05 Apr 2022
05 Apr 2022
Status: this preprint is currently under review for the journal GChron.

Supergene phases from ferruginous duricrusts: non-destructive microsampling and mineralogy prior to (U-Th)/He geochronological analysis

Karina Patricia Prazeres Marques1,2, Thierry Allard2, Cécile Gautheron3, Benoît Baptiste2, Rosella Pinna-Jamme3, Guillaume Morin2, Ludovic Delbes2, and Pablo Vidal-Torrado1 Karina Patricia Prazeres Marques et al.
  • 1Luiz de Queiroz” College of Agriculture, University of São Paulo, 13418-900, Piracicaba, São Paulo, Brazil
  • 2Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR CNRS 7590, Sorbonne Université, F-75252, Paris Cedex 05, France
  • 3Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay, France

Abstract. Interpreting the ages of supergene mineralogical phases in laterite is complex because they consist of polycrystalline mixtures of different phases at the microscopic scale that could be crystalized at different epochs. Among the geochronometers, the (U-Th)/He method on hematite and goethite is more used, but ages can be difficult to interpret due to phases mixing. To resolve this issue, this study proposes a methodology for performing detailed mineralogical analysis of hematite and goethite single grains prior to their dating using the (U-Th)/He method. Strictly non-destructive mineralogy of single grains is not achievable by classical tools, such as conventional powder XRD (requiring at least some mg of powder) or SEM (that can contaminate the grain by coating or fixing). Therefore, we have performed X-ray diffraction patterns of single grains using high-flux X-ray beams from both a rotating anode (XRD_rotat) laboratory diffractometer and a synchrotron beamline (XRD_synch) facility, and compared the results in order to design a method based on XRD_rotat only. For this purpose, two samples from the pisolitic facies of a Brazilian ferruginous duricrust (Alto Paranaíba region, Minas Gerais State, Brazil) were chosen because they presented a usual heterogeneity. Rietveld refinements of the XRD patterns obtained from both XRD_rotat and XRD_synch yielded similar results for weight percentage ratio of the main phases, Mean Coherent Domain sizes, and less for Al-substitution rates, thus validating the XRD_rotat approach. No beam-damage was observed when increasing X-ray exposure time, neither on XRD patterns nor (U-Th)/He ages. Hence, inframillimetric, undisturbed grains can be used to analyze the mineralogy of ferruginous duricrusts by XRD_rotat with a short exposure, and the same grains can subsequently be dated by the (U-Th)/He geochronology. The (U-Th)/He dating of pisolitic core and cortex grains also provided meaningful ages: they revealed two evolution phases of the ferruginous duricrust, which occurred at or before the Oligocene for pisolitic core and middle Miocene for pisolitic cortex, agreeing with the previous model for the development of pisoliths. The mineralogy of single grains selected for dating is helpful for discussing the crystallization ages, and the high-flux XRD approach may be applied to other supergene mineral parageneses used for absolute dating of weathering profiles.

Karina Patricia Prazeres Marques et al.

Status: open (until 29 May 2022)

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Karina Patricia Prazeres Marques et al.

Karina Patricia Prazeres Marques et al.


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Short summary
We proposed a new non-destructive mineralogical methodology on inframilimetric grains that allows to quantify the hematite and goethite content and hematite/goethite ratio of grains prior to (U-Th)/He geochronological analysis. (U-Th)/He data performed on different aliquots with different acquisition time shows no remarkable differences in age, opening a new way to investigate the (U-Th)/He data evolution in supergene lateritic duricrusts.