Neutron‐capture ³⁶Cl, ⁴¹Ca, ³⁶Ar, and ¹⁵⁰Sm in large chondrites: Evidence for high fluences of thermalized neutrons

Abstract

We have measured significant concentrations of 36 Cl, 41 Ca, 36 Ar from decay of 36 Cl, and 150 Sm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 36 Cl and 41 Ca, corrected for a high‐energy spallogenic component and a terrestrial age of ∼50 ka, give average neutron‐capture production rates of 208 atoms/min/g‐Cl and 1525 atoms/min/kg‐Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm 2 /s and 4.3 n/cm 2 /s, respectively. If sustained for the ∼65 Ma single‐stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ∼1.3×10 16 and 0.8 × 10 16 n/cm 2 for 36 Cl and 41 Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 36 Ar/ 38 Ar ratios as large as ∼9. The correlation of high 36 Ar/ 38 Ar with high Cl/Ca phases in neutron‐irradiated Chico indicates that the excess 36 Ar above that expected from spallation is due to decay of neutron‐produced 36 Cl. Excess 36 Ar in Chico requires a thermal neutron fluence of 0.9–1.7×10 16 n/cm 2 . Decreases in 149 Sm/ 152 Sm due to neutron‐capture by 149 Sm correlate with increases in 150 Sm/ 152 Sm for three samples of Chico, and one of the Torino H‐chondrite. The 0.08% decrease in 149 Sm/ 152 Sm shown by Chico corresponds to a neutron fluence of 1.23×10 16 n/cm 2 . This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 36 Ar identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of ∼0.2–0.6×10 16 n/cm 2 . Depletion of 149 Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8×10 16 n/cm 2 and 0.25×10 16 n/cm 2 , respectively. Neutron fluences of ∼10 16 n/cm 2 in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron‐capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron‐capture has the potential for identifying large meteorites with complex exposure histories.