HR: 11:40h
AN: T32A-06    [Abstracts]
TI: The nature of the Lehmann discontinuity from its seismological Clapeyron slopes
AU: * Deuss, A
EM: deuss@esc.cam.ac.uk
AF: University of Cambridge, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: Woodhouse, J
EM: john.woodhouse@earth.ox.ac.uk
AF: University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AB: While the existence of the Lehmann discontinuity at 220 km in Earth's mantle has been known for over 40 years from seismology, it is still debated what causes this discontinuity. The nature of the Lehmann discontinuity is of major importance for our understanding of upper mantle composition and flow. Here we report measurements of seismological Clapeyron slopes for the Lehmann discontinuity, which are key to explaining the seismic discontinuity as either a manisfestation of phase transitions or of other mineral physical processes. The Clapeyron slopes are measured by correlating discontinuity depths with local velocity perturbations from a tomographic model, assuming that the velocity perturbations are solely due to temperature variations. We find that in most regions the Lehmann discontinuity is characterised by a regionally varying negative seismological Clapeyron slope. Known phase transitions in the upper mantle above 400 km depth all have positive Clapeyron slopes. In the case of the Lehmann discontinuity the only remaining hypothesis for a negative Clapeyron slope is that it represents the transition in deformation mechanism from dislocation to diffusion creep. This corresponds to a change from anisotropic structure above, to isotropic structure below the discontinuity and thus is independent evidence for the maximum depth extend of upper mantle anisotropy. The depth at which this transition occurs is dependent on water content, grain size, stress level and temperature (Karato, 1992). In a dry upper mantle, the transition will appear at 340-380 km depth, which is too deep to explain our observations of the Lehmann discontinuity from 220 km depth. The transition depth is shallower in a wet mantle, so our observations are also an indicator of the existence of a significant amount of water in the Earth's upper mantle. Alternatively, a smaller grain size or lower stress level would lead to a shallower discontinuity. The regional variability in the size of the Clapeyron slope could thus be due to variations in water content, grain size or stress level.
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting