Observation of phase transformations in serpentine at high pressure and high temperature by in situ X ray diffraction measurements

In situ X ray diffraction measurements on natural serpentine have been performed at high pressure and high temperature using a combination of a double-stage multianvil system and synchrotron radiation. No clear evidence for the amorphization of serpentine was obtained upon compression to 28 GPa at room temperature, in contrast to an earlier study using a diamond anvil cell and rotating anode X ray source. However, when the temperature was increased under pressure, amorphization was observed in a limited interval between about 200 and 400 degrees C, at pressures greater than 14 GPa. At higher temperatures, X ray diffraction peaks became apparent quickly, suggesting rapid crystal growth of high pressure phases. In runs at pressures greater than 20 GPa, the final run products consisted of a phase assemblage with an unknown hydrous phase. The chemical composition of this phase is close to Mg2Si3O6(OH)(4), which is nearly identical to that of phase F. However, its X ray diffraction pattern is different from phase F and is rather similar to the pattern of phase D reported by Liu. It was demonstrated that serpentine completely dehydrate to form forsterite + enstatite + water at relatively low temperatures less than 700-800 degrees C and pressures less than 10 GPa, while it transforms to a phase assemblage including the hydrous phases, phase D and superhydrous phase B, at pressures greater than 20 GPa. This assemblage with dense hydrous magnesium silicate persists to high temperatures of at least 1200 degrees C at pressures corresponding to the lower parts of the mantle transition region and the uppermost lower mantle.