Abstract

The water content of cordierite was analyzed in seven samples of pelitic hornfels from successive contact-metamorphic zones in the Etive thermal aureole, Scotland, using secondary ion mass spectrometry. Together with independent estimates of peak metamorphic pressure and temperature, these results were used to calculate an H2O activity (aH2O) in each sample. The calculated aH2O values are considered to reflect peak metamorphic conditions. In the middle aureole calculated aH2O values are high, close to unity, suggesting fluid-present conditions. In the innermost aureole they are low, <0.5, suggesting fluid-absent conditions. The transition from high to low aH2O appears to be abrupt and occurs at higher grade than the melt-in isograd. The results are compatible with the operation of an up-temperature sequence of reactions in pelitic and semipelitic rocks involving, progressively, (1) dehydration, followed by (2) fluid-present partial melting, followed by (3) fluid-absent partial melting. The consumption of H2O by fluid-present melting reactions provides a mechanism for reducing aH2O at the transition. A xenolith within the Quarry Diorite appears to have crystallized in the presence of magmatically derived hydrous fluid, but wall-rock hornfels as close as 15 m from the contact have not.

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