Barium isotopic composition of the upper continental crust

Abstract 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 δ 137/134 Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ 137/134 Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛ Nd (t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites with high SiO 2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ 137/134 Ba (−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 δ 137/134 Ba (−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 δ 137/134 Ba similar to that of loess. The δ 137/134 Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ 137/134 Ba (−0.19‰ to 0.35‰), while those with low CIA ( 137/134 Ba 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 δ 137/134 Ba, ranging from −0.47‰ to 0.35‰. Using the weighted average of samples in this study, the δ 137/134 Ba 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 δ 137/134 Ba of seawater.