Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v41b..01v&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V41B-01
Mathematics
Logic
1025 Composition Of The Mantle, 7207 Core And Mantle, 8120 Dynamics Of Lithosphere And Mantle: General
Scientific paper
The seismological challenges for a better understanding of the dynamics, composition, and evolution of Earth's mantle are many but focus on (1) the upper mantle transition zone (wavespeed variations, radial gradients, and nature of and depth to seismic discontinuities), (2) the fate of deep sinking slabs and the relative wavespeed variations in the lower mantle, and (3) the seismologically inferred complexity just above the core mantle boundary. In the past decade our ability to map lateral variations in seismic wavespeed in Earth's mantle has improved significantly mainly as a result of dramatic increases in computer power and quantity and quality of seismic data, and results of seismic imaging are now routinely interpreted with constraints from mineral physics. However, the interpretation of the inferred wavespeed variations (e.g. in terms of temperature, composition, volatiles, anisotropy) is difficult because the coverage of data used to construct S wave models differs from that used for P models and the magnitude of wavespeed variations is often poorly constrained owing to uneven sampling, regularization of seismic inversions, and wave propagation effects such as wavefront healing. But some interesting observations can still be made. We - and others - have inferred changes in the character of wavespeed variations near 1500-2000 km depth, but there is no convincing evidence for any interface in that depth range. Recent studies have detected differences in the behavior of P and S wavespeed, e.g., the increase of the ratio dlnVs/dlnVp with depth, in particular beneath 1500 km depth. This is unlikely to be a global phenomenon and may occur primarily in regions away from zones of recent subduction (e.g., Saltzer et al, GRL, 2001). Analysis of the available theoretical and experimental data on elastic parameters suggest that these observations cannot be explained by thermal variations alone, but for a more specific and quantitative interpretation we need a better description (or extrapolations) of the elastic properties as a function of temperature, pressure, and composition, including the effects of Al and Ca, for silicates at lower mantle conditions. Furthermore, an adequate (multi component) description of the phase transformations (e.g., element partitioning, pressure and Clapeyron slope of the transition) is required to understand the seismological observations pertinent to the discontinuities and the deformation of subducted slabs in the upper mantle transition zone. Finally, numerical models based on thermal convections are likely to oversimplify the processes in Earth's interior, and more realistic thermochemical modeling should be considered to investigate, for instance, the fate of the slab fragments that have sunk into the lower mantle (e.g., compositional buoyancy), the presence of any pressure-induced compositional gradients, and the effects on dynamics of the temperature and pressure dependence of such physical parameters as thermal expansion and diffusivity. These pose some of the challenges for a concerted effort to understand to composition and evolution of our planet over geological time.
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