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 and at least one radiogenic isotope. The non-radiogenic isotopes are residues, either of the Big Bang or of supernovas. They arrived on Earth during accretion and were in great part degassed from the mantle during the episodes of partial or total melting of the very early history of our planet. Whatever fraction remained was stored in geochemical reservoir(s) inside the Earth; there they had little or no contact with the surface, though they continuously escaped at a very slow rate into deep melts that eventually bring them to the surface. The amount of non-radiogenic isotopes decreases over geological time as they are leaked from their reservoirs.
The noble gases are such important geological keys because they are incompatible elements and chemically inert. They preserve pristine information that relates directly to the formation of our planet and they accumulate information about the geological evolution.
The HIDDEN project was designed to reconstruct this elusive record. By combining first-principles molecular dynamics simulations with thermodynamic modeling, we computed the partitioning of noble gases between silicate and metallic melts across the full evolution of the early Earth. This includes the initial stages of core formation, the progression of the magma ocean, and the last moments of equilibration between silicate liquids and the metallic core. We extended this approach to volatile exchange between distinct geological reservoirs, and to model devolatilization during planetary melting events. Our results provide a quantitative foundation for interpreting noble gas signatures in mantle-derived rocks, and for reconstructing the deep volatile history of our planet.
Here is a selection of some of the most remarkable results:
The magma ocean was a huge helium reservoir in the early Earth
We determined the chemical behaviour of He in the magma ocean (MO) during core formation. We perform ab initio molecular dynamics simulations at temperatures and pressures along the MO adiabat. Our results show that primordial He was largely trapped in MO. At the top, under the hot, dense early atmosphere, He remained mainly in the MO and degassed only later, at low atmospheric pressure. At the bottom, He partitions preferentially into the MO rather than the liquid core. The origin of the OIBs reflects a significant contribution and contamination from mantle sources, with no contribution from the top of the outer core, which is depleted in primordial He. We suggest that the search for the He reservoirs be conducted at the base of the solid mantle.
The magma ocean was a huge helium reservoir in the early Earth
Ozge Ozgurel, Razvan Caracas
Geochemical Perspective Letters, 25, 46-590 (2023)