In situ observation of chrysotile decomposition in the presence of NaCl-bearing aqueous fluid up to 5 GPa and 400 °C

Natural occurrence of chrysotile, a rock-forming serpentine mineral, is largely associated with fluid-rich environments, thus making the fluid chemistry an important factor governing chrysotile growth and stability. Paper examines the effect of alkali chlorides, lowering the H2O activity in fluid, on chrysotile stability at high pressure. The behavior of natural chrysotile (Thetford) in the presence of concentrated NaCl-H2O fluids was studied up to 5 GPa and 400 degrees C using in-situ Raman spectroscopy and X-ray diffraction combined with resistively heated diamond anvil cell. Reference experiments were also performed on salt-free samples. In salt-free H2O-saturated conditions, chrysotile is stable within the whole temperature range at 1.5-5 GPa. The presence of NaCl in the fluid at X-NaCl = mole NaCl/(NaCl+H2O) approximate to 0.1-0.15 strongly suppresses the temperature of chrysotile dehydration at 2-4 GPa to 380 degrees C, which is about 170 degrees C lower compared to the equilibrium data for H2O-saturated salt-free system. Talc/talc-like phase only crystallizes at HP-HT conditions as product of the reaction "chrysotile -> forsterite + talc", whereas forsterite appears after the P,T release. This indicates an enhanced solubility of forsterite in supercritical NaCl-H2O fluid at HP-HT conditions. Chrysotile shows disordering/amorphization at unusually low temperature of 150 degrees C. The shift of the chrysotile dehydration to lower temperatures in saline environment makes this process more operable in natural metamorphic processes in serpentinites. This effect can be extended to other serpentine minerals, including antigorite, a common participant of HP-HT metamorphic reactions in subduction zones.