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

地球外核的粘性是地球最重要的物理性质之一，因为它对地球深部的热输运和物质输运有着重要的影响。尽管它很重要，但它可能是地球上最不为人所知的财产。本研究的目的是通过高压实验对外核的粘度进行约束。研究了压力对熔体物理性质和结构的影响。在2和3 GPa1450ﾟC下测定了玄武岩和流纹岩熔体中的水扩散系数。与以往在低压下的研究相比，在3 GPa时水扩散系数对水含量不敏感，所测得的总水浓度分布实际上可以用总水的单一恒定扩散系数来拟合。在3 GPa1450ﾟC下，玄武岩熔体(0.14-2.3wt%)的水扩散系数为4.7x10^-10；-10/S，流纹岩熔体(1.5-3.1wt%)的水扩散系数为1.8x10^-10/S。另外，2 GPa1450ﾟC下的水扩散系数与水浓度有关。M^2/S，含水率0.4wt%；m^2/S，含水率1.6wt%。然而，在压力较低的情况下，这种依赖性比之前的研究预期的要小。目前的实验结果表明，随着压力的增加，OH物种的扩散系数增加，而分子H_2O的扩散系数减小，因此前者的扩散系数在3 Gpa左右与后者相当。Ni在熔融Au中的扩散系数测量到3 Gpa时，在1 Gpa和1200ﾟC下的扩散系数为2.3+-0.4×10^&lt；-9&gt；m^2/S，在3 Gpa和1300ﾟC下的扩散系数为2.3-3.7×10^&lt；-9&gt；S。合成的粘度与使用安德拉德公式等理论估算的粘度是一致的。

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.