Effect of H2O on upper mantle phase transitions in MgSiO3: Is the depth of the seismic X-discontinuity an indicator of mantle water content?

Steven D. Jacobsen*, Zhenxian Liu, Tiziana Boffa Ballaran, Elizabeth F. Littlefield, Lars Ehm, Russell J. Hemley

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

29 Scopus citations


The mantle X-discontinuity, usually assigned to positive seismic velocity reflectors in the 260-330km depth range, has proved difficult to explain in terms of a single mineralogical phase transformation in part because of its depth variability. The coesite to stishovite transition of SiO2 matches deeper X-discontinuity depths but requires 5-10% free silica in the mantle to match observed impedance contrast. The orthoenstatite (OEn) to high-pressure clinoenstatite (HPCen) transformation of MgSiO3 also broadly coincides with depths of the X but requires chemically depleted and orthoenstatite-rich lithology at 300km depth in order to match observed seismic impedance contrast. On the basis of high-pressure infrared spectroscopy, X-ray diffraction, and Raman spectroscopy, we show that 1300ppm variation of H2O content in MgSiO3 can displace the transition of low-pressure clinoenstatite (LPCen) to HPCen by up to 2GPa, similar to previous quench experiments on the OEn to HPCen phase transition, where about 30-45km (1.0-1.5GPa) of deflection could occur per 0.1wt% H2O. If the mantle X-discontinuity results from pyroxene transitions in a depleted harzburgite layer, because of the strong influence of minor amounts of water on the transformation boundary, the depth of the mantle X-discontinuity could serve as a potentially sensitive indicator of water content in the upper mantle.

Original languageEnglish (US)
Pages (from-to)234-244
Number of pages11
JournalPhysics of the Earth and Planetary Interiors
Issue number1-2
StatePublished - Nov 2010


  • Clinoenstatite
  • Enstatite
  • MgSiO
  • Water
  • X-discontinuity

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science


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