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

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,