Origin of the metamorphosed clasts in the CV3 carbonaceous chondrite breccias of Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato-86009

摘要

We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato-86009. These clasts are coarse-grained polymineralic rocks composed of Ca-bearing ferroan olivine (Fa(24-40), up to 0.6 wt% CaO), diopside (Fs(7-12)Wo(44-50)), plagioclase (An(52-75)), Cr-spinel (Cr/[Cr + Al]=0.4, Fe/[Fe + Mg]=0.7), sulfide and rare grains of Fe-Ni metal, phosphate, and Ca-poor pyroxene (Fs(24)Wo(4)). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE 3.80-13.83xCI) and plagioclase (Eu 12.31-14.67xCI) but are low in olivine (REE 0.01-1.44xCI) and spinel (REE 0.25-0.49xCI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato-86009 breccias (Jogo etal. ) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 degrees C, the clast parent body should have accreted by 2.5-2.6Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800-1100 degrees C. If the body has a peak temperature of >1100 degrees C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.