A Research On Geochemical Characteristics Of Geothermal Fluids In Southeast Yunnan Province,China

The southeast Yunnan is affected by late Palaeozoic,Mesozoic and Cenozoic magmatism and deep faults,resulting strong geothermal activities and forming a large number of hot springs.Those springs are the best"window"for observation of deep-derived fluids.The fluid isotopic geochemical characteristics observed on the surface can reveal the magma fluid activity in the deep crust,which are of great significance for understanding the evolution of the lithosphere and conducting the seismic observational study.Based on the geochemical characteristics of geothermal fluids(water and outgassing)in Southeast Yunnan,this paper analyzes the origin of ions in the geothermal water,the thermal reservoir temperature,the origin of gases,the release intensity of mantle-derived fluids,and the carbon isotopic equilibrium fractionation temperature.The relationship between geothermal anomalies and seismicity,the volcanic activity near the Pingbian county,the mechanism of deep source fluids and mantle heat flow in the process of earthquake formation,etc.,have been discussed.The results have scientific and practical significance for selecting a batch of observation objects and chemical species or parameters with depth-derived features.Chemical characteristics of geothermal water in southeastern Yunnan show that hot spring water mainly originates from the replenishment of precipitation,and the main and trace ions in the water source from the dissolution of minerals during the circulation process.Stratum lithology and fault structure have obvious control over water chemistry characteristics.The components SO₄^2-,F^-,and Cl^-originating from the Red River fault zone have depth-derived features.The content and distribution of trace elements are related to stratum properties and sedimentary minerals.The distribution characteristics of the geothermal field have been expressed by the geothermal reservoir temperatures,showing a weak seismicity in geothermal anomaly area,and earthquakes often occur in the geothermal gradient zone.This phenomenon's main reason is that the decay of heat-generating elements(e.g.,²³⁸U,²³²Th,and ⁴⁰K)mainly generates the thermal source in the crust.The heat source is relatively stable,and the thermal stress field and the regional stress field are adjusted to a balanced state.The intense seismicity in the wedge-shaped structural zone is presumed to be caused by that the rigid rock in the depth of intersection of the Red River fault and Xiaojiang fault obstructs the Sichuan-Yunnan block continuing to move to the southwest.The gas geochemical characteristics show that the bubbling gases from hot springs are mainly derived from the crust or atmosphere,and the mantle-derived helium release intensity in the south segment of the Red River fault zone is only about 5%,indicating that the fault is a deep and large one connecting the crust and mantle.Stable carbon isotopes show that CO₂ and CH₄ are mainly derived from crustal limestone and thermogenic sources,and the characteristics of mantle source are not prominent.In combination with the lithosphere structure and tectonic activity background,it is believed that weak fault activity and dilution of radiogenic He are the main factors leading to the minor release of mantle-derived fluids.The isotopic fractionation equilibrium temperatures between CO₂ and CH₄ gas that show gases source temperatures in the Pingbian volcanic field are lower than the melting temperature of the material in the crust,indicating that there is no basaltic magmatism at present in the crust.By the analysis of the release intensity of mantle-derived fluids and the temperatures of the carbon-containing gases source,it can be concluded that the activity of Pingbian volcano is weak at present,but the fluid from deep is still worthy of long-term attention.Compared with the mantle-derived fluids released from the main tectonic boundaries on the southeastern margin of the Qinghai-Tibet Plateau and the Cenozoic volcanic area,the release of mantle-derived helium from Pingbian volcanic field shows a lower level,and the terrestrial heat flow is dominated by crustal heat flow.By the study of the distribution characteristics of seismic activity(M?6.0)on the southeastern margin of the Qinghai-Tibet Plateau,it is found that the regions with seismic activity are often accompanied by intense mantle heat flow,indicating that mantle fluids and thermal convection play a critical role in the formation and occurrence of earthquakes.According to the four-period observations of geothermal fluid in southeastern Yunnan,the mantle-derived helium and the carbon-containing gases source temperatures in the springs located on the Red River fault zone have precursors to the seismic activity(M?4.0)in the area.Therefore,the thermal springs with mantle-derived characteristics can be used as observation objects for earthquake monitoring and prediction,and the mantle-derived ions,gases and temperatures that reflect the deep thermal state can be employed as the components or parameters of observations.