Evidence for oxygen isotopic exchange in chondrules from Kaba (CV3.1) carbonaceous chondrite during aqueous fluid-rock interaction on the CV parent asteroid.

ABSTRACT

We report on the mineralogy, petrography, and oxygen isotopic compositions of primary olivine and plagioclase/feldspathic mesostases in chondrules and of secondary magnetite and fayalite in chondrules and matrix of an oxidized Bali-like CV3.1 carbonaceous chondrite, Kaba. In this meteorite, compositionally nearly pure fayalite (Fa98-100) associates with hedenbergite (Fs~50Wo~50), magnetite, and Fe,Ni-sulfides. There are several textural occurrences of this mineral paragenesis (i) coarse-grained intergrowths in interchondrule matrix, (ii) veins starting at the opaque nodules in the peripheries of type I chondrules and crosscutting fine-grained rims around them, and (iii) rims overgrowing olivine of type I and type II chondrule fragments. Oxygen isotopic compositions of fayalite and magnetite are in disequilibrium with chondrule olivines. On a three-isotope oxygen diagram, δ17O vs. δ18O, compositions of olivine plot along primitive chondrule minerals (PCM) line having a slope of ~1.0; deviations from the terrestrial fractionation line, Δ17O = δ17O - 0.52 × Î´18O, range from ~-8‰ to ~-5‰. In contrast, fayalite and magnetite plot along mass-dependent fractionation line with a slope of ~0.5; their δ18O values range from -1 to ~+9‰; Δ17O is nearly constant (average ± 2SE = -1.5±1‰). Oxygen isotopic compositions of chondrule plagioclase and feldspathic mesostases are in disequilibrium with chondrule olivines they deviate to the right from the PCM line by ~12‰ and plot close to the mass-dependent fractionation line defined by fayalite and magnetite. Based on the mineralogy, petrography, oxygen isotopic compositions of fayalite and magnetite, and the previously published thermodynamic analysis of the fayalite-bearing assemblages in ordinary and carbonaceous chondrites, we conclude that Kaba fayalite and magnetite formed during aqueous fluid-rock interaction at low water/rock ratio (0.1-0.2) and elevated temperatures (~200-300°C) on the CV chondrite parent asteroid. The Δ17O values of Kaba fayalite and magnetite (-1.5±1‰) correspond to Δ17O of aqueous fluid that operated on the CV chondrite parent asteroid and resulted in its alteration. Plagioclase and feldspathic mesostases in Kaba chondrules experienced postcrystallization oxygen isotopic exchange with this 16O-depleted fluid; olivine grains retained their original compositions acquired during chondrule melts crystallization. The inferred oxygen isotopic exchange in Kaba chondrules appear to have not affected their Al-Mg isotope systematics.