12:30 PM - 12:45 PM
[PPS22-13] Al-Mg chronology and oxygen isotope distributions of multiple melting for a Type C CAI from the Allende
The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequences of the core minerals are spinel, anorthite, olivine, and pyroxene. The mantle has the same mineral assemblages as the core, and shows incomplete melting and solidification textures.
Oxygen isotopic compositions of the minerals are distributed along with a carbonaceous chondrite anhydrous mineral (CCAM) line (δ18O = -44 to +9‰), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows 16O-rich signature (δ18O ~ -43‰), while anorthite shows 16O-poor signature (δ18O ~ +8‰). Olivine and pyroxene in the core have the same oxygen isotopic composition (δ18O ~ -15‰), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16O (δ18O = -13 to -4‰) compared with the same minerals in the core. The 26Al-Mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (26Al/27Al)0 = (3.5±0.2) x 10-5, anorthite is (-1±5) x 10-7, and olivine and pyroxene in the core are (-1±7) x 10-6. Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core.
The coordinated study of the oxygen and magnesium isotopes and the petrography indicates that the EK1-04-2 Type C CAI underwent multiple heating events after a precursor CAI formation. The precursor CAI was formed at 0.43 Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6 Myr after the precursor CAI formation, the CAI was partially melted and the partial melting melt exchanged oxygen isotopes with surrounding 16O-poor nebular gas. 16O-poor olivine and pyroxene in the core were recrystallized from the partial melting melt. Subsequently, Al-rich chondrules accreted on the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6 Myr and underwent multiple melting events in the nebula, and oxygen and 26Al-Mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.