H ₂O and Cl in deep crustal melts: the message of melt inclusions in metamorphic rocks

The use of NanoSIMS on primary melt inclusions in partially melted rocks is a powerful approach to clarify the budget of volatiles at depth during crust formation and its reworking. Anatectic melt inclusions are indeed gateways to quantify Hinline-formula₂O, halogens and other species (e.g. COinline-formula₂, N) partitioned into the deep partial melts generated during metamorphism of the continental crust. Here we present new datasets of NanoSIMS measurements of Hinline-formula₂O and Cl in preserved melt inclusions from metamorphic rocks with different protoliths – magmatic or sedimentary – which underwent partial melting at different pressure–temperature–fluid conditions. These new datasets are then compared with similar data on natural anatectic melts available in the literature to date. Our study provides novel, precise constraints for the Hinline-formula₂O content in natural melts formed at high pressure, a field previously investigated mostly via experiments. We also show that Hinline-formula₂O heterogeneities in partial melts at the microscale are common, regardless of the rock protolith. Correlations between Hinline-formula₂O contents and inline-formulaP–inline-formulaT values can be identified merging new and old data on anatectic inclusions via NanoSIMS. Overall, the data acquired so far indicate that silicate melt generation in nature always requires Hinline-formula₂O, even for the hottest melts found so far (inline-formula>1000 inline-formula^∘C). Moreover, in agreement with previous work, preserved glassy inclusions always appear to be poorer in Hinline-formula₂O than crystallized ones, regardless of their chemical system and/or inline-formulaP–inline-formulaT conditions of formation. Finally, this study reports the very first NanoSIMS data on Cl (often in amounts inline-formula>1000 ppm) acquired in situ on natural anatectic melts, showing how anatectic melt inclusions – additionally to magmatic ones – may become a powerful tool to clarify the role of halogens in many geological processes, not only in crustal evolution but also in ore deposit formation.