Paragonite stability at 700 C in the presence of H2O–NaCl fluids: constraints on H2O activity and implications for high pressure metamorphism

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H2O at 700 C, 1.5–2.5 GPa, in the presence of H2O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H2O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H2O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H2O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30– 2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293–301, 1979). Lowering X(H2O) decreases the pressure of paragonite breakdown to 2.10–2.20 GPa at X(H2O)=0.90 and 1.85–1.90 GPa at X(H2O)=0.75. The experiments at X(H2O) = 0.62 yielded the assemblage albite + corundum at £ 1.60 GPa, and jadeite + kyanite at ‡1.70 GPa. This constrains the position of the isothermal paragonite–jadeite–kyanite– albite–corundum–H2O invariant point in the system Na2O–Al2O3–SiO2–H2O to be at 1.6–1.7 GPa and X(H2O) 0.65±0.05. The data indicate that H2O activity, a(H2O), is 0.75–0.86, 0.55–0.58, and <0.42 at X(H2O)=0.90, 0.75, and 0.62, respectively. These values approach X(H2O) , and agree well with the a(H2O) model of Aranovich and Newton (Contrib Miner Petrol 125:200–212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H2O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7–2.0 GPa, 550–650 C. The absence of paragonite requires a fluid with low a(H2O) of 0.3–0.6, which could be due to the presence of saline brines.