The formation and chronology of the PAT 91501 impact-melt L chondrite with vesicle-metal-sulfide assemblages |
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Authors: | GK Benedix RA Ketcham TJ McCoy DH Garrison S Xue R Middleton |
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Institution: | a Impact and Astromaterials Research Centre (IARC)/Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK b Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA c Environmental Science Department, Lancaster University, Lancaster LA1 4YQ, UK d Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA e ARES, NASA-JSC, Houston, TX 77058, USA f ESCG/Barrios Technology, Houston, TX 77058, USA g Rutgers University, Piscataway, NJ 08854-8087, USA h University of Pennsylvania, Philadelphia, PA 19104, USA |
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Abstract: | 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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