Transport Properties of Liquid Iron-heavy Element Alloys at Earth's Outer Core Conditions

Iron-rich metallic liquid in the Earth's outer core is thought to contain many elements. While the incorporation of light elements in liquid iron has been widely studied, liquid iron-heavy element mixtures, particularly considering Co, Mo, and W which are relevant in constraining core-forming conditions need more studies. We have recently investigated the thermodynamic and structural behavior of these siderophile elements dissolved in liquid iron using first-principles molecular dynamics over the pressure-temperature conditions of Earth's core. Here we extend the first-principles simulation to investigate transport properties of these alloy liquids. Our results show that the diffusion coefficient of Fe being almost insensitive to the impurity varies modestly over the outer core regime from ~3 x 10-9 m2s-1 at the core-mantle boundary condition to ~5 x 10-9 m2s-1 at the inner core boundary condition. The impurity elements particularly W tend to diffuse slower than host iron atoms by almost a factor of two. Interestingly, the calculated viscosity of pure and alloyed iron liquids remains almost unchanged taking low value of 13±3 mPa.s at the outer core conditions. Our results seem to imply that outer core convection may involve small-scale turbulent circulation and barodiffusion possibly causing heavy elements to accumulate near the inner core boundary.