Synthesis and high-pressure synchrotron-infrared studies of OH-bearing silicate perovskite in the laser-heated diamond cell

A small amount of water incorporated as hydroxyl into the dominant silicate perovskite of the lower mantle has the potential to constitute Earth's largest H2O reservoir. Current estimates for the water storage capacity of silicate perovskite are controversial, mainly surrounding the interpretation of broad IR-absorption features in the 3200-3600 cm-1 range. Previously, basic experimental procedures have involved synthesis in large- volume presses, with subsequent recovery and analysis of samples for water by IR and/or SIMS at ambient conditions. For pure-Mg and Fe and Al-bearing perovskite, this procedure has resulted in estimates of water contents spanning three orders of magnitude; from essentially no water [Bolfan-Casanova et al. 2003, Geophys. Res. Lett. 30, 1905] to almost 4000 ppm by weight H2O [Murakami et al. 2002, Science 295, 1885-1887]. Here, we synthesized hydrous (Mg,Fe)- and (Mg,Fe,Al)-silicate perovskite in the laser- heated diamond cell by directly transforming of OH-bearing precursors to perovskite such as ringwoodite and enstatite. IR-measurements were carried out at synthesis pressures of 28-35 GPa on the U2A beamline of the National Synchrotron Light Source. After synthesis at temperatures corresponding to cold subduction geotherms, we observe sharp bands in the OH stretching region unlike those previously reported, which we attribute to ordered hydroxyl groups in silicate perovskite. We find at least 1000 ppm H2O can be incorporated into silicate perovskite, indicating that the lower mantle may represent a potentially vast reservoir of hydrogen in the Earth.