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 (10Be, 26Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic 22Ne/21Ne ratio were determined and compared with recent model predictions of production rates. The measured 22Ne/21Ne ratios (1.06‐1.08) and significant variations observed in concentrations of cosmogenic 21Ne and 3He 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 10Be (average = 20.7 dpm/kg) and 26Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either 22Ne/21Ne or 21Ne. Activities of 10Be and 26Al suggest irradiation in a smaller object (∼40–80 cm radius). The 26Al activity and the 26Al/10Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two‐stage irradiation with t1 ∼ 104 Ma and t2 ∼ 4 Ma and a second‐stage body the size of Knyahinya. The single stage, 10Be/21Ne exposure age for Chico is 65 Ma. The 22Ne/21Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with 22Ne/21Ne ratios significantly less than ∼ 1.055.