Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano
Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano
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维加拉诺 (Vigarano) 蓬松 A 型 CAI 中记录的太阳原行星盘氧同位素组成的年代学研究
DOI：
10.1016/j.gca.2015.12.031
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
Yurimoto H.
中科院分区：
文献类型：
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作者：
Kawasaki N.;Itoh S.;Sakamoto N.;Yurimoto H.
含有反向分带黄长石晶体的A型蓬松富钙铝包裹体(CAI)被认为是太阳星云气体直接凝聚的集合体。我们对来自维加拉诺的蓬松A型矿物V2-01的26Al-−26 mg系统进行了研究，获得了已知的反向分带黄长石晶体的氧同位素分布，并对共生矿物进行了氧同位素测量。六个反向分带的黄长石晶体中有两个显示出镁同位素组成的连续变化，δ25 mg沿推断的晶体生长方向减少，这支持了它们来自于凝聚的观点。岩石学研究表明，V2-01的组成矿物形成顺序为：先是黄长石包裹的尖晶石和方沸石，然后是反向分带的黄长石晶体，以及华克-罗弗林边缘的尖晶石和透辉石。黄长岩包体的尖晶石具有16O富组分(Δ17O、∼和−24‰)，Al-−-Mg演化图上沿模型等时线作图，初始值为(26Al/27Al)0=(5.6Al/27Al)0×10−5。黄长岩包体的片麻岩具有不同的氧同位素组成(Δ17O、∼、−12‰和−17‰)和(26Al/27Al)0=0(5.6±0.2)×10−5等时线图。逆分带黄长岩的氧同位素组成显示Δ17O沿晶体生长方向连续变化在反向分带黄长岩形成过程中，由贫16O(Δ17O值大于−10‰)变为富16O(Δ17O、∼和−25‰)。6个反向分带黄长石晶体的26Al/27Al初始值难以区分，平均(26Al/27Al)0为(4.7%±100.3)×10−5，与包体尖晶石和方沸石的值明显不同，表明其形成年龄比包体的尖晶石和方沸石年轻。Wark-Lovering边缘的尖晶石和透辉石显示富16O组分(Δ17O和∼和−23‰)，(26Al/27Al)0=0(4.50±40.4)×10−5。(26Al/27Al)0的值与岩相学推断的形成序列一致。根据26Al/27Al初始值的差异，估计V2-01CaI的形成周期为0.18±0.07 MYR。这些数据表明，在第一个∼0.2Myr太阳系形成过程中，围绕CaI的太阳星云气体的氧同位素组成从富16O转变为贫16O，再回到富16O。
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 of26Al−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 has16O-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 from16O-poor (Δ17O values larger than −10‰) to16O-rich (Δ17O ∼ −25‰). The six reversely zoned melilite crystals show indistinguishable initial26Al/27Al values with an average (26Al/27Al)0of (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 show16O-rich compositions (Δ17O ∼ −23‰) with (26Al/27Al)0= (4.5 ± 0.4) × 10−5. The values of (26Al/27Al)0are 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 initial26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from16O-rich to16O-poor and back to16O-rich during the first ∼0.2 Myr of Solar System formation.