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

Wu, Shitou; Yang, Yueheng; Roberts, Nick M.W.; Yang, Ming; Wang, Hao; Lan, Zhongwu; Xie, Bohang; Li, Tianyi; Xu, Lei; Huang, Chao; Xie, Liewen; Yang, Jinhui; Wu, Fuyuan. 2022 In situ calcite U−Pb geochronology by high-sensitivity single-collector LA-SF-ICP-MS. Science China Earth Sciences, 65 (6). 1146-1160.

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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 N2 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 µm) and for samples containing low contents of 238U (<1 µg g−1) and/or 207Pb (i.e., young samples <10 Ma). Using a spot size of 85 µm with a low fluence (∼2.0 J cm−2), the laser-induced elemental fractionation of 206Pb/238U 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 207Pb/206Pb calibration and instrument optimization because of its moderate contents of U (∼3.75 µg g−1) and Pb (∼12.7 µg g−1). We further demonstrate that the image-guided approach using LA-ICP-MS elemental mapping is an efficient tool in obtaining robust ages.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 1674-7313
Date made live: 14 Jun 2022 09:38 +0 (UTC)

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