A high-pressure and high-temperature study of serpentine and its implications to Earth's lower mantle.

摘要

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.