Cosmochemical study on parent body evolution of chondritic meteorites

1. 稀土元素、钍和铀的分布这些元素主要分布在平衡的普通球粒陨石中的磷酸钙矿物中。考虑到磷酸钙可溶于酸，球粒陨石分为酸溶相和酸残相。然后用电感耦合等离子体质谱法精确测定了酸残相中的稀土元素、钍元素和铀元素。酸残相由辉石、斜长石和玻璃组成；在不平衡的普通球粒陨石中，玻璃比斜长石更丰富。稀土含量及其与酸残相宇宙丰度归一化的丰度模式与母岩热变质球粒陨石的程度有关。钍和铀的含量也发生了类似的变化。这些结果表明，稀土元素、钍元素和铀元素的含量是描述球粒陨石在母体上的热活动的良好参数。此外，这些含量还与稀有气体，特别是重惰性气体的含量相关，表明这些稀有气体是从球粒陨石中释放出来的，在加热过程中由于热活动而从母体中脱气。普通球粒陨石金属相中难熔亲铁元素丰度普通球粒陨石主要由硅酸盐、硫化物和铁镍合金相组成。铁-镍相的重量从百分之几到百分之二十不等，亲铁元素定量分布在金属相中。对难熔亲铁元素的精确测定表明，它们的丰度在普通球粒陨石群之间有系统的变化。这种变化被认为是由普通球粒陨石母体的吸积温度差异引起的。

1. Distribution of rare earth elements, thorium and uraniumThese elements are largely distributed in Ca-phosphate minerals in equilibrated ordinary chondrites. Considering that Ca-phosphates are soluble in acid, chondritic meteorites were separated into acid-soluble and acid-residual phases. Then, rare earth elements, thorium and uranium were precisely determined for the acid-residual phases by inductively-coupled plasma mass spectrometry. The acid-residual phases consist of pyroxene, plagioclase and glass; glass is more abundant than plagioclase in unequilibrated ordinary chondrites. Rare earth contents and their abundance pattern normalized to their cosmic abundance for acid residual phases are correlated with the degree of thermal metamorphism chondrites experienced on the parent body. Similar change was observed for thorium and uranium contents. From these results, it was observed that contents of rare earth elements, thorium and uranium are good parameters for describing the thermal activity chondrites suffered on the parent body(ies). Furthermore, these contents are also correlated with contents of noble gases, especially heavy noble gases, suggesting that these noble gases were released from chondrites and were degassed from the parent body(ies) upon heating due to thermal activity.2. Refractory siderophile element abundances in metallic phases of ordinary chondritesOrdinary chondrites mainly consist of silicates, sulfides and Fe-Ni alloy phases. The weight of Fe-Ni phases ranges from several % up to 20% and siderophile elements are quantitatively distributed in metal phases. Precise determination of refractory siderophile elements revealed that their abundances systematically change from group to group of ordinary chondrites. Such a change was suggested to be caused by the difference in accretion temperature of ordinary chondrite parent bodies.