Density and Viscosity of Hydrous Magmas and Related Fluids and their Role in Subduction Zone Processes

We have developed density-viscosity-composition (ρ-µ-X) models for natural aqueous fluids and hydrous melts, based on experimental data for silicate + H2O, especially for the pressure (P) and temperature (T) conditions above subduction zones. We examine hydrothermal and melt pathway systematics above subducting slabs into the Earth's mantle, back up along the top-of-slab, and downward with the subduction. Aqueous slab fluids and hydrous mantle melts show distinct flow properties (as observed in activation energy in viscosity data) despite continuity in solute-polymerization characteristics. Buoyancy changes are small for fluids except in the localized vicinity of critical behaviour and at solidi where H2O partitions also into melt. Our model predicts dilute high-PT potassic haplogranite fluids to be less viscous than sodic varieties whereas for concentrated fluids a deep viscosity minimum occurs in mixed K/Na (c. 1:1 molar) compositions. Higher dissolved silicate concentrations increase fluid density and viscosity leading to slower less-buoyant flow with increasing PT. Thus ascent rates of slab fluid increase by about an order of magnitude (from c. 10−3·5 to 10−4·3 m s−1 for porous flow; c. 1 to 7 m s−1 for flow through 1 mm wide fractures) with decompression from 5 to 3 GPa, as a result of decreasing solute loads, ρ and µ. Mantle fluid viscosities are predicted (10−4 to 10−3·7 Pa s) to be approximately half those of crustal fluids (10−3·9 to 10−3·1 Pa s) and of lower density (e.g. 1·4 compared to 1·6 g cm−3), reflecting their compositional differences (here mainly SiO2). Thus, ascending slab fluids tend to accelerate as they move back up the slab and also moving from slab to porous mantle. Slab melts are up to c. 6 orders of magnitude less viscous (e.g. c. 10−0·5 to 102·5 Pa s) and therefore faster flowing than hydrous deep crustal granitoids (e.g. c. 106·5 to 103·5 Pa s), reflecting higher water contents of the former (e.g. 30 vs 10 wt %). Concentrated crustal fluids migrate 5-6 orders of magnitude faster than hydrous melt, mostly because of calculated viscosity differences. We find that fluids flow faster in the mantle than in the crust, and that most of the mass transfer through the mantle occurs via hydrous melt.