喜马拉雅山南地区地热水和钙华地球化学特征与成因机制

Geochemical characteristics and genetic mechanism of geothermal water and travertine in the southern Himalayas

关键词:  水文地球化学  温泉钙华  热储温度  氢氧同位素  碳同位素

Key words:  hydrogeochemistry  hot spring travertine  reservoir temperature  hydrogen and oxygen isotope  carbon isotope

摘要:

喜马拉雅山南地区拥有丰富的地热资源。开展地热水和钙华成因机制的研究，有助于了解地热资源特征和古气候变迁信息，对丰富山南地热资源的系统性研究和青藏高原气候环境变化研究均具有重要理论意义。本文以喜马拉雅东段山南地区的邛多江、古堆和曲卓木的三个温泉为研究对象，通过采集温泉地热水和钙华数据，综合分析了地热水水-岩作用特征、热储温度估算、补给来源追溯和温泉钙华的成因类型、形成年代、古气候意义等。结果表明：邛多江温泉的水化学类型为HCO₃·Cl-Na·Ca型；古堆日若沸泉的水化学类型为HCO₃·SO₄-Ca·Na型；古堆茶卡沸泉的水化学类型为Cl-Na型；曲卓木热泉的水化学类型为Cl·SO₄-Na·Ca型。温泉地热水中的阴阳离子来源主要是硅酸盐岩的溶解以及部分碳酸盐岩和盐岩的溶解。由于温泉地热水均未达到水-岩平衡状态，利用石英地热温标得出浅部热储温度为129～148℃，利用硅-焓图解得出深部热储温度和冷水混合比例为181～221℃和58%～65%；氢氧同位素显示地热水补给高程为4467～5303 m。在山南地区，地热水受到高海拔大气降水和冰雪融水的补给，通过主要断裂构造运移到深部加热并在高温高压下沿着裂隙、节理上升，然后与浅层冷水混合，最后沿浅部地表松散破碎带出露形成温泉。温泉钙华CaO占比43.43%～56.66%，且显示出轻稀土元素富集的特点；δ¹³C指示温泉钙华为热成因，钙华中的碳主要来自于深部碳酸盐岩的变质成因，仅古堆日若沸泉有部分地幔碳；¹⁴C测年显示钙华的年龄为21280±70～43500年，Mg/Ca、Mg/Sr比值指示降雨量在43.50 ka年由峰值迅速减弱，并在42 ka～21.28 ka年期间降雨量又逐渐增强。

Abstract:

The southern Himalayas is rich in geothermal resources. The study of the formation mechanism of geothermal water and travertine is helpful to understand the characteristics of geothermal resources and the information of paleoclimate change. It is of great theoretical significance to enrich the systematic study of geothermal resources in Shannan and the study of climatic and environmental changes in Qinghai-Tibet Plateau. In this study, three hot springs in Qiongduojiang, Gudui and Quzhuomu in Shannan area of eastern Himalayas are taken as the research objects. By collecting the data of hot spring geothermal water and travertine, the characteristics of geothermal water-rock interaction, the estimation of geothermal reservoir temperature of geothermal water, the traceability of geothermal water supply source, the genetic type and formation age of hot spring travertine and the climatic significance of travertine are comprehensively analyzed. The results show that the hydrochemical type of Qiongduojiang hot spring is HCO₃·Cl-Na·Ca type; The hydrochemical type of Guduiriruo boiling spring is HCO₃·SO₄-Ca·Na type. The hydrochemical type of Guduichaka boiling spring is Cl-Na type; The hydrochemical type of Quzhuomu hot spring is Cl·SO₄-Na·Ca. The source of anions and cations in hot spring geothermal water is mainly the dissolution of silicate rocks and the dissolution of some carbonate rocks and salt rocks. Since the hot spring geothermal water has not reached the water-rock equilibrium state, the shallow thermal reservoir temperature is 129～148℃ by using the quartz geothermal temperature scale, and the deep thermal reservoir temperature and cold water mixing ratio are 181～221℃ and 58%～65% by using the silicon-enthalpy diagram. Hydrogen and oxygen isotopes show that the geothermal water supply elevation is 4467～5303 m. In Shannan area, geothermal water is recharged by high-altitude atmospheric precipitation and ice and snow melt water. It migrates to the deep through the main fault structure to heat and rises along the fissures and joints under high temperature and high pressure. Then it is mixed with shallow cold water, and finally exposed along the shallow surface loose fracture zone to form a hot spring. Hot spring travertine CaO accounts for 43.43%～56.66%, and shows the characteristics of light rare earth element enrichment; the δ¹³C indicates that the hot spring travertine is thermogenic, and the carbon in travertine is mainly derived from the metamorphic origin of deep carbonate rocks, and only part of mantle carbon is found in Guduiriruo boiling spring; ¹⁴C dating shows that the age of travertine is 21280±70~>43500 years, the ratios of Mg/Ca and Mg/Sr indicate that the rainfall decreased rapidly from the peak in 43.50 ka, and gradually increased from 42 ka to 21.28 ka.