Constraining the Water Activity during Peak Metamorphism in a Thermal Aureole; a Mineral Equilibria Approach

In order to assess the presence or absence of fluids under peak metamorphic conditions within the inner aureole of the Etive igneous complex in west Scotland, eight suitable metabasic hornfels samples and one suitable pelitic hornfels were used to calculate water activities using a mineral equilibria. Calculated activities for water are lower than unity in the pelitic sample and extremely low in the metabasic rocks. These low H2O activities due to dilution of water by other fluid species are unlikely, since no mineralogical evidence for the presence of other fluids were found in the studied rocks. As a consequence, the low H2O activities support fluid-absent conditions during the peak metamorphism of the inner aureole rocks. The findings from this study are in good agreement with calculated water activities using water contents of cordierite in the pelitic rocks for the same part of the aureole, suggesting that cordierite in metapelites retained its syn-metamorphic volatiles.