安徽亳县球粒陨石的铅同位素研究

本文报道了安徽亳县LL4型球粒陨石的全岩、球粒、橄榄石、辉石和陨硫铁的铅同位素比值,以及上述部分样品的铀、钍、铅的含量.由陨石的铅-铅等时线求得的年龄为45.3±0.2亿年,这是安徽陨石母体的形成年龄.而南安徽陨石的内部等时线求得的年龄为44.4±0.2亿年,此年龄暗示着安徽陨石母体在形成后的大约1亿年以前产生过局部或全部的熔融.从安徽陨石的铀-铅一致曲线图解中得到的上交点年龄为45.4±0.2亿年,此与陨石母体形成的铅-铅年龄一致.而下交点的年龄为4亿年,这可能意味着在较近期内安徽陨石又经历了一次热扰动(如热冲击).全岩、球粒和陨硫铁都呈现出过剩的放射性成因铅,这与陨石经受过热事件有关. 

Isotopic Lead Investigations on Bo County Chondrite of Anhui Province,China

The isotopic compositions of lead were determined on the samples of whole rock, chondrule, olivine, pyroxene and troilite from Bo County chondrite (class LL4) of Anhui prov., China. Uraium, Thorium, and Lead concentrations of those samples were partially analysed. Observed 206Pb/204Pb ratios varied from 9.87 to 20.38; 207Pb/204Pb ratios from 10.51 to 16.61; 208Pb/204 Pb ratios from 30.20 to 40.58. In 207Pb/204Pb-206 Pb/204Pb diagram, a regression line fitted to most of data from Bo County and Bru-derheim-III and Modoc meteorites, has a slope of 0.6095 + 0.0095, corresponding to the model age of 4.53 ± 0.02 AB and the line is through the primordial lead of Canyon Diablo troilite. However, the age, corresponding to the internal isbehron line of Bo County chondrite, is 44.4±0.02 AE. It is reasonable to think that Bo County chondrite was formed at 4.53±0.02 AE ago and suffered in the early melting event at about 0.1 AE ago after formed. In a coreordia diagram the data plot a chord intersecting concordia at 4.54±0.02 AE and 0.4 AE. This indicates disturbance of the U-Pb systems happened relatively recently. The Th/U ratio from the 208Pb/204Pb- 206Pb/204 Pb diagram of the Bo County samples is 4.0 + 0.1, which is slightly higher than the normal value of 3.8. It is possible that Bo County chondrite might have suffered a thermal event.