Boron Geochemistry Of The Geothermal Waters From Typical Hydrothermal Systems In Tibet

Boron is a necessary nutrient element,but the excessive consumption of boron can also cause people and animals chronic intoxication or hinder the growth of plants.At present,a large amount of research has focused on the source of boron in environment related natural process,such as volcanic activity,surface rock weathering,boron minerals dissolution in the groundwater system and atmospheric precipitation,etc.,while the research degree of boron geochemistry from geothermal system is relatively low.In view that the discharge of geothermal water is one of the main source of boron in the environment and often contaminates other types of drinking water around the geothermal field,we selected several typical high-temperature hydrothermal systems from Tibet?Yangyi,Ningzhong,Qucai,Quzhuomu,Yangbajain,Daggyai,Semi and Gudui?.Based on the accurate identification and quantitative test methods of special forms of boron in geothermal water?polyborate anion and fluoroborate complexes?,the speciation,main controlling factors and mutual transformation of boron in Tibet geothermal waters were analyzed.At last,the geochemical geneses of boron in geothermal waters have also been studied,as well as its migration,transformation and accumulation processes in the surface environment.The main research contents of the thesis include:1.The water chemistry and hydrogen,oxygen,carbon,sulfur and boron isotope characteristics of geothermal waters in study area were analyzed,and the sources of the main chemistry components were discussed.According to the geothermal water boron content,the study area was divided into low boron,medium boron and high boron geothermal areas.The low boron geothermal area includes Yangyi,Ningzhong,Qucai and Quzhuomu.The medium boron geothermal area includes Yangbajain.The high boron geothermal area includes Daggyai,Semi and Gudui.Most of the geothermal water samples in the study area were neutral/weakly alkaline,and only a few acidic hot springs?DG02,DG03 and SM03?were distributed in Daggyai and Semi geothermal area.The hydrochemical types of geothermal waters in the low boron geothermal area are mainly Ca-HCO₃-Cl and Ca-Na-SO₄-Cl types.The hydrochemical type of the medium boron geothermal area is mainly Na-Cl-HCO₃ type,and the waters in high boron geothermal area are mainly Na-Cl and Na-SO₄ types.In the major components,from the low-boron,medium-boron to high-boron geothermal areas,Na⁺content increased while Ca²⁺and Mg²⁺contents decreased,and the three major anions(HCO₃^-,SO₄^2-and Cl^-)did not change significantly.Among the characteristic components,the contents of As,F,Si,Li,Rb,and Cs in the low-boron,medium-boron,and high-boron geothermal areas all increased sequentially.The main reason was that the contents of the above-mentioned components were closely related to the magmatic activity.The characteristics of hydrogen and oxygen isotopes show that the geothermal water is mainly derived from local meteoric waters recharge,and some samples in the high boron geothermal area show significant oxygen drift,indicating that geothermal water is also supplied by magmatic fluids.The carbon isotope characteristics indicate that the carbon in the high-boron geothermal water is mainly magmatic origin;in the low-boron geothermal area,the carbon in the hot water of the Quzhuo geothermal water mainly comes from the leaching of the marine carbonate rock,and Yangyi,Ningzhong,Qucai may have both of the above sources;the carbon in the Yangbajain geothermal water of medium boron geothermal area is mainly derived from the metamorphism of the Nyainqentanglha complex.The sulfur isotope features indicate that the dissolution of marine evaporate?anhydrite?is the main source of sulfur in low boron geothermal water,and the sulfur in high-boron geothermal water may come from dissolving of marine evaporates,contaminated atmospheric precipitation and mantle sulfur input.The boron isotope characteristics indicate that the boron in the low boron geothermal water mainly originates from the leaching of marine carbonate rocks and granites,while the boron in the high boron geothermal water may have other sources?such as magmatic fluid recharge?.2.The characteristics of the spatial distribution of boron in the geothermal water system are characterized,the geothermal environment characteristics conducive to boron enrichment are summarized,and the significance of the geothermal activity in the enrichment of boron in the environment is explained.The concentration of boron in geothermal water samples in the study area varied from 1.1 to 530.8 mg/L?average:89.4 mg/L?,which is much higher than the boron content in shallow groundwater and surface water.The geothermal environment conducive to boron enrichment is characterized by alkaline,high temperature,high TDS,and strong reduction.However,the formation of geothermal water with high boron content is fundamentally dependent on the geologic origin of geothermal systems and geochemical geneses of geothermal fluids.The boron-rich hot springs were distributed along the Brahmaputra suture zone,indicating that the abundance of boron in geothermal water is closely related to magmatic activity.There is a significant positive correlation between Cl,As,Li,Rb,Cs and B in geothermal water,indicating that B is equivalent to Cl,As and so on as partially conservative component,with similar geochemical behavior.Ca,SO₄ and B are not significantly related,because the Ca and SO₄ in the geothermal fluid are non-conservative components,and the Ca content is controlled by the solubility of the altered minerals such as calcite,wairakite,epidote,while the SO₄ content is affected by the precipitation of minerals such as alunite and anhydrite from geothermal fluids.At the hot spring vents,the boron contents in the sediments ranged from 22.5 to 1653.9 ppm with an average value of 656.9 ppm,which is much higher than the range of boron contents in the soils?2 to 100 ppm?.The distribution of total boron in sediments is as follows:Daggyai>Gudui>Semi>Yangbajain.The boron contents of each fraction in sediment vary:residual fraction>exchangeable fraction>Fe-Mn oxide-bound fraction>carbonate-bound fraction>organic and sulfide-bound fraction,indicating that the boron in the hot spring waters after excretion may be removed from the liquid phase primarily by precipitation or co-precipitation.3.Based on the geochemical comparison of the typical high-boron geothermal system Daggyai and the typical?relative?low-boron geothermal system Quzhuomu,the enrichment mechanism of boron from geothermal waters in Tibet was revealed.The water chemistry and boron isotope studies indicate that the acid springs in the Daggyai are typical steam-heated hot springs,which are actually shallow groundwater,and their sulfate and acidity are derived only from the oxidation of hydrogen sulfide separated from deep geothermal fluids,thus the B content is very low.The neutral/weakly alkaline hot springs of Daggyai are formed by the deep fluids ascending to the surface through different cooling types.The B contents are much higher than that of Quzhuomu,and the?¹¹B is lower than Quzhuomu.The hydrochemical and B isotopic evidence imply that the boron in Daggyai geothermal waters are contributed by both magmatic fluids input and host rocks leaching.In contrast,marine carbonates and granites,especially the former,are confirmed as the major boron sources of the Quzhuomu geothermal waters in view that the?¹¹B-B relations for these samples can be well fitted by a binary mixing model and the contribution from the marine carbonates endmember is estimated to be up to 90%.In other words,Daggyai is a typical representative of magmatic heat source geothermal system in Tibet,and the processes including partial melting of the crust,crystallization of magma melt and interaction of geothermal water with surrounding rocks contribute to the boron in geothermal waters.For Tibet geothermal systems without magmatic heat source,such as Quzhuomu,the water-rock interaction during geothermal fluid upflow is inevitably the main controlling factor for boron content.In geothermal systems,the adiabatic cooling of geothermal fluids?ie,water-vapor separation?,the adsorption of boron into altered minerals in fluids,or their co-precipitation,etc.,can result in the fractionation of boron isotopes in the liquid phase.The water-steam separation process can lead to H₃BO₃ rich in ¹¹B into the gas phase,resulting in the reduction of the?¹¹B value of geothermal waters,while the CO₂ removal can also cause the precipitation of carbonate minerals and B?OH?₄^-rich in ¹⁰B into the lattice of carbonates,thus increasing the?¹¹B value of geothermal waters.Another example is that the adsorption or co-precipitation of boron in the liquid phase by altered minerals results in a higher degree of removal of B?OH?₄^-from the liquid phase,resulting in an increase of?¹¹B value in the residual geothermal fluid.4.The form and main controlling factors of boron in geothermal water were identified,and the migration and transformation of polyborate anions and fluoroborate complexes in hot spring environment were discussed.The form of boron in geothermal water is affected by pH,total boron concentration,temperature and Eh.pH is the main controlling factor for the distribution of boron in geothermal water.When it is less than 7,the form of boron is dominated by H₃BO₃ with a percentage of up to 99%,while the contents of B?OH?₄^-,polyborate anions and fluoroborate are extremely low.As the pH value increases,the B?OH?₄^-content increases correspondingly,reaching up to 40%at pH 8.8.The effect of total boron concentration is relatively slim.When the total boron concentration in geothermal water is low,the H₃BO₃ is the main form.The B?OH?₄^-content is increased when the boron concentration is medium,and when the total boron concentration exceeds 0.1 mol/L,polyborate anions may form.The relationship between temperature and Eh values and the form distribution of boron in geothermal water actually reflects the geochemical geochemical origin.Hot springs with low Eh values are generally those in which deep geothermal fluids are less likely to be formed by shallow water mixing in the upflow process.The total boron content is high,and is often affected by near-surface adiabatic cooling?ie,water-steam separation?.Due to the escape of acid gases?CO2 and H2S?,the geothermal fluids are weakly alkaline,thus the boron is in the form of H₃BO₃,B?OH?₄^-,co-existence of polyborate anions and fluoroborate complexes.Hot springs with high Eh values are formed by the mixing of deep geothermal fluids and a large amount of shallow groundwater,or steam-heated acidic water?that is,shallow oxidation water?,so the boron concentration is relatively low and the form of boron is mainly H3BO3.In addition,the relative concentrations of fluorine and boron will also affect the boron speciation in geothermal water.Fluoroborate will only appear when the total fluorine content is equal to or greater than the total boron concentration.5.Based on the laboratory simulation,the adsorption and co-precipitation processes between carbonates?such as calcite?and boron in fluids were analyzed,and migration,transformation and accumulation of geothermal boron in different environmental conditions were evaluated.Although boron is a partially conservative component,due to the unusually high boron content in the geothermal water in some geothermal areas,it can still be largely precipitated out from the liquid phase after hot spring excretion.The existence of high levels of boron in some hot spring sediments is evidence.Carbonate minerals are widely present in hot spring sediments.In view of the similarities in the chemical structure and geochemical behavior of borate ions and carbonate ions,boron may be co-precipitated with carbonate minerals?or adsorbed in carbonate minerals?and migrate into the solid phase.A systematic simulation study was conducted to analyze the contribution of boron in liquid phase migrating into solid phase by both adsorption and co-precipitation with calcite.It was found that the amount of boron adsorbed and co-precipitation elevated along with the increase of liquid-phase boron content and reaction temperature.The amount of adsorption and co-precipitation increased first and then decreased with increasing pH,and peaked at pH 9.3.The pH has a controlled effect on the boron speciation in the solution and the surface potential of calcite.When the pH is raised,the surface negative potential of the calcite increases,which can inhibit the adsorption of boron.At the same time,the contents of the more easily adsorbed speciation,such as B?OH?₄^-,B₃O_3??OH4^-,B₄O₅?OH?₄^2-and B₅O₆?OH?₄^-,also increases,which will promote the boron adsorption.At the beginning of pH increase,the effect of the surface potential of calcite is weaker than that of boron speciation,so the adsorption of boron is generally enhanced.However,when pH value rises further than 9.3,the effect of boron speciation becomes weakened,resulting in reduction of boron adsorption.The key scientific problems solved by the thesis are:?1?Established an effective identification or quantitative test method for different types of polyborate anions and fluoroborate complexes in geothermal waters;?2?Calculated the quantitative contribution of boron in geothermal waters between deep magma fluid recharge and water-rock Interaction.The innovations of the dissertation are as follows:?1?The geochemical origin of boron in geothermal waters is identified;?2?The migration,form transformation and fate of boron from geothermal fluids in the environment are systematically studied.