HR: 0800h
AN: U41A-0734    [Abstracts]
TI: Testing Models of Core-Mantle Exchange
AU: * McDonough, W F
EM: mcdonough@geol.umd.edu
AF: University of Maryland, Dept. of Geology, College Park, MD 20742 United States
AB: Compositional estimates of Earth reservoirs (i.e., crust, mantle core) allow us to critically evaluate models of core-mantle exchange from an elemental and isotopic perspective. For a significant number of elements the composition of the Earth's core and mantle is well constrained by the data of chondritic meteorites (undifferentiated, primitive materials) and mantle samples. These constraints are most robust for elements with high condensation temperatures $>$1100 K (e.g., Fe, Mg, Si, etc), assuming a chondritic bulk planet. Isotopic models are the most sensitive to assessing small core contributions to the mantle (e.g., $<$1%), whereas chemical and isotopic data can be brought to bear on assessing larger amounts of core additions to the mantle (e.g., $<$0.5%). Chemically, the core and mantle have been isolated from each other since core separation, which occurred during and immediately following accretion (i.e., $<$30 Ma of solar system formation). Support for this isolation is demonstrated by W isotopic composition of the mantle and the constancy of mantle ratios of lithophile/siderophile elements (mantle/core stored elements). However, neither of these approaches have sufficient resolution to decisively test the alternative hypothesis that argues for limited core-mantle exchange in the sources of hotspot basalts based on Os isotopic evidence. A series of models are tested that encompass both elemental and isotopic consequences of core-mantle exchange for a range of plausible outer core compositions. Compositional models for the outer core are dependent on inner/outer core fractionation parameters and growth curves for the formation of the inner core. Results from these models permit limited transfer of core material (up to ~0.5%, by mass) into the mantle source regions of hotspot magmas. Future measurements and developments with improvements in precision will provide greater resolution and evaluation of these models.
DE: 1025 Composition of the mantle
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1015 Composition of the core
SC: Union [U]
MN: 2004 AGU Fall Meeting