Lithium, uranium, and noble gas Salar de Atacama

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.

锂（Lithium, Li）富集卤水广泛分布于干旱大陆盆地中，是支撑全球能源转型的关键战略资源，然而驱动其极端锂富集的地质过程至今仍存在争议。蒸发浓缩作用长期被认为是锂富集的主导机制，但长期地壳滞留过程所发挥的作用尚未得到定量约束。本研究整合了智利阿塔卡马盐沼（Salar de Atacama）补给水体、过渡含水层与中心卤水的锂浓度与锂同位素（δ⁷Li）、铀活度比（²³⁴U/²³⁸U）、稀有气体同位素体系（³He/⁴He、⁴He、²⁰Ne、⁴⁰Ar）以及氯氟烃示踪剂数据。演化卤水中的放射成因⁴He浓度超过2×10⁻⁶ 标准立方厘米每克（ccSTP g⁻¹），同时R/Ra值降至0.03以下，表明研究区以地壳氦累积为主，地幔输入可忽略不计。假设上地壳氦产生速率为1–3×10⁻¹² 标准立方厘米每克每年（ccSTP g⁻¹ yr⁻¹），由此计算得到的表观滞留时间约为0.5–2百万年（Myr）。沿盆地水流路径，铀活度比逐渐趋近于长期平衡，记录了持续的水-岩相互作用过程。朝向水文孤立的卤水区域，锂浓度提升超过两个数量级，但δ⁷Li的变化范围相对狭窄，表明锂富集经历了早期分馏作用后，进入了保守浓缩阶段。锂富集与地幔氦同位素特征解耦，以及²⁰Ne的保守行为，共同表明富锂卤水形成于结构开放但水文封闭的盆地中，经历了数百万年的地壳储集过程。蒸发作用实现了溶质浓缩，但长期累积的地壳相互作用才是控制锂富集程度的关键因素。本研究结果为大陆盐沼矿产资源的形成提供了定量的时间约束。