HR: 0800h
AN: U21B-0416    [Abstracts]
TI: Revised estimate of radioactive energy in the Earth
AU: * McDonough, W F
EM: mcdonoug@umd.edu
AF: University of Maryland, Dept of Geology, College Park, MD 20742, United States
AU: Arévalo, R
EM: rarevalojr@gmail.com
AF: University of Maryland, Dept of Geology, College Park, MD 20742, United States
AU: Luong, M
EM: mluong@umd.edu
AF: University of Maryland, Dept of Geology, College Park, MD 20742, United States
AB: The radioactive elements, K, Th and U, play a paramount role in chemical geodynamics by providing a significant proportion of the energy that drives mantle convection. A major unknown is the absolute abundance and distribution of these elements in the mantle. For example, a mantle with a barren upper mantle and a radioactively hot core-mantle boundary (CMB) implies a significantly different mode of mantle convection than typically invoked for a more internally-heated mantle. The canonical K/U ratio of the silicate Earth (12,700), which is based on N-MORB samples, has been challenged in recent years by alternative models suggesting lower values for the bulk silicate Earth. Support for a lower K/U value for the silicate Earth comes from reconciling the missing Ar-40 problem, a re-examination of the continental crust composition, and evaluating the relative composition and contributions of different components in the mantle. In addition, a commonly held view is that the abundances of the most-depleted elements in the Depleted Mantle (MORB-source), the highly incompatible elements, are kinetically controlled by fluxes from sediment recycling and/or plume additions. We have analyzed >50 MORB, spanning a range from N- (normal) to (enriched) E-types from the Indian, Pacific and Atlantic oceans, and report an average K/U of 18,500 and Th/U of 3.1. In addition, we also observe a weak correlation of decreasing K/U with higher 6/4Pb isotope ratios, with the E-type MORB (K/Ti>0.14) having, on average, lower K/U values. Our findings are inconsistent with these incompatible elements being controlled by additions from either a continental or plume flux, both of which are believed to have lower K/U and higher Th/U values, and indicate that the Depleted Mantle component has an even higher K/U value. We will present the implications of these data with (1) revised compositional model (and uncertainties) of the DM and the silicate Earth, (2) a model for the distribution of heat producing elements in the mantle and (3) constraints on the proportion of recycled crustal and plume components in the DM.
DE: 1025 Composition of the mantle
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3621 Mantle processes (1038)
DE: 5418 Heat flow
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Union [U]
MN: 2007 Fall Meeting