KINETICS AND MECHANISM OF OXYGEN ISOTOPE EXCHANGE BETWEEN ANALCIME AND WATER VAPOR AND ASSESSMENT OF ISOTOPIC PRESERVATION OF ANALCIME IN GEOLOGICAL FORMATIONS

Abstract

The purpose of this study is to delineate the mechanism of isotopic exchange between zeolites and fluid. Two size fractions of analcime were exchanged with water vapor at 400°C and the isotopic change of framework oxygen with time was measured. The rate of exchange was faster for the smaller size fraction yet neither fraction reached equilibrium. SEM results indicate that dissolution plays no significant role in isotopic exchange between analcime and water vapor. The difference in rate with grain size indicates that isotopic exchange is limited by diffusion. The fact that equilibrium is not reached may be explained by structural changes upon dehydration which cause Na cations to migrate into former water sites. The isotopic exchange results for both size fractions can be fit by a diffusive-reactive model, which yields a diffusion coefficient of 2.8 +/- .3 x 10-9 cm2/s and a combined rate/effective surface area constant of 0.011 +/- .002 cm3/mol/s. The model predicts that the oxygen isotopic composition of most geologic analcime will have been changed since formation unless it is found in a young deposit (<10 my) or has only been exposed to low temperatures (<30°C).