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

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.