Early crust on top of the Earth's core |
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| Institution: | 1. Max Plank Institute für Chemie, Abt. Geochemie, Joh-J.-Becher-Weg 27, Postfach 3060, 55020 Mainz, Germany;2. Geological Institute, Kola Scientific Center of Russian Academy of Sciences, 184200 Apatity, Russia |
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| Abstract: | In an effort to resolve the current conflict between geochemical requirements for an apparently isolated mantle reservoir and recent geophysical evidence for whole-mantle convection, we investigate the possibility that the region above the core-mantle boundary, termed D″, serves as an early-isolated, rare-gas- and incompatible-element-bearing reservoir, and we propose a mechanism for its formation that is a likely outcome of Earth accretion models. In these models, the most cataclysmic event in Earth history, the moon-forming giant impact on the proto-Earth of a Mars-size planet (perhaps as early as 4540 Ma ago) was followed by accretion of smaller bodies long afterwards (until ～3900 Ma). Some collisions probably triggered melting, metal segregation and degassing. However, the small bodies, fragments, particles, dust, including those of chondrite-like composition, existed on near-earth orbits, were irradiated by intense solar wind, and finally fell on an early-formed, incompatible-element-enriched basaltic crust without causing extensive melting. The respective regions of the crust, loaded with chondrite-like debris, were therefore significantly enriched in iron. When this mixed material was subducted, the bulk density of the subducted lithosphere exceeded that of the bulk silicate mantle, which had already lost its metallic iron to the core. Segregation of this denser material at the base of the mantle was facilitated by the high temperatures at the core-mantle boundary, which greatly reduce the viscosity, as was quantitatively modelled by Christensen and Hofmann (Christensen, U.R., Hofmann, A.W., 1994. Segregation of subducted oceanic-crust in the convecting mantle. J. Geophys. Res.-Solid Earth 99 (B10), 19867–19884). Assuming a basalt/chondrite mass ratio of about 4/1, we obtain a density contrast of ～7%, which would stabilize the subducted material between the metal core and silicate mantle.Mass balance considerations and preliminary results of geochemical modelling of the above scenario (similar to that performed by Tolstikhin and Marty Tolstikhin, I.N., Marty, B., 1998. The evolution of terrestrial volatiles, A view from helium, neon, argon and nitrogen isotope modeling. Chem. Geol. 147, 27–52]) show the potential geochemical importance of D″. (1) Modelling of Pu–U–I–Xe isotope systematics predicts formation of this reservoir early in Earth history, ～100 Ma after formation of the Solar system. (2) The total amount of heat-generating U, Th, K (and other highly incompatible elements) in D″ exceeds 20% of the Earth inventory, and a similar portion of terrestrial heat is being transferred from the core + D″ into the base of the overlying convecting mantle. (3) D″ is enriched in solar implanted rare gases because the small (re)-accreting debris with high surface/mass ratios will have been subjected to intense radiation by the early sun. (4) Rare gases diffuse from D″ into the overlying mantle and are then transferred into upwelling plumes, which originate above D″. In addition, small amounts of D″ material may be entrained by the mantle convective flow as was recently discussed by Schott et al. Schott, B., Yuen, D.A., Braun, A., 2002. The influences of composition and temperature-dependent rheology in thermal-chemical convection on entrainment of the D″ layer. Physics Earth Planet. Inter. 129, 43–65]. From the rare-gas modelling it follows that initially (～4500 Ma ago) D″ could have been more massive by a factor of ～1.2 than at present (about 2 × 1026 g). The present-day mass flux from D″ into the convecting mantle is estimated to be ≤0.05 × 1016 g year?1, a factor of ～100 less than the rate of ridge magmatism. This small contribution of D″ material makes it difficult to trace fingerprints of D″ even using such sensitive tracers as Pb isotope ratios. (5) The density contrast that stabilizes D″ is maintained by its higher intrinsic density due to the iron-rich chondrite-like component. Subduction of this material, its entrainment by convective mantle flow and mixing could also account for the preservation of the chondritic relative abundances of siderophile elements in the mantle. If D″ is partially molten, the density contrast may be caused by a high-density melt fraction. |
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