The L3-6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides

Kees Welten, Marc W. Caffee, Luitgard Franke, A. J. Timothy Jull, Marlene D. Leclerc, Knut Metzler, Ulrich Ott

Research output: Contribution to journalArticle

10 Scopus citations

Abstract

We measured cosmogenic radionuclides and noble gases in the L3-6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of 10Be, 26Al, and 36Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0-2.5m. The 14C and 10Be concentrations in three fragments yield a terrestrial age of 4.4±0.7kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 105yr ago, that excavated material up to approximately 80cm deep and exposed previously shielded material to higher cosmic-ray fluxes. This scenario is supported by the low cosmogenic 3He/21Ne ratios in these two samples, indicating recent loss of cosmogenic 3He. Most NWA samples, except for clasts of petrologic type 4-6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped 4He/20Ne ratio of approximately 7. Trapped planetary-type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of 129Xe from decay of now extinct 129I are highest. Cosmogenic 21Ne varies between 0.55 and 1.92×10-8cm3 STPg-1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (22Ne/21Ne)c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined 10Be and 21Ne concentrations indicate that NWA 869 had a 4π cosmic-ray exposure (CRE) age of 5±1Myr, whereas elevated 21Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10-30Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic 4He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8Gyr ago that may have also resulted in loss of solar helium.

Original languageEnglish (US)
Pages (from-to)970-988
Number of pages19
JournalMeteoritics and Planetary Science
Volume46
Issue number7
DOIs
StatePublished - Jul 2011

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science

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