On the nature of oxygen-isotope heterogeneity of igneous calcium-aluminum-rich inclusions in cv carbonaceous chondrites

Abstract

Coarse-grained igneous Ca,Al-rich inclusions (CAIs) in CV (Vigarano group) carbonaceous chondrites have typically heterogeneous O-isotope compositions with melilite, anorthite, and high-Ti (>10 wt% TiO2) fassaite being 16O-depleted (Δ17O up to ∼ − 3 ± 2‰) compared to hibonite, spinel, low-Ti (<10 wt% TiO2) fassaite, Al-diopside, and forsterite, all having close-to-solar Δ17O ∼ − 24 ± 2‰. To test a hypothesis that this heterogeneity was established, at least partly, during aqueous fluid-rock interaction, we studied the mineralogy, petrology, and O-isotope compositions of igneous CAIs CG-11 (Type B), TS-2F-1, TS-68, and 818-G (Compact Type A), and 818-G-UR (davisite-rich) from Allende (CV > 3.6), and E38 (Type B) from Efremovka (CV3.1–3.4). Some of these CAIs contain (i) eutectic mineral assemblages of melilite, Al,Ti-diopside, and ± spinel which co-crystallized and therefore must have recorded O-isotope composition of the eutectic melt; (ii) isolated inclusions of Ti-rich fassaite inside spinel grains which could have preserved their initial O-isotope compositions, and/or (iii) pyroxenes of variable chemical compositions which could have recorded gas–melt O-isotope exchange during melt crystallization and/or postcrystallization exchange controlled by O-isotope diffusivity. If these CAIs experienced isotopic exchange with an aqueous fluid, O-isotope compositions of some of their primary minerals are expected to approach that of the fluid.We find that in the eutectic melt regions composed of highly-åkermanitic melilite (Åk65−71), anorthite, low-Ti fassaite, and spinel of E38, spinel, fassaite, and anorthite are similarly 16O-rich (Δ17O ∼ − 24‰), whereas melilite is 16O-poor (Δ17O ∼ − 1‰). In the eutectic melt regions of CG-11, spinel and low-Ti fassaite are 16O-rich (Δ17O ∼ − 24‰), whereas melilite and anorthite are 16O-poor (Δ17O ∼ − 3‰). In TS-2F-1, TS-68, and 818-G, melilite and high-Ti fassaite grains outside spinel have 16O-poor compositions (Δ17O range from − 12 to − 3‰); spinel is 16O-rich (Δ17O ∼ − 24‰); perovskite grains show large variations in Δ17O, from − 24 to − 1‰. Some coarse perovskites are isotopically zoned with a 16O-rich core and a 16O-poor edge. Isolated high-Ti fassaite inclusions inside spinel grains are 16O-rich (Δ17O ∼ − 24‰), whereas high-Ti fassaite inclusions inside fractured spinel grains are 16O-depleted: Δ17O range from − 12 to − 3‰. In 818-G-UR, davisite is 16O-poor (Δ17O ∼ − 2‰), whereas Al-diopside of the Wark-Lovering rim is 16O-enriched (Δ17O < − 16‰). On a three-isotope oxygen diagram, the 16O-poor melilite, anorthite, high-Ti fassaite, and davisite in the Allende CAIs studied plot close to O-isotope composition of an aqueous fluid (Δ17O ∼ − 3 ± 2‰) inferred from O-isotope compositions of secondary minerals resulted from metasomatic alteration of the Allende CAIs.We conclude that CV igneous CAIs experienced post-crystallization O-isotope exchange that most likely resulted from an aqueous fluid-rock interaction on the CV asteroid. It affected melilite, anorthite, high-Ti fassaite, perovskite, and davisite, whereas hibonite, spinel, low-Ti fassaite, Al-diopside, and forsterite retained their original O-isotope compositions established during igneous crystallization of CV CAIs. However, we cannot exclude some gas–melt O-isotope exchange occurred in the solar nebula. This apparently “mineralogically-controlled” exchange process was possibly controlled by variations in oxygen self-diffusivity of CAI minerals. Experimentally measured oxygen self-diffusion coefficients in CAI-like minerals are required to constrain relative roles of O-isotope exchange during aqueous fluid–solid and nebular gas–melt interaction.