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.

Marianna Corre et al.

Status: final response (author comments only)

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

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.