鲜水河断裂带高温地热水的惰性气体定年与地热系统成因研究

Dating of geothermal water is useful not only for better understanding the formation of geothermal system, but also the evaluation of geothermal resources, because it indicates the residual time and renewal ability of the hot water underground. However, the age of geothermal water on Tibet Plateau is still unclear since dating of high-temperature geothermal water is one of the difficulties in geothermal research. This phenomenon could be attributed to the complicated origins of compositions which invalidate the conventional groundwater dating methods (3H, 14C, 36Cl, etc). Radio-krypton and argon dating could be the best choice when the detection of 81Kr, 85Kr and 39Ar can be improved by the Atom Trap Trace Analysis (ATTA) with high precision and efficiency. The dating of high-temperature geothermal water is designed to concentrate on the geothermal systems along the Xianshuihe fault zone. In this regard, a new sampling device suitable for geothermal gas extraction will be built after selection of the representative sampling points. After analyzing all the possible influence factors, the age of geothermal waters will be determined by establishing reasonable correction models upon comprehensive analysis of 81Kr, 85Kr and 39Ar together with the conventional dating results of 14C. Furthermore, the genesis of high-temperature geothermal systems along the Xianshuihe fault zone will be analyzed based on the geologic setting and the evolution of geothermal fluid. The potential findings will expand the application of advanced groundwater dating methods in geothermal studies and help to understand the genesis of high-temperature geothermal systems on the Tibetan Plateau.

地热水年龄指示热水在地壳中的平均驻留时间，是刻画地热水循环的重要参数，也是评价地热资源量的基本依据。但高温地热水定年是地热研究的难点之一，迄今青藏高原地热水年龄仍不清楚。其原因是高温地热流体的组分来源复杂，常用定年示踪剂（3H、14C、36Cl等）易被稀释或富集，导致测年结果失真且校正结果的准确性无从验证。放射性氪、氩同位素的来源相对单一，近年来原子阱痕量分析（ATTA）技术的发展实现了85Kr、39Ar、81Kr的高精度、高效率测试，可以为高温地热水定年提供新方法。本项目拟以鲜水河断裂带的地热系统为研究对象，针对地热井搭建满足ATTA测试要求的样品采集装置；综合开展85Kr、39Ar、81Kr、14C等多种同位素测年，分析定年结果的影响机制，建立校正模型，确定地热水年龄；结合地质背景及流体地球化学演化，分析鲜水河断裂带地热系统成因。项目成果为揭示青藏高原的地热成因提供科学依据。

本项目以鲜水河断裂带的地热系统为研究对象，针对个地热井搭建满足一起测试需求的地热流体样品采集装置，开展了温泉水、温泉气体样品采集，测试水化学、气体化学与同位素数据，基于3H、85Kr、39Ar、81Kr和14C等放射性同位素数据初步获得鲜水河断裂带地热水表观年龄；结合区域地质背景及流体地球化学特征，分析了鲜水河断裂带地热系统成因和流体循环机制。研究结果指示，多数川西温泉的地热气体以二氧化碳为主要组分，主要来源于壳内变质成因和放射性成因；温泉水接受大气降水补给，在鲜水河断裂带的康定地热田热储温度达到260℃，其成因是鲜水河断裂带作为深大走滑断裂延展深度大，为深部热量和热流体上涌提供运移通道，进而生成高温地热系统。放射性同位素定年显示鲜水河断裂带地热水年龄在4万年以上，但Kr、Ar等因空气、地质源等混入影响，测试结果有待进一步校正。本项目成果为揭示青藏高原地热成因提供新的依据。