Effect of pressure on the viscosity of molten iron : experimental study on the viscosity of the outer core

点击翻译按钮获取中文摘要

The viscosity of the earth's outer core is one of the most important physical property of the earth because it has significant influence on the thermal and material transport in the deep earth. In spite of its importance, it is probably the least well-known property of the earth. The purpose of the present study is to give a constraint on the viscosity of outer core from high pressure experiments. Effect of pressure on the physical property and structure of melt was studied.Water diffusivities in basaltic and rhyolitic melts were determined at 2 and 3 GPa and 1450ﾟC.Contrary to previous studies at low pressures, water diffusivity is not sensitive to the water content at 3 GPa, and the measured total water concentration profiles can be practically fitted by a single constant diffusion coefficient for total water. Water diffusivities are 4.7_x10^<-10>m^2/s in the basaltic melt (0.14-2.3 wt% water) and 1.8_x10^<-10>m^2/s in the rhyolitic melt (1.5-3.1 wt% water) at 3 GPa and 1450ﾟC.On the other hand, water diffusivity at 2 GPa and 1450ﾟC in the basaltic melt depends on the water-concentration ; 1.4_x10^<-10>m^2/s at 0.4 wt% water and 5.6_x10^<-10>m^2/s at 1.6 wt% water. However, this dependence is smaller than that expected from previous studies at lower pressures. Present experimental results suggest that the diffusivity of OH species increases with increasing pressure whereas that of molecular H_2O decreases, and consequently the former becomes comparable to the latter at about 3 GPa.Diffusion of Ni in molten Au was measured up to 3 GPa and it is 2.3+-0.4_x10^<-9>m^2/s at 1 GPa and 1200ﾟC,and 2.3-3.7_x10^<-9>m^2/s at 3 GPa snd 1300ﾟCApplying Sutherland-Einstein equation to the measured diffusivity, viscosity of Au at high pressure was calculated. The resultant viscosity is consistent with viscosities using theoretical estimation such as Andrade's formula.