Limited V and Fe isotopic fractionations in a lateritic profile formed by extremely weathered basalts

Vanadium is a multivalent element (+2, +3, +4, +5) with two stable isotopes (50V and 51V). Theoretical calculations predicted significant fractionation of V isotopes during redox reactions, which indicates the potential application of V isotopes to tracking the redox variation of geochemical processes [1]. To investigate the transportation mechanism of V and Fe during extreme weathering and pedogenesis process, we analyzed V and Fe isotopes of soil samples from a lateritic profile in Zhanjiang, South China. The δ51V values of laterites range from -0.77 ± 0.06‰ to -0.94 ± 0.05‰, indistinguishable from the parent basalt rock with a δ51V value of -0.79 ± 0.06‰. The δ56Fe values of lateritic soils and base rock basalts are within a narrow range (0.04 ± 0.03‰ to 0.14 ± 0.02‰). Contrast with the limited V and Fe isotopic fractionations, the whole profile exhibits significant V and Fe loss, indicating that losing V and Fe during strong oxidization weathering of basalts does not significantly fractionate V and Fe isotopes in laterites. The possible explanation is that the the primary minerals may be weathered layer by layer during laterite formation, and V and Fe of each layer could be completely oxidized, forming secondary minerals without significant fractionation of V and Fe isotopes. The good correlation between V and Fe contents further suggests that after weathering of the basalts, V species with pentavalence were probably completely adsorbed onto colloidal of iron (hydro)oxides, and transported by aqueous solution downward. This study provides new insights into the behaviour of V isotopes during extreme weathering processes, which is helpful to understand the geochemical cycle of V on the Earth's environment. [1] Wu et al., (2015) EPSL, 426, 216-224