Viscosity of NaAlSi3O8−H2O fluids and implications for arc magmatism

Abstract

Water plays a crucial role in the transport properties of silicate liquids and is especially important in subduction-zone magma source regions where conditions promote complete miscibility between water and silicate melts. However, accurately tracing the movement of fluids and volatile-rich magmas is prevented by poor knowledge of the variation of viscosity across silicate- H 2 O joins at relevant pressure-temperature conditions. We used molecular dynamics driven by a machine learning potential of ab initio quality to investigate supercritical NaAlSi 3 O 8 − H 2 O fluids from 0 - 100 % H 2 O at 1200–3000 K (at 2 GPa) and 1–5 GPa (at 2500 K). Our results provide atomic-level insight into how H 2 O disrupts the silicate melt network to change viscosity. We find smaller fluid viscosity at a given pressure, temperature, and H 2 O content than obtained in previous models. We fit our results to an empirical viscosity model that allows application to transport in geologic settings. For model subduction zones, we find that the mean percolation velocity of fluids ascending by Darcian flow varies strongly with H 2 O content and that, at constant H 2 O , the fluids accelerate as they rise from slab source into the mantle wedge, and decelerate on further ascent. For plausible water contents, we find that ascent times from the slab to the Moho may be as little as 8000 years, satisfying constraints from U-Th disequilibria.