First evidence for silica condensation within the solar protoplanetary disk

摘要

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 (Delta(17) O similar to -24 parts per thousand) compositions, most likely near the protosun. Here we describe a O-16-rich (Delta O-17 similar to -22 +/- 2 parts per thousand) 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 similar to 1,800 to similar 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 similar to 10x 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.