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

研究无球粒陨石中的原始惰性气体成分，特别关注 U-Xe

基本信息

批准号：
07454141
负责人：
NAGAO Keisuke
金额：
$ 4.1万
依托单位：
Okayama University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1996
项目状态：
已结题

项目摘要

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.

我们测量了许多无球粒陨石，eucrites、diogenites、howardites、火星陨石，以及硅酸盐和钯矿的金属相。尝试了几种不同的方法从陨石中提取惰性气体，以分离陨石中捕获的不同成分。然而，由于被捕获的惰性气体的浓度比陨石中的浓度低2-3个数量级，当我们测量少量陨石样品时，很难精确测定同位素比率。因此，在本报告中仅介绍了通过全熔化和逐步加热实验获得的惰性气体数据。本实验室正在研制适用于小样品的技术和仪器，取得了以下显著成果：（1）捕集的辉长岩同位素组成与辉长岩相比，缺乏较重的同位素。球粒陨石中的H-Xe成分富含钻石中捕获的较重的H-Xe同位素。另一方面，无球粒陨石中的Xe具有类似于太阳型Xe或暗示的U-Xe的同位素组成。2）Xe同位素比表明，<244>在大多数eucrite中，在eucrite母体的固溶作用中存在绝灭的^ Pu（T_<1/2>=8.2x10^7y）。（<244>3）地幔岩中重稀有气体Ar、Kr和Ar的相对丰度变化趋势与地幔岩明显不同。地球和火星的惰性气体也有无球粒陨石的趋势; 4）无球粒陨石母体形成时可能不含太阳前颗粒，如金刚石，或者在母体形成时，太阳前颗粒中的气体可能在高温下流失; 5）太阳型惰性气体可能是从太阳星云中获得的。