WATER IN GRANULITES REVISITED: EVIDENCE FROM RAMAN ANALYSIS OF FLUID INCLUSIONS

Over the past few decades there has been much discussion in the literature concerning the composition of fluids associated with granulite-facies metamorphism. Much of the available information concerning compositions of granulite fluids comes from studies of fluid inclusions, and "pure" carbon dioxide fluids are nearly ubiquitous in these rocks. Importantly, microthermometric and Raman spectroscopic analysis of these inclusions at room temperature has failed to detect water in the inclusions. Some have questioned whether these fluid inclusions were formed at granulite P-T conditions and have suggested that they formed during later retrograde processes, and others still have suggested that water that was originally present in the inclusions was subsequently lost by diffusion or leakage. Recently, Berkesi et al. (2009, Journal of Raman Spectroscopy) detected the presence of water in fluid inclusions from mantle xenoliths that had previously been thought to contain only carbon dioxide. Analysis of the inclusions at either low temperature (-100°C) or at elevated temperature (150°C) using Raman spectroscopy revealed characteristic peaks for either carbon dioxide clathrate (-100°C) or water vapor (150°C). These same techniques were applied to carbon dioxide-rich fluid inclusions in granulite-grade rocks from the Adirondack Mountains, USA. Although no water had been detected previously during microthermometry, small and variable amounts of water were detected in every inclusion when they were analyzed by Raman spectroscopy at 150°C. These results suggest that water may be a common component in the previously reported "pure" carbon dioxide inclusions associated with granulite-facies rocks.