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，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。本研究的目的是从高压实验中给出外核粘度的约束。本文研究了压力对熔体物理性质和结构的影响，测定了玄武质和流纹岩熔体在2和3GPa、1450 ℃下的水扩散系数。与以往低压下的研究相反，在3GPa下，水扩散系数对水含量不敏感，测量的总水浓度剖面可以用一个总水扩散系数来拟合。在<-10>3 GPa和1450 ° C下，玄武质熔体（0.14-2.3 wt%水）的水扩散率为4.7_x10 ^m^2/s，<-10>流纹质熔体（1.5-3.1 wt%水）的水扩散率为1.8_x10^ m^2/s。另一方面，玄武质熔体在2 GPa和1450 ° C下的水扩散率取决于水的浓度; <-10>0.4 wt%水时为1.4_x10^ m^2/s<-10>，1.6 wt%水时为5.6_x10^ m^2/s。然而，这种依赖性小于先前在较低压力下研究的预期。实验结果表明，OH物种的扩散系数随压力的增加而增加，而H_2O的扩散系数则随压力的增加而减小，在3GPa左右，OH物种的扩散系数与H_2O的扩散系数相当。在3GPa以下测量了Ni在Au熔体中的扩散系数，在1GPa和1200 ℃时为2.3 ± 0.4 × <-9>10 ~（-4）m ~ 2/s，在3GPa和1300 ℃时为2.3- 3.7 × 10 ~<-9>（-4）m ~ 2/s。所得粘度与使用理论估计（例如Andrade公式）的粘度一致。

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.