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 Delta O-17 similar to 23 parts per thousand and 1 parts per thousand. Hibonite and spinel are uniformly O-16-rich (Delta O-17 = similar to 23 parts per thousand) irrespective of their occurrences, while melilite crystals exhibit wide O-isotope variations ranging between 4 O-17 similar to -23 parts per thousand and 1 parts per thousand. The O-isotopic composition in a melilite crystal changes abruptly within similar to 2 mu m, indicating that disturbances of O-isotopes in melilite after condensation are less than similar to 2 mu m. Because the melilite in the FGI typically has grain sizes of 5-10 mu m, 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 mu m) and oscillatory chemical zoning, exhibit O-16-poor compositions with small variations ranging between Delta O-17 similar to -4 and 1 parts per thousand. Among them, a large melilite crystal (similar to 20 mu m) with homogeneously O-16-poor composition (Delta O-17 similar to 0 parts per thousand) across the single crystal was found. The coexistence of O-16-poor and O-16-rich melilite crystals without O-isotope disturbances in the FGI reveals that O-16-poor (Delta O-17 similar to 0 parts per thousand) nebular gas existed in the formation region of the FGI HKD01 in addition to O-16-rich (Delta O-17 similar to-23 parts per thousand) nebular gas.