Transport of melt and volatiles in magmas inferred from kinetic experiments on the partial melting of granitic rocks

International audiencePartial melting of crustal rocks is responsible for the formation of silicic magmas and crustal differentiation. Determiningthe mechanisms and effects of partialmelting is necessary to understand the geochemical signature ofthemagmas and the transport of melt and volatiles in the crust. To this end, we have experimentally reproducedthe partial melting of granitic rock under three different conditions: i) partial melting in a closed system (dry andhydrous conditions); ii) partial melting in the presence of a discontinuity, represented by a channel of synthetichaplogranite and iii) assimilation of granitic rock by a hydrous trachyandesitic melt. Experiments were performedat temperatures between 750 and 1000 °C and pressures between 300 and 500 MPa, with durationsranging from24 to 240 h. Partialmelting initiates at temperatures higher than 750 °C as a result of both dehydrationmelting of hydrous minerals and melting at grain boundaries. The interplay between the two mechanismsdetermines the complex evolution of melt composition at the various degrees of partial melting. Core-to-rimcompositional profiles of residual feldspars in contact with the melt attest to a rapid mineral-melt re-equilibration(240 h). When a melt channel intersects the granite, the melt produced at the grain boundaries is rapidlysegregated within the channel and mineral-melt exchange reactions are inhibited. In turn, themelt in the channelis enriched in H2O and incompatible trace elements. When in contact with a hydrous, partially moltentrachyandesite, complete dissolution of the granite occurs at 900 °C, assisted by the migration of H2O fromthecrystallizing trachyandesiticmelt. The melting-assimilation process results in a systemcharacterized by a crystallinetrachyandesite enclosing a hybrid trachyticmelt, produced by the chemicalmixing of the granitic melt withthe melt segregated from the crystalline trachyandesite.Our experimental results indicate that partialmelting could be extremely common in granitic bodies and yield notraces in the residual rock after segregation of the interstitial melt. In contrast, core-to-rim profiles of feldsparsmay preserve a record of the interaction between residual minerals and the extracted melt. Fast segregation ofmelts enriched in volatiles and incompatible trace elements, produced by breakdown of hydrous minerals,may represent a possible mechanism for generation of pegmatites