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

为了找出主要小行星甜菜中最常见的S小行星与太阳系原始球粒陨石源区物质的关系，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。所研究的样品包括五个具有四种粗晶结构(Lodran，MAC88177，Y74357，Y791491，EET84302)和两种细晶结构(Acapulco，ALH81187)的铁陨石-无钛陨石，以及两种铁陨石中的硅酸盐包裹体(卡多县和兰德斯)。利用电子探针显微分析仪和微焦点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为矿物分布图。这些陨石由普通球粒陨石中的橄榄石、斜辉石、辉石、斜长石、FeNi金属、硫铁矿等常见成分矿物组成，其化学成分在同一颗粒内基本一致，分布在H和E球粒陨石之间。然而，从矿物分布图中获得的它们的模式丰度在不同样品之间存在着极大的差异。罗丹石主要由橄榄石和斜方辉石组成，亏损斜长石，而阿卡普尔科和EET84302等非斜长岩和卡多县铁陨石则富含斜长石、铁镍金属和硫铁矿。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区熔体的耗尽，以及熔体在衰减区周围的迁移和再结晶来解释。热源可能是~(26)Al的衰变和小行星的碰撞。这样的过程是在小行星碰撞和碰撞形成相当大的S小行星的过程中所预期的。

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.