Barium isotopic composition of the upper continental crust

The upper continental crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (≤±0.05‰, 2SD) Ba isotopic data for 71 samples (including granites, granodiorites, loess, glacial diamictites, and river sediments) to constrain the Ba isotopic composition of the UCC. I-type granites from the Fogang batholith, Southeastern (SE) China, exhibit variable δ137/134Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ137/134Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛNd (t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites with high SiO2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ137/134Ba (−0.47‰ to −0.33‰), which possibly arose from magmatic differentiation or assimilation of crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous δ137/134Ba (−0.02‰ to 0.03‰) that shows no correlation with bulk compositions, sample locations, or degree of chemical weathering, suggesting that loess is representative of the average Ba isotopic composition of the UCC. Three river sediments from northern China have δ137/134Ba similar to that of loess. The δ137/134Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ137/134Ba (−0.19‰ to 0.35‰), while those with low CIA (<60) have δ137/134Ba around 0‰, identical to the values of loess and river sediments, suggesting that Ba isotopes can be fractionated during chemical weathering. In all, samples from the UCC have highly heterogeneous δ137/134Ba, ranging from −0.47‰ to 0.35‰. Using the weighted average of samples in this study, the δ137/134Ba of the UCC is estimated to be 0.00 ± 0.03‰ (2SD/√n, n = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than δ137/134Ba of seawater.