Behavior of uranium and thorium isotopes in soils of the Boréon area, Mercantour Massif (S.E. France): Leaching and weathering rate modeling

Abstract

Four cores were collected in weathered rocks and soils in the Boréon forest area (1765 m, Mercantour Massif, France). The samples were analyzed for the isotopes ²³⁰Th, ²³²Th, ²³⁴U and ²³⁸U. The activity and isotopic ratio profiles suggest that uranium was mobilized (leaching and precipitation) during the weathering process, as well as thorium but in a much less proportion. A model was drawn up to evaluate the U leaching rate and the time that some levels of the weathered rocks have been subjected to weathering. It utilizes LATHAM and SCHWARCZ’s two equations,¹⁵ expressed as ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios, which assume that the alpha recoil effect allows easier leaching for ²³⁴U than ²³⁸U and no Th mobility. But this last assumption does not correspond to the observations made in the Boréon area, since it appears that in some soil deeper layers ²³⁰Th and ²²⁸Th are in radioactive deficit relatively to their parents. As there are four unknown quantities (the time, the leaching rates of ²³⁸U, ²³⁴U, ²³⁰Th), the problem to be solved requires two more equations; these can be obtained utilizing the U activity ratio in water, and taking into account the ²³²Th behavior. In some sites the ²³⁸U leaching rate is high in deeper soil levels (near the fresh rocks); this would correspond to a loss of half the U amount in less than 24 000 years.