The formation of Type B CAIs: Evolution from Type A CAIs

被引:17
作者
MacPherson, G. J. [1 ]
Krot, A. N. [2 ]
Kita, N. T. [3 ]
Bullock, E. S. [4 ]
Nagashima, K. [2 ]
Ushikubo, T. [3 ,5 ]
Ivanova, M. A. [6 ]
机构
[1] Smithsonian Inst, Museum Nat Hist, Dept Mineral Sci, Washington, DC 20560 USA
[2] Univ Hawaii Manoa, Hawaii Inst Geophys & Planetol, Sch Ocean & Earth Sci & Technol, Honolulu, HI 96822 USA
[3] Univ Wisconsin, Dept Geosci, WiscSIMS, Madison, WI 53706 USA
[4] Carnegie Inst Sci, Earth & Planets Lab, 5241 Broad Branch Rd NW, Washington, DC 20015 USA
[5] Kochi Inst Core Sample Res, JAMSTEC, Nankoku, Kochi 7838502, Japan
[6] Vernadsky Inst, Kosygin St 19, Moscow, Russia
基金
美国国家航空航天局; 美国国家科学基金会;
关键词
CAI; Oxygen isotopes; Al-Mg istopes; Type B; Type A; Pyroxene; GRAINED REFRACTORY INCLUSIONS; OXYGEN-ISOTOPE EXCHANGE; RICH INCLUSIONS; SOLAR NEBULA; CRYSTALLIZATION SEQUENCES; BULK COMPOSITIONS; ALLENDE; AL-26-MG-26; CHONDRITE; ORIGIN;
D O I
10.1016/j.gca.2021.12.033
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Five Type A CAIs from three CV3 chondrites (Vigarano, Northwest Africa 3118, Allende), which differ in age by no more than -105 years, show mineralogical and textural evidence of gradual transition into Type Bs, indicating that Type B inclusions formed by evolution of Type A CAIs in the solar nebula. This model differs from the conventional condensation model in which aggregates of condensate grains form different kinds of CAIs depending on the relative populations of different kinds of grains. In our model the pyroxene forms nearly isochemically by reaction of perovskite with melilite under highly reducing conditions. Anorthite requires the addition of silica from the gas, and originally forms as veins and reaction rims on gehlenitic melilite within Fluffy Type As. Later partial re-melting of these assemblages results in the formation of poikilitic pyroxene and anorthite that enclose rounded (partially melted) tablets of melilite. Oxygen isotopes in four of the CAIs support the formation of Ti-rich 16O-depleted pyroxene from 16O-depleted perovskite, but not in the fifth CAI. An alternative possibility is that Ti-rich 16O-depleted pyroxene is the result of later solid-state exchange that preferentially affects the most Ti-rich pyroxene. Regardless of the origin of the 16O-depleted pyroxene, we give a model for nebular reservoir evolution based on sporadic FU-Orionis flare-ups in which the 16O-rich region near the proto-Sun fluctuated in size depending on whether the protoSun was in flare-up stage or quiescent. Published by Elsevier Ltd.
引用
收藏
页码:343 / 374
页数:32
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