Abrupt transition in H₂O activity in the melt-present zone of a thermal aureole: Evidence from H₂O contents of cordierites

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.