A High-Pressure Compound of Argon and Nickel: Noble Gas in the Earth’s Core?

Abstract

Various geophysical models have shown that Ar, a natural decay product of 40K, is depleted in Earth’s main reservoirs (i.e., continental crust, atmosphere, and silicate mantle). This indicates that a large amount of Ar may be present in the Earth’s core, but such consideration is hindered by the lack of knowledge on the reactability of Ar with the core’s main constituents (Ni and Fe). Here we demonstrate the synthesis of a thermodynamically stable compound of Ar and Ni under thermodynamic conditions representative of the Earth’s core. Using in situ synchrotron X-ray diffraction and density functional calculations, we identified the compound as ArNi with a L11 Laves structure. The ArNi compound is stabilized by notable electron transfer from Ni to Ar, changing their electron configurations toward 3d7 and 4s1. The present results suggest that the abundance of Ar in the Earth’s core is controlled, beyond a simple solubility of Ar in molten Ni–Fe, by chemical bonding, which is truly extraordinary considering the inert nature of Ar under ambient conditions. Moreover, establishing the Earth’s core as a viable reservoir for Ar helps to postulate the natural decay of 40K in the core as a heating source for Earth’s inner dynamics.