ABSTRACT:

Lithium (Li) rich brines in arid continental basins are critical resources for the global energy transition, yet the processes governing extreme Li enrichment remain debated. Evaporative concentration is widely invoked as the dominant mechanism, but the role of long-term crustal residence has not been quantitatively constrained. Here, we integrate Li concentrations and isotopes (δ⁷Li), uranium activity ratios (²³⁴U/²³⁸U), noble gas systematics (³He/⁴He, ⁴He, ²⁰Ne, ⁴⁰Ar), and chlorofluorocarbon tracers from recharge waters, transitional aquifers, and central brines of the Salar de Atacama, Chile. Radiogenic ⁴He concentrations exceed 2 × 10⁻⁶ ccSTP g⁻¹ in evolved brines, while R/Ra values decrease to <0.03, indicating dominant crustal helium accumulation and negligible mantle input. Assuming upper crust production rates of 1–3 × 10⁻¹² ccSTP g⁻¹ yr⁻¹, these concentrations imply apparent residence times of ~0.5–2 Myr. Uranium activity ratios progressively approach secular equilibrium along basin flow paths, recording sustained water–rock interaction. Li concentrations increase by more than two orders of magnitude toward hydrologically isolated brine domains, yet δ⁷Li remains comparatively restricted, indicating early-stage fractionation followed by conservative concentration. The decoupling of Li enrichment from mantle helium signatures and the conservative behavior of ²⁰Ne demonstrate that Li rich brines form through multimillion-year crustal storage within a structurally open but hydrologically closed basin. Evaporation concentrates solutes, but time integrated crustal interaction governs the magnitude of enrichment. These results provide quantitative temporal constraints on mineral resource formation in continental salars.