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

Porcelli, D Woolum, D Cassen, P

D. Porcelli et al., Deep Earth rare gases: initial inventories, capture from the solar nebula,and losses during Moon formation, EARTH PLAN, 193(1-2), 2001, pp. 237-251

63

INGLESE

Article

Earth Sciences

EARTH AND PLANETARY SCIENCE LETTERS

0012-821X → ACNP

193

1-2

2001

237 - 251

ISI

0012-821X(20011130)193:1-2<237:DERGII>2.0.ZU;2-A

The implications of mantle rare gas characteristics for both the acquisitio n of rare gases from the solar nebula and subsequent losses to space are ex amined. There is at least one deep mantle reservoir rich in He-3 and Ne tha t was trapped early in Earth history. with minimum concentrations obtained by closed system calculations. Ne isotopes indicate the presence of a compo nent that has a solar composition. Xe isotopes indicate that extensive late losses occurred from the mantle as well as from the atmosphere. Calculatio ns based on a simple two-stage evolution provide times of losses of up to s imilar to 100 Ma after the formation of the solar system from both the mant le 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 t wo 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 de ep Earth (or Earth-forming materials) must provide these high initial conce ntrations, presumably in the high-energy environment of planetary accretion where strong degassing of solids might be expected to have occurred. A mec hanism that satisfies these requirements is the dissolution in a magma ocea n of rare gases from a dense primary atmosphere. A massive atmosphere of so lar composition would have been captured if the Earth had formed prior to d ispersal of the solar nebula. The underlying mantle would have melted due t o the energy of accretion and the blanketing effect of this atmosphere. Rar e gases would then have entered the molten Earth by dissolution at the surf ace and downward advection. For typical solubility coefficients, a total pr essure 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 com position, therefore., somewhat less Xe must be trapped prior to the late lo ss event for longer periods of exchange. As solidification of the mantle pr oceeded outward during cooling, the distributions of retained rare gases wo uld have been determined by the history of surface pressure and temperature during the coupled cooling of the Earth and atmosphere. The giant impact p roposed for Moon formation may have been responsible for the inferred subst antial and late gas losses from the deep mantle as well as from the atmosph ere. Constraints on the timing of Moon formation derived from Hf-W systemat ics and simulations of the giant impact are consistent with the Xe isotope constraints for gas loss. (C) 2001 Elsevier Science B.V. All rights reserve d.