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FRIPRO-Fri prosjektstøtte

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

Alternative title: Er jordens kjerne det skjulte reservoaret av edelgasser?

Awarded: NOK 12.0 mill.

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2021 - 2025

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What makes the noble gases special from any other group of chemical elements is that they are chemically inert – they do not form any chemical bonds or only very weak bonds with other atoms. Consequently, in the deep Earth they are trapped between the atoms forming the crystal structures of various minerals and rocks of the interior, like in small atomic-size cages. From there, they escape when their host minerals and rocks melt. As a result, they are entrained from the deep to the surface by uprising magmas, and then they are released in the atmosphere. Like many other elements the noble gases have both radiogenic and non-radiogenic isotopes. All radiogenic ones are by-products of nuclear reactions of decay of unstable radioactive isotopes of specific elements, like uranium and thorium, which are found in the entire Earth’s mantle. Over geological time, the decay of the radiogenic isotopes continuously produces more and more radiogenic isotopes. All non-radiogenic isotopes were brought to Earth during its formation in the times of the accretion. Over geological time, their amounts in the convective mantle continuously decreased as they escaped into the atmosphere via sub-surface fluids and volcanic eruptions. Only the isotopes stored in deep isolated layers, like the outer core or specific regions of the lowermost mantle, were preserved over geological time. From there today they can be brought to the surface by deep upwelling magmas. Consequently, the isotopic composition of noble gases can tell us if a rock or lava originated from one of these deep reservoirs, if it was in contact with them, or none of the above. Here we use atomistic simulations to calculate which were the most plausible parts inside the Earth that have stored in the most efficient way the noble gases during the accretion. With these results we estimate the origins and ascent paths of deep magmas occurring in various places at the surfaces, like ocean ridges and volcanic islands.

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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FRIPRO-Fri prosjektstøtte

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