HR: 17:00h
AN: U34A-05    [Abstracts]
TI: Mass Balance Considerations for Highly Siderophile Elements Between the Upper and Lower Mantle
AU: * Walker, R J
EM: rjwalker@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20815 United States
AB: The highly siderophile elements (HSE) are strongly partitioned into metal relative to silicates. In the terrestrial planets these elements are concentrated in metallic cores. Earth's mantle has sufficiently high abundances of the HSE that it has been hypothesized approximately 0.1-0.8 % of the mass of the Earth was added following the last major interactions between the core and mantle. The "late veneer" likely consisted of both differentiated and undifferentiated planetesimals of moderate size. Uncertainties in the amount of mass added to Earth by late accretion largely stem from our lack of understanding of the distribution of the HSE within the mantle. If the HSE abundances present in the upper mantle are representative of the entire mantle, then the upper end of the mass requirements (~0.4 to 0.8 % of the mass of Earth) must have been added to the Earth subsequent to the conclusion of substantial metal-silicate equilibration. This is considerable mass (roughly 0.3 to 0.6 % the mass of the Moon). Assuming the HSE comprising the late veneer were added to the mantle as additions to the outer Earth, it is conceivable that HSE were vertically stratified in the terrestrial mantle during early Earth history, possibly extending to the present. The latter option, however, is only possible if there has been limited chemical transport between the upper and lower mantles. Thus, independent constraints on the mass of the late veneer may be useful to assess how well the modern mantle is mixed, at least with respect to HSE. Constraints on the mass of the late veneer for the Earth-Moon system can potentially best be generated from studies of the Moon. The giant impact that created the Moon likely marked the last period of major core-mantle interaction for Earth. Hence, the Moon probably experienced the same flux of late accretionary materials as Earth. If HSE are uniformly distributed throughout the entire terrestrial mantle, then the minimum required influx mass is approximately 2 x 10$^{22}$kg. Using the latter mass and a commonly reported Earth/Moon mass influx ratio of 35, suggests a late veneer comprising ~6 x 10$^{20}$ kg for the Moon. Our new estimates for the concentrations of HSE in the lunar mantle suggest that such large amounts of mass were not added to the lunar mantle, and are also likely not present in the lunar crust. The mass of the late veneer added to the Moon was probably no more than 8 x 10$^{19}$ kg. This limited amount of mass may be difficult to reconcile with concentrations of HSE present in the terrestrial mantle. Options include the possibilities that high abundances of the HSE are present in only the upper mantle, indicative of limited whole mantle mixing, or that the estimate of the Earth/Moon influx ratio has been incorrectly estimated.
DE: 6250 Moon (1221)
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1015 Composition of the core
SC: Union [U]
MN: 2004 AGU Fall Meeting