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

Abstract

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~3.80-13.83 9 CI) and plagioclase (Eu~12.31-14.67 9 CI) but are low in olivine (REE~0.01-1.44 9 CI) and spinel (REẼ 0.25-0.49 9 CI). 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 et al. 2012) 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°C, the clast parent body should have accreted by~2.5-2.6 Ma 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°C. If the body has a peak temperature of >1100°C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.

Fumarolic-like activity on carbonaceous chondrite parent body

“…In this model, the various CV lithologies, which may be in part applied to CO chondrites, are inferred to be fragments of an asteroid percolated heterogeneously via porous flow of hot and reduced hydrothermal fluids. To sum up, although there is a general agreement that parent body alteration is a fundamental process in the evolution and the formation of secondary phases of CV and CO carbonaceous chondrites, there is, however, no consensus neither on the composition and the nature of the fluids involved nor if we should consider one or several generations of parent bodies (18).…”