Melilite condensed from an 16O-poor gaseous reservoir: Evidence from a fine-grained Ca-Al-rich inclusion of Northwest Africa 8613

Abstract

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates from the solar nebular gas and thus captured O-isotopes from the gas. We conducted a combined study of petrographic observations and in situ O-isotope analysis using secondary ion mass spectrometry for an FGI, named HKD01, from the reduced CV chondrite Northwest Africa 8613. HKD01 has an irregular shape and petrographically three-layered structures: a hibonite-rich core, a spinel-rich core, and a mantle. Each petrographic domain contains melilite, hibonite, and spinel with variable proportions of those minerals. The O-isotopic compositions of the constituent minerals plotted along the slope-1 line on an O three-isotope diagram ranged between Δ17O ∼ −23‰ and 1‰. Hibonite and spinel are uniformly 16O-rich (Δ17O = −23‰) irrespective of their occurrences, while melilite crystals exhibit wide O-isotope variations ranging between Δ17O ∼ −23‰ and 1‰. The O-isotopic composition in a melilite crystal changes abruptly within ∼2 µm, indicating that disturbances of O-isotopes in melilite after condensation are less than ∼2 µm. Because the melilite in the FGI typically has grain sizes of 5–10 µm, the abrupt change of O-isotopic composition demonstrates that melilite crystals in the FGI preserve the O-isotopic composition of the nebular gas from which they condensed. In the mantle, aggregates of melilite crystals, having relatively large grain sizes (10–25 µm) and oscillatory chemical zoning, exhibit 16O-poor compositions with small variations ranging between Δ17O ∼ −4 and 1‰. Among them, a large melilite crystal (∼20 µm) with homogeneously 16O-poor composition (Δ17O ∼ 0‰) across the single crystal was found. The coexistence of 16O-poor and 16O-rich melilite crystals without O-isotope disturbances in the FGI reveals that 16O-poor (Δ17O ∼ 0‰) nebular gas existed in the formation region of the FGI HKD01 in addition to 16O-rich (Δ17O ∼ –23‰) nebular gas.