Oxygen isotopic composition of the solar nebula gas inferred from high‐precision isotope imaging of melilite crystals in an Allende CAI

Abstract– High‐precision isotope imaging analyses of reversely zoned melilite crystals in the gehlenitic mantle of Type A CAI ON01 of the Allende carbonaceous chondrite reveal that there are four types of oxygen isotopic distributions within melilite single crystals: (1) uniform depletion of ¹⁶O (δ¹⁸O ≈ ?10‰), (2) uniform enrichment of ¹⁶O (δ¹⁸O ≈ ?40‰), (3) variations in isotopic composition from ¹⁶O‐poor core to ¹⁶O‐rich rim (δ¹⁸O ≈ ?10‰ to ?30‰, ?20‰ to ?45‰, and ?10‰ to ?35‰) with decreasing åkermanite content, and (4) ¹⁶O‐poor composition (δ¹⁸O ≥ ?10‰) along the crystal rim. Hibonite, spinel, and perovskite grains are ¹⁶O‐rich (δ¹⁸O ≈ ?45‰), and adjoin ¹⁶O‐poor melilites. Gas‐solid or gas‐melt isotope exchange in the nebula is inconsistent with both the distinct oxygen isotopic compositions among the minerals and the reverse zoning of melilite. Fluid‐rock interaction on the parent body resulted in ¹⁶O‐poor compositions of limited areas near holes, cracks, or secondary phases, such as anorthite or grossular. We conclude that reversely zoned melilites mostly preserve the primary oxygen isotopic composition of either ¹⁶O‐enriched or ¹⁶O‐depleted gas from which they were condensed. The correlation between oxygen isotopic composition and åkermanite content may indicate that oxygen isotopes of the solar nebula gas changed from ¹⁶O‐poor to ¹⁶O‐rich during melilite crystal growth. We suggest that the radial excursions of the inner edge of the protoplanetary disk gas simultaneously resulted in both the reverse zoning and oxygen isotopic variation of melilite, due to mixing of ¹⁶O‐poor disk gas and ¹⁶O‐rich coronal gas. Gas condensates aggregated to form the gehlenite mantle of the Type A CAI ON01.