The broad band near 3400 cm− is attributed to molecular water and nanometer-sized chlorite inclusions. Anthophyllite exsolutions may be formed by decomposition of hydrous garnet from ultrahigh-pressure eclogites during exhumation. Significant amounts of water can be stored in garnet from massif eclogites in the forms of hydroxyl in garnet and nanometer-sized inclusions of anthophyllite and clinochlore, as well as fluid inclusions.