Determination of the massdependence of cadmium isotope fractionation during evaporation
Abstract
Mass fractionation laws relate the fractionation factor α _{A} for one isotope ratio to the fractionation factor α _{B} for a second isotope ratio of the same element, with a fractionation exponent β such that α _{A} = α _{B}^{β}. The exponent β defines the massdependence of the mass fractionation law and thus determines the slope of a mass fractionation line in linearized three isotope space. The generalized power law (GPL) defines β as a function of a variable exponent n. The laws that aim to describe equilibrium and kinetic isotope fractionations are special cases of the GPL with n = 1 and n → 0, respectively. Large isotope fractionations (up to 10% for ^{106}Cd/ ^{114}Cd) were found to accompany the evaporation of molten Cd into vacuum at about 180°C. The slopes of the fractionation lines (βvalues) were obtained by analyzing the Cd isotope compositions of the evaporation residues relative to the starting material with two different multiple collectorICPMS instruments. For the most fractionated sample, the difference between the theoretical βvalues, that describe kinetic and equilibrium isotope fractionation, is 10 to 20 times larger than the measurement uncertainty. A massdependence with n = 0.35 was determined for this sample. This result differs significantly from the value that would be expected for simple kinetic evaporation (n → 0), which is governed by the diffusion of monatomic Cd from the melt into vacuum. The observed "nonkinetic" massdependence probably results from partial recondensation (back reaction) of Cd vapor into the melt phase. This interpretation requires that equilibrium evaporation of Cd at about 180°C is associated with significant isotope fractionation. The present study demonstrates that the mechanism of isotope fractionation can be investigated by studying the associated massdependence, which can be determined by measuring the isotope ratios of a fractionated product relative to the starting material. The quantification of mass fractionation line slopes with the GPL should aid the interpretation of massdependent and small massindependent isotope effects.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 May 2004
 DOI:
 10.1016/j.gca.2003.12.013
 Bibcode:
 2004GeCoA..68.2349W