Surface-Scanning U-Pb Dating of Carbonate Minerals and Its Application to Samples Altered by Complex Fluids

Carbonate U-Pb dating provides direct temporal constraints on basin-fluid evolution and hydrocarbon accumulation. Although the spot-ablation method has been widely applied to carbonate dating, carbonate minerals commonly exhibit low U concentrations, high common-Pb contents, and multistage fluid alteration. These characteristics result in insufficient dispersion of U/Pb ratios among analytical spots and consequently lead to low dating success rates, limiting the application of this technique. In this study, three calcite reference materials—WC-1, GX-5, and Tarim—and three natural samples—HY2, high common-Pb calcite from Permian shale in central Sichuan; GLC-1, medium- to low-U calcite veins hosted in the Permian limestones of the central Sichuan Basin; and DH1-3, fracture-filling calcite affected by multistage fluid alteration in Devonian limestone from Guizhou—were investigated using a direct U-Pb dating method based on laser-ablation sector-field inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) mapping. Comparisons between spot ablation and the mapping method were conducted to evaluate the applicability of the latter to complex carbonate samples and its effectiveness in suppressing downhole fractionation. To address these dating challenges, the mapping method reduced the single-point dwell time to ＜5 s and decreased the number of effective ablation pulses from approximately 200 to ＜50, thereby reducing downhole fractionation. Combined with continuous line scanning and virtual spot construction, this method expanded the range of U/Pb ratios and improved the geochronological constraints for such samples. Validation using the reference materials showed that the age uncertainties obtained using the mapping method were substantially smaller than those obtained using spot ablation, decreasing from an uncertainty of ±2.6 Ma to ±1.3 Ma for WC-1 and from ±6.5 Ma to ±3.4 Ma for Tarim. Analyses of the three natural samples further demonstrated the applicability of this method. For HY2, the high common-Pb calcite from Permian shale in central Sichuan, spot ablation failed to yield a valid age, whereas the mapping method produced a lower-intercept age of 239 ± 19 Ma, consistent with the Early Indosinian Movement in the Sichuan Basin. For the low-U sample GLC-1 from Permian limestone in central Sichuan, the age uncertainty decreased from ±41 Ma to ±2.9 Ma. For DH1-3, the sample affected by multistage fluid activity in Devonian limestone from Guizhou, a single mapping run simultaneously yielded three independent ages for the host rock (384.4 ± 9.9 Ma), high-Mg calcite (379 ± 44 Ma), and calcite vein (247 ± 15 Ma). This method significantly improves the dating accuracy and success rate of complex carbonate samples, providing reliable technical support for geochronological studies of basin-fluid evolution and hydrocarbon accumulation.