Oxygen isotope fractionation between analcime and water: An experimental study

The fractionation of oxygen isotopes between natural analcime (~100 μm) and water has been determined at 300, 350, and 400°C at fluid pressures ranging from 1.5 to 5.0 kbar. Isotope ratios were obtained for the analcime framework, the channel water, and bulk water. Analcimes from Surtsey (145°C), DSDP Hole 417A (30 to 55°C), and Guam (25°C) were used to constrain the fractionation factors below 300°C. Analcime channel water exchanged completely with external water in all runs. Although some retrograde exchange may have occurred during quenching, the results indicate that the channel water is depleted in ¹⁸O relative to bulk water by a constant value of ~5%, nearly independent of temperature. Analcime is the first hydrated mineral found to have water of hydration depleted in ¹⁸O. Analcime framework oxygen exchanged 80, 90, and 96% at 300°C for 412 h, 350°C for 178 h, and 400°C for 120 h, respectively. Equilibrium Δ¹⁸O (%) are as follows: 2.9 (400°C), 4.5 (350°C), and 5.8 (300°C) for the experimental runs and 12.2 (145°C) and 24.2 to 28.2 (30-55°C) for the empirical data. The analcime-water fractionation curve is within experimental error of that of calcite-water. The exchange had little effect on grain morphology and does not involve recrystallization. This is the fastest exchange observed for a silicate. The rapid exchange rates indicate that zeolites in active high-temperature geothermal areas are in oxygen isotopic equilibrium with ambient fluids. Once calibrated, zeolites may be among the best low-temperature oxygen isotope geothermometers.