ABSTRACT

We developed a non-matrix matched U-Pb dating method for calcite by using excimer laser ablation-multiple Faraday collector-inductively coupled plasma-mass spectrometry (LA-MFC-ICP-MS). The excimer LA was set to generate a low-aspect-ratio crater of 100 μm diameter × 50 μm depth to minimize downhole U-Pb fractionation. We used He sample ablation gas mixed with Ar carrier gas and additional trace N₂ gas to create a robust plasma setup in MFC-ICP-MS. The use of N₂ additional gas allowed for low oxide molecular yield (UO⁺/U⁺ < 1%) for high-sensitivity JET-sampler and X-skimmer interface cones with the ICP shield electrode disconnected. Moreover, this resulted in robust ICP plasma against different matrixes in LA aerosols owing to efficient dissociation-ionization of the aerosols by increased plasma temperature. The above setup helped accomplish accurate U-Pb dating of calcite samples by using SRM 612 glass from the National Institute of Standards and Technology (NIST) as the standard. We applied this method to the following calcite samples: (1) recently-proposed reference material named WC-1 with a determined U-Pb age of 254.6 ± 3.2 Ma and (2) a well-preserved fossil specimen of blastoid Pentremites sp. with an estimated age of ~339–318 Ma. Prior to the U-Pb dating, quantitative two-dimensional maps of U, Th, and Pb isotope abundances of the calcite samples were obtained using a LA-ICP-MS imaging technique to select suitable areas for dating. Obtained U-Pb age of the WC-1 sample was 260.0 ± 6.7 Ma with an anchored point of initial ²⁰⁷Pb/²⁰⁶Pb ratio of 0.85 ± 0.02 determined by Roberts et al. (2017), which was close to the reported U-Pb age of 254.4 ± 0.8 Ma. The regression-based U-Pb discordant line age of the Pentremites sample was dated 332 ± 12 Ma, which indicate accurate U-Pb dating by this method.