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

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.