Viscosity of Depolymerized Dunite Melts under Medium and High Pressures

— Some properties of depolymerized ultramafic (pyroxenite, peridotite, kimberlite, and dunite) melts under P – T parameters of the upper mantle and crust are still known inadequately poorly. These properties are, first of all, the concentration, temperature, and pressure functions of the viscosity of these melts. This publication reports the first experimental–theoretical data on the temperature and pressure dependences of the viscosity of model dunite melts (degree of depolymerization 100NBO/T = 340) within broad ranges of temperature (1300–1950°C) and pressure 100 MPa to 7.5 GPa in comparison with such dependences for more strongly polymerized basalt melts (100NBO/T = 58). Our experimental data (accurate to ±30 relative %) on the viscosity of model dunite melts are compared with analogous calculated dependences of the viscosity of dunite melts obtained with a practically experimental error using a modified physicochemical model for predicting the viscosity of magmatic melts. The experimentally determined viscosity of extremely depolymerized dunite melts is very low at both medium and high pressures: 0.09–0.63 Pa s. The viscosity of model dunite melts is demonstrated to exponentially decrease with increasing temperature under medium (100 MPa) and high (up to 7.5 GPa) pressures and, conversely, exponentially increase with increasing pressure by approximately one order of magnitude as the pressure increases from 100 MPa to 7.5 GPa at a constant temperature. The pressure function of the viscosity of basalt melts has an minimum at ~5.5 GPa. Our first experimental data prove that the viscous flow activation energy of dunite melts linearly increases with increasing pressure. Based on analysis of newly obtained and preexisting literature data, we developed a generalized concentration dependence of the viscous flow activation energy of depolymerized ultramafic melts over the whole compositional range of melts from pyroxenite to dunite.