Raman spectroscopic characterization of H₂O in CO₂-rich fluid inclusions in granulite facies metamorphic rocks

Abstract

Water (H2O) has been identified during Raman analyses at ~ 150–200 °C of fluid inclusions from granulites that were previously thought to contain pure CO2. At room temperature, H2O in a CO2-rich fluid inclusion forms an optically unresolvable liquid wetting the inclusion walls. During heating, the H2O phase evaporates into the CO2 phase to produce a homogeneous H2O–CO2 fluid throughout the inclusion volume, and the H2O Raman peak at ~ 3640 cm− 1 is clearly resolvable at elevated temperatures. To test and confirm the method, synthetic H2O–CO2 FI of known composition (87.5 mol% and 95 mol% CO2) were also analyzed. Previously studied granulite samples from the Adirondack Mountains, USA, and from the Kerala Khondalite Belt, Sittampundi Complex and Sevitturangampatti, India were tested for the presence of H2O in CO2-rich fluid inclusions. Traces of H2O were found in CO2-rich fluid inclusions in quartz crystals from the Adirondacks and from the Kerala Khondalite Belt. H2O was not detected in CO2-rich secondary fluid inclusions in garnet from the Sittampundi Complex and from Sevitturangampatti. Rather, hydrogarnet was detected along the walls of inclusions in garnet, as evidenced by a peak at ~ 3661 cm− 1. The hydrogarnet is interpreted to be a “step-daughter” phase that formed by reaction of the H2O in the fluid inclusion with the garnet host during retrogression. These observations confirm the presence of an H2O-bearing, CO2-rich fluid at some time during granulite petrogenesis, but do not resolve the question of whether these CO2-rich fluids were trapped at peak metamorphic conditions or during retrogression, or whether the fluid inclusions have reequilibrated following entrapment.