Cosmogenic nuclides in core samples of the Chico L6 chondrite: Evidence for irradiation under high shielding

Abstract

Abstract— Core samples were obtained from various locations of the ∼ 105‐kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic‐ray‐produced nuclides. Relations between measured abundances of cosmogenic nuclides ( 10 Be, 26 Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic 22 Ne/ 21 Ne ratio were determined and compared with recent model predictions of production rates. The measured 22 Ne/ 21 Ne ratios (1.06‐1.08) and significant variations observed in concentrations of cosmogenic 21 Ne and 3 He suggest an ∼40‐cm shielding gradient across Chico and irradiation within a large object (> 100‐cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of 10 Be (average = 20.7 dpm/kg) and 26 Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either 22 Ne/ 21 Ne or 21 Ne. Activities of 10 Be and 26 Al suggest irradiation in a smaller object (∼40–80 cm radius). The 26 Al activity and the 26 Al/ 10 Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two‐stage irradiation with t 1 ∼ 104 Ma and t 2 ∼ 4 Ma and a second‐stage body the size of Knyahinya. The single stage, 10 Be/ 21 Ne exposure age for Chico is 65 Ma. The 22 Ne/ 21 Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with 22 Ne/ 21 Ne ratios significantly less than ∼ 1.055.