HR: 16:15h
AN: DI14A-02    [Abstracts]
TI: Constraining the Bulk Major Element Composition and Thermal State of the Earth's Lower Mantle from a Joint Inversion of Electromagnetic Sounding, Seismic and Gravity Data
AU: * Khan, A
EM: amir@gfy.ku.dk
AF: Niels Bohr Institute, Juliane Maries Vej 30, Copenhagen Oe, 2100, Denmark
AU: Connolly, J
EM: james.connolly@erdw.ethz.ch
AF: Swiss Federal Institute of Technology, Sonneggstr. 5, Zurich, 8092, Switzerland
AB: We jointly invert global seismic travel time data, mean mass, mean moment of inertia and deep electromagnetic sound data directly for the Earth's mantle composition and thermal state, using a, by us, recently developed method. The chemical composition of the silicate Earth is modeled within the system CaO-FeO-MgO-Al₂O₃-SiO₂. Given these parameters, in addition to a geotherm, we calculate stable mineral modes, elastic properties, bulk density (ρ) at the prevailing physical conditions using Gibbs free energy minimisation. Voigt-Reuss-Hill averaging is subsequently employed to compute bulk seismic P and S wave velocity profiles (V_P, V_S). The mineral modes are combined with laboratory-based models for the conductivity of individual minerals to estimate the bulk Earth electrical conductivity structure (σ). Assuming shear attenuation to be a thermally activated process, we used our thermodynamic calculations to estimate radial bulk and shear attenuation profiles, and employed these to assess anelastic contributions to V_P and V_S. From these radial profiles, seismic travel times, mean mass, mean moment of inertia and electromagnetic responses at the surface of the Earth are calculated. Given this scheme, the data are jointly inverted using a Markov chain Monte Carlo algorithm, from which a range of compositions and temperatures fitting data within uncertainties are obtained. More specifically, we find a more chondritic lower mantle composition with Mg/Si~1.15, in comparison to 1.27 for a pyrolitic upper mantle, providing a sink in addition to the core for the missing Si from the upper mantle and at the same time making it possible to derive the Earth from cosmic abundances. The lower mantle geothermal gradient is superadiabatic, attaining a most probable value of around 0.56°C/km, with core mantle boundary temperatures of ~2900 °C. Our results also imply that the 660 discontinuity is most probably related to a change in chemistry, while evidence for deep chemical layering is not found. The calculated physical properties (σ, ρ, V_P and V_S) agree excellently with purely geophysically-derived models such as PREM, AK135 and electrical conductivity models obtained by various workers.
DE: 8020 Mechanics, theory, and modeling
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8125 Evolution of the Earth (0325)
DE: 8160 Rheology: general (1236, 8032)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting