1Alexander N.Krot,1Kazuhide Nagashima,2Glenn J.MacPherson,3Alexander A.Ulyanov
Geochimica et Cosmochimica Act a(in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.013]
1Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
2Department of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA
3Department of Geology, Moscow State University, Moscow, 119992, Russia
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.