First evidence for silica condensation within the solar protoplanetary disk

Area of Science:

• Cosmochemistry
• Planetary Science
• Meteoritics

Background:

• Calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) are refractory components of chondritic meteorites.
• These formed in high-temperature regions of the protoplanetary disk near the protosun with solar chemical and oxygen isotopic compositions.

Purpose of the Study:

• To investigate the formation conditions of a unique 16O-rich AOA from the CR chondrite Yamato-793261.
• To provide direct evidence for gas-solid condensation of silica in a CAI/AOA-forming region.

Main Methods:

• Analysis of a 16O-rich AOA containing an ultrarefractory CAI, forsterite, low-Ca pyroxene, and silica.
• Thermodynamic modeling of nebular gas condensation under specific pressure and cooling conditions.

Main Results:

• The AOA formed via gas-solid reactions over a wide temperature range (approx. 1,800 to 1,150 K).
• Silica condensation occurred in a gas with a fractionated Mg/Si ratio, likely due to prior forsterite grain condensation.
• Thermodynamic modeling indicated silica formed from nebular gas depleted in H and He, cooled at 50 K/hour at 10^-4 bar.

Conclusions:

• This AOA provides direct evidence for silica condensation in CAI/AOA-forming regions.
• The formation conditions suggest a chemically fractionated solar nebula.
• This process may explain silica observations in remote protostellar disks.