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

doi:10.1126/science.1155822

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Geological Sciences

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

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

Alexander R. Hutko,¹^* Thorne Lay,¹ Justin Revenaugh,² Edward J. Garnero³

Earth's lowermost mantle has thermal, chemical, and mineralogicalcomplexities that require precise seismological characterization.Stacking, migration, and modeling of over 10,000 P and S wavesthat traverse the deep mantle under the Cocos plate resolvestructures above the core-mantle boundary. A small –0.07± 0.15% decrease of P wave velocity (V_p) is accompaniedby a 1.5 ± 0.5% increase in S wave velocity (V_s) neara depth of 2570 km. Bulk-sound velocity Formula decreases by –1.0 ± 0.5% at this depth.Transition of the primary lower-mantle mineral, (Mg_1-x-y Fe_xAl_y)(Si,Al)O₃perovskite, to denser post-perovskite is expected to have anegligible effect on the bulk modulus while increasing the shearmodulus by $~$ 6%, resulting in local anticorrelation of V_b andV_s 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.

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