The retention of dust in protoplanetary disks: Evidence from agglomeratic olivine chondrules from the outer Solar System

1Devin L. Schrader, 2Kazuhide Nagashima, 3Scott R. Waitukaitis, 4Jemma Davidson, 5Timothy J. McCoy, 6,7Harold C. Connolly Jr., 7Dante S. Lauretta
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.12.014]
1Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
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
3Leiden University, Huygens Laboratory, Niels Bohrweg 2, 2333 CA Leiden, Netherlands
4Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, District of Columbia 20015–1305, USA
5Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10[th] & Constitution Avenue NW, Washington, DC 20560-0119, USA
6Department of Geology, School of Earth & Environment, Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028, USA
7Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
Copyright Elsevier

By investigating the in situ chemical and O-isotope compositions of olivine in lightly sintered dust agglomerates from the early Solar System, we constrain their origins and the retention of dust in the protoplanetary disk. The grain sizes of silicates in these agglomeratic olivine (AO) chondrules indicate that the grain sizes of chondrule precursors in the Renazzo-like carbonaceous (CR) chondrites ranged from <1 to 80 µm. We infer this grain size range to be equivalent to the size range for dust in the early Solar System. AO chondrules may contain, but are not solely composed of, recycled fragments of earlier formed chondrules. They also contain 16O-rich olivine related to amoeboid olivine aggregates and represent the best record of chondrule-precursor materials.

AO chondrules contain one or more large grains, sometimes similar to FeO-poor (type I) and/or FeO-rich (type II) chondrules, while others contain a type II chondrule core. These morphologies are consistent with particle agglomeration by electrostatic charging of grains during collision, a process that may explain solid agglomeration in the protoplanetary disk in the micrometer size regime. The petrographic, isotopic, and chemical compositions of AO chondrules are consistent with chondrule formation by large-scale shocks, bow shocks, and current sheets.

The petrographic, isotopic, and chemical similarities between AO chondrules in CR chondrites and chondrule-like objects from comet 81P/Wild 2 indicate that comets contain AO chondrules. We infer that these AO chondrules likely formed in the inner Solar System and migrated to the comet forming region at least 3 Ma after the formation of the first Solar System solids. Observations made in this study imply that the protoplanetary disk retained a dusty disk at least ∼3.7 Ma after the formation of the first Solar System solids, longer than half of the dusty accretion disks observed around other stars.

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