Big difference in 87 Sr/ 86 Sr ratios of basalt and basin water: higher 87 Sr/ 86 Sr ratios in plagioclase

We analyzed the 87Sr/86Sr 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 87Sr/86Sr ratios than bulk basalt and basin water, which might explain the difference in 87Sr/86Sr ratios of basalt and basin water. We infer that low-temperature hydrothermal alteration increased the 87Sr/86Sr 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 87Sr/86Sr ratios than basaltic rocks.