Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

摘要

The H2O and CO2 content of cordierite was analysed in 34 samples from successive contact metamorphic zones of the Etive thermal aureole, Scotland, using Fourier-transform infrared spectroscopy (FTIR). The measured volatile contents were used to calculate peak metamorphic H2O and CO2 activities. Total volatile contents are compared with recently modelled cordierite volatile saturation surfaces in order to assess the extent of fluid-present v. fluid-absent conditions across the thermal aureole. In the middle aureole, prior to the onset of partial melting, calculated aH(2)O values are high, close to unity, and measured volatile contents intersect modelled H2O-CO2 saturation curves at the temperature of interest, suggesting that fluid-present conditions prevailed. Total volatile contents and aH(2)O steadily decrease beyond the onset of partial melting, consistent with the notion of aH(2)O being buffered to lower values as melting progresses once free hydrous fluid is exhausted. All sillimanite zone samples record total volatile contents that are significantly lower than modelled H2O-CO2 saturation surfaces, implying that fluid-absent conditions prevailed. The lowest recorded aH(2)O values lie entirely within part of the section where fluid-absent melting reactions are thought to have dominated. Samples within 30 m of the igneous contact appear to be re-saturated, possibly via a magmatically derived fluid. In fluid-absent parts of the aureole, cordierite H2O contents yield melt-H2O contents that are compatible with independently determined melt-H2O contents. The internally consistent cordierite volatile data and melt-H2O data support the conclusion that the independent P-T estimates applied to the Etive rocks were valid and that measured cordierite volatile contents are representative of peak metamorphic values. The Etive thermal aureole provides the most compelling evidence, suggesting that the cordierite fluid monitor can be used to accurately assess the fluid conditions during metamorphism and partial melting in a thermal aureole.