HR: 0800h
AN: S41C-1020    [Abstracts]
TI: Seismic constraints on the physical reference structure of the Earth's mantle
AU: Cobden, L
EM: l.cobden@imperial.ac.uk
AF: Dept. of Earth Science and Engineering, Imperial College, London, SW7 2AZ United Kingdom
AU: * Cammarano, F
EM: fabio@seismo.berkeley.edu
AF: Berkeley Seismological Lab., University of California, Berkeley, CA 94720 United States
AU: Goes, S
EM: s.goes@imperial.ac.uk
AF: Dept. of Earth Science and Engineering, Imperial College, London, SW7 2AZ United Kingdom
AU: Connolly, J
EM: james.connolly@erdw.ethz.ch
AF: Inst. of Mineralogy and Petrography, ETH, Zurich, 8092 Switzerland
AB: Improved seismic and elastic and anelastic mineral data warrant increasingly quantitative interpretations of tomographic models in terms of the dynamically important variables, temperature and composition. For a physical interpretation it is important to understand the one-dimensional structure that any velocity anomalies are referenced to. This structure is generally assumed to be well constrained and to correspond to adiabatic, pyrolitic conditions. Indeed, at a first glance, the comparison between the predicted seismic structure of a pyrolitic mantle with a 1300°C adiabatic geotherm (compatible with MOR temperatures) and seismic observations appears good. However, an inversion of common seismic reference models for physical parameters yields erratic structures, which may well be the result of interpreting non-unique models with data with significant uncertainties. An inversion where we searched for sets of mineral parameters (within published uncertainty bounds) that can reconcile an adiabatic, pyrolitic structure, for the mantle down to 2500 km, with seismic travel-time and fundamental-mode-frequency data, showed that this was almost impossible. This allows two possible interpretations (1) the used extrapolation to high pressure and temperature is significantly flawed, or (2) the average physical structure of the mantle deviates from adiabatic pyrolite. With even further expanded uncertainty bounds to address point (1), we test a range of alternative average thermal structures for upper and lower mantle with different potential temperatures and gradients that deviate from that of an adiabat in various depth intervals. For each physical structure we generate a large number of synthetic velocity structures within the mineral parameter uncertainties. The number of models that gives a satisfactory fit to the global seismic data reflects the likelihood that a physical model is seismically acceptable, and allows us to map out which structures are more, or less, acceptable than adiabatic pyrolite.
DE: 3909 Elasticity and anelasticity
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Seismology [S]
MN: Fall Meeting 2005