A redox effect on the viscosity of molten pyrolite

Abstract

The viscosity of molten Earth mantle has been determined for various redox states using very high temperature viscometry at 1 bar. The viscosity-temperature relationship of the most oxidised pyrolite studied here is comparable to that of a previous determination of the viscosity of a peridotite melt. For the first time, the effect of iron redox state on molten mantle viscosity has been determined by calorimetric analysis of the glass transition on extremely carefully characterised glassy samples quenched from conditions of variable fO2 using gas-mixing levitation. There is a clear trend of decreasing viscosity with increasing redox state over the range of Fe3+/ΣFe. = 0.07–0.29. We also observe an increase in glass transition temperatures with increasing melt depolymerisation (i.e., higher NBO/T ratios). Both the negative oxidation dependence of viscosity and glass transition behaviour upon depolymerisation are contradictory to the trends observed from redox viscometry of more silica-rich melts but may be consistent with the broadly diminishing positive redox viscometry trends of melts with increasing basicity.