Mobility of Magmas Within the Earth: Insights From the Elasticity and Transport Properties of Hydrous Albitic Melts

Abstract

AbstractThe continental crust is produced by the solidification of aluminosilicate‐rich magmas which are sourced from deep below the surface. Migration of the magma depends on the density (ρ) contrast to source rocks and the melt viscosity (η). At the surface, these silica‐rich melts are typically sluggish due to high η > 1,000 Pa s. Yet at their source regions, the melt properties are complexly influenced by pressure (P), temperature (T), and water contents (). In this study, we examined the combined P‐T‐ effects on the behavior of melts with an albite stoichiometry (NaAlSi3O8). We used first‐principles molecular dynamics simulations to examine anhydrous (0 wt % H2O) and hydrous (5 wt % H2O) melts. To constrain the P and T effects, we explored P ≤ 25 GPa across several isotherms between 2500 and 4000 K. The melts show anomalous P‐ρ relationships at low P ∼ 0 GPa and high T ≥ 2500 K, consistent with vaporization. At lithospheric conditions, melt ρ increases with compression and is well described by a finite‐strain formalism. Water lowers the melt density (ρhydrous < ρanhydrous) but increases the compressibility, that is, 1/Khydrous >1/Kanhydrous or Khydrous < Kanhydrous. We also find that the melt η decreases with pressure and then increases with further compression. Water decreases the viscosity (ηhydrous < ηanhydrous) by depolymerizing the melt structure. The ionic self‐diffusivities are increased by the presence of water. The decreased ρ and η by H2O increase the mobility of magma at crustal conditions, which could explain the rapid eruption and migration timescales for rhyolitic magmas as observed in the Chaitén volcano in Chile.