Preprints
https://doi.org/10.5194/egusphere-2022-520
https://doi.org/10.5194/egusphere-2022-520
 
10 Aug 2022
10 Aug 2022

U and Th content in magnetite and Al-spinel obtained by wet chemistry and laser ablation methods: implication for (U-Th)/He thermochronometer

Marianna Corre1, Arnaud Agranier2, Martine Lanson1, Cécile Gautheron3,1, Fabrice Brunet1, and Stéphane Schwartz1 Marianna Corre et al.
  • 1ISTerre, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, 38000 Grenoble, France
  • 2UBO, IUEM, Place Nicolas Copernic, 29820 Plouzané, France
  • 3GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, 91405 Orsay, France

Abstract. Magnetite and spinel thermochronological (U-Th)/He dates obtained in different geological contexts often present significantly dispersed values, that could be related to the low concentration of U and Th isotopes, the lack of standard samples or other parameters. For this purpose, this study focuses on the analysis of U and Th content variability. U and Th in magnetite (natural and synthetic) and in natural Al-spinel samples containing different amounts of U and Th, from 0.02 to 116 µg/g, are analyzed using both wet chemistry and in-situ laser ablation extraction methods. To increase the number of reference samples, two U-Th doped nanomagnetite powders were synthesized and the U and Th concentration were firstly determined using wet chemistry extraction (U and Th of NMA is ~40 µg/g and NMB ~0.1 µg/g). We show that for both U and Th analyses, the obtained reproducibility of the wet chemistry protocol depends on their concentration and is below 11 % for U-Th values higher than 0.4 µg/g and reaches 22 % for U-Th content lower than 0.1 µg/g. It implies that (U-Th)/He thermochronological dates cannot be more reproducible than 24 % for magnetite containing less than 0.1 µg/g of U and Th, explaining part of the natural date variability. Secondly, U and Th concentration extracted by laser ablation on natural magnetite and Al-spinel samples were calibrated using both silicate glass standards and synthetic magnetite samples. The determined U and Th content using NMA sample give similar values than the one obtained by wet chemistry extraction but is 30 % overestimated using the glass standard samples. These results highlight the impact of matrix effect on the determination of the U-Th content and we recommend to use a well-characterized magnetite sample for calibrating the U-Th signals by laser ablation. In addition, the scatter on the (U-Th)/He magnetite dates can be expected to be ~20 % if the U and Th contents are determined by laser ablation. Such a precision level is not that different to the one obtained using wet chemistry extraction, opening the use the use of laser ablation extraction method for determining (U-Th)/He dates. In the absence of spinel reference for U and Th, the silicate glasses and NMA samples was used for laser ablation calibration and U and Th content and are of ~30 % lower compared to values obtained using wet chemistry extraction. This discrepancy underlines the importance of using a standard with a composition close to that of the mineral of interest. Although magnetite and Al-spinel have related crystal-structures, the magnetite standard is not appropriate for U and Th analysis in Al-spinel.

Journal article(s) based on this preprint

22 Nov 2022
U and Th content in magnetite and Al spinel obtained by wet chemistry and laser ablation methods: implication for (U–Th) ∕ He thermochronometer
Marianna Corre, Arnaud Agranier, Martine Lanson, Cécile Gautheron, Fabrice Brunet, and Stéphane Schwartz
Geochronology, 4, 665–681, https://doi.org/10.5194/gchron-4-665-2022,https://doi.org/10.5194/gchron-4-665-2022, 2022
Short summary

Marianna Corre et al.

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-520', Emily Cooperdock, 09 Sep 2022
  • RC2: 'Comment on egusphere-2022-520', Florian Hofmann, 17 Sep 2022

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-520', Emily Cooperdock, 09 Sep 2022
  • RC2: 'Comment on egusphere-2022-520', Florian Hofmann, 17 Sep 2022

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision
ED: Publish subject to minor revisions (further review by editor) (17 Oct 2022) by Daniela Rubatto
AR by Marianna Corre on behalf of the Authors (26 Oct 2022)  Author's response    Author's tracked changes    Manuscript
ED: Publish subject to technical corrections (31 Oct 2022) by Daniela Rubatto
ED: Publish subject to technical corrections (08 Nov 2022) by Greg Balco(Editor)

Journal article(s) based on this preprint

22 Nov 2022
U and Th content in magnetite and Al spinel obtained by wet chemistry and laser ablation methods: implication for (U–Th) ∕ He thermochronometer
Marianna Corre, Arnaud Agranier, Martine Lanson, Cécile Gautheron, Fabrice Brunet, and Stéphane Schwartz
Geochronology, 4, 665–681, https://doi.org/10.5194/gchron-4-665-2022,https://doi.org/10.5194/gchron-4-665-2022, 2022
Short summary

Marianna Corre et al.

Marianna Corre et al.

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Short summary
This contribution is focused on the accurate measurement of U and Th by wet chemistry and laser ablation to improve the (U-Th)/He dating of magnetite and spinel. The low U-Th content and the lack of specific U-Th standards limit significantly the accuracy of (U-Th)/He dating. Obtained U and Th results on natural and synthetic magnetite and aluminous spinel samples analyzed by both extraction methods, have important implications for the (U-Th)/He method and dates interpretation.