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

分化的天体(分化的陨石和行星)被认为是由原始陨石(=球粒陨石)之类的物质形成的。然而，关于它们与分化过程之间的关系的细节仍是个问题。在本研究项目中，我们试图在惰性气体同位素研究的基础上对它们有更好的理解。为此，用质谱仪测定了几类分异陨石(奥布里特陨石、角闪石、角闪石和月球陨石)和球粒陨石中的稀有气体元素和同位素组成。取得了以下显著成果：(1)奥布里特成因：奥布里特只是氧同位素组成为球粒陨石对的非球粒陨石。因此，研究奥辉铜矿与球粒陨石(顽辉石)的关系，弄清奥辉铜矿的演化过程具有重要意义。我们测量了9个奥布里特的惰性气体和5个奥布里特的氧同位素，从中我们得出结论：(I)所以…更多的测量到的奥布里特含有球粒状原始稀有气体，但它很可能来自球粒陨石碎屑，而不是土著特征。(Ii)在一些奥布里特中观察到了钚衍生的裂变Xe。Pu/U比值与球粒陨石和其他非球粒陨石的Pu/U比值一致，表明奥布里特陨石是在太阳系形成后几百英里内形成的。(3)球粒陨石与顽火辉石球粒陨石的喜剧射线年龄分布不同，表明这些母体小行星的轨道可能不同。(2)球粒陨石中的原始稀有气体：虽然球粒陨石中的大多数原始稀有气体通常被认为是“Q”型稀有气体，但我们在一些(或大多数)不平衡的普通球粒陨石中发现了另一种主要成分。具有轻元素富集度(Ar/Xe和Kr/Xe值高于Q值)、低释放温度(~1100℃，Q值约为1300℃)和HCl可溶相捕集等特点。它能够在小功率的铂丝上熔化样品(例如，用φ0.6 mm铂丝熔化~30W)。较少

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