First evidence for silica condensation within the solar protoplanetary disk

Mutsumi Komatsu, Timothy J. Fagan, Alexander N. Krot, Kazuhide Nagashima, Michail I. Petaev, Makoto Kimura, and Akira Yamaguchi

PNAS July 2, 2018. 201722265; published ahead of print July 2, 2018.


“The oldest solar system solids dated are refractory inclusions [Ca-Al–rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], which occur in chondritic meteorites and provide records of high-temperature processes in the early solar system. An ultrarefractory CAI and the silica-phase quartz occur in an AOA from the carbonaceous chondrite Yamato-793261, indicating formation over a temperature range exceeding 650 K. The minerals have 16O-rich compositions consistent with the nebular setting associated with refractory inclusions. This AOA provides direct evidence that silica condensed from gas in a CAI/AOA-forming region in our solar system indicates that gas became Si-rich as Mg condensed and may explain the origin of silica detected from infrared spectroscopy of T Tauri and asymptotic giant branch stars.”

“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.”