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Geochronology Advances in geochronological science
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https://doi.org/10.5194/gchron-2020-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2020-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  27 Jul 2020

27 Jul 2020

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This preprint is currently under review for the journal GChron.

The use of ASH-15 flowstone as a matrix-matched reference material for laser-ablation U-Pb geochronology of calcite

Perach Nuriel1, Jörn-Frederik Wotzlaw2, Maria Ovtcharova3, Anton Vaks1, Ciprian Stremtan4, Martin Šala5, Nick M. W. Roberts6, and Andrew R. C. Kylander-Clark7 Perach Nuriel et al.
  • 1Geological Survey of Israel, 32 Yeshayahu Leibowitz St. Jerusalem, 9692100, Israel
  • 2Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25 I 8092 Zurich, Switzerland
  • 3Department of Earth Sciences, University of Geneva, Geneva, Switzerland
  • 4Teledyne Photon Machines, 384 Gallatin Park Drive, Bozeman, MT 59715, USA
  • 5Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
  • 6Geochronology and Tracers Facility, British Geological Survey, Environmental Science Centre, Nottingham, NG12 5GG, UK
  • 7Department of Earth Science, University of California, Santa Barbara, CA 93106, USA

Abstract. Latest advances in laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) allow for accurate in-situ U-Pb dating of carbonate material, with final age uncertainties usually > 3 % 2σ. Cross-laboratory reference materials (RMs) used for sample-bracketing are currently limited to WC1 calcite with an age of 254.4 ± 6.5 (2σ). The minimum uncertainty on any age determination with the LA-ICPMS method is therefore ≥ 2.5 %, and validation by secondary RMs are usually performed on in-house standards. This contribution present a new reference material, ASH-15, a flowstone that is dated here by high-precision Isotope Dilution (ID) TIMS analysis using 36 sub-samples, 1–7 mg each. Age results presented here are slightly younger compared to previous ID dating of ASH-15, but in agreement with in-situ analyses (using WC1 as a RM). We provide new correction parameters to be used as primary or secondary standardization. The suggested 238U/206Pb apparent age, not corrected for disequilibrium and without common-lead anchoring, is 2.965 ± 0.011 Ma (2σ). The new results could improve the propagated uncertainties on the final age with a minimal value of 0.4%, which is approaching the uncertainty of typical ID analysis on, for example, zircon (< 1 % 2 s). We show that although LA-ICPMS spot analyses of ASH-15 exhibits significant scatter in their isotopic ratios, the down-hole fractionation of ASH-15 is similar to that of other reference materials. For LA work, we recommend the use of the new ID-TIMS ages that are 1.3&ndasdh;1.5 % younger than previously suggested, because of the lower uncertainties (0.4 %), the large number of sub-samples (n = 36), the use of the EARTHTIME isotopic tracers, and the small aliquots (1–7 mg) that are more representative of laser-ablation spot analysis.

Perach Nuriel et al.

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Short summary
This contribution present a new reference material, ASH-15 flowstone with an age of 2.965 ± 0.011 Ma (2σ), to be used for in-situ U-Pb dating of carbonate material. The new age analyses include the use of the EARTHTIME isotopic tracers and a large number of sub-samples (n = 36) with small aliquots (1–7 mg) each that are more representative of laser-ablation spot analysis. The new results could improve the propagated uncertainties on the final age with a minimal value of 0.4 %.
This contribution present a new reference material, ASH-15 flowstone with an age of 2.965 ±...
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