First evidence for silica condensation within the solar protoplanetary disk

1,2Mutsumi Komatsu, 2Timothy J. Fagan, 3Alexander N. Krot, 3Kazuhide Nagashima, 4,5Michail I. Petaev, 6,7Makoto Kimura, 6,8Akira Yamaguchi
Proceedings of the National Academy of Sciences of the United States of America
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1The Graduate University for Advanced Studies (SOKENDAI), Hayama, 240-0193 Kanagawa, Japan
2Department of Earth Sciences, Waseda University, Shinjuku, 169-8050 Tokyo, Japan
3Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822
4Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
5Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138
6National Institute of Polar Research, Tachikawa, 190-8518 Tokyo, Japan
7Ibaraki University, 310-8512 Mito, Japan
8Department of Polar Science, School of Multidisciplinary Science, SOKENDAI, Tachikawa, 190-8518 Tokyo, Japan

Calcium-aluminum–rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), a refractory component of chondritic meteorites, formed in a high-temperature region of the protoplanetary disk characterized by approximately solar chemical and oxygen isotopic (Δ17O ∼ −24‰) compositions, most likely near the protosun. Here we describe a 16O-rich (Δ17O ∼ −22 ± 2‰) AOA from the carbonaceous Renazzo-type (CR) chondrite Yamato-793261 containing both (i) an ultrarefractory CAI and (ii) forsterite, low-Ca pyroxene, and silica, indicating formation by gas–solid reactions over a wide temperature range from ∼1,800 to ∼1,150 K. This AOA provides direct evidence for gas–solid condensation of silica in a CAI/AOA-forming region. In a gas of solar composition, the Mg/Si ratio exceeds 1, and, therefore, silica is not predicted to condense under equilibrium conditions, suggesting that the AOA formed in a parcel of gas with fractionated Mg/Si ratio, most likely due to condensation of forsterite grains. Thermodynamic modeling suggests that silica formed by condensation of nebular gas depleted by ∼10× in H and He that cooled at 50 K/hour at total pressure of 10−4 bar. Condensation of silica from a hot, chemically fractionated gas could explain the origin of silica identified from infrared spectroscopy of remote protostellar disks.


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