Deep Earth rare gases: initial inventories, capture from the solar nebula, and losses during Moon formation

The implications of mantle rare gas characteristics for both the acquisition of rare gases from the solar nebula and subsequent losses to space are examined. There is at least one deep mantle reservoir rich in He-3 and Ne that was trapped early in Earth history. with minimum concentrations obtained by closed system calculations. Ne isotopes indicate the presence of a component that has a solar composition. Xe isotopes indicate that extensive late losses occurred from the mantle as well as from the atmosphere. Calculations based on a simple two-stage evolution provide times of losses of up to similar to 100 Ma after the formation of the solar system from both the mantle and the atmosphere, These losses appear to have depleted the rare gases by greater than or equal to 97%, therefore, there originally was at least two orders of magnitude more rare gases than now present. Mechanisms for the capture of rare gases soon after the start of the solar system into the deep Earth (or Earth-forming materials) must provide these high initial concentrations, presumably in the high-energy environment of planetary accretion where strong degassing of solids might be expected to have occurred. A mechanism that satisfies these requirements is the dissolution in a magma ocean of rare gases from a dense primary atmosphere. A massive atmosphere of solar composition would have been captured if the Earth had formed prior to dispersal of the solar nebula. The underlying mantle would have melted due to the energy of accretion and the blanketing effect of this atmosphere. Rare gases would then have entered the molten Earth by dissolution at the surface and downward advection. For typical solubility coefficients, a total pressure of similar to 100 atm and surface temperatures of > similar to 2500 degreesC are required to dissolve sufficient rare gases to account for the initial lower mantle concentrations. While Xe in the mantle is isotopically exchanging with the primary atmosphere. it will be buffered to a solar composition, therefore., somewhat less Xe must be trapped prior to the late loss event for longer periods of exchange. As solidification of the mantle proceeded outward during cooling, the distributions of retained rare gases would have been determined by the history of surface pressure and temperature during the coupled cooling of the Earth and atmosphere. The giant impact proposed for Moon formation may have been responsible for the inferred substantial and late gas losses from the deep mantle as well as from the atmosphere. Constraints on the timing of Moon formation derived from Hf-W systematics and simulations of the giant impact are consistent with the Xe isotope constraints for gas loss. (C) 2001 Elsevier Science B.V. All rights reserved.