Accurate potassium/argon dating of and preservation of oxygen in zeolites

K/Ar dating and isotopic measurement of feldspars and clay minerals has been used with success to constrain the timing and source of fluids. The major setback in applying this method to zeolites is their structure. “Channels” are responsible for extremely rapid diffusion through the mineral which brings into question the retention of K/Ar and oxygen ratios.; K/Ar dates of zeolites are too young indicating that Ar has been lost. Diffusion of Ar through the channels in clinoptilolite is extremely rapid, yet clinoptilolites do retain Ar over geologic timescales. Heating and cation exchange have no effect upon the radiogenic Ar content suggesting that Ar must not be located in the channels. Cation exchange capacity is high in clinoptilolites but 25% of cations are not exchangeable. Removal of exchangeable K results in accurate K/Ar ratios and dates.; Experimental studies suggest that the rate of oxygen isotope exchange between zeolites and water vapor is limited by diffusion (Feng and Savin, 1993). Exchange between two sizes of analcime and water vapor support this idea. A size fraction with a mean diameter of 12.5μm exchanges oxygen more quickly than a fraction with a mean size of 182μm indicating that diffusion through larger grains limits the exchange rate.; Oxygen isotope ratios in 11 m.y. old clinoptilolite near the surface at Yucca Mountain, Nevada, are in equilibrium with temperatures and fluid isotopic compositions calculated from illite/smectite suggesting that clinoptilolite like clay will retain its oxygen isotope ratio. The difference between paleo and modern conditions is too minimal at depth to distinguish whether clinoptilolite oxygen ratios have been altered at higher temperatures.; Reconstruction based upon hydrogen and oxygen ratios in illite/smectite from Yucca Mountain, Nevada, shows that the isotopic ratio in precipitation 11 m.y. ago did not differ from precipitation trapped in fluid inclusions in calcite 2 m.y. ago (Winograd et al., 1995). As an isotopic effect accompanies the rain shadow effect of mountains, it is concluded that the Sierra Nevada Mountains to the west did not change in elevation between 11 and 2 m.y. ago.