Non-conservative behaviour of lithium isotopes during estuarine transport: Impact on seawater δ7Li composition

This study presents uranium (U) and lithium (Li) isotope and major and trace element data for the dissolved load, suspended particles and bedload from rivers draining dominantly basaltic catchments on the island of Sao Miguel in the Azores Archipelago. Uranium activity ratios are at secular equilibrium in the bedload, but are higher for suspended material, suggesting sorption of uranium from the solution onto particle surfaces. The ( 234U/ 238U) of the riverine dissolved phase varies between 1.02 and 1.86, and lies on a mixing trend between the values expected from chemical weathering of basalt (high [U]; ( 234U/ 238U) at secular equilibrium) and an endmember with low [U] and high ( 234U/ 238U), which probably reflects α-recoil effects associated with physical weathering and/or soil formation. Lithium isotope ratios are consistently lighter in suspended material than the corresponding bedload, suggesting preferential retention of 6Li in the suspended load. In turn, the δ 7Li of the dissolved load is always isotopically heavier, ranging from 5.9 to 36.2‰, although the lightest values are affected by hydrothermal input. The lithium isotopic composition of the dissolved load reflects the balance of primary mineral dissolution to secondary mineral formation. Comparison of the behaviour of U and Li isotopes between Sao Miguel, Iceland, and other basaltic terrains suggests that whilst U isotopes are dominated by weathering regimes dependent on factors such as weathering rates, temperature, runoff and climate, neither weathering intensity nor climate exerts a direct influence on Li isotope behaviour; rather the formation of secondary minerals, which is indirectly controlled by climate, weathering, primary mineralogy and biology, appears to play the dominant role.

Lithium and its isotopes behavior during incipient weathering of granite in the eastern Tibetan Plateau, China

Introduction Lithium (Li) isotopes are powerful tracers of silicate weathering processes. However, the geochemical behavior of lithium isotopes in granite leachates remains unclear. Methods Here we report Li isotope compositions of leachates from a granite weathering profile in southeastern China. The parameter tau (τ Si and τ Al ) and the Chemical Index of Alteration (CIA) were also used to characterize weathering intensity and related geochemical processes. Results The Li concentration in the leachate varied from 0.05 to 1.03 mg/kg, and the δ 7 Li leachate values were −15.0‰ to +6.0‰ (mean = −0.85‰, n = 28). Below 0.8 m depth, the leachate had similar Li isotopic composition (−0.80‰ to +6.0‰, mean = +2.5‰, n = 19) with parent granite (+3.7‰). Discussion The leachate δ 7 Li values exhibit distinct vertical variations, reflecting contrasting geochemical processes along the profile. Below 0.8 m, δ 7 Li values are comparable to those of the parent granite, indicating limited isotopic fractionation during early weathering process. In contrast, markedly lower δ 7 Li values above 0.8 m suggest the release of 6 Li from the dissolution of secondary minerals. This interpretation is supported by increased τ Si and τ Al values and their negative correlations with δ 7 Li leachate , implying co-migration of 6 Li, Si, and Al during mineral dissolution. A positive correlation between δ 7 Li and CIA in the upper profile further indicates enhanced secondary mineral dissolution under intensified weathering. Our results suggest that as weathering progressed, the Li isotopic composition of the leachate from the upper weathering profile became gradually heavier toward the top, positively correlating with weathering intensity and indicating the dissolution of surface secondary minerals under intense weathering conditions.

Equilibrium lithium isotope fractionation in Li-bearing minerals

Extreme lithium isotopic fractionation during continental weathering revealed in saprolites from South Carolina

Size-fractionated thorium isotopes (228Th, 230Th, 232Th) in surface waters in the Jiulong River estuary, China

Thorium isotopes in the Southeast Atlantic Ocean: Tracking scavenging during water mass mixing along neutral density surfaces

Geochemical and cosmochemical studies on lithium are reviewed and the potential of lithium isotope systematics as a geological tracer is discussed in this article. Lithium, the smallest of the alkaline elements, possesses two stable isotopes, 6Li and 7Li. It also displays the following unique physicochemical characteristics: (1) an extremely high cross-section of the lithium isotope 6Li for thermal neutrons; (2) high incompatibility in the mafic silicate minerals in the magmatic processes; (3) high solubility in fluid phases; and (4) large isotopic variations in the 6Li/7Li ratio of natural samples exceeding 30‰. The content and isotopic composition of lithium in meteoritic and terrestrial materials may provide important clues to understanding nucleosynthesis theories and many geological phenomena including water/rock interactions, metamorphic and magmatic processes. However, the isotopic behavior of lithium in geologic processes is still not well understood, since only a few reliable isotopic measurements of lithium are available for natural samples due essentially to analytical difficulties; derived from the extremely large mass difference between the two isotopes and no internally-derived correction for the mass fractionation in mass spectrometry. In order to apply lithium isotope systematics for understanding geochemical and water-related processes in the Earth and other planets, the development of a high precision technique for lithium isotope analysis is urgently required so as to accumulate the much needed fundamental data set of natural samples.