Water transport by subduction: Clues from garnet of Erzgebirge UHP eclogite

Abstract

A key question concerning the water budget of Earth’s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43–84 ppm), omphacite (400–820 ppm), and in the bulk coesite eclogite (ca. 280–460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5–10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500–3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals—as in the Erzgebirge—and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole.

On the survival of intergranular coesite inUHPeclogite

“…All analysed spots on garnet and omphacite showed two or more hydroxyl absorption bands, which is consistent with results from previous studies (e.g. Katayama & Nakashima, ; Schmädicke & Gose, ; Sheng et al., ; Zhang, Jin, Green, & Jin, ; Zhang et al., ).…”

“…For convenience, henceforth we refer to structural hydroxyl and molecular H 2 O colloquially as “water.” Although molecular water might be present in omphacite, the amount is likely to be small (cf. Schmädicke & Gose, ).…”

“…These features precluded assessment of the spatial heterogeneity of water contents in garnet. Sample analysis and calculation of the water concentrations in garnet and omphacite followed the methods described in Schmädicke and Gose (; see also Sheng, Xia, Dallai, Yang, & Hao, ). All analysed spots on garnet and omphacite showed two or more hydroxyl absorption bands, which is consistent with results from previous studies (e.g.…”

“…Garnet and omphacite are important nominally anhydrous minerals (NAMs) that contain structural water in point defects in the crystal lattice and H 2 O both as clusters of molecules and in fluid inclusions that are generally connected to dislocations (Zhang et al., ; Zheng, ). These minerals have total water contents that may reach several thousand ppm H 2 O, although measured values are highly variable (Gong, Chen, & Zheng, ; Schmädicke & Gose, ; Zhang et al., ; Zheng, ). Therefore, NAMs are potentially an important source of fluid that may become available during exhumation by leakage of H 2 O along dislocations, decrepitation of fluid inclusions and exsolution of structural water.…”

Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

“…Comparing orthopyroxene from mantle xenoliths and tectonically exhumed mantle rocks, no difference seems to exist regarding its ability to preserve the original content of water. On the other hand, tectonic mantle slices cool during ascent, in contrast to xenoliths, and Schm€ adicke and Gose (2017) suggested this being the reason why orogenic eclogite and peridotite bodies, depending on their specific decompression-cooling history, are equally well, if not even better, suited as proxies for mantle water compared xenoliths. Exactly the same maximum amount was found in xenolithic samples (Peslier et al, 2002).…”