Mineralogy and precipitation controls on saprolite lithium isotopes during intensive weathering of basalt

Abstract The fractionation of lithium (Li) isotopes in saprolites under intense weathering conditions, particularly in relation to precipitation and mineral types, remains a crucial yet complex aspect of geochemical studies. This study reports Li isotope compositions from saprolites and unaltered bedrock within a basalt weathering profile on Hainan Island, China, a region known for its high rainfall and low dust levels. We observe that Li concentrations in weathering products (ranging from 2.36 to 10.65 μg g−1) are generally higher than in the unaltered bedrock (6.54 μg g−1). Correspondingly, the δ7Li values of these products range from −9.3‰ to +7.4‰, typically lower than those of unaltered bedrock (+4.3‰). Our findings suggest that the impact of rainwater Li on saprolite Li isotopes is not a straightforward mixing of rainwater and bedrock, but rather is modulated by adsorption and desorption processes involving secondary minerals. Notably, the δ7Li values in saprolites correlate with the proportion of specific secondary minerals; saprolite δ7Li values rise with increased kaolin minerals and drop with more Fe and Al oxide/hydroxide minerals. Integrating these results with previous studies on suspended sediment leaching, we propose that the higher dissolved δ7Li values in basaltic catchments, compared to granitic ones, are due to a greater abundance of Fe-Al (hydro)oxides in basaltic catchments. This mineralogical control on saprolite Li isotopes also implies a potential explanation for the observed variations in river δ7Li values under different precipitation regimes, influenced by diverse secondary minerals in riverine suspended loads.