Tin stable isotope analysis of geological materials by double-spike MC-ICPMS

Abstract

Tin is a volatile as well as chalcophile and siderophile element, and this geochemical behaviour gives rise to a broad range of potential applications for Sn as a stable isotope tracer in geological processes. We present the first high-precision method to analyse the stable isotopic composition of Sn in geological materials using ion-exchange chromatography and double-spike multi-collector inductively-coupled-plasma mass-spectrometry (MC-ICPMS). We apply these methods to analyse the Sn stable isotopic composition of four geological reference materials, including a number of replicate digestions of BHVO-2 and BCR-2 to assess the reproducibility of the technique. Tin was purified prior to analysis using TRU resin to remove matrix and interfering elements. Isotopic ratios were measured using a Thermo-Fischer Neptune Plus MC-ICPMS combined with a 117Sn–122Sn double-spike to correct for any mass dependent fractionation resulting from instrumental mass bias or incomplete yields from chemical purification. Results are expressed in delta notation as δ122/118Sn, representing the per mil (‰) difference in the 122/118Sn ratio of the sample relative to our in-house standard Sn_IPGP. Based on replicate analyses of the USGS reference materials BHVO-2 and BCR-2, we estimate the external reproducibility to be ca. ± 0.065‰ (2 sd) on the δ122/118Sn ratio, or ca. ± 0.016‰ per amu. Analyses of these plus two additional USGS reference materials, AGV-2 and GSP-2, show a large range (>0.2‰ on δ122/118Sn) of Sn stable isotopic compositions that are correlated with igneous differentiation indicators (e.g. SiO2 content), indicating that Sn isotopes are fractionated during igneous processes. These results indicate significant potential for Sn stable isotopes as a tracer of magmatic differentiation and the redox state of the mantle. In addition, Sn stable isotopes may prove useful in tracing diverse geological processes such as volatilisation/condensation and metal–silicate/metal–sulphide differentiation.