Evolutional history of eucrite parent body

我们对具有不同热史和震荡史的玄武岩榴辉岩中的锆石进行了岩石学和U-Th-Pb同位素研究。大多数情况下，产出的锆石与钛铁矿有关。在许多情况下，榴晶锆石在阴极发光本质图像中是由多个区域组成的亚面体形状，可能是由于初始结晶后的变质效应。未变质的榴辉岩中的锆石中铀和钍的浓度高于变质的榴辉石，这表明未变质的大量榴辉石可能富含不相容的元素，代表了更多的演化物质。榴晶状锆石的U-Pb年龄基本一致。然而，细小的锆石颗粒表现出相反的不一致性，表明放射性成因的铅成分过多。所分析的大多数榴辉岩中的锆石的^&lt；207&gt；Pb206&gt；Pb值为4551±8 Ma(n=24，误差95%置信限)，这强烈地表明榴辉岩母体上存在早期和短期的火山活动。获得的锆石U-Pb年龄大于硅酸盐的U-Pb、Sm-ND和Rb-SR年龄，表明各同位素体系或硅酸盐的闭合温度差异更易发生冲击变质作用/陆源风化作用。

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.