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The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides
Author(s) -
WELTEN Kees,
CAFFEE Marc W.,
FRANKE Luitgard,
Timothy JULL A. J.,
LeCLERC Marlene D.,
METZLER Knut,
OTT Ulrich
Publication year - 2011
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2011.01204.x
Subject(s) - breccia , regolith , meteorite , noble gas , meteoroid , chondrite , geology , radionuclide , astrobiology , cosmogenic nuclide , geochemistry , carbonaceous chondrite , radiochemistry , mineralogy , cosmic ray , chemistry , physics , astrophysics , nuclear physics , organic chemistry
– 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 10 Be, 26 Al, and 36 Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The 14 C and 10 Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 10 5 yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic‐ray fluxes. This scenario is supported by the low cosmogenic 3 He/ 21 Ne ratios in these two samples, indicating recent loss of cosmogenic 3 He. 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 4 He/ 20 Ne ratio of approximately 7. Trapped planetary‐type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of 129 Xe from decay of now extinct 129 I are highest. Cosmogenic 21 Ne varies between 0.55 and 1.92 × 10 −8 cm 3 STP g −1 , with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic ( 22 Ne/ 21 Ne) c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined 10 Be and 21 Ne concentrations indicate that NWA 869 had a 4π cosmic‐ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated 21 Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr 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 4 He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.