Gallium isotope fractionation during granite weathering: Insights from two profiles in contrasting climatic conditions

The recently developed gallium (Ga) isotope systematics provides valuable insights into continental weathering, particularly under conditions of intense chemical weathering. However, the relationship between Ga isotope fractionation and the factors influencing the intensity of silicate weathering has yet to be systematically explored. In this study, we report Ga isotope compositions from two granite weathering profiles developed under contrasting climates: a temperate semiarid climate in the Beijing profile and a tropical moist monsoon climate in the Guangdong profile. Our study shows that the majority of Ga in these weathering profiles is retained within the crystal-lattice of Al-bearing minerals, followed by Fe-bearing minerals, with minimal surface adsorption. Mössbauer spectra and sequential extraction results show that the influence of Fe on Ga concentrations and isotope compositions becomes more significant as weathering intensity increases, driven by the transformation of dissolved ferrous Fe into ferric Fe at advanced weathering stages. Both granite weathering profiles exhibit a similar trend of slightly decreasing δ71Ga values from bedrock to surface (Beijing profile: from 0.71 ‰ to 0.61 ‰; Guangdong profile: from 0.70 ‰ to 0.54 ‰). This suggests that the lighter Ga isotope, 69Ga, is preferentially enriched in regolith relative to the coexisting aqueous solution in the pore spaces of weathered rocks, consistent with observations from a basalt weathering profile. In the Beijing profile, a strong correlation between τGa,Th (a weathering index) and δ71Ga values suggests that Ga isotope compositions are mainly controlled by the dissolution of primary minerals. This correlation indicates that variations in δ71Ga values can serve as effective tracers of weathering intensity during early stages of chemical weathering. However, this pattern is absent in the Guangdong profile, where Ga isotope compositions appear to be influenced not only by mineral dissolution but also by processes such as mineral adsorption and coprecipitation. A broader comparison of our results with previously published Ga isotope composition suggests the lithology of bedrock provides the major control on the extent of Ga isotope fractionation. Other factors, including the composition of secondary minerals and the climatic conditions, also play significant roles in influencing Ga mobility and Ga isotope fractionation during chemical weathering.