Effect of H 2 O on upper mantle phase transitions in MgSiO 3 : Is the depth of the seismic X-discontinuity an indicator of mantle water content?

Details Effect of H 2 O on upper mantle phase transitions in MgSiO 3 : Is the depth of the seismic X-discontinuity an indicator of mantle water content? Effect of H 2 O on upper mantle phase transitions in MgSiO 3 : Is the depth of the seismic X-discontinuity an indicator of mantle water content?

Nov 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010pepi..183..234j&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 183, Issue 1-2, p. 234-244.

Physics

Scientific paper

The mantle X-discontinuity, usually assigned to positive seismic velocity reflectors in the 260-330 km 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 SiO 2 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 MgSiO 3 also broadly coincides with depths of the X but requires chemically depleted and orthoenstatite-rich lithology at 300 km 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 1300 ppm variation of H 2 O content in MgSiO 3 can displace the transition of low-pressure clinoenstatite (LPCen) to HPCen by up to 2 GPa, similar to previous quench experiments on the OEn to HPCen phase transition, where about 30-45 km (1.0-1.5 GPa) of deflection could occur per 0.1 wt% H 2 O. 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.

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