Cosmochemical study on parent body evolution of chondritic meteorites

1.稀土元素、钍和铀的分布平衡的普通球粒陨石中，这些元素主要分布在钙磷矿物中。考虑到钙磷酸盐易溶于酸，球粒陨石可分为酸溶相和酸残留相。然后用电感耦合等离子体质谱对酸渣相中的稀土元素、钍和铀进行了精确测定。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，玻璃比斜长石更为丰富。稀土含量及其与酸性残留相的宇宙丰度的归一化模式与母体上经历的球粒陨石热变质程度相关。钍和铀的含量也有类似的变化。从这些结果可以看出，稀土元素、钍和铀的含量是描述球粒陨石在母体上遭受热活动的良好参数。此外，这些含量还与惰性气体，特别是重惰性气体的含量相关，这表明这些惰性气体是从球粒陨石中释放出来的，并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Fe-Ni合金相组成。Fe-Ni相的重量从几%到20%不等，亲铁元素定量地分布在金属相中。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地变化。这种变化可能是由于普通球粒陨石母体的吸积温度不同所致。

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.