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https://doi.org/10.5194/gchron-2019-20
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2019-20
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Jan 2020

Submitted as: research article | 27 Jan 2020

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This preprint is currently under review for the journal GChron.

New analytical and data evaluation protocols to improve the reliability of U-Pb LA-ICP-MS carbonate dating

Marcel Guillong, Jören-Frederik Wotzlaw, Nathan Looser, and Oscar Laurent Marcel Guillong et al.
  • Department of Earth Sciences, ETH, Zürich, 8092, Switzerland

Abstract. We document that the reliability of carbonate U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is improved by matching the aspect ratio of the LA single hole drilling craters and propagating of long-term excess uncertainty and systematic uncertainties. We investigated the impact of different matrices and ablation crater geometries using U-Pb isotope analyses of one primary (WC-1) and two secondary reference materials (RMs). Validation RMs (VRM) include a previously characterized (ASH-15D) and a new candidate (JT), characterized by ID-TIMS (intercept age: 13.797 ± 0.031 Ma) with excellent agreement to pooled LA-ICP-MS measurements (13.81 ± 0.11 ¦ 0.30 Ma), U concentration of approx. 1 μg/g and 238U/206Pb ratios from 5 to 460, well defining the isochron. Differences in ablation crater depth to diameter ratios (aspect ratio) introduce an offset due to downhole fractionation and/or matrix effects. This effect can be observed either when the crater size between U-Pb RM and sample changes or when the ablation rate for the sample is different than for the RM. Observed deviations are up to 20 % of the final intercept age depending on the degree of crater geometry mismatch. The long-term excess uncertainty was calculated to be in the range of 2 % (ASH-15D) to 2.5 % (JT), and we recommend propagating this uncertainty into the uncertainty of the final results. Additionally, a systematic offset to the ID-TIMS age of 2–3 % was observed for ASH-15D but not for JT. This offset might be due to different ablation rates of ASH-15D compared to the primary RM or remaining matrix effects, even when chosen aspect ratios are similar.

Marcel Guillong et al.

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Marcel Guillong et al.

Marcel Guillong et al.

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Latest update: 01 Apr 2020
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Short summary
The dating of carbonates by laser ablation ICP-MS is improved by an additional, newly characterised reference material, and adapted data evaluation protocols: The shape (diameter to depth) of the ablation crater has to be as similar as possible between the reference material used and the unknown samples. Otherwise an offset is introduced. Different carbonates have different ablation rates per laser pulse. With robust uncertainty propagation, precision can be as good as 2–3 %.
The dating of carbonates by laser ablation ICP-MS is improved by an additional, newly...
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