藏南新生代火山-地热区温室气体的释放通量与成因——以谷露-亚东裂谷为例  

Flux and genesis of greenhouse gases emissions from Cenozoic volcanic-geothermal fields,South Tibet:A case study of volcanic-geothermal fields in Gulu-Yadong rift

作　　者：张丽红[1] 郭正府[1] 郑国东[2] 张茂亮[1] 孙玉涛[1] 成智慧[1] 马向贤[2] ZHANG LiHong GUO ZhengFu ZHENG GuoDong ZHANG MaoLiang SUN YuTao CHENG ZhiHui MA XiangXian(Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China)

机构地区：[1]中国科学院地质与地球物理研究所新生代地质与环境重点实验室,北京100029 [2]中国科学院地质与地球物理研究所油气资源研究重点实验室,兰州730000

出　　处：《岩石学报》2017年第1期250-266,共17页Acta Petrologica Sinica

基　　金：中国科学院战略性先导科技专项(B类)(XDB03010600);国家自然科学基金项目(41602341;41572321)联合资助

摘　　要：印度-亚洲大陆俯冲带是地球深部碳循环的重要场所,在全球气候变化研究中占有至关重要的地位。谷露-亚东裂谷是青藏高原南部规模最大、延伸最长的近NS向拉张性裂谷,裂谷内出露新生代火山岩和多处大规模地热区,正在向大气圈释放巨量的温室气体。利用密闭气室法测量并计算得出的谷露和羊应火山-地热区的土壤微渗漏CO_2的平均释放通量分别为436.8g·m^(-2)·day^(-1)和99.0g·m^(-2)·day^(-1),接近具有潜在喷发危险的美国黄石公园火山-地热区(410g·m^(-2)·day^(-1))和加利福尼亚州Mammoth Mountain火山区(107.9g·m^(-2)·day^(-1))的CO_2平均释放通量,表明谷露-亚东裂谷的深部地热流体活动异常强烈。此外,在野外考察的基础上,通过实验室测试和理论计算估算了谷露-亚东裂谷每年以土壤微渗漏的形式向当今大气圈释放的CO_2总量约为1.76×107t,与西特提斯俯冲带前缘的意大利Apennine火山区CO_2的释放总量(1.32×10~7t·a^(-1))属于同一个数量级。因此,印度-亚洲大陆俯冲带是全球地质碳脱气的重要场所。气体地球化学特征显示,谷露-亚东裂谷内火山-地热气体的He同位素比值(0.10~0.29RA,RA为大气3He/4He比值)和δ^(13)CCO_2值(-7.20‰^-0.78‰)在空间分布上表现为,从南至北呈逐渐递增的变化特征,显示裂谷内火山-地热区气体在南北方向上可能具有不同的动力学成因机制。具体来讲,δ^(13)CCO_2值自南向北递增的趋势,可能与裂谷南北方向地层岩性存在差异有关;裂谷内火山-地热气体的He同位素比值高于0.1RA,揭示"重要的地幔组分",同时裂谷内地幔来源的流体组分从南向北逐渐递增的趋势表明,印度大陆岩石圈的向北俯冲作用是导致谷露-亚东裂谷内剧烈的水热活动和大规模温室气体释放最终的动力来源和成因机制。Gulu-Yadong rift （GYR） is the largest and longest extensional rift in Lhasa terrane, South Tibet. There are a lot of volcanic-geothermal fields in the rift, making it one of the strongest degassing areas in the India-Asia continental subduction zone. Following accumulation chamber method, we estimated the soil CO2 fluxes of Gulu and Yangying volcanic-geothermal fields. The results show that the average soil CO2 flux of Gulu geothermal field is 436.8g·m-2·day-1 which is similar with Yellowstone in America （410g·m-2·day-1）, while the average soil CO2 flux of Yangying is 99g·m-2·day-1 which is close to Mammoth Mountain volcano （107.9g·m-2·day-1） in California, USA. According to the soil CO2 flux of the volcanic-geothermal fields in the GYR, we also estimate the total CO2 emission of the Gulu-Yadong rift to be 1.76×107t·a-1; these high soil CO2 emission suggests significant geological carbon degassing in India-Asia continental subduction zone at present. Based on the geochemical characteristics of the volcanic-geothermal gases from the GYR, both 3He/4He ratios （0.10~0.29RA, RA is the 3He/4He ratio of air） and δ13CCO2 values （-7.20‰ to -0.78‰） of soil and bubbling gases display gradually increasing trends from south to north, implying significant difference in geodynamics processes along the GYR. In details, the increasing δ13CCO2 values from south to north might be related with variations of crustal lithology from south to north along the GYR. Additionally, most of the 3He/4He is higher than 0.1RA, implying significant mantle volatile contribution, and the progressively increasing 3He/4He from south to north might be related with the northward subduction of the Indian continental lithosphere.

关 键 词：CO2释放通量 火山 地热区 大陆俯冲带 谷露 亚东裂谷 青藏高原 

分 类 号：P317[天文地球—固体地球物理学]