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

Céline Defouilloy, Daisuke Nakashima, David J. Joswiak, Donald E. Brownlee, Travis J. Tenner, Noriko T. Kita

Earth and Planetary Science Letters,
Volume 465, 1 May 2017, Pages 145-154


• 7 silicates grains from comet Wild 2 were analyzed for their oxygen isotope ratios.
• 16O-poor particles show Δ17O-Mg# range and variations similar to CR chondrites.
• We found the first 16O-rich pyroxenes.
• 16O-rich pyroxenes show similarities with AOAs enstatites and are likely condensates.”

“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 (View the MathML source=δO17−0.52×δO18) values from −2‰ to +1‰, similar to previous studies, while one enstatite (En99) particle shows lower Δ17O value of −7±4‰−7±4‰(2σ)(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‰−51.2±1.5‰(2σ)(2σ) and −43.0±1.3‰−43.0±1.3‰(2σ)(2σ), respectively, and Δ17O values of −22.3±1.9‰−22.3±1.9‰(2σ)(2σ) and −21.3±2.3‰−21.3±2.3‰(2σ)(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.”