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Science 23 May 2008:
Vol. 320. no. 5879, pp. 1070 - 1074
DOI: 10.1126/science.1155822
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Reports

Anticorrelated Seismic Velocity Anomalies from Post-Perovskite in the Lowermost Mantle

Alexander R. Hutko,1* Thorne Lay,1{dagger} Justin Revenaugh,2 Edward J. Garnero3

Earth's lowermost mantle has thermal, chemical, and mineralogical complexities that require precise seismological characterization. Stacking, migration, and modeling of over 10,000 P and S waves that traverse the deep mantle under the Cocos plate resolve structures above the core-mantle boundary. A small –0.07 ± 0.15% decrease of P wave velocity (Vp) is accompanied by a 1.5 ± 0.5% increase in S wave velocity (Vs) near a depth of 2570 km. Bulk-sound velocity Formula decreases by –1.0 ± 0.5% at this depth. Transition of the primary lower-mantle mineral, (Mg1-x-y FexAly)(Si,Al)O3 perovskite, to denser post-perovskite is expected to have a negligible effect on the bulk modulus while increasing the shear modulus by ~6%, resulting in local anticorrelation of Vb and Vs anomalies; this behavior explains the data well.

1 Earth and Planetary Sciences Department, University of California, Santa Cruz, CA 95064, USA.
2 Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive Southeast, Minneapolis, MN 55455, USA.
3 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287–1404, USA.

* Present address. U.S. Geological Survey, Mail Stop 966 Box 25046, Denver Federal Center, Denver, CO 80225, USA.

{dagger} To whom correspondence should be addressed. E-mail: thorne{at}pmc.ucsc.edu

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