Experimental studies of mantle petrology find that small concentrations of
water and carbon dioxide have a large 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 melt transport
and reactive melting in the shallow asthenosphere. Here we present theory and
computations indicating that low-degree, reactive, volatile-rich melts cause
channelisation of magmatic flow at depths approximately corresponding to the
anhydrous solidus temperature. These results are obtained with a novel method
to simulate the thermochemical evolution of the upper mantle in the presence of
volatiles. The method uses a thermodynamically consistent framework for
reactive, disequilibrium, multi-component melting. It is coupled with a system
of equations representing conservation of mass, momentum, and energy for a
partially molten grain aggregate. Application of this method in two-phase,
three-component upwelling-column models demonstrates that it reproduces
leading-order features of hydrated and carbonated peridotite melting; in
particular, it captures the production of low-degree, volatile-rich melt at
depths far below the volatile-free solidus. The models predict that segregation
of volatile-rich, deep melts promotes a reactive channeling instability that
creates fast and chemically isolated pathways of melt extraction. Reactive
channeling occurs where volatile-rich melts flux the base of the silicate
melting region, enhancing dissolution of fusible components from the ambient
mantle. We find this effect to be similarly expressed for models of both
hydrated and carbonated mantle melting. These findings indicate that despite
their small concentrations, water and carbon dioxide have an important control
on the extent and style of magma genesis, as well as on the dynamics of melt
transport.
physics.geo-ph
,physics.geo-ph
