LA-MC-ICP-MS calcite U-Pb dating technique

Calcite is widely formed in various geological environments such as diagenesis, biological processes and hydrothermal processes. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) calcite U-Pb dating technique has a broad application prospect in solving key geological problems. Calcite has relatively low U content (5.0x10(-6)) and high common lead proportion (U-238/Pb-206 < 1), which requires high sensitivity of the instrument. Compared with Q-ICP-MS and SF-ICP-MS, MC-ICP-MS has the advantage of simultaneous isotope determination and higher analysis accuracy. Predecessors optimized instrument parameters and made some attempts to improve sensitivity. To maximize sensitivity, based on Neptune Plus model MC-ICP-MS, this study gets the best sensitivity of U and Pb by optimizing cone and auxiliary nitrogen systematically. The research shows that the instrument has the highest sensitivity under the condition of Jet+X cone combined with nitrogen (N-2=4.0mL.min(-1)). Compared with the conventional condition (S+H cone combined without nitrogen), the signal intensities of U and Pb are increased by about 4 times and 3 times respectively. At the same time, we find that the nitrogen sensitization has a great relationship with the cone combination. When X sample cone is used, the introduction of nitrogen can improve the signal intensity of U, while when H sample cone is used, the introduction of nitrogen does not change the signal intensity. Based on the optimized instrument parameters, we established a high spatial resolution (similar to 85 mu m) LA-MC-ICP-MS calcite U-Pb dating technique. Three calcite U-Pb dating reference materials are used to verify our technique, and the results are consistence with ID-TIMS value within error, which proves that our method is effective for the analysis of low U (similar to 0.03x10(-6)) sample. Two reference materials in the laboratory are preliminary determined: TLM and LSJ07. The results of one measurement of TLM are 219.8 +/- 2.3Ma (2s, n=30), and the results of two measurements of LSJ07 are 26.4 +/- 0.2 Ma (2s, n=30) and 26.7 +/- 0.2Ma (2s, n=30). The proportion of common lead in these two reference materials is relatively low, and they are potential calcite U-Pb dating reference materials. We find that the change of laser spot size significantly affected the accuracy of the results, which may be related to the difference of fractionation effect under different laser spots, especially when the laser spot is smaller than 60 mu m. Therefore, we suggest that the laser parameters should be strictly matched between the reference materials and the unknown samples in the U-Pb dating of calcite. Finally, we preliminarily discussed the closure of calcite U-Pb system.