Isotope Geochemistry Of Rare Gases In Groundwater And Their Volcanic Geothermal Gensis

Noble gas isotopes have been used to detect mantle signal in deep aquifers in hydrology.In this thesis,the dissolved noble gases(He,Ne,Ar,Kr,Xe)in groundwater are used to identify the heat sources of geothermal system,characterize the thermal superposition state of mantle sources,and evaluate the mixing and migration of fluids from different sources.Guangdong Province is located in the southeast edge of the Eurasian plate and has been affected by the subduction of the Indian Ocean plate,the Pacific plate and the Philippine Sea plate for a long time.Deep and large fault zones have developed in the territory since Mesozoic and Cenozoic period,accompanied by crustal uplifting and multiple periods of intense magmatic activities,thus creating very favorable conditions for the occurrence of geothermal resources.Huizhou and Shantou are in the areas where mantle upwellings occur in Guangdong,which make Guangdong an ideal place for studying the relationship among fluid geochemistry,deep structure and heat source in hydrothermal cycling.Wudalianchi is a well-known Quaternary volcanic group in China with modern volcanic eruptions.However,there is no strong geothermal indication,like hot springs or high geothermal gradient,be obeserved near the surface of Wudalianchi.It is a modern volcanic group accompanied by cold mineral springs,which makes it rare in China and even in the world.Geophysical data show that there is still suspected magma reservoir at the depth of 10-15 km in Wudalianchi area.The study of the noble gas in Wudalianchi Weishan in this thesis explain the existence of magma reservoir in Wudalianchi from a geochemical perspective.Shallow groundwater samples from Chaozhou,Huizhou and Guangzhou are collected to test the concentrations and isotopic ratios of noble gases.The recharging temperatures,recharging sources and resident time of groundwater samples are calculated according to the calculated noble gas temperatures(NGTs)and the ratio of noble gas isotopes.In addition,the contribution of different sources of noble gas components to the sample components is also calculated.In Huizhou area,according to 1/Xe VS Ne/Xe and NGT data,the shallow sandstone confined water samples from Shiba area and the phreatic water samples from huangshadong area indicate different temperature ranges,showing that they have different(time or space)recharge sources of atmospheric precipitation.The He_eq and He_ea in the samples are calculated,the radioactive ⁴He contents in the crust are also obtained,and the groundwater ages under two conditions(closed condition and open condition)are simulated.The geothermal water samples of Huangshadong in Huizhou contain high concentrations of radioactive ⁴He.The high contribution rate of ⁴He produced by radioactive decay of uranium and thorium in the crust indicates that the heat source of this geothermal system contains radioactive heat supplied by granite basement.The isotope ratios of noble gases indicate that in addition to atmospheric and crustal sources of He in basalt aquifers and sandstone aquifers in Chaozhou and Huizhou areas,there is a certain proportion of mantle-derived components in the shallow underground cold water in Huizhou and Chaozhou areas.The noble gases in groundwater have significant mantle-derived characteristics,and it is speculated that there are fluids that carry mantle characteristics in the deep.The upwelling of mantle derived material may be caused by the collision between India and Eurasia plate or Philippine Sea plate and Eurasia plate,which will result in asthenosphere activities.The hydro-chemical characteristics,hydrogen and oxygen isotope characteristics and the ratio of noble gas isotopes of the sandstone aquifer and basalt aquifer in Wudalianchi Weishan area,are used to calculate the recharge temperature,recharge source and residence time of groundwater.The contributions of noble gases from different sources are also calculated.The recharge elevation and recharge temperature of the two aquifers were estimated based on the characteristics of hydrogen and oxygen isotopes and the contents of Xe and Ne,and the recharge temperature fitted by the NGT model was verified.The results show that the main recharge elevations of groundwater around Wudalianchi are 500-600 m,and recharge temperatures are 2-7?.The He_eq and He_ea in the sample were simulated by the OD model,the radioactive ⁴He content in the crust was obtained,and the groundwater ages under two conditions(closed condition and open condition)were simulated,and it was found that water samples from sandstone layer under both conditions were older than water samples from basalt layer.Hydro-chemical characteristics and noble gas isotope ratios indicate that in addition to atmospheric and crustal sources of He in basalt aquifer and sandstone aquifer,there is also mantle-derived helium input.The faults constitute the upper mantle magma and gas channels.The mantle source components in water samples near the fault are much higher than those in the non-fault areas.³He/⁴He ratios and geophysical evidence indicate that there may be intrusive magma containing mantle helium heat in the shallow crust of Weishan in Wudalianchi.Mantle-derived material upwelling exists in regional fault zone in southeast China with Cenozoic basalts exposed.These signs indicate that the tectonic channels of Cenozoic basaltic magmas may have been active up to now and continue to provide upwelling mantle material.Similar to the Changbaishan and Wudalianchi,there are magmatic activities in the crust of Songliao basin.Along the lithosphere fracture,mantle material transfer into different parts of the crust.Meanwhile,the magmatic reservoirs can transfer heat to the shallow crust through molten state convection,which provides favorable conditions for the formation of high temperature geothermal resources in Wudalianchi-Changbaishan-Songliao basin area.With a large number of sample points that can collect escaped gases,CO₂-He system can be applied to research of geothermal system in a large scale.But for small regional geothermal systems,as there are not enough locations for the collection of escaped gases,the dissolved noble gases in groundwater can be useful tools for the research.Despite the higher requirements for sampling and testing than collecting escaping gases,geochemistry of dissolved noble gases can provide more information than CO₂-He system.