HR: 0800h
AN: U41A-0722    [Abstracts]
TI: An Early Formed D'' Reservoir Reconciles Geochemical Mass Balance With Whole Mantle Convection Models
AU: Tolstikhin, I N
EM: tolstikin@yahoo.com
AF: Max-Planck-Inst. Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Tolstikhin, I N
EM: tolstikin@yahoo.com
AF: Geological Institute, Kola Scientific Center Russian Academy Sci., Apatity, 184200 Russian Federation
AU: Kramers, I
EM: kramers@geo.unibe.ch
AF: Geological Institute, University of Bern, Bern, 3012 Switzerland
AB: One of the most intriguing present-day problems in Earth sciences is reconciling whole mantle convection models (that follow from seismic tomography and dynamic modeling) and the chemical and isotopic mass balance of continents and depleted mantle, which favor partial-mantle convection. Specifically, geochemical observations point to an apparently isolated, early-formed reservoir deep in the Earth. The most important of these observations are: (1) The occurrence of solar noble gases in the mantle, which is in contrast with the extreme degassing of this reservoir indicated by mantle xenology; (2) specific isotopic compositions of mantle He, Ne and Xe point to a reservoir with low U/3He and 136Xe(Pu)/129Xe(I) ratios, implying both early formation and low degassing of this reservoir. We suggest that the core-mantle transition zone (termed D'') is the reservoir indicated by these observations. The material of D'' could comprise an early gabbroic-basaltic crust loaded with chondrite-like, late-accreting matter including a solar-wind irradiated regolith. If subducted, this material should accumulate above the metal core due to an intrinsic density contrast. Provided that it was not hydrated at the surface, so that subduction did not entail volatile loss, it could have retained its geochemical characteristics. We examined the consequences of this scenario by transport models envisaging: (1) Earth accretion accompanied by mantle melting and fractionation, core segregation, formation and recycling of mafic crust, degassing, and gas loss from the atmosphere, followed by (2) crust-mantle evolution involving continent growth and recycling. Comparison of calculated and observed parameters allows a solution of the model. The D'' is formed within a time interval from 40 to 80 Ma after formation of the solar system and comprises about 20% of the BSE inventory of incompatible (including heat-producing) elements. Because the bulk of the D'' material (basalt) is fractionated, its apparent isolation allows a mass balance for 147Sm-144Nd and U-Th-Pb systematics to be achieved with whole-mantle convection (apart from D''. D'' is an important 40Ar*-, 129Xe*- and 3He- bearing reservoir in the Earth. After accretion a small amount of D'' material was entrained by convective flow thus contributing noble gases to the mantle: Rare gas modeling yields a low flux from D'' into the overlying mantle, about 20% of the D'' mass per 4.5 Ga, which is about 100 times lower than the rate of ridge magmatism. These and other results of the modeling characterize D'' as a geochemically important reservoir. formed within a time interval from 40 to 80 Ma after formation of the solar system and comprises about 20% of the BSE inventory of incompatible (including heat-producing) elements. Because the bulk of the D'' material (basalt) is fractionated, its apparent isolation allows a mass balance for 147Sm-144Nd and U-Th-Pb systematics to be achieved with whole-mantle convection (apart from D''). D'' is an important 40Ar*-, 129Xe*- and 3He- bearing reservoir in the Earth. After accretion, a small amount of D'' material was entrained by convective flow thus contributing noble gases to the mantle: Rare gas modeling yields a low flux from D'' into the overlying mantle, about 20% of the D'' mass per 4.5 Ga, which is about 100 times lower than the rate of ridge magmatism. These and other results of the modeling characterize D'' as a geochemically important reservoir.
DE: 8130 Heat generation and transport
DE: 7207 Core and mantle
DE: 3319 General circulation
DE: 1212 Earth's interior--composition and state (8105)
DE: 1507 Core processes (8115)
SC: Union [U]
MN: 2004 AGU Fall Meeting