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

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Title
Melilite condensed from an 16O-poor gaseous reservoir: Evidence from a fine-grained Ca-Al-rich inclusion of Northwest Africa 8613
Other Titles
태양계 최초 물질내 멜릴라이트의 산소동위원소 조성: 산소-16이 적은 가스에서 응축되었다는 동위원소 증거 발견
Authors
Wada, Sohei
Kawasaki, Noriyuki
Park, Changkun
Yurimoto, Hisayoshi
Subject
Geochemistry & Geophysics
Keywords
Ca-Al-rich inclusions; Oxygen isotope; Al?Mg systematics; Early solar system; Secondary ion mass spectrometry
Issue Date
2020-11
Citation
Wada, Sohei, et al. 2020. "Melilite condensed from an 16O-poor gaseous reservoir: Evidence from a fine-grained Ca-Al-rich inclusion of Northwest Africa 8613". GEOCHIMICA ET COSMOCHIMICA ACTA, 288(1): 161-175.
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.
URI
https://repository.kopri.re.kr/handle/201206/11883
DOI
https://doi.org/10.1016/j.gca.2020.08.004
Appears in Collections  
2020-2020, Antarctic geology and meteorite research based on the Jangbogo station: Crustal evolution of Victoria Land and characteristics of asteroidal materials (20-20) / Lee, Mi Jung (PE20200)
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