激光质谱测定南极乔治王岛北海岸A635玄武岩微区^(40)Ar-^(39)Ar的等时年龄  

^(40)Ar－(39)Ar Isochron Dating on a Microscope Scale of A635 Basalt from the Northern Coast of King George Island，Antarctica by Using a Continuous Laser System and a Mass－Spectrometer

作　　者：胡世玲[1] 郑祥身[1] 戴撞漠 蒲志平[1] 

机构地区：[1]中国科学院地质研究所,中国科学院广州地球化学研究所

出　　处：《岩石学报》1995年第3期306-311,共6页Acta Petrologica Sinica

基　　金：国家自然科学基金

摘　　要：采用连续激光直接对岩石光片样品微区熔样，以高灵敏度、高精确度的质谱计测试，对南极乔治王岛玄武岩样品进行（４０）Ａｒ－（３９）Ａｒ法定年，取得了成功。样品置反应堆中心Ｅ－５孔道，以１．８×１０（１８）ｎ／ｃｍ２的快中子总通量照射，时间为５０ｈ，放射性冷却后进行测定。样品的加热源为自行设计的Ｎｄ：ＹＡＧ固体激光器，以Ｈｅ－Ｎｅ准激光器导光。测定中对４个２５０μｍ直径的微区分别在１５００℃高温熔融，经Ｚｒ－Ａｌ泵纯化的气体用ＭＭ１２００质谱计测量氩同位素各峰值。经计算得到该样品的等时年龄为５２．４±１．１Ｍａ，远远低于江个点的视年龄值，而（（４０）Ａｒ／（３６）Ａｒ）＞＞２９５．５。这不仅证明该样品含有过剩氩，同时也证明了由Ｋ－Ａｒ稀释法得到的偏高年龄值亦是样品中存在过剩氩造成的。激光微区等时年龄的获得进一步厘定了南极乔治王岛北海岸火山岩时代为第三纪，同时证明了我们建立的激光质谱微区、微粒熔样和测定技术是可行的和成功的。An (4o)Ar-(39)Ar isochron dating of a microscope scale of A635 basalt from the northern coast of King George Island, Antarctica, was performed with success by using a continuous laser system for melting sample on a high precision and high sensitive mass-spectrometer. Sample had been irradiated in the E-5 passage at the center of reactor for 50 hours with fast neutron, of which the total flux was 1. 8 0133(18)n/cm2, and was determined after radioactive cooling. Heating source was a Nd: YAG solid laser illuminator designed by authors and a He-Ne quasi-laser illuminator had been chosen as the light conductor. 4 microscope scale areas,250pm in diameter,of the sample were seperatively melted at 1500℃. Released gas passed a Zr-Al pump for purification,then entered into MM1200 mass-spectrometer for determining of each peak value of Ar isotopes.The calculated (4o)Ar-(39)Ar isochron age of the studied sample is 52. 4 ± 1. 1Ma,which is far lower than the apparent ages of 4 micro areas. However,the ((40)Ar/)36)Ar).value is much higher than 295. 5.The result proves that the sample contains some excess argon and hence gives the high K-Ar age. The (40)Ar-(39)Ar isochron age determined by a continuous laser system and mass spectrometer further collates and stipulates the Tertiary volcanic rocks in the northern coast of King George Island, and has, at the same time, fully certificated the feasibility and success of thetechnique established by authors.

关 键 词：玄武岩 氩同位素 等时年龄 激光质谱 测定 

分 类 号：P588.145[天文地球—岩石学]