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Mineralogy and petrology of Stardust particles encased in the bulb of track 80: TEM investigation of the Wild 2 fine-grained material

Abstract : We have characterized by transmission electron microscopy the mineralogy of samples extracted from the walls of the Stardust track 80. More than 500 fragments were studied using conventional imaging, electron diffraction and EDX microanalysis. Two categories of particles are distinguishable in equal proportions (wt%). The first one is comprised of relatively large crystalline grains (≈1 μm on average), dominated by silicates (olivine and pyroxene). They display a wide range of compositions and microstructures comparable to those found in terminal particles. Minor phases including magnetite and apatite are also present. Their occurrence suggests that the Wild 2 material underwent aqueous alteration to some extent. The second type of particle, called GEMS-like material, is made of silica-rich glassy clumps embedding iron sulfide beads and vesicles. Their microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. This melted material may form by shedding of melted and vaporized material, but given the shape of the impact track and high diversity of surviving mineral compositions, much of it originated from fine-grained aggregates that disaggregated during the collection. Chemical mapping at the nano-scale allowed the localization of individual components within the silica-rich glass. They are dominated by Mg-rich components with a size less than 300 nm. The average composition of this thermally modified material is close to the solar abundance for the major elements Fe, Mg and S. The fine-grained material has probably not been chemically fractionated in the protoplanetary disk before its incorporation in comet Wild 2 unlike the sulfur depleted matrix of chondrites. From these two categories of particles, we deduce that Wild 2 is likely made of an assemblage of relatively large evolved grains (first category) cemented by a fine-grained material with primitive chemistry (second category). The pre-impact configuration of the incident material deduced from this study seems comparable to the matrix of the most primitive chondrites (3.0) or to chondritic porous interplanetary dust particles.
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Julien Stodolna, Damien Jacob, Hugues Leroux. Mineralogy and petrology of Stardust particles encased in the bulb of track 80: TEM investigation of the Wild 2 fine-grained material. Geochimica et Cosmochimica Acta, Elsevier, 2012, Geochimica et Cosmochimica Acta, 87, pp.35-50. ⟨10.1016/j.gca.2012.03.026⟩. ⟨hal-03147291⟩



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