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

We have developed density^viscosity^composition (r^m^X) models for natural aqueous fluids and hydrous melts, based on experimental data for silicate þ 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 .1 0 � 3·5 to 10 � 4·3 ms � 1 for porous flow; c. 1 to 7 m s � 1 for flow through 1mm wide fractures)