HR: 08:00h
AN: U41A-01 INVITED     [Abstracts]
TI: Chemical interaction of the core and mantle
AU: * Humayun, M
EM: humayun@magnet.fsu.edu
AF: Dept. of Geological Sciences & National High Magnetic Field Laboratory, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, United States
AB: New geochemical techniques, available only since the last decade, have opened the study of the mantle geochemistry of siderophile elements, i.e., those elements that are preferentially present in the metallic core (Fe, Ni, W, Re, Os, Pt, etc.). Evidence that the core and mantle chemically interact was first recognized by the presence of radiogenic Os isotope ratios in Hawaiian lavas. Such Os isotope signals are generated by the growth of the solid inner core due to preferential partitioning of the parental Re and Pt isotopes in the outer core. Successful models of Os isotope evolution for the outer core require an early differentiation of the inner core and strong solid metal-liquid metal partitioning of Re, Os and Pt. An early growth (pre-3.5 Ga) of the inner core is supported by Os isotope analysis of 2.7 Ga Kostomuksha komatiites. Such models represent an extreme endmember among models of the timing of inner core differentiation. The partitioning of Os and Pt are testable, with some difficulty, by ingenious new experiments using a Fe-Ru solid solution to stabilize the hcp-Fe phase at pressures accessable by the multi-anvil cell. Measurements of the platinum group element abundances coupled with Os isotope ratios in the Kostomuksha, and other komatiites, indicate that the mechanism of core-mantle interaction takes place by isotopic and chemical equilibration between the outer core and lower mantle. No evidence is found for the large- scale addition of outer core material to the base of the mantle. A chemical exchange near equilibrium is possibly consistent with Fe/Mn data for Hawaii, as well. The mantle is composed of Fe-Mg silicates and variation of the ratio of Fe/Mg is an important potential source of density variation observed by seismic tomography. In the lower mantle, the mineralogy is Mg-silicate perovskite or post-perovskite phase and ferropericlase, an FeO-MgO solid solution. The core is composed of Fe-Ni-X alloy, where X is a light element. Potentially, oxygen enters the core as FeO. If FeO is a component of the core, the chemical exchange of FeO should be controlled by equilibrium between ferropericlase and metallic liquid. Evidence for this is apparent in the systematically higher Fe/Mn ratios observed in many mantle plume-derived basalt lavas. Mantle geochemistry is finally coming to grips with the single most important question that geophysicists may ask of it: major element variation in mantle plumes that reach the surface to form volcanic islands.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Joint Assembly