Analysis of mass dependent and mass independent selenium isotope variability in black shales

Abstract

The measurement of selenium isotope ratios is of increasing interest for understanding redox conditions in present and past surface environments. Se has six stable isotopes, and is therefore well suited for isotope analysis by double spiking. However due to relatively large interferences on every isotope, and complex chemical purification methods that frequently do not generate 100% yields, rigorously determining the accuracy of measurements is critical. Here we present analyses of USGS shale standards (SCo-1 and SGR-1b), as representatives of material which might be of interest to Se isotope studies. We have made analyses using two separate double spikes (⁷⁴Se–⁷⁸Se and ⁷⁸Se–⁸²Se), and compare them to previously published results. In addition, we present models of the effects of uncorrected interferences on double spike inversions. This leads us to propose δ^82/76Se (parts per thousand deviation of ⁸²Se/⁷⁶Se from NIST SRM-3149) values of −0.22 ± 0.15 for SCo-1, and +0.25 ± 0.17 for SGR-1b. Further, we present a new method of measuring Se isotopes by desolvation nebulisation. Se sensitivity is enhanced by a factor of 100–200 times by doping solutions with pure Mg, leading to almost a factor of two less material required compared to the more standard hydride generation. Interferences are different compared to our standard hydride generation protocol, but analyses of double spiked NIST-3149 shows that this method can generate accurate isotope ratios. Finally, mass independent fractionation (MIF) of sulphur isotopes has generated considerable interest for constraining the early oxygenation of the atmosphere. Given the chemical similarities between S and Se, Archean shales with S MIF might be expected to exhibit Se MIF. However, within our analytical uncertainty of ±0.4–0.5‱ (parts per 10 000), there is no resolvable Se MIF in these samples, indicating different atmospheric cycling of Se and S.