Age and rate of weathering determined using uranium-series isotopes: Testing various approaches

Field biotite weathering rate determination using U-series disequilibria

“…Scanning Electron Microscopy also reveals some traces of accessory minerals such as Ti oxides (< 2 %) and chlorite (< 1 %) (Voinot, 2012). In view of the low abundance of these accessory minerals or of the low U concentrations generally observed in quartz (0.1 -1 ppm, Bosia et al, 2018;Dosseto et al, 2019), these impurities are not expected to significantly impact U-series activity ratios measured in the separated grains, wherein measured U concentrations range from 31 to 65 ppm. With a view to consider the amount of weathered biotite present within the analyzed grains, the proportion of remaining biotite and the weathering products were therefore recalculated from the results of Voinot (2012) without quartz contribution, i.e.…”

“…Based on all these features, and despite the potential presence of slight initial heterogeneities, the observed U and Th contents variability seems therefore most likely attributable to a preferential loss of Th relative to U Several previous studies have identified Th mobility in soil, usually linked to organic matter complexation (Dosseto et al, 2008;Rezzoug et al, 2009;Rihs et al, 2011;Keech et al, 2013;Gontier et al, 2015, Dosseto et al, 2019. In the present study, however, a similar Th mobility is observed from samples positioned at the bottom of the soil profile as well as at the surface.…”

“…Additional in-situ SIMS analyses of the Gandak River samples demonstrate that the Th depletion is not restricted to the grain surface, but instead continues 15-20 µm below the mineral surface while the ( 234 U/ 238 U) ratios remain equal to 1 (Bosia, 2016). Menozzi (2016) and Dosseto et al (2019) also reported a significant leaching of 230 Th from quartz sampled in both granitic and sandstone saprolites at a depth of 2.5 m and in apparently pristine bedrocks (New South Wales, Australia). In contrast, a gain of 232 Th (as well as U) was observed by these authors in the biotite separated grains from the granitic profile, highlighting, if needed, the influence of external-to-the-mineral factors on nuclides behavior in weathering profiles.…”

“…Dosseto et al (2014) used the U-series derived weathering ages of bulk sediments to indirectly determine catchment-wide mineral-specific weathering rates. Even more recently, Dosseto et al (2019) investigated the use of separated biotite and quartz to infer the mineral dissolution rates as well as the regolith production rate. Except this recent study, the potential for U-series to determine the longterm weathering rate of a specific mineral remains largely unexplored.…”

Determination of regolith production rates from 238U-234U-230Th disequilibrium in deep weathering profiles (Longnan, SE China)

“…The latter approach has been applied to various alteration contexts: centimetric weathering rinds developed on basaltic clasts (Engel et al, 2016;Ma et al, 2012;Pelt et al, 2008), metric size spheric alteration systems (Chabaux et al, 2013;Claude et al, 2016), decimetric to metric alteration profiles (Ackerer et al, 2016;Dosseto et al, 2008a;Ma et al, 2013;Schoonejans et al, 2016;Rihs et al, 2020). These works have led to the definition of a theoretical and conceptual frame for the application of the radioactive disequilibrium methodology for alteration profiles, which comprises (a) the analysis of the variations of U-series nuclides along the main weathering direction and (b) the interpretation of these variations using a realistic model of radionuclide mobility during water-rock interactions, commonly, a model assuming both gain and loss of elements along the profile (e.g., Chabaux et al, 2008Chabaux et al, , 2011Chabaux et al, , 2013Dosseto et al, 2008bDosseto et al, , 2012Dosseto et al, , 2019Ma et al, 2013). A key assumption behind this approach is to be able to define, within the studied profile or the alteration system, a sufficiently extended zone for which the mobility parameters may be assumed constant.…”