A baseline for the Sn isotopic composition of the upper continental crust
Edith Kubik, Frédéric Moynier, Jia-Xin She, Roberta L. Rudnick
Abstract
We report high precision Sn isotopic compositions, expressed as δ 122/118 Sn 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 122/118 Sn, ranging from 0.15 to 0.32 ‰ with an average δ 122/118 Sn 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.