Effects of volatiles on melt production and reactive flow in the mantle

Magmatism in the Earth interior has a significant impact on its dynamic, thermal and compositional evolution. Experimental studies of petrology of mantle melting find that small concentrations of water and carbon dioxide have a significant effect on the solidus temperature and distribution of melting in the upper mantle. However, it has remained unclear what effect small fractions of deep, volatile-rich melts have on melting and melt transport in the shallow asthenosphere. We present a method to simulate the thermochemical evolution of the upper mantle in the presence of volatiles. The method is based on a novel, thermodynamically consistent framework for reactive, disequilibrium, multi-component melting/crystallisation. This is coupled with a system of equations representing conservation of mass, momentum, and energy for a partially molten grain aggregate. Application of this method to upwelling-column models demonstrates that it captures leading-order features of hydrated and carbonated peridotite melting. Our findings suggest that volatiles promote a reactive channelling instability that creates fast and chemically isolated pathways of melt extraction. Reactive channelling occurs where volatile-rich melts, formed at depth, flux into the silicate melting region, enhancing dissolution of fusible minerals from the ambient mantle. These findings indicate that despite their small concentrations, mantle volatiles have an important control on the extent and style of magma genesis, as well as the dynamics of melt transport.