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Formation and evolution processes of S-asteroids as inferred from primitive achondrites

从原始无球粒陨石推断S-小行星的形成和演化过程

基本信息

批准号：
05833005
负责人：
TAKEDA Hiroshi
金额：
$ 1.47万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1993
资助国家：
日本
起止时间：
1993 至 1994
项目状态：
已结题

项目摘要

In order to find relationship between the most common S-asteroids in the main asteroid beet and primitive chondritic source materials in the Solar System, mineralogical distributions of several new primitive achondrites have been investigated by electron and X-ray microanalysis techniques. The samples studied include five lodranites-acapulcoites with four coarse-grained recrystallized textures (Lodran, MAC88177, Y74357, Y791491, EET84302) and two with fine-grained ones (Acapulco, ALH81187) and two silicate inclusions in iron meteorites (Caddo County and Landes) . Electron probe microanalyzer and microfocus X-ray fluorescent analysis techniques have been applied to obtain two-dimensional elemental distribution maps of the above samples. The maps were then converted into mineral distribution maps. These meteorites consist of common component minerals in ordinary chondrites such as oliveine, orthopyroxene, augite, Plagioclase, FeNi metal, troilite etc. Their chemical compositions are nearly uniform within a grain and distribute within a small range between H and E chondrites. However, their modal abundances obtained from the mineral distribution maps differ geatly between the samples. Lodranites consist mainly of olivine and orthopyroxene and are depleted in plagioclase and some acaplucoites such as Acapulco and EET84302 and the Caddo County iron meteorite are enriched in plagioclase and FeNi metal and troilites. The processes of their formation and evolution can be explained by partial melting of low temperature melts and depletion of the melt from a high temperature region, and by migration of the melts and recrystallization around the depleted area. The heat source may be decay of ^<26>Al and collisions of planetesimals. Such process is what is expected during the growth of planetesimals by collisions and accreation to from fairly large S-asteroids.

为了寻找太阳系中最常见的S型小行星与原始无球粒陨石源物质之间的关系，利用电子和X射线微区分析技术研究了几种新的原始无球粒陨石的矿物学分布。所研究的样品包括五个具有四种粗粒重结晶结构（Lodran，MAC 88177，Y 74357，Y 791491，EET 84302）和两个具有细粒结构（Acapulco，ALH 81187）的lodranites-accountcoites和铁陨石中的两个硅酸盐包裹体（卡多县和Landes）。采用电子探针和微聚焦X射线荧光分析技术，获得了上述样品的二维元素分布图。这些地图随后被转换成矿物分布图。这些陨石由普通陨石中常见的组分矿物组成，如橄榄石、斜方辉石、普通辉石、斜长石、FeNi金属、陨硫铁等，其化学成分在一个颗粒内几乎是均匀的，分布在H和E陨石之间的一个小范围内。然而，从矿物分布图上得到的它们的模态丰度在样品之间存在很大差异。洛长岩主要由橄榄石和斜方辉石组成，斜长石和某些无水陨石（如Acapulco和EET 84302）和卡多县铁陨石中斜长石和FeNi金属和陨硫铁富集。它们的形成和演化过程可以用低温熔体的部分熔融和高温熔体的亏损以及亏损区周围熔体的迁移和重结晶来解释。热源可能是Al的衰变<26>和星子的碰撞。这一过程是小行星通过碰撞和吸积成长为相当大的S-小行星的过程。