HIDDEN

Is the Earth’s core the hidden reservoir of noble gases?

The noble gases bear in their isotopic ratios the traces of the Earth differentiation, degassing, and long-term geodynamic evolution. They have the particularity that each one of them has at least one stable non-radiogenic isotope, which continuously escapes to the atmosphere at a very slow rate, and at least one radiogenic isotope, which is partially replenished over geological time. When mixtures arrive at the surface their ratios hold the key to deciphering the isolation and mixing of Earth’s internal reservoirs over the course of Earth’s history. The signatures in the mid-ocean ridge basalts are distinct from ocean island basalts, as their parent magmas have different sources. Stable noble gas isotopes may be stored in proposed hidden reservoirs in the deep Earth, which are so far hypotheses that need to be verified by experiments or calculations. Here we propose to compute the partitioning of noble gases between silicate magmas and iron metallic melts from first-principles molecular-dynamics calculations coupled with thermodynamic modeling. We cover the core formation moment, the evolution of the magma ocean, and the last droplets of highly evolved silicate liquid equilibrating with the liquid outer core. In particular, we test if the Earth’s core is the plausible hidden reservoir for all or some of the noble gases. We compare how the core fractioned He and Ne. If the fractionation is similar then the 3He/22Ne signature recorded in ocean island basalts is consistent with mantle plumes tapping into the core. We compare the partitioning of Xe with that of Ne, Ar, and Kr. If it is much higher, then the missing Xe could be trapped and stored in the core. We identify the mechanisms of the leakage of each of the noble gases, whether related to chemical potentials or secular cooling.

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