Mushmicrophysics and the reactivation of crystal-rich magma reservoirs

Abstract

Reactivation and eruption of upper crustal crystal-rich magma reservoirs ("crystal mushes") following recharge has recently been invoked in numerous volcanic systems worldwide. Over the last few years, several models have been proposed for the reactivation of such mushes prior to or during eruptions. These models vary significantly in terms of predicted timescales associated with reactivation, because they assume that different physical mechanisms control the dynamics of this process. A common limitation of all the proposed models is that they parameterize the complex nonlinear multiphase dynamics that govern the evolution of these magmas in their open system reservoirs and rely on simple empirical laws. We argue that microscale physical models are a necessity if one wants to better constrain the evolution of these complex systems and the conditions that lead to eruption. As petrological observations of erupted mushes strongly support a thermal and fluid input from wet magma recharges, we have developed a pore-scale multiphase heat and fluid transport model to understand the effect of a percolating fluid phase on the partial melting and reactivation of crystal mushes. Specifically, we use lattice Boltzmann calculations to reveal a counterintuitive feedback between volatile transport and melting in crystal-rich environments. We find that partial melting, even at a low degree, can significantly reduce the efficiency of the buoyant migration of exsolved volatiles in the mush and therefore negatively impact the heat transfer upward during reactivation. This negative feedback between melting and volatile transport is expected to significantly affect the distribution of exsolved volatiles in the reservoirs, as well as the transport of trace species carried by the volatile phase (e.g., S, metals). The presence of a disperse magmatic volatile phase (unconnected bubbles) will also affect the thermomechanical properties of the mush during reactivation, making it more compressible and thermally less conductive.