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Evidence of denser MgSiO3 glass above 133?gigapascal (GPa) and implications for remnants of ultradense silicate melt from a deep magma ocean

  1. Jay D. Bassb
  1. aDepartment of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan; and
  2. bDepartment of Geology, University of Illinois, 1301 West Green Street, Urbana, IL 61801
  1. Edited by David Walker, Columbia University, Palisades, NY, and approved August 31, 2011 (received for review June 16, 2011)

Abstract

Ultralow velocity zones are the largest seismic anomalies in the mantle, with 10–30% seismic velocity reduction observed in thin layers less than 20–40?km thick, just above the Earth’s core-mantle boundary (CMB). The presence of silicate melts, possibly a remnant of a deep magma ocean in the early Earth, have been proposed to explain ultralow velocity zones. It is, however, still an open question as to whether such silicate melts are gravitationally stable at the pressure conditions above the CMB. Fe enrichment is usually invoked to explain why melts would remain at the CMB, but this has not been substantiated experimentally. Here we report in situ high-pressure acoustic velocity measurements that suggest a new transformation to a denser structure of MgSiO3 glass at pressures close to those of the CMB. The result suggests that MgSiO3 melt is likely to become denser than crystalline MgSiO3 above the CMB. The presence of negatively buoyant and gravitationally stable silicate melts at the bottom of the mantle, would provide a mechanism for observed ultralow seismic velocities above the CMB without enrichment of Fe in the melt. An ultradense melt phase and its geochemical inventory would be isolated from overlying convective flow over geologic time.

Footnotes

  • ?1To whom correspondence should be addressed. E-mail: motohiko{at}m.tohoku.ac.jp.
  • Author contributions: M.M. designed research; M.M. performed research; M.M. and J.D.B. contributed new reagents/analytic tools; M.M. analyzed data; and M.M. and J.D.B. 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.1109748108/-/DCSupplemental.

Freely available online through the PNAS open access option.

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