U-Pb dating of calcite using LA-ICP-MS: Instrumental setup for non-matrix-matched age dating and determination of analytical areas using elemental imaging

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 lowaspect-ratio crater of 100 mu m diameter x 50 mu m depth to minimize downhole U-Pb fractionation. We used He sample ablation gas mixed with Ar carrier gas and additional trace N-2 gas to create a robust plasma setup in MFC-ICP-MS. The use of N-2 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 similar to 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 (207)pb/Pb-206 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.