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Oxygen isotopic composition of the solar nebula gas inferred from high‐precision isotope imaging of melilite crystals in an Allende CAI
Author(s) -
PARK Changkun,
WAKAKI Shigeyuki,
SAKAMOTO Naoya,
KOBAYASHI Sachio,
YURIMOTO Hisayoshi
Publication year - 2012
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12032
Subject(s) - melilite , allende meteorite , isotopes of oxygen , anorthite , geology , isotopic shift , mineralogy , analytical chemistry (journal) , oxygen , chondrite , carbonaceous chondrite , pyrope , geochemistry , chemistry , isotope , meteorite , spinel , astrobiology , environmental chemistry , physics , paleontology , organic chemistry , quantum mechanics
– 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 16 O (δ 18 O ≈ −10‰), (2) uniform enrichment of 16 O (δ 18 O ≈ −40‰), (3) variations in isotopic composition from 16 O‐poor core to 16 O‐rich rim (δ 18 O ≈ −10‰ to −30‰, −20‰ to −45‰, and −10‰ to −35‰) with decreasing åkermanite content, and (4) 16 O‐poor composition (δ 18 O ≥ −10‰) along the crystal rim. Hibonite, spinel, and perovskite grains are 16 O‐rich (δ 18 O ≈ −45‰), and adjoin 16 O‐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 16 O‐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 16 O‐enriched or 16 O‐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 16 O‐poor to 16 O‐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 16 O‐poor disk gas and 16 O‐rich coronal gas. Gas condensates aggregated to form the gehlenite mantle of the Type A CAI ON01.