The Study Of Geochemical Characteristics Of Thermal Springs And Carbon Degassing Fluxes From Typical Geothermal Fields In The Central Gangdise-Himalayan Orogenic Belt,Tibet,China

Quantification of the CO2emission from the solid earth is very important for either the study of long-time scale carbon cycle or secular atmospheric C budget.In collision origin,accompanied with large scale of magmatic activity during the process of collision considerable of CO2released to the atmosphere from the magmatic volatile constituent.Concurrently,the sedimentary that is rich in organic material and carbonates may be buried at the depth where metamorphism could happen,then formed CO2degassing and released as geothermal gases and fluids.Since Cenozoic the India plate collided with the Europe-Asia plate,which made the orogenisis happened in south and central Tibet intensively and formed the great Gangdise-Himalayan fold mountain system.The study area is located in the center of the orogenic belt system,where the tectonic sites are special and folds and faults develop well,and the intensive compression made the crust thicken abnormally and derived strike-slip activity,which made t some parts of this region get split property.All the faults and split structures afford the access to the substance to transport from the upper mantle and lower crust to the surface,and the wide spread igneous rock to be the heat source of the geothermal fields.The temperature range of thermal springs is15.5-86.5℃,pH range of which is7.00-8.70.Electricity conductivity of most water samples are ranged between1000-4500μS/cm,the main cations in waters are K+、Na+,the main anions in the waters are HCO3-and Cl-.The salinity of the thermal springs is low,main TDS range of which is800-1900mg/L.Most of the waters are peripheral water,main hydrochemical type of which is Na—HCO3,just a few samples with the type of Ca—HCO3or Na—Cl.Silica geothermalmeters suggested that the main temperature of thermal waters in shallow geothermal reservoirs is ranged between70～150℃.K—Mg geothermalmeter indicated that the temperature range of the geothermal reservoirs is110～210℃and with average of 148℃.The log(K2/Mg)vs log(K2/Ca)diagram was applied to estimate the CO2partial pressure(pCO2),which suggested that the range of pCO2is0.01～5.05×105Pa.The median value of saturation index(SI)of22minerals was calculated,SI of albite,aragonite,Ca-Montmorillonite,calcite,chalcedony,hydroxyapatit,illite,K-feldspar,K-mica,kaolinite,quartz and talc is positive.Whereas the median value of SI of alunite,anhydrite,anorthite,chlorite,chrysotile,dolomite,fluorite,gibbsite,sepiolite and amorphous SiO2is negative.Activity plot was used to discuss the equilibrium between different mineral phases in the thermal waters.In the Na2O—Al2O3—SiO2—H2O system,all the samples were located in the paragonite and albite phase region,and all the samples were located in the muscovite and K-feldspar phase region in K2O—Al2O3—SiO2—H2O system.In the CaO—K2O—Al2O3—SiO2—H2O system all the samples were located in the K-feldspar phase region.In the MgO—K2O—Al2O3—SiO2—H2O system all the samples were located in the muscovite and K-feldspar phase region.The range of total dissolved inorganic carbon(TDIC)in the waters of study area is0.0003～0.0255mol/L,the range of δ13CTDIC of the thermal spring(vs.PDB)is-9.519‰～+6.59‰,and which of the surface waters is-14.34‰～+1.54‰.In δ13CTDIC vs.TDIC diagram all the samples were located in the theoretical regions of soil CO2and deep source CO2.The geothermal fields were classified into two kinds(travertine type and geyserites type)by the different hydrothermal sediments.Langjiu geothermal field is a typical travertine type geothermal field and Dagejia geothermal field is the geyserites type,they were taken as the case study to assume the CO2degassing flux.The results showed that the CO2degassing flux of Langjiu geothermal field was2.57kg/m2·a-1,which is31times more than the CO2flux uptake by the rock chemical weathering.CO2degassing flux of Dagejia geothermal field was2.79kg/m2·a-1,which is278times more than the CO2flux uptake by the rock chemical weathering.