Pressure-, temperature- and water-dependent melting in silica-rich rocks – new parameterization and its application in models of porous melt flow

Abstract. Melting of the continental crust and transport of melt through it are key processes in the continental crust. Progress in numerical modeling of these processes is hindered by challenges in implementing the complex melting reactions that occur in crustal rocks. Here, we propose a description of the melting of micaceous silica-rich rocks based on a simplified parameterization of existing thermodynamic models and experimental data. In this parameterization, the equilibrium melt fraction depends only on the temperature, pressure, and bulk rock water content. It can be implemented in numerical models, is computationally efficient, and can be tuned to approximate the melting of different metasedimentary and metaigneous rock types. Besides melt productivity, it determines the water content in the solid matrix and melt, which can be used to estimate melt viscosity. We demonstrate the applicability of such parameterization on one-dimensional models representing porous melt flow through vertical crustal columns at various conditions. We show that long-lasting porous melt flow typically results in stratification with a low melt fraction in deep levels and melt accumulation at shallow levels of the partially molten column. In the melt source region, the water content decreases and heterogeneities disappear. In contrast, in shallow levels of the partially molten column, melt inflow from below causes heterogeneous enrichment in water content. In addition, due to the decrease of melt viscosity with pressure, the porous melt flow efficiency increases with depth. At deep-crustal conditions, the melt flow may be efficient already at ~5–10 % melt fraction.