The key controls of thallium isotopic fractionation in soil

The purpose of this study was to investigate the key geochemical and mineralogical factors that could affect the fractionation of stable thallium (Tl) isotopes in soil. A set of grassland soil samples enriched in geogenic Tl in combination with selected Tl-containing mineral materials from the Czech Republic (Kluky) were investigated for this purpose. A combination of X-ray diffraction analysis (XRD), chemical extractions and stable isotope analysis were used to understand the behaviour of Tl and its isotope systematics within the soil profile. The results demonstrate significant incorporation of Tl in pedogenic Mn-oxide, which led to a large accumulation of the heavy ²⁰⁵Tl isotope (~+14 ε²⁰⁵Tl units), presumably resulting from continuous redox reactions with Mn-oxides and systematic accumulation of heavy isotope fraction onto the oxide surface(s). Consequently, we concluded that the Mn-oxide-controlled Tl uptake is the primary cause of the observed ²⁰⁵Tl enrichment in the middle profile zone, at the A/B soil horizon interface, with up to +4 of ε²⁰⁵Tl. Furthermore, our results displayed a clear relationship between the Tl isotopic fractionation degree and the Mn-oxide soil concentration (R² = 0.6), as derived from the oxalate-extractable data. A combination of soil and mineralogical considerations suggests that ²⁰⁵Tl enrichment in the soil samples is also partly due to the Tl present in micaceous clay minerals, mainly illite, which is the predominant pedogenic Tl host phase. Supported by our previous results, this Tl behaviour can be inferred from systematic Mn-oxide degradation and the associated Tl (enriched in ²⁰⁵Tl) cycling in the studied soils and therefore, presumably in the redoximorphic soils in general.