摘要
对43个球粒陨石进行了稀有气体分析,其中34个为我国球粒陨石,9个为国外球粒陨石。43个球粒陨石包括17个H、15个L、6个LL、3个L/LL、1个EH3及1个CV3-an化学群。给出了43个球粒陨石的宇宙射线暴露年龄及气体保存年龄,在此基础上讨论了二者(T_3/T_(21)与T_4/T_(40))之间的关系,划分出~3He_c和~4He_r同步丟失及~4He_r单独丢失两种类型的球粒陨石。
Described in this paper are the noble gases of 43 chondrites (34 from China).The cosmic-ray exposure ages and gas retention ages of the 43 chondrites (17H, 15L, 6LL, 3L/LL, 1EH3 and 1CV3-an) have been obtained. For the H chondrites two main peaks at 7 and 33 Ma appear even in the case of a relatively small sample set. The L chondrites are clustered in the range of 20-30 Ma and the Junan is consistent with the 40-Ma peak. Only the Rugao(16.8±2.0 Ma) falls on the prominent 15-Ma peak of the LL chondrites. All the other LL and L/LL chondrite ages are distributed quite evenly within the range of 3-40 Ma. 4He retention ages, T4, are generally lower than 40Ar retention ages, T40 as usually determined for the chondrites. Some of the 40Ar ages are in the range 4400-4600 Ma, indicating that these meteorites have retained all radiogenic 40Ar since their formation. Such is the case for the Xingyang (H5), the Wuan (H6), the Taonan (L6), the Xi Ujimgin (L/LL6), and the Ningqiang (CV3-an). Some meteorites studied by us show 3Hec loss, as is identified from a T3 value beyond the experimental errors. Thus lower exposure ages are expected. All these meteorites yield T4 < T40 On the other hand, some chondrites show T4 < T40 but their T3 is concordant with the exposure ages calculated from other nuclides. These two types of chondrite are plotted in T3/T21 vs. T4/T40 diagram. The chondrites that have lost 3Hec and 4Her roughly follow a line passing through the origin with slope = 1, indicating that the two components of He were equally lost. The majority of the meteorites in the T3/T21 vs. T4/ T40 diagram, within the error limits of ±15%, have concordant T3 and T21 ages, but some of them have lost radiogenic 4He. This 4He lost, must therefore, have occurred before or at the break-up of the meteoroid from its parent body, whereas the meteorites with correlated 3Hec and 4Her deficiencies lost He during their cosmic-ray exposure presumably as a result of solar heating or a collision event.
出处
《地球化学》
CAS
CSCD
北大核心
1992年第4期313-332,共20页
Geochimica
基金
国家自然科学基金
