Water incorporation in garnet: Coesite versus quartz eclogite from Erzgebirge and Fichtelgebirge

Abstract

The water content of garnet was determined for eclogite from two Variscan complexes in Germany: the Erzgebirge (EG), Saxony, and the Fichtelgebirge (FG), Bavaria. Erzgebirge eclogites occur in three units, each of which experienced specific peak conditions (unit 1: 840-920°C/ ≥ 30 kbar, unit 2: 670-730°C/24-26 kbar, unit 3: 600-650°C/20-22 kbar). Peak conditions of the FG eclogite (690-750°C/25-28 kbar) are close to those of eclogite from EG unit 2. Coesite eclogite is restricted to the EG ultra-high pressure (UHP) unit 1. Garnet shows infrared absorption bands at ca. 3650, 3580-3630, and 3570cm-1, ascribed to structural water. Many garnets also contain molecular water (in sub-microscopic fluid inclusions), which is irregularly distributed on the grain scale and of secondary origin. Grain volumes with molecular water invariably reveal a band at 3580-3630cm-1 attributed to a hydrogarnet substitution. Because structural water due to this substitution positively correlates with molecular water, the primary content of structural water can only be deduced from grain volumes that are free of molecular water as demonstrated by Schmädicke & Gose (2017; American Mineralogist 102, 975-986). This primary content is typically low in garnet from quartz eclogite ( < 2-50ppm); averages for most samples fall in the range of 8-28ppm. Garnet from coesite eclogite hosts more water (50-180 ppm) except for garnet from an unusual, phlogopite-bearing coesite eclogite that contains only 19-55 ppm. Structural water in garnet is unrelated to metamorphic peak pressure but governed by the presence (or absence) of eclogite-facies hydrous minerals such as calcic amphibole, zoisite, and, or, phlogopite. In the case that hydrous minerals were stable at peak metamorphism-as in quartz eclogite-garnet hosts little or no water. If hydrous minerals are not part of the peak assemblage- as in common coesite eclogite-garnet contains distinctly more water. The latter was apparently derived from eclogite-facies hydrous minerals, which decomposed and liberated their H2O due to overstepping their stability field during UHP metamorphism. Moreover, garnet in coesite eclogite is more Ca-rich than garnet in quartz eclogite. This is ascribed to the breakdown of prograde zoisite, liberating Ca and facilitating a higher grossular content, which, in turn, enhances the garnet's capacity for water storage. This study further suggests: (1) post-peak metamorphic introduction of secondary fluid; (2) relatively dry conditions prior to fluid influx, because only water-deficient garnet is able to incorporate additional structural water; (3) The determined primary contents of structural water were probably not modified by decompressional water loss, because the latter should only occur if the water content at peak pressure is ≥ 75 % of the maximum storable amount; (4) Since garnet from both eclogite types was water-deficient at the metamorphic peak it is unlikely that the different water contents are related to pressure; (5) The mineral assemblage and the dehydration of hydrous minerals is definitely more important in this context; (6) Garnet and, by implication, omphacite from both eclogite types was able to incorporate only part of the water liberated by hydrous minerals, a great part must have been released to hanging-wall rocks; and (7) The study points to a moderate amount of water (several hundred ppm) that is transported by subducting coesite eclogite to depths of > 100 km, an amount equivalent to that in ca. 1-2 % calcic amphibole.