Externally- vs. internally-derived H2O as control on reaction progress during high-pressure metamorphism

The progress of metamorphic reactions at high-pressure is commonly limited by kinetics. Relatively low temperatures, absence of deformation, short timescales and particularly the paucity of fluid may hamper transformation, thereby promoting the preservation of lower-pressure assemblages at eclogite facies conditions. However, minute amounts of H2O can trigger the prograde breakdown of plagioclase to denser high-pressure assemblages. In the case of dry protoliths such as gabbro/granulite, H2O must be added to the system to cause metamorphism, for example via fluid infiltration. The ‘eclogitization’ of granitoids however can occur in a closed system due to the partial breakdown of igneous biotite. Both the addition of external H2O as well its liberation from biotite leads a localized increase in chemical potential (µH2O), causing H2O to be transported into nearby anhydrous minerals. If external H2O is added to metagranitoids at high pressure, it is mainly accommodated in phengite growing at the expense of K-feldspar. Upon decompression, such hydrated rocks would dehydrate, thereby causing fluid-assisted retrogression and loss of diagnostic eclogite-facies assemblages at lower pressure. By contrast, mafic eclogites have a limited capacity to store H2O in minor hydrous minerals such as amphibole. Even when fully saturated at high pressure, the crystallographically-bound H2O is insufficient for dehydration to occur during exhumation, thus favoring the preservation of garnet–omphacite-bearing assemblages. These mechanisms explain the discrepancy between peak pressures recorded in cogenetic mafic eclogites and orthogneisses observed in classic (U)HP terranes such as the Western Gneiss Region (Norway) and Dabie Sulu (China).