[PPS07-P05] O and Al-Mg isotope systematics of a hibonite-melilite-rich fine-grained CAI in the reduced CV chondrite NWA8613
The FG-CAI has an irregular shape with a size of approximately 10 × 12 mm and composed mainly of melilite, hibonite, and spinel. O-isotope compositions of the constituent minerals plot along the CCAM line, ranging between Δ17O ~ −23‰ and 0‰. The FG-CAI is petrographically divided into hibonite-rich core, spinel-rich core, melilite-rich inner-mantle, and hibonite-spinel-rich outer mantle. The CAI is rimmed by thin spinel and diopside layers. Melilite crystals are contained in core and mantle. Melilite crystals in the core exhibit normal chemical zoning and O-isotope compositions from Δ17O ~ −23‰ to −10‰ with increasing Åk composition. The melilite crystals in the inner-mantle show chemically complex, oscillatory zoning patterns, but are uniformly 16O-poor (Δ17O ~ 0‰) despite their chemical variations (Åk2−14). Melilite crystals in the outer-mantle exhibit normal chemical zoning and variable O-isotope compositions from Δ17O ~ −17‰ to 0‰ with increasing Åk composition. The spinel and hibonite grains in the FG-CAI are uniformly 16O-rich (Δ17O = −23‰). Al−Mg isotopic compositions of hibonite in the outer-mantle and in the core and melilite in the inner-mantle plot on a single straight line within error; an Al−Mg mineral isochron can be defined to give an initial 26Al/27Al value of (4.50 ± 0.09) × 10−5. Given that the constituent minerals formed by condensation and were accumulated to form the FG-CAI, the nebular gas, from which they condensed, had variable O-isotope compositions between Δ17O ~ −23‰ and 0‰. The Al−Mg systematics of the FG-CAI indicate that these formation events occurred at 0.16 ± 0.02 Myr after the formation of canonical CAIs , if 26Al was homogeneously distributed. Our data support the presence of the nebular gas with variable O-isotope compositions during the first ~0.2 Myr of the Solar System formation .
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