Water Solubility Studies in Lower Mantle Perovskite by Fourier Transform Infrared Spectroscopy

Although the water solubility in lower mantle minerals is critical for understanding of global water circulation, the water solubility in lower mantle minerals is poorly constrained. The water solubility in MgSiO3-perovskite as well as Al-bearing MgSiO3-perovskite synthesized at 1500 - 1600 K and 24 - 25 GPa under SiO2 or MgO saturated conditions have been investigated by FTIR spectroscopy both at ambient as well as modest pressures of up to 10 GPa in a diamond-anvil cell using KBr as a pressure medium. The FTIR spectra show one dominant band at 3440 cm-1 in MgSiO3-perovskite corresponding to about 50 - 70 ppm wt water in the perovskite. This is consistent with the results by Litasov et al. (2003) who obtained solubility of about 100 ppm wt water in MgSiO3-perovskite, but much larger than the values reported by Bolfan-Casanova et al. (2000). Based on the in-situ FTIR in a diamond-anvil cell using KCl as a pressure medium, Reid et al. (2006) reported infrared absorption peaks at 3160 and 3066 cm-1 at high pressures that broaden and weaken at low (<3 GPa) pressures. They interpreted that these peak are caused by unquenchable hydroxyl-related species. However, we did not find these peaks but instead we found these peaks from the FTIR spectroscopy of KCl. We conclude that 3160 and 3060 cm-1 peaks are due to KCl, and MgSiO3 perovskite has small but finite water solubility (~50-70 ppm wt) that is expected to increase with Al content. We also present corresponding high-pressure x-ray diffraction measurements on the nominally hydrous MgSiO3-perovskite carried out up to 30 GPa.