The mass-dependence of cadmium isotope fractionation during experimental evaporation

Details The mass-dependence of cadmium isotope fractionation during experimental evaporation The mass-dependence of cadmium isotope fractionation during experimental evaporation

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....4942w&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #4942

Computer Science

Scientific paper

Mass fractionation laws relate the fractionation factor αj/i for one isotope ratio to the fractionation factor αk/i for a second isotope ratio of the same element, with a fractionation exponent β such that αj/i = αk/iβ (Young et al. 2002, GCA 66, 1095-1104). The exponent b defines the mass-dependence 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 = (m_i^n-m_j^n)/(m_i^n-m_k^n) (Maréchal et al. 1999, Chem. Geol. 156, 251-273). The equilibrium and the kinetic fractionation law are special cases of the GPL, with n = -1 and n rightarrow 0, respectively. Large isotope fractionation (up to 10% for 106Cd/114Cd) was found to accompany the evaporation of molten Cd into vacuum at about 200^oC. Cadmium evaporates as monatomic species, therefore, isotopic weights can be used to determine the slopes of the fractionation lines (β-values). The β-values were obtained by analyzing the Cd isotope composition of the evaporation residues relative to the starting material using multiple collector-ICPMS (Nu Plasma in Zürich and IsoProbe in Münster). For the most fractionated sample, the difference between theoretical β-values describing kinetic and equilibrium isotope fractionation is 10 to 20 times larger than the measurement uncertainty. The GPL with n = -0.35 accurately describes the observed mass-dependence for all Cd isotopes. This indicates that 35% of the isotope fractionation result from equilibrium, and 65% from kinetic evaporation. Batch equilibrium evaporation models and mass balance considerations suggest that about 2/3 of the Cd evaporated via an equilibrium pathway with an equilibrium isotope fractionation factor αeq (110Cd/114Cd) ≈ 1.005. The present study (1) shows that the mechanism of isotope fractionation (e.g., the extent of equilibrium in a system) can be investigated by measuring the isotope ratios of the fractionated product relative to the starting material, (2) suggests that the fractionated Cd isotope compositions of chondritic meteorites (e.g., Rosman and De Laeter 1976, Nature 261, 216-218) could also result from equilibrium evaporation and (3) further shows that different processes can result in variable slopes of mass fractionation lines. This should be considered in the evaluation of small mass-independent isotope fractionations (Miller 2002, GCA 66, 1881-1889; Young et al. 2002, GCA 66, 1095-1104).

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