Fluid Inclusions in Granulites: Peak vs. Retrograde Formation

Abstract

Various lines of evidence may indicate that CO2-rich fluid inclusions in granulites contain samples of the peak metamorphic fluid. This evidence includes: (1) textural relations, (2) fluid inclusion thermobarometry, and (3) correlation of inclusion type with metamorphic grade (CO2-rich in the granulite facies and H2O-rich in lower grade rocks). However, many CO2-rich inclusions in granulites are texturally secondary, and inclusion thermobarometry often yields scattered results. Furthermore, the correlation of fluid inclusion composition with metamorphic grade may be explained by retrograde processes. Thus, the evidence for peak metamorphic formation of granulite facies fluid inclusions is often ambiguous. In some samples collected from the Adirondack Mountains, equilibrium mineral assemblages require that the peak metamorphic fluids (if any existed) were CO2-poor. These same samples contain high density CO2-rich fluid inclusions. This disagreement between two methods commonly applied to determine the composition of peak metamorphic fluids shows that the fluid inclusions do not contain samples of the peak metamorphic fluid. Rather, the investigation of these inclusions has shown that the retrograde pressure temperature path in the Adirondacks was initially more nearly isobaric than isothermal.