Mn–Cr ages and formation conditions of fayalite in CV3 carbonaceous chondrites: Constraints on the accretion ages of chondritic asteroids

Abstract

Chondritic planetesimals are among the first planetary bodies that accreted inside and outside water snow line in the protoplanetary disk. CV3 carbonaceous chondrite parent body accreted relatively small amount of water ice, probably near the snow line, and experienced water-assisted metasomatic alteration that resulted in formation of diverse secondary minerals, including fayalite (Fa80–100). Chemical compositions of the CV3 fayalite and its Mn–Cr isotope systematics indicate that it formed at different temperature (10–300°C) and fluid pressure (3–300bars) but within a relatively short period of time. Thermal modeling of the CV3 parent body suggests that it accreted ∼3.2–3.3Ma after CV3 CAIs formation and had a radius of >110–150km. The inferred formation age of the CV3 parent body is similar to that of the CM2 chondrite parent body that probably accreted beyond the snow line, but appears to have postdated accretion of the CO and ordinary chondrite parent bodies that most likely formed inside the snow line. The inferred differences in the accretion ages of chondrite parent bodies that formed inside and outside snow line are consistent with planetesimal formation by gravitational/streaming instability.