Evolutional history of eucrite parent body

对不同热冲击历史的玄武岩长辉长岩锆石进行了岩石学和U-Th-Pb同位素研究。锆石多与钛铁矿伴生。在阴极发光图像中，欧几里氏锆石多为由多个畴组成的亚面体形状，可能是由于初始结晶后的变质作用所致。未变质长辉长岩锆石中铀和钍的含量高于变质长辉长岩锆石中铀和钍的含量，说明未变质长辉长岩可能富含不相容元素，代表了更进化的物质。铕质锆石的U-Pb年龄基本一致。而小的锆石颗粒则表现出相反的不一致性，表明存在过量的放射性铅成分。本研究分析的大部分长长辉长岩锆石的^<207>Pb-^<206>Pb年龄为4551±8 Ma （n = 24，误差为95%置信限），强烈提示长长辉长岩母体发生过早期、短暂的火山活动。锆石U-Pb年龄大于硅酸盐的U-Pb、Sm-Nd和Rb-Sr年龄，说明各同位素体系的闭合温度存在差异，说明硅酸盐更容易受到冲击变质作用/陆地风化作用的影响。

We have undertaken petrologic and U-Th-Pb isotopic studies on zircons from basaltic eucrites with different thermal and shock histories. Zircon occurs are associated with ilmenite in most cases. Eucritic zircon is in many cases subeuhedral shape composed of several domains in the cathodeluminessence images probably due to metamorphic effects after initial crystallization. Concentrations of uranium and thorium in zircons from unmetamorphosed eucrite are higher than those from metamorphosed eucrites, suggesting bulk unmetamorphosed eucrites may be enriched in incompatible elements representing more evolved material. The U-Pb ages of eucritic zircons are almost concordant. However, small zircon grains show reverse discordance, suggesting excess radiogenic lead components. The ^<207>Pb-^<206>Pb age of zircons from most eucrites analyzed in this study are 4551 ± 8 Ma (n = 24, error is 95% confidence limit), which strongly suggest early and short volcanic activities occurred on the eucrite parent body. The obtained zircon U-Pb ages are older than the U-Pb, Sm-Nd and Rb-Sr ages of silicates, suggesting the difference of closure temperatures of each isotopic systems or silicates are more susceptible shock metamorphism/terrestrial weathering.