40Ar-39Ar age of Northwest Africa 091: More evidence for a link between L chondrites and fossil meteorites

Authors

  • J. R. WEIRICH,

    Corresponding author
    1. Department of Planetary Sciences, The University of Arizona, 1629 E. University Blvd., Tucson, Arizona 85721, USA
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    • Present address: School of Earth and Space Exploration, Arizona State University, 781 E. Terrace Rd., Tempe, Arizona 85287, USA.

  • T. D. SWINDLE,

    1. Department of Planetary Sciences, The University of Arizona, 1629 E. University Blvd., Tucson, Arizona 85721, USA
    2. Department of Geosciences, The University of Arizona, 1040 E. 4th St., Tucson, Arizona 85721, USA
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  • C. E. ISACHSEN

    1. Department of Geosciences, The University of Arizona, 1040 E. 4th St., Tucson, Arizona 85721, USA
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Corresponding author. E-mail: jweirich@asu.edu

Abstract

Abstract— Most 40Ar-39Ar ages of L chondrites record an event at approximately 500 Ma, indicating a large collisional impact at that time. However, there is a spread in ages from 400 to 600 Ma in these meteorites that is greater than the analytical uncertainty. Identification of, and correction for, trapped Ar in a few L chondrites has given an age of 470 ± 6 Ma. This age coincides with Ordivician fossil meteorites that fell to Earth at 467 ± 2 Ma. As these fossil meteorites were originally L chondrites, the apparent conclusion is that a large impact sent a flood of L chondrite material to Earth, while material that remained on the L chondrite parent body was strongly heated and reset. We have reduced 40Ar-39Ar data for Northwest Africa 091 using various techniques that appear in the literature, including identification and subtraction of trapped Ar. These techniques give a range of ages from 455 to 520 Ma, and show the importance of making accurate corrections. By using the most straightforward technique to identify and remove a trapped Ar component (which is neither terrestrial nor primordial), an 40Ar-39Ar age of 475 ± 6 Ma is found for Northwest Africa 091, showing a temporal link to fossil meteorites. In addition, high temperature releases of Northwest Africa 091 contain evidence for a second trapped component, and subtraction of this component indicates a possible second collisional impact at approximately 800 Ma. This earlier age coincides with 40Ar-39Ar ages of some H and L chondrites, and lunar samples.

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