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

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.