The Stable Isotope Geochemistry of Copper and Zinc

摘要

Copper and zinc are the last two elements of the first row of transition metals (d-block). Interest in these elements arises because they are both strongly chalcophile and, thanks to a rich coordination chemistry, participate in a large number of important biological compounds and reactions. Copper has two isotopes, ~(63)Cu and ~(65)Cu with respective abundances of 69.174% and 30.826% in the reference metal SRM-NIST 976 (Shields et al. 1964). Zn has five stable isotopes ~(64)Zn, ~(66)Zn, ~(67)Zn, ~(68)Zn, and ~(70)Zn with average natural abundances of 48.63, 27.90, 4.10, 18.75, and 0.62%, respectively (Rosman and Taylor 1998). The first attempts at describing the stable isotope geochemistry of copper go back to Walker et al. (1958) and Shields et al. (1965) who identified isotopic variations in the range of several per mil but the analytical difficulties were then such that these pioneering investigations remained isolated. With the exception of a paper by Rosman (1972) who determined the isotopic abundances of Zn isotopes and concluded that there is no noticeable isotopic fractionation in terrestrial samples, the stable isotope geochemistry of this element remained essentially unexplored. It was not until the adyent of inductively-coupled plasma mass spectrometry (ICP-MS) instruments equipped with a magnetic sector and multiple collection that precise isotopic measurements became possible and that the isotope geochemistry of these two elements took off. Maréchal et al. (1999) published the first measurements of Cu and Zn isotope compositions in a variety of minerals and biological materials. Maréchal et al. (2000) and Pichat et al. (2003) demonstrated the variability of Zn isotopes in ferromanganese nodules, sediment trap material, and marine carbonates. Maréchal et al. (1999) and Zhu et al. (2000) confirmed the broad range of isotopic variations in copper ores observed by the earlier workers. The same authors and Zhu et al. (2002) observed that biological material has isotopic compositions which vary and differ from the solution in which organisms grow. Marechal et al. (1999) and Zhu et al. (2000) found the first evidence of isotope fractionation of transition element isotopes on chromatographic columns and this important observation was extended to Fe isotopes by Anbar et al. (2000) and to Zn isotopes by Maréchal and Albarède (2002). So far, very little is known about isotopic fractionation of Cu and Zn isotopes between coexisting phases in controlled environments and most of the existing work is still scattered in abstracts and PhD memoirs (Rouxel 2002; Maréchal and Sheppard 2002; Young 2003; Ehrlich et al., submitted). There is no doubt anymore that the broad variability of Cu and Zn isotopic compositions in the mineral and biological worlds is an enormous potential resource awaiting more experimental and theoretical work.