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.

¹Junya MATSUNO,^1,2,3Akira TSUCHIYAMA,⁴Akira MIYAKE,⁵Keiko NAKAMURA-MESSENGER,^5,6Scott MESSENGER
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.031]
¹Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-7, Japan
²CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
³CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
⁴Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
⁵Johnson Space Center, NASA, Houston, TX 77058, United States
⁶Present address: Blue 22 Software, Houston, TX
Copyright Elsevier

GEMS (Glass with Embedded Metal and Sulfides) grains found in interplanetary dust particles are considered one of the most primitive materials in the Solar System, yet questions remain on how they formed. It has been suggested that GEMS grains are products of radiation processing and amorphization of sulfide and silicate mineral grains in the interstellar medium. Alternatively, GEMS grains are proposed to be disequilibrium condensation products in late-stage protosolar disks. We examined the 3D distributions of elements and inclusions within GEMS grains using TEM (transmission electron microscopic)-tomography to better constrain their possible formation processes. We found some core-shell particles composed of metals and amorphous silicates and observed a binary distribution of Mg/Si in amorphous silicates of GEMS grains. These properties are highly similar to the features of experimental condensation products. Furthermore, the location of sulfides only on the surface of GEMS and their larger sizes than metals are also consistent with the condensation experiments, where sulfides formed by sulfidation of metal grains with S-bearing gas species. Textures showing aggregation and possible coalescence of primary grains were also observed. Therefore, we conclude that GEMS grains are condensates from gas at high temperatures and some of them were aggregated.