||Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano
Kawasaki, Noriyuki ,
Itoh, Shoichi ,
Sakamoto, NaoyaYurimoto, Hisayoshi
Geochimica et cosmochimica acta
102 , 2017-03-15 , Elsevier
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 Al-26-Mg-26 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 delta Mg-25 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 O-16-rich compositions (Delta O-17 similar to -24 parts per thousand) and on an Al-Mg evolutionary diagram plots along model isochron with an initial value of (Al-26/Al-27)(0) = (5.6 +/- 0.2) X 10(-5). The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Delta O-17 similar to -12 parts per thousand and -17 parts per thousand) and plots on an isochron with (Al-26/Al-27)(0) = (5.6 +/- 0.2) X 10(-5). The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Delta O-17 along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from O-16-poor (Delta O-17 values larger than -10 parts per thousand) to O-16-rich (Delta 17 O similar to -25 parts per thousand). The six reversely zoned melilite crystals show indistinguishable initial Al-26/Al-27 values with an average (Al-26/Al-27)(0) of (4.7 +/- 0.3) X 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 O-16-rich compositions (Delta O-17 similar to -23 parts per thousand) with (Al-26/Al-27)(0) = (4.5 +/- 0.4) X 10(-5). The values of (Al-26/Al-27)(0) are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18 +/- 0.07 Myr from the difference in initial (Al-26/Al-27) values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from O-16-rich to O-16-poor and back to O-16-rich during the first similar to 0.2 Myr of Solar System formation. (C) 2016 Elsevier Ltd. All rights reserved.