Geochemical Characteristics Of Hot Spring Fluids In The Minjiang Fault Zone

Geothermal fluid geochemistry in active fault is a powerful tool for research coupling of mantle-derived and crust-derived fluid.The hydrology cycle model of hot springs may be established to explore the relationship between hydrogeochemical changes and earthquakes in a hot spring.It is very significant for the judgment of short impending fluid anomaly of the moderate-strong earthquake,using hydrogeochemical data.Hot springs are widely exposed in the vicinity of Minjiang fault zone,with well-developed tectonic activities and many earthquakes in history.Typically anomalies before earthquakes nearby the Epicenter appeared more early with a complicated shape,which can be divided into early term trend change,mid term vibrant change and sudden jump before earthquake in the mainshock.There are many hot springs in the Minjiang fault belt,which was less than 100 kilometers away from the epicenter of the 2017 Jiuzhaigou earthquake.Therefore,concentrations of dissolved species including the major,trace elements and gas compositions,stable isotopes in water and gas samples collected from thermal springs of Minjiang fault zone were used to characterise their hydrogeochemical feature and to reveal the relationships between fluid geochemistry and regional seismic activity.Four hot springs in Minjiang fault were seven times investigated systematically(from June 2010 to July 2018).The helium and carbon isotope(3He/4He,?13CCO2)and gas compositions of 16 gas samples and major and trace element composition and stable isotope values(?18O–?D)of 27 water samples were determined.The results show that:(1)it was mainly divided into four hydrochemical types: Ca-HCO3?Mg-HCO3?Ca·Mg-HCO3 and Mg·Ca-HCO3 of the thermal springs in the Minjiang fault zone.(2)?D and ?18O values(-13.2~-19.8 ‰ and-95.6~-113.33 ‰)of the geothermal waters indicated that the hot springs were a meteoric origin,where the recharge elevation range is 3436~4506 m.(3)Si O2 in the region is 2.49~5.92 mg/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 1.17~2.67 km.(4)The Na-K-Mg triangle diagram clears that the spring water was immature water.(5)The concentration ratio of Sr in the spring water was above 1,and strontium isotopes were ranged from 0.70862 to 0.70870.These data indicated strontium isotopes and Sr of spring water in the Minjiang fault zone were mainly came from carbonate minerals.(6)The concentration coefficient all less than 1 of the trace element expects B?Sr and Ba in the spring water of the Minjiang fault zone,which indicated the contents of trace element were low,with less water-rock interaction.(7)The 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 0.68 Ra(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.The fluid geochemical data were obtained of the second day after the Jiuzhaigou MS7.0 earthquake.There were an obvious increased before the Jiuzhaigou earthquake of Cl-and SO42-in the hot springs of kakagou and chuanpanqiao,the SO42-in Mounigou,and to be a decline of Cl-and SO42-in the hot springs of kakagou,chuanpanqiao and Mounigou after the earthquake.Hydrogeochemical model was proposed on the basis of geological,structural,hydrochemical and isotopic data.This model indicated the geothermal water recharge of the Minjiang Fault Zone was meteoric origin,and recharge elevation ranged from 3.44~4.50 km.The Minjiang fault which can be used as a fast migration channel for geothermal water.In this system,geothermal waters are heated by a crustal interior and circulation to 1.17~2.67 km undergrounds,followed by the water rising to the surface along faults and fractures that act as hydrothermal conduits recharge to hot springs.3He/4He ratio are exceptionally low can well be explained by this model.The downward convergence of the Minjiang fault zone to the detachment surface of 30 km hinders the upwelling of mantle derived He.Therefore,it is very significant for the judgment of precursory anomalies of the future medium-strong earthquake of Minjiang fault zone.