Preprints
https://doi.org/10.5194/gchron-2020-33
https://doi.org/10.5194/gchron-2020-33

  24 Nov 2020

24 Nov 2020

Review status: a revised version of this preprint was accepted for the journal GChron and is expected to appear here in due course.

Uranium incorporation in fluorite and exploration of U-Pb dating

Louise Lenoir1, Thomas Blaise1, Andréa Somogyi2, Benjamin Brigaud1, Jocelyn Barbarand1, Claire Boukari1, Julius Nouet1, and Maurice Pagel1 Louise Lenoir et al.
  • 1Université Paris-Saclay, CNRS, GEOPS, Orsay, 91405, France
  • 2Université Paris-Saclay, Synchrotron SOLEIL, Saint-Aubin, 91190, France

Abstract. The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U-Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). Uranium distribution within fluorite is mapped using induced fission-track and Synchrotron radiation X-Ray Fluorescence nano-imaging, showing that higher uranium content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr implies that neither solid-state diffusion nor dissolution-recrystallization did occur. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰ V-CDT. We propose that the U incorporation in the fluorite lattice results from its reduction mediated by H2S release during bacterial sulphate reduction. Flog generation sampled and analyzed by LA-ICP-MS on four different crystals provides identical U-Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U-Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

Louise Lenoir et al.

 
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Louise Lenoir et al.

Louise Lenoir et al.

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Short summary
To explore the U-Pb geochronometer in fluorite, the spatial distribution of uranium and other substituted elements in natural crystals is investigated using induced fission-track and synchrotron radiation X-ray fluorescence mapping. LA-ICP-MS U-Pb dating on four crystals, that preserved micrometer-scale variations in U concentrations, yield identical ages within analytical uncertainty. Our results show that fluorite U-Pb geochronology has potential for dating distinct crystal growth stages.