TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52 (No. 6), pp. 531-540, 2018

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

Tatsunori Yokoyama,1* Jun-ichi Kimura,2* Takehiro Mitsuguchi,1 Toru Danhara,3 Takafumi Hirata,4 Shuhei Sakata,5 Hideki Iwano,3 Seiji Maruyama,3 Qing Chang,2 Takashi Miyazaki,2 Hiroaki Murakami1 and Yoko Saito-Kokubu1

1Tono Geoscience Center, Japan Atomic Energy Agency (JAEA), Jorinji, Izumicho, Toki 509-5102, Japan
2Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
3Kyoto Fission-Track Co., Ltd., Omiya, Kita-ku, Kyoto 603-8832, Japan
4Geochemistry Research Center, Graduate School of Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
5Department of Chemistry, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan

(Received March 5, 2018; Accepted September 19, 2018)

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.
Key words: U-Pb dating, calcite, laser ablation ICP-MS, elemental imaging

*Corresponding authors E-mail: yokoyama.tatsunori@jaea.go.jp; jkimura@jamstec.go.jp

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