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Mineralogical effects on the detectability of the postperovskite boundary

  1. Vitali Prakapenkab
  1. aDepartment of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; and
  2. bCenter for Advanced Radiation Sources, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637
  1. Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved December 23, 2011 (received for review June 14, 2011)

Abstract

The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature (P?-?T) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400?km above the core-mantle boundary, hereafter the D′′ discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe2+ and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv?-?pPv boundary (400–600?km) to much larger than the D′′ discontinuity (≤?70?km). These results challenge the assignment of the D′′ discontinuity to the Pv?-?pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv?→?pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv?→?pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P?-?T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv?-?pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv?→?pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv?-?pPv boundary due to their distinct mineralogy. Therefore, the observation of the D′′ discontinuity may be related to the Pv?→?pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.

Footnotes

  • ?1To whom correspondence should be addressed. E-mail: b.grocholski{at}gmail.com.
  • Author contributions: B.G., K.C., and S.-H.S. designed research; B.G., K.C., and S.-H.S. performed research; B.G., K.C., S.-H.S., and V.B.P. contributed new reagents/analytic tools; B.G., K.C., and S.-H.S. analyzed data; and B.G., K.C., and S.-H.S. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1109204109/-/DCSupplemental.

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