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Seismic Velocities in Mantle Minerals and the Mineralogy of the Upper Mantle

Duffy, Thomas S. and Anderson, Don L. (1989) Seismic Velocities in Mantle Minerals and the Mineralogy of the Upper Mantle. Journal of Geophysical Research B, 94 (B2). pp. 1895-1912. ISSN 0148-0227. doi:10.1029/JB094iB02p01895.

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Comparison of seismic velocities in mantle minerals, under mantle conditions, with seismic data is a first step toward constraining mantle chemistry. The calculation, however, is uncertain due to lack of data on certain physical properties. “Global” systematics have not proved very useful in estimating these properties, particularly for the shear parameters. A new approach to elasticity estimation is used in this study to produce estimates of unknown quantities, primarily pressure and temperature derivatives of elastic moduli, from the structural and chemical trends evident in the large amount of elasticity data now available. These trends suggest that the derivatives of unmeasured high-pressure phases can be estimated from “analogous” low-pressure phases. Using these predictions and the best available measurements, seismic velocities are computed along high-temperature adiabats for a set of mantle minerals using third-order finite strain theory. The calculation of density and moduli at high temperature, to initiate the adiabat, must be done with care since parameters such as thermal expansion are not independent of temperature. Both compressional and shear seismic profiles are well-matched by a mineralogy dominated by clinopyroxene and garnet and with an olivine content of approximately 40% by volume. Between 670 and 1000 km, perovskite alone provides a good fit to the seismic velocities. Combining seismic velocities with recent phase equilibria data for a hypothetical pure olivine mantle suggests that a mineralogy with a maximum of 35% olivine (shear profile) or 40–53% olivine (compressional profile) by volume can satisfy the constraint imposed by the 400-km discontinuity. Other features of the upper mantle can then be matched by appropriate combinations of pyroxenes, garnets, and their high-pressure equivalents. While mantle models with a substantially larger fraction of olivine cannot be ruled out, they are acceptable only if the derivatives of the spinel phases are substantially different from olivine and deviate from trends in the larger data set.

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Additional Information:© 1989 American Geophysical Union. Received November 12, 1987; revised October 14, 1988; accepted September 8, 1988. We would like to thank Jean-Paul Montagner, Ian Jackson, and Don Weidner for useful discussion and comments. This research was supported by the National Science Foundation grant EAR-8509350. Contribution 4619, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
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Caltech Division of Geological and Planetary Sciences4619
Issue or Number:B2
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Official Citation:Duffy, T. S., and D. L. Anderson (1989), Seismic velocities in mantle minerals and the mineralogy of the upper mantle, J. Geophys. Res., 94(B2), 1895–1912, doi:10.1029/JB094iB02p01895.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45584
Deposited On:07 May 2014 21:33
Last Modified:10 Nov 2021 17:14

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