Sn isotope data for UCC

Stable Sn isotope ratios are emerging as a novel tracer for a wide range of geological processes, however, the Sn isotopic baseline of the upper continental crust (UCC) is not yet well-constrained. Here, we report high-precision Sn isotope data of a wide range of UCC samples, including granites, pegmatites, and sediments, to document the Sn isotopic composition of the UCC. Significant variations in δ122/118Sn3161a (per mil deviation in 122Sn/118Sn relative to NIST 3161a) values are revealed for I-type (δ122/118Sn3161a = 0.025±0.026 to 0.495±0.046‰, N=20) and S-type (δ122/118Sn3161a = 0.156±0.018 to 0.501±0.075‰, N=22) granites. More extreme Sn isotope variability is observed from pegmatites, which have δ122/118Sn3161a of 0.256±0.047 to 0.930±0.049‰ (N=13). The δ122/118Sn3161a of I-type granites decrease with declining TFe2O3 (total Fe as Fe2O3) and MgO contents and are attributed to the segregation of Fe-bearing minerals. The Sn isotope variation of S-type granites likely arises from source heterogeneity. The Sn isotope variability of pegmatites may reflect fluid activities. In contrast, the loess samples display homogeneous δ122/118Sn3161a (0.132±0.034‰ to 0.239±0.020‰, N=20) that show no correlation with the degree of chemical weathering, suggesting that loess is representative of the average Sn isotope composition of the UCC. The δ122/118Sn3161a of modern sediments and sedimentary rocks range from 0.080‰ to 0.490‰ (N=25). The Sn isotope variations of modern sediments and sedimentary rocks may be related to chemical weathering or sediment provenance. Based on the lithology-weighted average δ122/118Sn3161a of UCC samples (41 granites and 45 sediments) in this study, the δ122/118Sn3161a value of the UCC is estimated to be 0.233±0.099‰, providing a reference point for further applications.锡稳定同位素比值正逐步成为诸多地质过程的新型示踪剂，然而上地壳（upper continental crust, UCC）的锡同位素基准值目前尚未得到精准约束。本文报道了一系列涵盖花岗岩、伟晶岩与沉积物的上地壳样品的高精度锡同位素数据，以厘定上地壳的锡同位素组成。研究显示，I型花岗岩的δ¹²²/¹¹⁸Sn₃₁₆₁a（即相对于NIST 3161a标样的¹²²Sn/¹¹⁸Sn比值千分偏差）值范围为0.025±0.026‰至0.495±0.046‰（样品数N=20），S型花岗岩的该值范围为0.156±0.018‰至0.501±0.075‰（N=22），二者存在显著差异。伟晶岩的锡同位素分异更为极端，其δ¹²²/¹¹⁸Sn₃₁₆₁a值介于0.256±0.047‰至0.930±0.049‰之间（N=13）。I型花岗岩的δ¹²²/¹¹⁸Sn₃₁₆₁a值随全铁（以Fe₂O₃计，TFe₂O₃）与MgO含量降低而减小，该现象可归因于含铁矿物的分离结晶作用。S型花岗岩的锡同位素分异可能源于源区物质的不均一性。伟晶岩的锡同位素变化或可反映流体活动的影响。与之形成对比的是，黄土样品的δ¹²²/¹¹⁸Sn₃₁₆₁a值均一，分布范围为0.132±0.034‰至0.239±0.020‰（N=20），且与化学风化程度无相关性，表明黄土可代表上地壳的平均锡同位素组成。现代沉积物与沉积岩的δ¹²²/¹¹⁸Sn₃₁₆₁a值范围为0.080‰至0.490‰（N=25），其锡同位素分异可能与化学风化或沉积物物源有关。基于本研究中41件花岗岩与45件沉积物的上地壳样品岩性加权平均δ¹²²/¹¹⁸Sn₃₁₆₁a值，本文估算得到上地壳的δ¹²²/¹¹⁸Sn₃₁₆₁a为0.233±0.099‰，可为后续相关研究应用提供参考基准。