Water Content in Garnet from Eclogites: Implications for Water Cycle in Subduction Channels

Yiren Gou,Yan Li,Richard Wirth,Qin Wang

doi:10.3390/min10050410

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 concentration in garnet. The broad band near 3400 cm−1 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. Amphibolite facies retrograde metamorphism can significantly increase both hydroxyl concentration and water heterogeneity in garnet from massif eclogites. These nano-inclusions in garnet provide a window to trace the water cycle in subduction channels.

Highlights

Water can widely exist in nominally anhydrous minerals (NAMs) such as olivine, pyroxene, and garnet, in the form of hydroxyl in lattice defects or molecular water in fluid inclusions (e.g., [1,2,3,4,5,6]).Subduction zones provide an important pathway to carry water into the earth’s interior and play a critical role in the earth’s water cycle
The unstable peaks around 3710 cm−1 in some grains were caused by varying water vapor during experiments [10], which were deleted in the estimation of hydroxyl concentration
The group III band is generally recognized as the vibration of clusters of molecular water in micro-inclusions (

Summary

Introduction

Water can widely exist in nominally anhydrous minerals (NAMs) such as olivine, pyroxene, and garnet, in the form of hydroxyl in lattice defects or molecular water in fluid inclusions (e.g., [1,2,3,4,5,6]).Subduction zones provide an important pathway to carry water into the earth’s interior and play a critical role in the earth’s water cycle. During subduction of the oceanic lithosphere, water released by breakdown of hydrous minerals will hydrate the mantle wedge and trigger arc volcanism, whereas subduction of the continental lithosphere is characterized by limited fluid activity and lack of coeval arc volcanism [7]. In both cases, several thousand ppm of water can be incorporated in garnet and omphacite, as evidenced by infrared (IR) spectroscopy analysis of eclogites from high-pressure (HP). Because water can remarkably decrease viscosity [16,17] but enhance electrical conductivity [18] and Fe-Mg interdiffusion in garnet [19], evaluation of water content in garnet is critical for dynamic modeling and interpretation of electrical conductivity anomalies, as well as for estimation of metamorphic P-T conditions of HP

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