Rare earth elements (REE) serve as effective tracers of water-rock interactions, a crucial process in the evolution of geothermal systems. However, the geochemical behavior of REE during their migration within geothermal systems, particularly the fractionation mechanism, remains poorly understood. In this study, we focus on the geochemical characteristics of REE in hot springs, host rocks, and sinters from various hot spring systems in the Yadong-Gulu rift (YGR), the largest extensional rift in the southern Tibetan Plateau. The objective is to explore the factors that control REE concentrations in hot springs and elucidate the fractionation of REE during their migration. The concentrations of REE are significantly influenced by pH, Fe oxides (oxyhydroxides), and HCO3− concentrations in central and southern hot springs, whereas high reservoir temperatures contribute to higher ΣREE concentrations in northern hyperthermal springs. REE speciation calculations show that LnO2− and LnO2H are dominant complexes in northern alkaline hot springs, while carbonate complexes (LnCO3+ and Ln(CO3)2−) prevail in central and southern spring samples. Additionally, weakly acidic hot springs exhibit a prevalence of Ln3+, LnF2+, and LnSO4+ species. The chondrite-normalized REE patterns of all hot spring samples display enrichment in MREE, which is significantly distinct from the patterns observed in host rocks and sinters enriched in LREE. The fractionation of REE in the YGR hot spring systems is primarily controlled by the following processes: (1) Temperature, pH, and relative abundance of complexing agents influence the complexation of REE and further regulate their fractionation in hot springs; (2) During water-rock interaction, preferential leaching of HREE and MREE from host rocks can lead to their enrichment in alkaline hot springs. The dissolution of iron-rich sediments plays a significant role in generating the distinctive MREE bulge pattern in acid-neutral hot springs compared to host rocks; (3) The higher stability of MREE and HREE complexes in water, as well as the preferential coprecipitation of unstable LnCO3+ complexes formed by LREE with carbonates, are key factors responsible for LREE enrichment in sinters.

中文翻译：

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青藏高原南部地热系统稀土元素迁移及控制因素


稀土元素 (REE) 是水-岩相互作用的有效示踪剂，而水-岩相互作用是地热系统演化的关键过程。然而，稀土元素在地热系统内迁移过程中的地球化学行为，特别是分馏机制，仍然知之甚少。在这项研究中，我们重点研究了青藏高原南部最大的伸展裂谷亚东-古鲁裂谷（YGR）的温泉、围岩和不同温泉系统的烧结矿中的稀土元素地球化学特征。目的是探索控制温泉中稀土元素浓度的因素，并阐明稀土元素在迁移过程中的分馏。中部和南部温泉的 REE 浓度受到 pH 值、Fe 氧化物（羟基氧化物）和 HCO3− 浓度的显着影响，而高储层温度导致北部热泉的 ΣREE 浓度较高。稀土元素形态计算表明，LnO2−和LnO2H是北部碱性温泉中的主要复合物，而碳酸盐复合物（LnCO3+和Ln(CO3)2−）在中部和南部温泉样品中占主导地位。此外，弱酸性温泉中普遍存在 Ln3+、LnF2+ 和 LnSO4+ 物种。所有温泉样品的球粒陨石归一化稀土元素模式都显示出 MREE 富集，这与在主岩和烧结矿中观察到的富含轻稀土元素的模式明显不同。 YGR温泉系统中稀土元素的分馏主要受以下过程控制：（1）温度、pH值和络合剂的相对丰度影响稀土元素的络合，并进一步调节其在温泉中的分馏； (2)在水-岩相互作用过程中，HREE和MREE从围岩中优先淋滤，导致它们在碱性温泉中富集。与主岩相比，富铁沉积物的溶解在酸中性温泉中产生独特的 MREE 隆起模式中发挥着重要作用； (3) MREE和HREE络合物在水中的较高稳定性以及LREE与碳酸盐形成的不稳定LnCO3+络合物的优先共沉淀是导致LREE在烧结矿中富集的关键因素。