Improvement of in situ LA-ICP-MS U-Pb dating method for carbonate minerals and its application in petroleum geology

In situ carbonate U-Pb dating is gaining popularity, and it has great potential for application in petroleum geology. However, the low U content (<10 mu g/g) and high common Pb content of carbonate minerals, along with the uneven distribution of U and Pb and the lack of matrix-matched reference material make carbonate U-Pb dating inaccurate and less successful, which limits the widespread application in geosciences. This study evaluated the limitations of in situ carbonate U-Pb dating and proposed a method to rationally determine the laser ablation parameters of samples by improving the experimental approach based on the laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS). By setting a different spot size and laser frequency for the reference material and unknown samples in the same session based on the U content of the sample, the ablation craters of the reference material and unknown samples were given the same depth/width ratio, avoiding systematic offset caused by differences in down-hole element fractionation and reducing the consume of reference material. Depending on the heterogeneous distribution of U and Pb contents in carbonate minerals, the method of grid screening and setting ablation spots during screening were used to quickly select domains with high U and low common Pb, which improves the efficiency of setting laser spots and the success rate of dating, as well as reduces the experimental time and economic cost. The accuracy and success rate of carbonate U-Pb dating were effectively improved by improving the experimental method, and the technique was applied to two carbonate samples with low U and high common Pb contents that were difficult to date by traditional methods. The two samples are the saddle dolomite in the central Sichuan Basin and the calcite cement in the sandstone reservoir of the Cretaceous Qingshuihe Formation in the South Junggar Basin. The robust ages have been obtained, which constrains the timing of the diagenetic and hydrocarbon accumulation process in the studied area.