Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment Anne-Désirée Schmitt ⁎, Stephen J.G. Galer, Wafa Abouchami Max-Planck-Institute für Chemie, Postfach 3060, D-55020 Mainz, Germany abstract article info Article history: Received 20 March 2008 Received in revised form 21 October 2008 Accepted 21 October 2008 Available online 25 November 2008 Editor: R.W. Carlson Keywords: cadmium cadmium isotopes double spike TIMS Fe–Mn deposits sulphides We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε 112/110 Cd, which are deviations in 112 Cd/ 110 Cd from our in-house JMC Cd standard in parts per 10 4 . The total ε 112/110 Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites. The MORB, OIB and loess ε 112/110 Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is - 0.95±0.12 relative to our Cd standard (ε 112/110 Cd=+0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε 112/110 Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/ redissolution of sulphide phases en route within the hydrothermal system. The ε 112/110 Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε 112/110 Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (N 2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε 112/110 Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Much progress has been made in recent years in exploring and understanding natural stable isotope fractionations of “non-traditional” elements and, more specifically, those of many transition metals. Cadmium possesses many physical characteristics leading to think that it might exhibit stable isotope variations. First, Cd is one of the most volatile of the metals – a group also including Tl, Zn and Pb – and second, Cd has a nutrient-like distribution in the oceans, close to that of phos- phate, from which some “biological” fractionation might be anticipated. On the other hand, cadmium possesses only a single, divalent oxidation state and is unlikely to exhibit as much stable isotope fractionation as multi-valent elements such as Fe and Mo, for example. Study of the stable isotope fractionation of Cd has had a surprisingly long history, beginning with the pioneering work of Rosman and co- workers (i.e., Rosman and de Laeter, 1975, 1976, 1978). The most im- portant finding of this corpus of work is undoubtedly the extreme stable isotope fractionation present in ordinary chondrites and lunar samples, which amounts to as much as ∼ 0.5%/u (i.e. half-percent per dalton, u) in some samples (Rosman et al., 1980a,b; Wombacher et al., 2008). Such effects are, reasonably, ascribed to evaporation/condensa- tion resulting from the volatility of cadmium, as known from the elemental inventories in chondrites (see Lipschutz and Woolum,1988). Indeed, recent experiments by Wombacher et al. (2004) evaporating Cd metal in vacuo have demonstrated that fractionations of several per cent per dalton can be generated in this manner. In the original papers by Rosman and co-workers, the Cd stable isotope fractionation was determined using a double-spike TIMS method with precisions of 0.006–0.1%/u (e.g. Rosman et al., 1980a). With the advent of MC-ICPMS over the past decade, there has been a renewed interest in Cd stable isotope fractionation starting from the comprehensive study by Earth and Planetary Science Letters 277 (2009) 262–272 ⁎ Corresponding author. Present address: Université de Franche-Comté, UFR des Sciences et des Techniques, Département des Géosciences, 16, Route de Gray, F-25030 Besançon Cedex, France. Tel.: +33 381666561; fax: +33 381666563. E-mail address: adschmit@univ-fcomte.fr (A.-D. Schmitt). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.10.025 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl