摘要
土壤微渗漏是地球深部的构造-岩浆活动向大气圈释放温室气体的重要形式之一。近年来,国外许多地热区已经开展了土壤微渗漏温室气体释放通量的定量研究,然而,目前我国尚无该方面的系统研究报道。本文阐述了利用密闭气室法测量地热区土壤微渗漏温室气体释放通量的方法和原理,并将其应用于青藏高原的羊八井地热田。根据研究区的地热活动强度,将羊八井地热田划分为土壤微渗漏释放较弱的A区和土壤微渗漏释放较强的B区。计算结果表明,A区和B区的CO2气体释放通量分别为6.7g·m-2·day-1和98.5g·m-2·day-1。两区释放通量的差异主要受控于断裂的发育程度,即A区位于裂谷的中部,断裂发育程度较差,B区靠近念青唐古拉正断层,断裂发育程度较高,为温室气体的逸出提供了良好的运移通道。羊八井地热田土壤微渗漏CO2气体的释放总量约为8.6×104t·a-1,接近于意大利Vulcano火山区的土壤微渗漏温室气体释放规模(1×105t·a-1)。研究区的气体同位素组分测试结果显示,He同位素值介于0.107RA^0.648RA(RA为大气3He/4He比值),δ13C值介于-11.33‰^-6.79‰(vs.PDB),表明羊八井地热田的温室气体可能主要来源于大陆俯冲环境下的加厚陆壳,其温室气体释放通量的规模主要受控于地壳的岩浆活动以及南北向裂谷拉张作用。羊八井地热田所在的拉萨地块是青藏高原温室气体释放活动最强烈的区域之一,以往的研究表明,青藏高原出露大量新生代火山、地热区,开展火山、地热区温室气体释放通量的研究将有助于深入理解地质因素向当今大气圈释放温室气体的规模等与深部碳循环相关的科学问题。
Soil micro-seepage associated with high temperature geothermal systems has long been regarded as one of the most important forms of deep earth degassing, especially for the CO2. Estimation of soil micro-seepage CO2 flux for geothermal systems could contribute to understanding the role of geothermal CO2 in global carbon cycle. The soil micro-seepage CO2 fluxes of geothermal areas have been well investigated in abroad, while few similar studies are available in China at present. In this paper, the closed chamber method for measurement and calculation of soil micro-seepage CO2flux is introduced and applied in Yangbajing geothermal field, South Tibet. The study area is divided into two parts according to the intensity of geothermal activity, i. e. , A area with weak soil micro- seepage CO2 emissions and B area with intense soil micro-seepage CO2 emissions. The results show that, the soil micro-seepage CO2 flux of A area and B area is 6. 7g ~ m-2 ~ day-t and 98. 5g ~ m-2 ~ day-1 , respectively. The total soil micro-seepage CO2 flux of Yangbajing geothermal field is up to 8.6 × 104t a-1 , which is comparable to the total CO2 emission（ 1 - 10st a-1 ） of the Vulcano volcanic field, Italy. The soil micro-seepage gas flux may be mainly controlled by NS trend rift and magmatism around the studied area. Yangbajing geothermal field is one of the most intense degassing areas in southern Tibet. Systematic studies on soil micro-seepage CO2 fluxes of geothermal field in southern Tibet would be significant to evalule the contribution of the geological CO2 to the global carbon cycle. Furthermore, the origin of the greenhouse gas from Yangbajing geothermal field is still poorly understood at present. Gas samples were collected for analyses of gas compositions, helium and carbon isotopic compositions, in order to unravel the genesis of soil micro- seepage CO2 emission. The geochemical signatures of gas samples show that, helium isotope ratios varies between 0. IRA and 0. 138RA （where RA = 3 He/4He in air）, significantly lower than that of mantle derived volatiles but higher than the crustal values; carbon isotope values（813 C） fall between -11.33%0 and -6.79%v（ vs. PDB）, indicating contamination by crustal organic sediments. The observed helium isotopic composition of the gas could be explained by the mixing of components from air （ 1 RA ）, crustal sediments （0. 03RA ） and little mantle volatiles（ 1% - 3% ）. The carbon isotopic compositions indicate contributions from granite gneiss that are heated by the crustal magma, as well as little contribution from mantle component（ 3.6% ）. In combination with geophysical studies, we suggest that, the geothermal gas might originate from the intrusive magmas containing little mantle-derived components in the Tibetan crust, which might be related with the thickening continental crust of Tibet since the Indo-Asia collision at 55Ma.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第12期3612-3626,共15页
Acta Petrologica Sinica
基金
中国科学院战略性先导科技专项(B类)(XDB03010600)
国家自然科学基金重大国际合作研究项目(41020124002)
国家自然科学基金重点项目(41130314)联合资助
