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

1G.J.MacPherson,2A.N.Krot,3N.T.Kita,4E.S.Bullock,2K.Nagashima,3,5T.Ushikubo,1,6M.A.Ivanova
Geochimica et Cosmochimica Acta (in Press) lIk to Article [https://doi.org/10.1016/j.gca.2021.12.033]
1Dept. of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, DC, USA 20560
2Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706
4Carnegie Institution for Science, Earth and Planets Laboratory, 5241 Broad Branch Rd., N.W., Washington, DC 20015, USA
5Kochi Institute for Core Sample Research, JAMSTEC, Nankoku, Kochi 783-8502, Japan
6Vernadsky Institute, Kosygin St. 19, Moscow, Russia
Copyright Elsevier

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, and the reaction may be triggered by influx of hydrogen from the gas. 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 proto-Sun was in flare-up stage or quiescent.

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