Rates and Mechanisms of Isotopic Exchange

Abstract

### Background The proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies. This trend is likely to continue because of the development of micro-analytical techniques (e.g. laser ablation, ion microprobe, femtomole-carrier gas methods, etc.) for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record (see McKibben et al. 1998). Equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems. Indeed, determination of the distribution of the stable isotopes of C-O-H-S in gases, fluids, and minerals has become a standard and extremely powerful approach in elucidating the temperatures, material fluxes, rock and fluid origins, and time scales associated with ancient and active fluid-rock interaction processes in the Earth’s crust (see Valley et al. 1986). Although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by Urey (1947), the calibration of fractionation factors has proven to be difficult. During the last four decades, a great deal of progress has been made in determining the equilibrium fractionations of 18O/16O, D/H, 13C/12C, and 34S/32S among fluid and mineral components at high temperatures (≥400°C), both from experimental exchange studies, theoretical calculations (using vibrational spectra), semi-empirical calculations (using bond-type considerations), and empirical calibrations based on natural assemblages (see references in O’Neil 1986; Kyser 1987; Criss 1991, 1999; Chacko et al., this volume). Of these, the experimental method is the most direct because it involves the least number of a priori assumptions. Despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water-rock …