The formation and chronology of the PAT 91501 impact-melt L-chondrite with vesicle-metal-sulfide assemblages.

Benedix, G. K. and Ketcham, R. A. and Wilson, Lionel and McCoy, T. J. and Bogard, D. D. and Garrison, D. H. and Herzog, G. F. and Xue, S. and Klein, J. and Middleton, R. (2008) The formation and chronology of the PAT 91501 impact-melt L-chondrite with vesicle-metal-sulfide assemblages. Geochimica et Cosmochimica Acta, 72 (9). pp. 2417-2428. ISSN 0016-7037

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The L chondrite Patuxent Range (PAT) 91501 is an 8.5-kg unshocked, homogeneous, igneous-textured impact melt that cooled slowly compared to other meteoritic impact melts in a crater floor melt sheet or sub-crater dike [Mittlefehldt D. W. and Lindstrom M. M. (2001) Petrology and geochemistry of Patuxent Range 91501 and Lewis Cliff 88663. Meteoritics Planet. Sci. 36, 439–457]. We conducted mineralogical and tomographic studies of previously unstudied mm- to cm-sized metal–sulfide–vesicle assemblages and chronologic studies of the silicate host. Metal–sulfide clasts constitute about 1 vol.%, comprise zoned taenite, troilite, and pentlandite, and exhibit a consistent orientation between metal and sulfide and of metal–sulfide contacts. Vesicles make up 2 vol.% and exhibit a similar orientation of long axes. 39Ar–40Ar measurements probably date the time of impact at 4.461 ± 0.008 Gyr B.P. Cosmogenic noble gases and 10Be and 26Al activities suggest a pre-atmospheric radius of 40–60 cm and a cosmic ray exposure age of 25–29 Myr, similar to ages of a cluster of L chondrites. PAT 91501 dates the oldest known impact on the L chondrite parent body. The dominant vesicle-forming gas was S2 (15–20 ppm), which formed in equilibrium with impact-melted sulfides. The meteorite formed in an impact melt dike beneath a crater, as did other impact melted L chondrites, such as Chico. Cooling and solidification occurred over 2 h. During this time, 90% of metal and sulfide segregated from the local melt. Remaining metal and sulfide grains oriented themselves in the local gravitational field, a feature nearly unique among meteorites. Many of these metal–sulfide grains adhered to vesicles to form aggregates that may have been close to neutrally buoyant. These aggregates would have been carried upward with the residual melt, inhibiting further buoyancy-driven segregation. Although similar processes operated individually in other chondritic impact melts, their interaction produced the unique assemblage observed in PAT 91501.

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Journal Article
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Geochimica et Cosmochimica Acta
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21 Dec 2009 12:19
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16 Sep 2023 00:22