Formation and preservation of carpholite in low blueschist facies assemblages: implications for the H ₂ O budget at seismogenic zone depth

Carpholite is indicative of blueschist facies conditions in metapelites. It is thought to be one of the main H 2 O carriers, contributing to the change in the mechanical properties of rocks deformed along plate interfaces. However, first-order parameters controlling the formation/preservation of carpholite in natural mineral assemblages are poorly constrained. We carried out forward modelling on a dataset of low blueschist facies metapelites, with the aim to elucidate which physio-chemical parameters govern the formation/preservation of carpholite in the observed mineral assemblages. Additionally, we attempted to evaluate the link between the hydrated phases in the mineral assemblages and the potentially released fluids during dehydration reactions. The results show that the formation/preservation of significant amounts of carpholite depends mainly on SiO 2 /Al 2 O 3 >2:1, and XMg [MgO/MgO + FeO total ] and A [Al 2 O 3 − 3 × K 2 O/Al 2 O 3 − 3 × K 2 O + FeO tot ] molar ratios of 0.40–0.60 and >0.40, respectively, and H 2 O-saturated conditions. Fe 3+ appears to have a minor role in carpholite stability. The amounts of fluids released by dehydration reactions along P–T paths typical of subduction zones are consistent for samples whose composition promote kaolinite, pyrophyllite and carpholite formation. Additionally, in samples with bulk compositions conducive to significant carpholite formation (>10 vol%), the predicted amount of potentially released aqueous fluids is higher than that predicted using a bulk composition that does not promote the abundant formation of this mineral. The implications for seismic behaviour are also discussed.