Study of the primordial noble gas components in achondrites, with special interest in U-Xe

我们测量了许多非球粒陨石，即真辉石、闪长岩、辉绿岩、火星陨石，以及硅酸盐和绿柱石的金属相。人们尝试了几种从陨石中提取惰性气体的不同方法，以分离陨石中捕获的不同成分。然而，由于捕获的稀有气体的浓度比球粒陨石中的低2-3个数量级，所以当我们测量少量陨石样品时，很难准确地测定同位素比值。因此，本报告仅提供了通过全熔化实验和逐步加热实验获得的惰性气体数据。本实验室目前正在开发适用于小样本量的技术和仪器。该项目取得的重要结果如下：1)与球粒陨石相比，捕获的Xe同位素组成在较重的同位素中是不足的。球粒陨石具有富含重Xe同位素的H-Xe组分，捕获在钻石中。另一方面，非球粒陨石中的Xe具有类似于太阳型Xe或暗示U-Xe的同位素组成。2)Xe同位素比值表明，在榴辉岩母体的固化期，大多数榴辉石中存在已灭绝的^&lt；244&gt；Pu(T_&lt；1/2&gt；=8.2×10~(-7)y)。Pu-Xe形成年龄约为45Ga，太阳系形成后为1.5Ga。3)球粒陨石中重惰性气体Ar、Kr、Xe的相对丰度与球粒陨石明显不同。地球和火星的稀有气体也有非球粒陨石的趋势。4)非球粒陨石在形成非球粒陨石母体时可能不包括钻石等太阳前颗粒，或者捕获在太阳前颗粒中的气体可能在母体形成过程中在高温下损失。5)太阳类型的稀有气体可能是从太阳星云获得的。

We measured many achondrites, i.e., eucrites, diogenites, howardites, martian meteorites, and both silicates and metal phases of pallasites. Several different methods for noble gas extraction from meteorites were tried to separate different components trapped in meteorites. However, because the concentrations of trapped noble gases are 2-3 orders of magnitude lower than those in chondrites, precise determination of isotopic ratios was difficult when we measure small amounts of meteorite samples. Hence, only the noble gas data obtained by total melt and stepwise heating experiments are presented in this report. Techniques and apparatus applicable to smaller sample size are now under developments in our laboratory.Remarkable results obtained from the project are followings :1) Trapped Xe isotopic compositions are dificient in heavier isotopes compared with those of chondrites. Chondrites have H-Xe component enriched in heavier Xe isotopes trapped in diamonds. On the other hand, Xe in achondrites has isotopic composition similar to the solar type Xe or suggestive U-Xe.2) Xe isotopic ratios indicate a presence of extinct ^<244>Pu(T_<1/2>=8.2x10^7y) in most eucrites at the sollidification of eucrite parent body. Estimated ^<244>Pu-Xe formation age are in the range of 1.5 Ga after the formation of solar system at about 45 Ga ago.3) Relative abundances of heavy noble gases, Ar, Kr and Xe, in chondrites show a trend clearly different from that of chondrites. Noble gases of the Earth and the Mars are also in the trend for achondrites.4) Achondrites might not have included pre-solar grains such as diamonds at the formation of achondrite parent body, or the gases trapped in pre-solar grains might have lost at high temperature during the formation of parent body.5) Solar type noble gases may have acquired from the solar nebula.