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

Abstract

Partial melting of crustal rocks is responsible for the formation of silicic magmas and crustal differentiation. Determining the mechanisms and effects of partial melting is necessary to understand the geochemical signature of the magmas 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 from 24 to 240 h. Partial melting 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, the melt 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 from the crystallizing trachyandesitic melt. The melting-assimilation process results in a system characterized by a crystalline trachyandesite enclosing a hybrid trachytic melt, produced by the chemical mixing of the granitic melt with the melt segregated from the crystalline trachyandesite.Our experimental results indicate that partial melting 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.