A baseline for the Sn isotopic composition of the upper continental crust

Abstract

We report high precision Sn isotopic compositions, expressed as δ^122/118Sn relative to the NIST3161a standard, for the fine grained matrix of 24 glacial diamictite composites. The diamictites were deposited from the Mesoarchean to the Palaeozoic and sampled from four continents (Africa, Asia, North and South America). They are relatively homogeneous in δ^122/118Sn, ranging from 0.15 to 0.32 ‰ with an average δ^122/118Sn value of 0.22 ± 0.14 ‰ (2 s.d., n = 24). The Sn isotopic composition of the diamictites is not influenced by chemical weathering, depositional age, geographic setting, atmospheric oxygen content or magmatic differentiation processes in the source region. As such, the average provides a robust estimate of the Sn isotopic composition of the bulk upper continental crust (UCC). This baseline is within the range of, but isotopically lighter than, the current depleted mantle estimate (0.37 ± 0.09 ‰; 2 s.d., n = 12) and bulk silicate Earth (0.38 ± 0.11 ‰; 2 s.d., n = 9). It also overlaps with the very few available measurements made on andesites, granites, a granodiorite and a rhyolite.