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

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Science  23 May 2008:
Vol. 320, Issue 5879, pp. 1070-1074
DOI: 10.1126/science.1155822

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Abstract

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 Embedded Image 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.

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