鲜水河断裂带富硼地热水中稀土元素分布特征及其对硼来源的指示意义

Characteristics of rare earth elements in boron-rich thermal waters of the Xianshuihe Fault Belt Zone and its indication to the sources of boron

关键词:  富硼地热水  稀土元素  Eu和Ce异常  硼来源  鲜水河断裂带

Key words:  Boron-rich geothermal water  REEs  Eu and Ce anomaly  B sources  The Xianshuihe Fault Belt Zone

摘要:

高温地热系统地热水中普遍富含高浓度的硼，其来源的研究一直是地热地质学者关注的热点问题。尽管众多的学者对地热水中硼形成机理开展了广泛的研究，但富硼地热水中稀土元素的分布特征及其迁移规律能否指示热水中硼的来源尚不清楚。本研究以鲜水河断裂带富硼地热水为研究对象，通过野外调查取样、室内测试分析、水文地球化学模拟和综合研究等技术手段和方法，探究断裂带地热水中硼和稀土元素的分布特征和迁移规律。研究结果显示：鲜水河断裂带地热水中硼含量90%超过我国饮用水标准规定值0.5 mg/L（地热水中硼含量最大值为10.50 mg/L）；地热水中稀土元素含量为0.08～3.49 μg/L，且主要以LnCO₃⁺和Ln(CO₃)₂^?的络合物形态存在。地热水稀土元素PAAS归一化模式表现为重稀土元素相对于轻稀土元素富集（(Nd/Yb)_SN均值为0.41），且具有较显著的Eu（均值为0.34）和Ce（均值为0.07）正异常特征。地热水中硼和稀土元素的迁移均受到赋热含水层长英质和碳酸盐岩类矿物溶解过程的影响，且地热水中稀土元素的地球化学特征一定程度上可以指示地热水中硼的富集过程。研究成果拓展了稀土元素在富硼地热水研究中的应用，能为揭示富硼地热水成因研究提供依据。

Abstract:

Geothermal water of high-temperature geothermal system is generally rich in high concentration of boron (B), and the research on B sources has been a hot topic for geothermal geologists. Although many researchers have conducted extensive research on the formation mechanism of high B geothermal water, it is unclear whether characteristics and migration laws of rare earth elements (REEs) in B-rich geothermal water can illustrate B sources in geothermal water. Our study focused on the B-rich geothermal water in the Xianshuihe Fault Belt Zone (XSHFZ) to explore the distribution and migration rules of B and REEs by combining with field survey and sampling, laboratory testing, hydrogeochemical modeling, and comprehensive analysis. The results showed that the maximum value of B in geothermal water was 10.50 mg/L, and the content of B in 90% of geothermal water samples was higher than 0.5 mg/L (China standard values for drinking water). The ∑REE value was 0.08-3.49 μg/L, mainly existed in the complex form of LnCO₃⁺ and Ln(CO₃)₂^?. PAAS-normalized model and (Nd/Yb)_SN value of geothermal water in the XSHFZ showed that HREEs is enriched relative to LREEs, with significant positive Eu (an average value of δEu was 0.34) and Ce (an average value of δCe was 0.07) anomaly. Dissolving felsic mineral and carbonate rock controlled the migration of B and REEs in geothermal water. The geochemical characteristics of REEs in geothermal water can expound B's enrichment process in geothermal water to a certain extent. Our research results can expand the application of REEs in the study of B-rich geothermal water and provide a basis for illustrating the genesis study of B-rich geothermal water in similar areas.