40Ar-39Ar chronology of lunar meteorites Northwest Africa 032 and 773
40Ar-39Ar chronology of lunar meteorites Northwest Africa 032 and 773
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DOI：
10.1111/j.1945-5100.2003.tb00026.x
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发表时间：
2003-04-01
影响因子：
2.2
通讯作者：
Turner, G
中科院分区：
文献类型：
--
作者：
Fernandes, VA;Burgess, R;Turner, G
40 Ar-39 Ar测年技术已被应用于月球陨石西北非洲032（NWA 032），一个unbrecciated月海玄武岩，和西北非洲773（NWA 773），（由累积和角砾岩岩性组成），以确定这些陨石可能经历的结晶年龄和冲击事件的时间。对NWA 032的几个不同样品进行的逐步加热分析给出了复杂的年龄谱，但无法区分总年龄，平均值为2.779 +/- 0.014 Gyr。从NWA 032获得的复杂年龄谱的可能原因包括反冲的39 Ar，或存在的冲击前40 Ar纳入冲击熔融脉。通过激光融合20个预计含有不同比例的休克静脉的小样本，研究休克静脉的影响。激光年龄显示出一个狭窄的年龄分布，在2.61-2.86 Gyr之间，平均值为2.73 +/- 0.03 Gyr，与获得的大样本的总年龄相似，为2.80 Gyr。扩散计算的基础上逐步加热的数据表明，Ar释放可以调和释放长石（和可能的冲击脉）在低温下，然后在较高的温度下辉石。NWA 032的暴露年龄为212 +/- 11 Myr，它包含低捕获的太阳Ar。NWA 773的堆积岩和角砾岩部分的逐步加热也给出了2.91 Gyr的相对年轻的年龄。角砾岩中被困的Ar的存在下，这一组成部分的年龄测定不太精确，但释放的Ar似乎是从同一矿物相，假设是斜长石，在这两种岩性。暴露年龄之间的显着差异2岩性也存在，与角砾岩花了81万年的时间在月球表面，这一发现是一致的较高的被困Ar含量的岩性。假设2.80 Gyr和2.91 Gyr分别是NWA 032和NWA 773的结晶年龄，这两块陨石是可供研究的最年轻的月海玄武岩。
The 40Ar-39Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 +/- 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of 39Ar, or the presence of pre-shock 40Ar incorporated into shock-melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61-2.86 Gyr and a mean of 2.73 +/- 0.03 Gyr, identical to the total age of similar to2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 +/- 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.