A study of relation between undifferentiated and differentiated meteorites based on noble gas elemental and isotopic compositions

分化体（分化的陨石和行星）被认为是由原始陨石（=球粒陨石）等物质形成的。然而，它们之间的关系和分化过程的细节仍然是一个问题。在这个研究项目中，我们试图在稀有气体同位素研究的基础上更好地了解它们。为此，用质谱仪测定了几类分异陨石（黄辉岩、黄辉岩、辉橄榄岩和月球陨石）和球粒陨石中的稀有气体元素和同位素组成。我们取得了如下显著的成果。(1)钠长辉石的成因：钠长辉石仅为无球粒陨石，氧同位素组成中有一对球粒陨石。因此，研究辉长岩与球粒陨石（顽辉石球粒陨石）的关系，明确辉长岩的演化过程具有重要意义。我们测量了9个稀有气体组分和5个氧同位素组分，得出以下结论：(1)更多的稀有气体组分含有球粒质原始稀有气体，但它们可能来自球粒质碎屑，而不是本地特征。（ii）^<244>在一些权威中观察到pu衍生的裂变Xe。在球粒陨石和其他无球粒陨石中所观察到的Pu/U比值范围相当，这表明黄岩形成于太阳系形成后的几百万年。（三）火辉石和顽辉石球粒陨石的宇宙射线暴露年龄分布不同，说明这两种母小行星的轨道可能不同。(2)球粒陨石中的原始惰性气体：虽然通常认为球粒陨石中的大多数原始惰性气体是“Q”型惰性气体，但我们发现另一种成分是某些（或大多数）不平衡普通球粒陨石的主要成分。它表现出一些特征；富集较轻元素（Ar/Xe和Kr/Xe比高于Q），释放温度较低（~ 1100℃，而Q为~ 1300℃），捕获于HCl可溶相。设计并构建了从mg或亚mg样品中提取惰性气体的微型烤箱系统。它可以用小功率（例如，φ0.6mm Pt-wire ~ 30W）在Pt-wire上熔化样品。少

Differentiated bodies (differentiated meteorites and planets) are thought to have been formed from materials like primitive meteorites (=chondrites). However, details about relation between them and differentiation processes are still questions. In this research project, we tried to have better understanding them based on noble gas isotope studies. For this, noble gas elemental and isotopic compositions in several classes of differentiated meteorites (aubrites, angrite, brachinite and lunar meteorites) and chondrites have been determined with a mass spectrometer. Remarkable results we obtained are as follows.(1)Origin of aubrites : Aubrites are only achondrites that have a chondrite-pair for oxygen isotopic compositions. Therefore, it is interesting to study relation of aubrites with the chondrites (enstatite chondrites) and to clear the process of aubrite evolution. We have measured nine aubrites for noble gases and five aubrites for oxygen isotopes, from which we concluded that (i)so … More me of the measured aubrites contain chondritic primordial noble gases, but it is likely originated from chondritic clasts and not a indigenous signature. (ii)^<244>Pu-derived fission Xe has been observed in some aubrites. The Pu/U ratios range in those observed in chondrites and other achondrites, which indicates that aubrites were formed within several hundreds m.y.after the solar system formation. (iii)The distribution of comic-ray exposure ages for aubrites is different from that for enstatite chondrites, implying that orbits of these parent asteroids are probably different.(2)Primordial noble gases in chondrites : Although the majority of primordial noble gases in chondrites are often believed to be "Q" type noble gases, we found another component that is the major component in some (or most) unequilibrated ordinary chondrites. It shows some characteristic features ; enrichments in lighter elements (Ar/Xe and Kr/Xe ratios are higher than those of Q), low release temperatures (〜1100℃ compared with〜1300℃ for Q) and trap in a HCl soluble phase.A mini-oven system for extracting noble gases from mg or sub-mg samples has been designed and constructed. It enables to melt a sample on a Pt-wire with small power (e.g., 〜30W with φ0.6mm Pt-wire). Less