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 ho rnfels from successive contact-metamorphic zones in the Etive thermal aureo le, Scotland, using secondary ion mass spectrometry. Together with independ ent estimates of peak metamorphic pressure and temperature, these results w ere used to calculate an H2O activity (aH(2)O) in each sample. The calculat ed aH(2)O values are considered to reflect peak metamorphic conditions. In the middle aureole calculated aH(2)O values are high, close to unity, sugge sting fluid-present conditions. In the innermost aureole they are low, <0.5 , suggesting fluid-absent conditions. The transition from high to low aH(2) O 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 sequenc e of reactions in pelitic and semipelitic rocks involving, progressively, ( I) dehydration, followed by (2) fluid-present partial melting, followed by (3) fluid-absent partial melting. The consumption of H2O by fluid-present m elting reactions provides a mechanism for reducing aH(2)O 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 c lose as 15 m from the contact have not.