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Geodynamically Consistent Seismic Velocity Predictions at the Base of the Mantle

Sidorin, I. and Gurnis, M. (1998) Geodynamically Consistent Seismic Velocity Predictions at the Base of the Mantle. In: The Core Mantle Boundary Region. Geodynamics Series. No.28. American Geophysical Union , Washington, D.C., pp. 209-230. ISBN 9780875905303.

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A model of thermoelastic properties for a chemically homogeneous adiabatic lower mantle is calculated. Constraints provided by this model are used in convection models to study dynamics of a chemically distinct layer at the bottom of the mantle. We find that the layer must be at least 2% denser than the overlying mantle to survive for a geologically significant period of time. Realistic decrease with depth of the thermal expansivity increases layer stability but is unable to prevent it from entrainment. Seismic velocities are computed for an assumed composition by applying the thermal and compositional perturbations obtained in convection simulations to the adiabatic values. The predicted velocity jump at the top of the chemical layer is closer to the CMB in the cold regions than in the hot. The elevation of the discontinuity above CMB in the cold regions decreases with increasing thermal expansivity and increases with increasing density contrast, while in the hot regions we find that the opposite is true. If the density contrast is small, the layer may vanish under downwellings. However, whenever the layer is present in the downwelling regions, it also exists under the upwellings. For a 4% density contrast and realistic values of expansivity, we find that the layer must be more than 400 km thick on average to be consistent with the seismically observed depth of the discontinuity. A simple chemical layer cannot be used to interpret the D″ discontinuity: the required change in composition is large and must be complex, since enrichment in any single mineral probably cannot provide the required impedance contrast. A simple chemical layer cannot explain the spatial intermittance of the discontinuity.

Item Type:Book Section
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Gurnis, M.0000-0003-1704-597X
Additional Information:© 1998 American Geophysical Union. We are very grateful to Don Anderson and Don Heimberger for stimulating discussions. We also thank Don Anderson and an anonymous reviewer for reading the manuscript and providing a number of valuable comments. This research was funded by NSF grant EAR-9629279. This represents contribution number 6217 of the Division of Geological and Planetary Sciences, California Institute of Technology.
Group:Seismological Laboratory
Funding AgencyGrant Number
Subject Keywords:Core-mantle boundary
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences6217
Series Name:Geodynamics Series
Issue or Number:28
Record Number:CaltechAUTHORS:20121126-134141766
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Official Citation:Sidorin, I., and M. Gurnis (1998), Geodynamically consistent seismic velocity predictions at the base of the mantle, in The Core-Mantle Boundary Region, Geodyn. Ser., vol. 28, edited by M. Gurnis et al., pp. 209–230, AGU, Washington, D. C., doi:10.1029/GD028p0209
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35640
Deposited By: Tony Diaz
Deposited On:27 Nov 2012 22:27
Last Modified:09 Nov 2021 23:16

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