Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano

Abstract

Fluffy Type A Ca–Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be aggregates of direct condensates from solar nebular gas. We conducted an investigation of 26Al−26Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals; we also conducted oxygen isotope measurements of coexisting minerals. Two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition, with δ25Mg decreasing along the inferred direction of crystal growth, which supports the idea that they originated through condensation. Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark–Lovering rim. The spinel enclosed by melilite has 16O-rich compositions (Δ17O∼−24‰) and on an Al−Mg evolutionary diagram plots along model isochron with an initial value of (26Al/27Al)0=(5.6±0.2)×10−5. The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ17O∼−12‰ and −17‰) and plots on an isochron with (26Al/27Al)0=(5.6±0.2)×10−5. The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ17O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16O-poor (Δ17O values larger than −10‰) to 16O-rich (Δ17O∼−25‰). The six reversely zoned melilite crystals show indistinguishable initial 26Al/27Al values with an average (26Al/27Al)0 of (4.7±0.3)×10−5, which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark–Lovering rim show 16O-rich compositions (Δ17O∼−23‰) with (26Al/27Al)0=(4.5±0.4)×10−5. The values of (26Al/27Al)0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18±0.07Myr from the difference in initial 26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16O-rich to 16O-poor and back to 16O-rich during the first ∼0.2Myr of Solar System formation.