HR: 10:20h
AN: T31G-01 INVITED     [PDF]
TI: Radiogenic Heat Production in the Core?
AU: * Humayun, M
EM: hum8@uchicago.edu
AF: University of Chicago, Dept. of the Geophysical Sciences, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AB: Radiogenic heat production in the Earth is dominated by three elements: K, Th and U, which mainly reside in the Earth's mantle and crust. The global budget is determined from the fact that Th and U are both refractory. Thus, it is believed that the Th/U ratio of the bulk Earth must be identical to those of chondritic meteorites. In contrast, K is a volatile element, but like Th and U is lithophile and incompatible. Relative to CI chondrites (K/U about 70,000), K is depleted by a factor of 6-7 in the Bulk Silicate Earth (BSE) due to volatility. New determinations of the chondritic Th/U ratio provide a value of 3.6, which may be taken to represent the bulk Earth Th/U ratio. The Th/U ratio (BSE) of 4.2-4.3 is determined by regression from lead isotope systematics. The discrepancy between the inferred bulk Earth and BSE Th/U ratios can be taken to imply that U is partially siderophile in the Earth, an observation that requires that D(U)= 0.3 between metal and silicate. The implied amount of U present in the core is sufficient to provide about 2-3 TW of power for the geodynamo. The presence of a uranogenic heat source significantly affects geophysical estimates of the timing of inner core crystallization. Recent Os isotope data imply that several plume sources have interacted with the outer core at the CMB. If this is correct, the high Pt/Os ratio inferred for the outer core would be most efficacious at explaining the observed Os isotope ratios if the inner core crystallized prior to 3.5 Ga. Such an early age of the inner core is not possible if heat loss at the CMB is dominated exclusively by latent heat release, but requires an additional source of radiogenic heating. It should be noted that most highly siderophile elements are siderophile at 1 atm conditions. However, many lithophile elements (V, Cr, Mn) become more siderophile at higher P and T. The mineral constituents of iron meteorites are notably lacking in U and Th, requiring that if U is present in the Earth's core it must change its metal-silicate partitioning at high P-T. While U is a prospective heat source for the Earth's core, resolution of the issue will require high P-T partitioning data for U and Th. Geophysical treatment of the crystallization rate of the inner core should include a discussion of the effect of radiogenic contribution to core heat production from U.
DE: 1015 Composition of the core
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Trace elements (3670)
DE: 8130 Heat generation and transport
DE: 8147 Planetary interiors (5430, 5724)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting