First evidence for silica condensation within the solar protoplanetary disk.
basic_science · Level V
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- Record sourced from PubMed, PMID 29967181.
- Also identified by DOI 10.1073/pnas.1722265115 and PMC identifier 6055204.
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Abstract
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 (Δ<sup>17</sup>O ∼ -24‰) compositions, most likely near the protosun. Here we describe a <sup>16</sup>O-rich (Δ<sup>17</sup>O ∼ -22 ± 2‰) AOA from the carbonaceous Renazzo-type (CR) chondrite Yamato-793261 containing both (<i>i</i>) an ultrarefractory CAI and (<i>ii</i>) 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<sup>-4</sup> bar. Condensation of silica from a hot, chemically fractionated gas could explain the origin of silica identified from infrared spectroscopy of remote protostellar disks.