恩施球粒陨石热历史及原始氩特征  

THERMAL HISTORY AND TRAPPED ^(40)Ar/^(36)Ar RATIO OF THE ENSHI CHONDRITE

作　　者：王松山[1] 桑海清[1] 裘冀[1] 

机构地区：[1]中国科学院地质研究所

出　　处：《地质论评》1997年第3期303-309,共7页Geological Review

基　　金：地质矿产部同位素地质开放研究实验室(编号931228-4);国家基金委资助项目(编号49373163)

摘　　要：H_s型恩施球粒陨石K-Ar年龄为4503±91Ma,^(40)Ar-^(39)Ar坪年龄为4518±8Ma,总气体年龄为4515±23Ma,等时线年龄为4505±16Ma,年龄的一致性表明该陨石未经受过明显的热扰动,它是我国首次发现的Ar保存年龄高达45亿年以上的陨石。陨石形成早期有过短期的快速冷却。陨石母体在4.66±0.04Ma时破裂。恩施球粒陨石原始捕获(^(40)Ar/^(36)Ar)_t为0.89±0.44,推测4.5Ga前太阳风中^(40)Ar/^(36)Ar近于l。The Enshi meteorite fell on Dec. 26,1974,in Enshi County,Hubei Province,China (109°51' 20'E,30°34'40'N). It consists of four fragments at least, of which the largest one is 4. 5kg,and total mass recovered was about 9. 57kg. The Enshi meteorite is an ordinary chondrite,classified as group H5. The principal minerals are chrysolite, bronzite, kamacite and troilite, with minor plagioclase ,chromite,ilmenite,clinopyroxene and glassy material.The 40Ar/39Ar age spectrum of the Enshi meteorite gives a plateau age of 4518±8 Ma for all gases,which is in agreement with the total gases age (4515±23 Ma),isochron age (4505±16 Ma) and K-Ar age (4503±91 Ma) within the range of errors. The result of ages shows that the parent body of the Enshi meteorite has not suffered collisional event probably. The low-middle temperature 40Ar is associated with Ca-poor high-K/Ca plagioclase,while the high-temperature 40Ar is related to Ca-rich,low-K/Ca bronzite. The plateau ages of plagioclase and bronzite are 4513±4 Ma and 4519±4 Ma respectively. Calculation indicates that the cooling rates of the En-shi meteorite are 4. 2 to 59. 2℃/Ma during the early stage evolution.The Ar correlation diagram constructed from the release of gases at over 1000℃ gives a trapped 40Ar/36Ar ratio of 0. 89±0. 44 for the Enshi meteorite, using the York equation. From the plot of the derived K/Ca ratio, it is concluded that the trapped Ar is derived from the Ca-rich, high-temperature phases with a low K/Ca rario,i. e. bronzite. Considering that most of the trapped 40Ar/36Ar ratios of lunar soil and fragments are about 1 ,it is possible that the trapped Ar of lunar and meteorites originated from solar wind.

关 键 词：恩施球粒陨石 陨石 热 同位素年龄 氩 钾 

分 类 号：P185.83[天文地球—天文学] P597