岷江断裂带北段温泉流体地球化学特征

Geochemical Characteristics of Hot Spring Fluids along the Minjiang Fault Zone

摘要:

温泉流体地球化学方法是研究活动断裂带深浅部流体耦合变化的有利手段。通过对2011年6月至2018年7月岷江断裂带内的7个采样点进行了7次系统的调查，测定了16个气体样品中的³He/⁴He和δ¹³C_CO2以及27个水样中的常量元素、微量元素和稳定同位素（δD、δ¹⁸O），得出以下结论：①岷江断裂带温泉水化学类型主要分为Ca-HCO₃、Mg-HCO₃、Ca·Mg-HCO₃、Mg·Ca-HCO₃四种；②δD、δ¹⁸O的测量结果表明岷江断裂带温泉水主要为大气降水的补给，补给高程为3.4～4.5km；③温泉水中SiO₂含量为2.49～5.92mg/L，热储温度为26.00～52.22℃，循环深度为1.17～2.67km；④Na-K-Mg三角图表明岷江断裂带温泉水均为未成熟水；⑤岷江断裂带温泉水中除B、Sr、Ba外，微量元素的富集因子均小于1，说明微量元素含量较低，主要来自于岷江断裂带的灰岩；⑥幔源和壳源之间的混合作用为控制He-C系统和He-Sr系统的主要因素，且研究结果表明³He/⁴He变化范围为0.02 Ra～0.68Ra（Ra为大气中³He/⁴He的比值，为1.39×10^-6），温泉水逸出气体中幔源He贡献率变化范围为0.07%～7.8%，表明温泉气体中的He主要来自壳源，岷江断裂带内温泉水逸出气中的CO₂主要来自地壳中的灰岩（75.00%～99.47%）。2017年发生了九寨沟M_S7.0地震，研究发现地震前后温泉水地球化学特征有明显变化，但幔源He较低，表明无明显幔源He增加。因此，根据岷江断裂带温泉流体地球化学数据以及相关研究资料，建立了岷江断裂带深浅部流体耦合模型，对于今后判定岷江断裂带未来中强地震的短临前兆流体异常具有一定的参考意义。

Abstract:

Geothermal fluid geochemistry along active fault is a powerful tool for studying coupling of mantle-derived and crust-derived fluid. Four hot springs along Minjiang fault were investigated systematically for seven times(from June 2010 to July 2018). The helium and carbon isotope(³He/⁴He, δ¹³C_CO2) and gas compositions of 16 gas samples and major and trace element composition and stable isotope values(δ¹⁸O、δD) of 27 water samples were measured. The results show that:①there are mainly four hydro-chemical types:Ca-HCO₃、Mg-HCO₃、Ca·Mg-HCO₃ and Mg·Ca-HCO₃ of the thermal springs in the Minjiang fault zone. ②δD and δ¹⁸O values(-13.2‰~-19.8‰ and-95.6‰~-113.33‰) of the geothermal waters indicated that the hot springs were with a meteoric origin, where the recharge elevation range is at 3436~4506m. ③SiO₂ in the region is 2.49~5.92mg/L, and thermal storage temperature estimated by aqueous geothermometry was 26.00℃~52.22℃. This data, along with estimated temperature, gave an estimated circulation depth for the spring waters of 1.17~2.67km. ④the Na-K-Mg triangle diagram indicates that the spring water was a type of immature water. ⑤almost all trace element concentration coefficients are less than 1 except for B、Sr and Ba in the spring water of the Minjiang fault zone, which suggested that the contents of trace element were low, and with a limestone origin. ⑥the He-C and He-Sr systematics were mostly controlled by the mixture between mantle and various crustal sources. The observed ³He/⁴He ratios varied from 0.02 to 0.68Ra(Ra is atmospheric ³He/⁴He ratio=1.39×10^-6) indicated that hot springs discharge gas with a low contribution of mantle-derived helium. The limestone was the primary source of carbon(75%~99.47% of the total carbon inventory) from the hot springs in the Minjiang fault zone. It was found that the geochemical characteristic of hot spring fluids before and after the earthquake has changed obviously. But the observed ³He/⁴He ratio indicates that hot springs discharge gas with a low contribution of mantle helium. Based on above observation, a fluid geochemistry model was proposed for the geochemical system combined with the geology and hydrogeology of the study area, it is very significant for the determination of precursory anomalies of the future medium-strong earthquake along Minjiang fault zone.