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Experimental studies of mantle petrology find that small concentrations ofwater and carbon dioxide have a large effect on the solidus temperature anddistribution of melting in the upper mantle. However, it has remained unclearwhat effect small fractions of deep, volatile-rich melts have on melt transportand reactive melting in the shallow asthenosphere. Here we present theory andcomputations indicating that low-degree, reactive, volatile-rich melts causechannelisation of magmatic flow at depths approximately corresponding to theanhydrous solidus temperature. These results are obtained with a novel methodto simulate the thermochemical evolution of the upper mantle in the presence ofvolatiles. The method uses a thermodynamically consistent framework forreactive, disequilibrium, multi-component melting. It is coupled with a systemof equations representing conservation of mass, momentum, and energy for apartially molten grain aggregate. Application of this method in two-phase,three-component upwelling-column models demonstrates that it reproducesleading-order features of hydrated and carbonated peridotite melting; inparticular, it captures the production of low-degree, volatile-rich melt atdepths far below the volatile-free solidus. The models predict that segregationof volatile-rich, deep melts promotes a reactive channeling instability thatcreates fast and chemically isolated pathways of melt extraction. Reactivechanneling occurs where volatile-rich melts flux the base of the silicatemelting region, enhancing dissolution of fusible components from the ambientmantle. We find this effect to be similarly expressed for models of bothhydrated and carbonated mantle melting. These findings indicate that despitetheir small concentrations, water and carbon dioxide have an important controlon the extent and style of magma genesis, as well as on the dynamics of melttransport.

The role of volatiles in reactive melt transport in the asthenosphere