Effect of Fe–Ti oxides on Mo isotopic variations in lateritic weathering profiles of basalt

Abstract

This study aimed to improve our understanding of processes fractionating Mo isotopic compositions during extreme weathering of basalt and to identify a long-term surface reservoir for light δ98Mo signatures, which are key to constraining the isotopic composition of riverine inputs to the oceans. A lateritic weathering profile of basalt from Hainan Island, South China, was studied by determining bulk δ98Mo values as well as δ98Mo values of extracted Fe–Mn (hydro) oxides and residual phases, and parent-rock minerals. δ98Mo values relative to NIST SRM 3134 = +0.25‰ in the lateritic weathering profile were in the ranges of −0.28‰ to +0.48‰, with the δ98Mo values of the upper and middle sections being heavier, and those of the lower section lighter than that of the parent basalt. Rainwater input and changing redox conditions are likely to be the essential factors controlling variations in bulk δ98Mo values in the upper and middle sections, respectively, resulting in their heavier δ98Mo values. In contrast, the lighter values in lower section are likely due to the leaching of primary minerals (e.g., pyroxene and plagioclase) with some heavier δ98Mo being released to aqueous media. The Fe–Mn (hydro) oxides phase in the sequential extraction procedure is thus characterized by heavy δ98Mo values in the lateritic weathering profile, which may be responsible for heavy Mo isotopic composition in bulk samples. Light δ98Mo signatures are preserved in the residual phase, where Mo hosted in Fe–Ti oxides is inherited mainly from parent basalt rock, possibly accounting for light Mo isotopic composition in bulk samples. Owing to their high resistance to weathering and chemical erosion, Fe–Ti oxides might be a potential long-term surface reservoir for light δ98Mo signatures. The difference in degrees of Mo isotopic fractionation between basalt and granite lateritic weathering profiles may reflect variations in Mo behavior in parent-rock host minerals during weathering. The results elucidate mechanisms of Mo isotopic fractionation during chemical weathering and improve understanding of the riverine input of Mo isotopes to oceans.