Fayalite in the Vigarano CV3 carbonaceous chondrite: Occurrences, formation age and conditions

We have performed petrographic characterization, ⁵³Mn-⁵³Cr age determination and thermodynamic stability evaluations of fayalite in Vigarano meteorite that belongs to the reduced subgroup of CV3 chondrites. Vigarano is a breccia consisting of clasts which are separate chondrules surrounded by olivine-rich fine-grained materials. Four out of twenty three explored clasts contain fayalites that represent materials of the Bali-like oxidized subgroup of CV3 chondrites. The fayalites (Fa_{> 80}) with grain sizes typically < 20 μm occurs in veins that extend from chondrules into the fine-grained materials. The fayalite commonly coexists with troilite and/or magnetite. The fayalite-bearing veins terminate at the boundaries of clasts. No evidence of strong impact enough to make melt veins is found in materials adjacent to the veins. These observations suggest that the fayalite-bearing veins in the Bali-like clasts formed through aqueous alteration in an asteroid prior to fragmentation and re-accretion to the Vigarano parent body. In saponite-rich fine-grained materials, we also found troilite-magnetite veins, which are similar to the fayalite-bearing veins in morphology. Morphological evidences and thermochemical equilibrium calculations suggest that fayalite replaced magnetite, and that replacement occurred at temperatures < 200 °C and low water/rock mass ratios from 0.07 to 0.18, which represent aqueous to metamorphic transition. Fayalite grains typically show iron-magnesium zoning (fayalite content decreases towards the grain edges). Based on equilibrium models, this zoning may have occurred at increasing temperature. The observed initial ratio of (⁵³Mn/⁵⁵Mn)₀ = (2.3 ± 0.5) × 10^{- 6} suggests that fayalite formed ~ 5 Ma before the timing when the Mn-Cr system was closed in angrite NWA 4801 and has an absolute age of ~ 4563 ± 1 Ma. The age of fayalite is identical within errors to that in Mokoia and Kaba CV3 chondrites, which belong to the Bali-like oxidized subgroup. The identical age implies that aqueous alteration occurred at the same time in parent asteroids of Bali-like subgroup materials. These fayalite-bearing materials may have been derived from a single CV3 asteroid or from separate CV3 asteroids where aqueous alteration simultaneously occurred.