Effects of water on the behavior of MgSiO3-clinoenstatite at high pressure

Just 1000 ppm of H2O stored in the major minerals of Earth's upper mantle would represent more than one ocean-mass equivalent of liquid water. The incorporation of water as hydroxyl into clinoenstatite (MgSiO3) was investigated using synchrotron-based IR spectroscopy on beamline U2A at NSLS, and the effects of water on the low-density clinoenstatite (LCEN) to high-density (HCEN) phase transformation were studied using high-pressure Raman spectroscopy. Single-crystal samples of LCEN with varying water contents were synthesized in a multianvil press at 16-18 GPa and around 1100C. IR-absorption bands in the region of O-H stretching at 3675, 3600, and 3550 cm-1 were monitored to 17 GPa. The bands shift to lower wavenumbers upon compression, but between 6 and 8 GPa a change in the pressure dependence of the band at 3675 cm-1 was observed, accompanied by broadening and disappearance of the 3600 cm-1 band. Changes in the FTIR spectra are attributed to the LCEN to HCEN phase transformation, and indicate that there are differences in the hydrogen bonding environment between the two structures. Raman spectroscopy was used to monitor the transformation pressure (PT) of hydrated and anhydrous crystals loaded together in the same diamond cell. On compression, PT is reduced from 8.1 GPa for the dry sample to about 5.7 GPa for the most hydrous sample containing about 1000 ppm H2O. Upon decompression, all samples back-transformed to LCEN at around 4.5 GPa. The results indicate that hydration narrows the transformation hysteresis and that water reduces the transformation pressure by about 1 GPa per 1000 ppm H2O, if PT is taken to be the mid-point of the hysteresis. If hydroxyl defects affect the orthoenstatite to HCEN transformation similarly in the mantle, a shift in PT of 1 GPa would uplift any corresponding seismic discontinuity by about 30 km. Thus, relatively small amounts of water may have seismically detectable effects on the behavior and properties of the upper mantle.