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 parts per thousand, 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 delta Ba-137/134 (-0.16 parts per thousand to 0.01 parts per thousand), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The delta Ba-137/134 of S-type granites from Darongshan-Shiwandashan, SE China (-0.03 parts per thousand to 0.11 parts per thousand) correlate with epsilon(Nd) (t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites with high SiO2 contents (similar to 76 wt%) from Nankunshan, SE China have remarkably low delta Ba-137/134 (-0.47 parts per thousand to -0.33 parts per thousand), which possibly arose from magmatic differentiation or assimilation of crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous delta Ba-137/134 (-0.02 parts per thousand to 0.03 parts per thousand) 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 delta Ba-137/134 similar to that of loess. The delta Ba-137/134 of glacial diamictites vary with CIA values: those with high CIA (>= 60) have heterogeneous delta Ba-137/134 (-0.19 parts per thousand to 0.35 parts per thousand), while those with low CIA (<60) have delta Ba-137/134 around 0 parts per thousand, 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 delta Ba-137/134, ranging from -0.47 parts per thousand to 0.35 parts per thousand. Using the weighted average of samples in this study, the delta Ba-137/134 of the UCC is estimated to be 0.00 +/- 0.03 parts per thousand (2SD/root n, n = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than delta Ba-137/134 of seawater. (C) 2018 Elsevier Ltd. All rights reserved.