In situ calcite U-Pb geochronology by high-sensitivity single-collector LA-SF-ICP-MS

U-Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method, with potential applications to a vast array of geological problems. Due to the low levels of U and Pb in calcite, measurement using higher-sensitivity instruments, such as sector field (SF) ICP-MS, have advantages over more commonly used quadrupole (Q) ICP-MS instruments. Using a Thermo Element XR ICP-MS, we demonstrate that the Jet+X cone combination with the N-2 enhancement technique provides the best sensitivity for measuring U and Pb isotopes. This higher sensitivity improves the precision of calcite U-Pb isotope measurements, and permits dating at high spatial resolutions (<110 mu m) and for samples containing low contents of U-238 (<1 mu g g(-1)) and/or Pb-207 (i.e., young samples <10 Ma). Using a spot size of 85 mu m with a low fluence (similar to 2.0 J cm(-2)), the laser-induced elemental fractionation of Pb-206/U-238 in the NIST SRM 614, ARM-3 and WC-1 reference materials are insignificant (<2.2%). Adopting the optimized instrument parameters, we analysed four commonly-used calcite U-Pb reference materials (WC-1, Duff Brown Tank, JT, and ASH-15). The results match well with published isotope dilution data, demonstrating the reliability of our technique. ARM-3, an andesitic glass, is shown to be an appropriate reference material for both Pb-207/Pb-206 calibration and instrument optimization because of its moderate contents of U (similar to 3.75 mu g g(-1)) and Pb (similar to 12.7 mu g g(-1)). We further demonstrate that the image-guided approach using LA-ICP-MS elemental mapping is an efficient tool in obtaining robust ages.

Automated summary

The study on U-Pb geochronology of calcite using LA-ICP-MS is an emerging method with improved sensitivity, resulting in precise measurements of U and Pb isotopes. The optimized instrument parameters allowed for accurate analysis of commonly-used calcite U-Pb reference materials, demonstrating the reliability of the technique in obtaining robust ages. The image-guided approach using LA-ICP-MS elemental mapping proved to be an efficient tool in age determination.