The Role of Volatiles in Reactive Melt Transport in the Asthenosphere

Abstract

Experimental studies of mantle petrology find that small concentrations of\nwater and carbon dioxide have a large effect on the solidus temperature and\ndistribution of melting in the upper mantle. However, it has remained unclear\nwhat effect small fractions of deep, volatile-rich melts have on melt transport\nand reactive melting in the shallow asthenosphere. Here we present theory and\ncomputations indicating that low-degree, reactive, volatile-rich melts cause\nchannelisation of magmatic flow at depths approximately corresponding to the\nanhydrous solidus temperature. These results are obtained with a novel method\nto simulate the thermochemical evolution of the upper mantle in the presence of\nvolatiles. The method uses a thermodynamically consistent framework for\nreactive, disequilibrium, multi-component melting. It is coupled with a system\nof equations representing conservation of mass, momentum, and energy for a\npartially molten grain aggregate. Application of this method in two-phase,\nthree-component upwelling-column models demonstrates that it reproduces\nleading-order features of hydrated and carbonated peridotite melting; in\nparticular, it captures the production of low-degree, volatile-rich melt at\ndepths far below the volatile-free solidus. The models predict that segregation\nof volatile-rich, deep melts promotes a reactive channeling instability that\ncreates fast and chemically isolated pathways of melt extraction. Reactive\nchanneling occurs where volatile-rich melts flux the base of the silicate\nmelting region, enhancing dissolution of fusible components from the ambient\nmantle. We find this effect to be similarly expressed for models of both\nhydrated and carbonated mantle melting. These findings indicate that despite\ntheir small concentrations, water and carbon dioxide have an important control\non the extent and style of magma genesis, as well as on the dynamics of melt\ntransport.\n