Evolutional history of eucrite parent body

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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.