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

ABSTRACT: In order to explore the link between comet 81P/Wild 2 and materials in primitive meteorites, seven particles 5 to 15 ?m 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 ?17O (= ?17O - 0.52 × ?18O) values from -2%0 to +1%0, similar to previous studies, while one enstatite (En99) particle shows lower ?17O value of -7±4%o (2?). 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 ?17O 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 ?18O values of -51.2 ± 1.5%0 (2?) and -43.0 ± 1.3% (2?), respectively, and ?17O values of -22.3 ± 1.9% (2?) and -21.3 ± 2.3%0 (2?), respectively. They are the first 16O-rich pyroxenes found among 81P/Wild 2 particles, with similar ?17O 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.

SUBMITTER: Defouilloy C

PROVIDER: S-EPMC6350803 | biostudies-literature | 2017 May

REPOSITORIES: biostudies-literature

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

Defouilloy Céline C Nakashima Daisuke D Joswiak David J DJ Brownlee Donald E DE Tenner Travis J TJ Kita Noriko T NT

Earth and planetary science letters 20170322

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 <sup>16</sup>O-poor cluster). Four particles extracted from Track 149 are <sup>16</sup>O-poor an ...[more]

PMID: 30705461

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Project description:A close relationship between CM and CO chondrites has been suggested by previous petrologic and isotopic studies, leading to the suggestion that they may originate from similar precursor materials or even a common parent body. In this study, we evaluate the genetic relationship between CM and CO chondrites using Ti, Cr, and O isotopes. We first provide additional constraints on the ranges of ε50Ti and ε54Cr values of bulk CM and CO chondrites by reporting the isotopic compositions of CM2 chondrites Murchison, Murray, and Aguas Zarcas and the CO3.8 chondrite Isna. We then report the ε50Ti and ε54Cr values for several ungrouped and anomalous carbonaceous chondrites that have been previously reported to exhibit similarities to the CM and/or CO chondrite groups, including Elephant Moraine (EET) 83226, EET 83355, Grosvenor Mountains (GRO) 95566, MacAlpine Hills (MAC) 87300, MAC 87301, MAC 88107, and Northwest Africa (NWA) 5958, and the O-isotope compositions of a subset of these samples. We additionally report the Ti, Cr, and O isotopic compositions of additional ungrouped chondrites LaPaz Ice Field (LAP) 04757, LAP 04773, Lewis Cliff (LEW) 85332, and Coolidge to assess their potential relationships with known carbonaceous and ordinary chondrite groups. LAP 04757 and LAP 04773 exhibit isotopic compositions indicating they are low-FeO ordinary chondrites. The isotopic compositions of Murchison, Murray, Aguas Zarcas, and Isna extend the compositional ranges defined by the CM and CO chondrites in ε50Ti versus ε54Cr space. The majority of the ungrouped carbonaceous chondrites with documented similarities to the CM and/or CO chondrites plot outside the CM and CO group fields in plots of ε50Ti versus ε54Cr, Δ17O versus ε50Ti, and Δ17O versus ε54Cr. Therefore, based on differences in their Ti, Cr, and O isotopic compositions, we conclude that the CM, CO, and ungrouped carbonaceous chondrites likely represent samples of multiple distinct parent bodies. We also infer that these parent bodies formed from precursor materials that shared similar isotopic compositions, which may indicate formation in regions of the protoplanetary disk that were in close proximity to each other.

Dataset Information

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

Publications

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

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