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The Formation and Chronology of the PAT 91501 Impact-Melt L-Chondrite with Vesicle-Metal-Sulfide AssemblagesThe L chondrite Patuxent Range (PAT) 41 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. 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 approximately 2 vol.% and exhibit a similar orientation of long axes. Ar-39-Ar-40 measurements date the time of impact at 4.461 +/- 0.008 Gyr B.P. Cosmogenic noble gases and Be-10 and Al-2l 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 (approximately 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 approximately 2 hours. During this time, approximately 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.
Document ID
20070023478
Acquisition Source
Johnson Space Center
Document Type
Other
Authors
Benedix, G. K. (Museum of Natural History London, United Kingdom)
Ketcham, R. A. (Texas Univ. Austin, TX, United States)
Wilson, L. (Lancaster Univ. United Kingdom)
McCoy, T. J. (Smithsonian Institution Washington, DC, United States)
Bogard, D. D. (NASA Johnson Space Center Houston, TX, United States)
Garrison, D. H. (Barrios Technology, Inc. Houston, TX, United States)
Herzog, G. F. (Rutgers Univ. Piscataway, NJ, United States)
Xue, S. (Rutgers Univ. Piscataway, NJ, United States)
Klein, J. (Pennsylvania Univ. Philadelphia, PA, United States)
Middleton, R. (Pennsylvania Univ. Philadelphia, PA, United States)
Date Acquired
August 23, 2013
Publication Date
January 1, 2007
Subject Category
Lunar And Planetary Science And Exploration
Funding Number(s)
CONTRACT_GRANT: NSF EAR-03-45710
CONTRACT_GRANT: NSF EAR-01-13480
Distribution Limits
Public
Copyright
Public Use Permitted.

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