Origin of crystalline silicates from Comet 81P/Wild 2: Combined study on their oxygen isotopes and mineral chemistry

摘要

In order to explore the link between comet 81P/Wild 2 and materials in primitive meteorites, seven particles 5 to 15 μm in diameter from comet 81P/Wild 2 have been analyzed for their oxygen isotope ratios using a secondary ion mass spectrometer. Most particles are single minerals consisting of olivine or pyroxene with Mg# higher than 85, which are relatively minor in 81P/Wild 2 particles (~1/3 of the ¹⁶O-poor cluster). Four particles extracted from Track 149 are ¹⁶O-poor and show Δ¹⁷O (= δ¹⁷O − 0.52 × δ¹⁸O) values from −2%0 to +1%0, similar to previous studies, while one enstatite (En99) particle shows lower Δ¹⁷O value of −7±4%o (2σ). This compositional range has not been reported among ¹⁶O-poor particles in 81P/Wild 2, but is commonly observed among chondrules in carbonaceous chondrites and in particular in CR chondrites. The distribution in Δ¹⁷O indicates that ¹⁶O-poor 81P/Wild 2 particles are most similar to chondrules (and their fragments) in the CR chondrites and Tagish Lake-like WIS91600 chondrite chondrule silicate grains, which indicates that they likely come from a reservoir with similar dust/ice ratios as CR chondrites and WIS91600. However, differences in the Mg# distribution imply that the 81P/Wild 2 reservoir was comparatively more oxidized, with a higher dust enrichment. Two nearly pure enstatite grains from track 172 are significantly enriched in ¹⁶O, with δ¹⁸O values of −51.2 ± 1.5%0 (2σ) and −43.0 ± 1.3% (2σ), respectively, and Δ¹⁷O values of −22.3 ± 1.9% (2σ) and −21.3 ± 2.3%0 (2σ), respectively. They are the first ¹⁶O-rich pyroxenes found among 81P/Wild 2 particles, with similar Δ¹⁷O values to those of ¹⁶O-rich low-iron, manganese-enriched (LIME) olivine and CAI (calcium and aluminum-rich inclusions) -like particles from 81P/Wild 2. The major element and oxygen isotopic compositions of the pyroxenes are similar to those of enstatite in amoeboid olivine aggregates (AOAs) in primitive chondrites, in which ¹⁶O-rich pyroxenes have previously been found, and thus suggest a condensation origin.