| High pressure, high temperature quench experiments were carried out on serpentine at pressures 13 to 53 GPa and temperatures between 800-1800{dollar}spcirc{dollar}C. Results show that the post-serpentine phase assemblages vary depending on the pressure and temperature. The sequence of phase transformation of DHMS phases is A, E, D + SB or D + brucite, then D + periclase with increasing pressure. The findings of phase E and superhydrous phase B as decomposition products of serpentine are supported by the recent report by Irifune et al. (1998). Phase E is found to be unstable beyond 18.5 GPa. Superhydrous phase B is stable up to the P-T condition at the top of the lower mantle ({dollar}sim{dollar}700-800 km). Phase D decomposes at pressures equivalent to about 1250 km depth, which may define the lowest limit for a dense hydrous magnesium silicate. The decomposition reaction sequence outlined above could be one of the mechanisms for transporting H{dollar}sb2{dollar}O within the mantle.; In-situ X-ray diffraction experiments on serpentine were carried out in this study at 22-35 GPa and 1000-1300{dollar}spcirc{dollar}C. Results show that phase D should be stable under the P-T conditions in the lower mantle. Besides phase D, there may be other unquenchable new phase(s), as observed under above high P-T conditions. However, these phases eventually break down into stishovite and brucite, as pressure is totally released. This indicates that the new phase may contain some H in the structure, thereby qualifying as a high P-T hydrous phase.; In-situ X-ray diffraction studies were carried out on single component phase, superhydrous phase B at about 43 GPa, 1340{dollar}spcirc{dollar}C and phase D at 42 GPa, 1300-1750{dollar}spcirc{dollar}C, respectively. Results show that they decompose at the given P-T conditions, where Superhydrous phase B transforms into phase D + stishovite + periclase; phase D decomposes into perovskite, stishovite and an unknown phase. These results on the single phase materials are in accord with the observations on serpentine, supporting the idea that superhydrous phase B and phase D will decompose under lower mantle conditions. |
