The irradiation history of the Ghubara (L5) regolith breccia

Abstract

We measured cosmic-ray products-noble gases, radionuclides, thermoluminescence, and nuclear tracks-and trace element contents and mineralogy of samples of three orthogonal and mutually intersecting cores (41-46 cm long) of a 101.6 kg Ghubara individual (1958, 805) at The Natural History Museum, London. The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked. While contents of noble gases and degree of shock-loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two-generation regolith breccia. Contents of cosmogenic 26Al and 10Be and low track densities indicate that the Ghubara individuals were located more than 15 cm below the surface of an 85 cm meteoroid. Because of its large size, Ghubara's cosmic-ray exposure age is poorly defined to be 15-20 Ma from cosmogenic nuclides. Ghubara's terrestrial age, based on 14C data, is 2-3 ka. Not only is Ghubara the first known case of a two-generation regolith breccia on the macroscale, it also has a complicated thermal and irradiation history.