Transport of melt and volatiles in magmas inferred from kinetic experiments on the partial melting of granitic rocks
Résumé
Partial melting of crustal rocks is responsible for the formation of silicic magmas and crustal differentiation. Determining
the mechanisms and effects of partialmelting is necessary to understand the geochemical signature of
themagmas and the transport of melt and volatiles in the crust. To this end, we have experimentally reproduced
the partial melting of granitic rock under three different conditions: i) partial melting in a closed system (dry and
hydrous conditions); ii) partial melting in the presence of a discontinuity, represented by a channel of synthetic
haplogranite and iii) assimilation of granitic rock by a hydrous trachyandesitic melt. Experiments were performed
at temperatures between 750 and 1000 °C and pressures between 300 and 500 MPa, with durations
ranging from24 to 240 h. Partialmelting initiates at temperatures higher than 750 °C as a result of both dehydration
melting of hydrous minerals and melting at grain boundaries. The interplay between the two mechanisms
determines the complex evolution of melt composition at the various degrees of partial melting. Core-to-rim
compositional 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 rapidly
segregated within the channel and mineral-melt exchange reactions are inhibited. In turn, themelt in the channel
is enriched in H2O and incompatible trace elements. When in contact with a hydrous, partially molten
trachyandesite, complete dissolution of the granite occurs at 900 °C, assisted by the migration of H2O fromthe
crystallizing trachyandesiticmelt. The melting-assimilation process results in a systemcharacterized by a crystalline
trachyandesite enclosing a hybrid trachyticmelt, produced by the chemicalmixing of the granitic melt with
the melt segregated from the crystalline trachyandesite.
Our experimental results indicate that partialmelting could be extremely common in granitic bodies and yield no
traces in the residual rock after segregation of the interstitial melt. In contrast, core-to-rim profiles of feldspars
may preserve a record of the interaction between residual minerals and the extracted melt. Fast segregation of
melts enriched in volatiles and incompatible trace elements, produced by breakdown of hydrous minerals,
may represent a possible mechanism for generation of pegmatites.