Tin isotope fractionation during magmatic processes and the isotope composition of the bulk silicate Earth
Résumé
Tin is a moderately volatile element whose isotope composition can be used to investigate Earth and planet differentiation and the early history of the Solar System. Although the Sn stable isotope composition of several geological and archaeological samples has been reported, there is currently scarce information about the effect of igneous processes on Sn isotopes. In this study, high-precision Sn isotope measurements of peridotites and basalts were obtained by MC-ICP-MS with a double-spike technique. The basalt samples display small variations in delta Sn-124/116 ranging from -0.01 +/- 0.11 to 0.27 +/- 0.11 parts per thousand (2 s.d.) relative to NIST SRM 3161a standard solution, while peridotites have more dispersed and more negative delta Sn-124 values ranging from -1.04 +/- 0.11 to -0.07 +/- 0.11 parts per thousand (2 s.d.). Overall, basalts are enriched in heavy Sn isotopes relative to peridotites. In addition, delta Sn-124 in peridotites become more negative with increasing degrees of melt depletion. These results can be explained by different partitioning behavior of Sn4+ and Sn2+ during partial melting. Sn4+ is overall more incompatible than Sn2+ during partial melting, resulting in Sn4+-rich silicate melt and Sn2+-rich residue. As Sn4+ has been shown experimentally to be enriched in heavy isotopes relative to Sn2+, the effect of melting is to enrich residual peridotites in relatively more compatible Sn2+, which results in isotopically lighter peridotites and isotopically heavier mantle-derived melts. This picture can be disturbed partly by the effect of refertilization. Similarly, the presence of enriched components such as recycled oceanic crust or sediments could explain part of the variations in Sn isotopes in oceanic basalts. The most primitive peridotite analyzed in this study was used for estimating the Sn isotope composition of the BSE, with delta Sn-124 = -0.08 +/- 0.11 parts per thousand (2 s.d.) relative to the Sn NIST SRM 3161a standard solution. Altogether, this suggests that Sn isotopes may be a powerful probe of redox processes in the mantle. (C) 2018 Elsevier Ltd. All rights reserved.