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

为了寻找主要小行星群中最常见的s型小行星与太阳系原始球粒陨石源物质之间的关系，利用电子和x射线显微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5种lodranite -acapulcoite，具有4种粗粒重结晶结构（Lodran, MAC88177, Y74357, Y791491, EET84302）和2种细粒结构（Acapulco, ALH81187）和铁陨石中的2种硅酸盐包裹体（Caddo County和Landes）。利用电子探针微量分析仪和微聚焦x射线荧光分析技术获得了上述样品的二维元素分布图。然后将这些地图转换成矿物分布图。这些陨石由橄榄石、正辉石、辉长石、斜长石、FeNi金属、三辉石等普通球粒陨石中常见的组分矿物组成。它们的化学成分在一个颗粒内几乎是均匀的，并且分布在H和E球粒陨石之间的小范围内。然而，从矿物分布图中获得的它们的模态丰度在样品之间差异很大。洛德长石主要由橄榄石和正辉石组成，在斜长石中富集，而Acapulco和EET84302等部分斜长石和Caddo县铁陨石则富集斜长石、FeNi金属和三苯乙烯。它们的形成和演化过程可以用低温熔体的部分熔化和高温区熔体的枯竭以及熔体在枯竭区周围的迁移和再结晶来解释。热源可能是^<26>Al的衰变和星子的碰撞。这样的过程是在星子通过碰撞和吸积从相当大的s型小行星成长的过程中所预期的。

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.