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We analyzed the Sr/Sr ratios of basaltic rocks and basin water in Xuyi, China, and found a big difference, which may challenge conventional wisdom on the contribution of basalt weathering in end-member analysis. Results of an in-house weathering experiment suggest that rainwater and dust are not responsible for the difference. By isolating the major minerals in basalt, we found that plagioclase has much higher Sr/Sr ratios than bulk basalt and basin water, which might explain the difference in Sr/Sr ratios of basalt and basin water. We infer that low-temperature hydrothermal alteration increased the Sr/Sr ratios of plagioclase. Future analyses of end-member contribution in a mixed-rock basin should take into account that basin water and plagioclase have higher Sr/Sr ratios than basaltic rocks. [ABSTRACT FROM AUTHOR]