NEUTRON-CAPTURE CL-36, CA-41, AR-36, AND SM-150 IN LARGE CHONDRITES - EVIDENCE FOR HIGH FLUENCES OF THERMALIZED NEUTRONS

We have measured significant concentrations of Cl-36, Ca-41, Ar-36 from decay of Cl-36, and Sm-150 produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of Cl-36 and Ca-41, corrected for a high-energy spallogenic component and a terrestrial age of similar to 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 similar to 65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of similar to 1.3x10(16) and 0.8x10(16) n/cm(2) for Cl-36 and Ca-41, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows Ar-36/Ar-38 ratios as large as similar to 9. The correlation of high Ar-36/Ar-38 With high Cl/Ca phases in neutron-irradiated Chico indicates that the excess Ar-36 above that expected from spallation is due to decay of neutron-produced Cl-36. Excess Ar-36 in Chico requires a thermal neutron fluence of 0.9-1.7x10(16) n/cm(2). Decreases in Sm-149/Sm-152 due to neutron-capture by Sm-149 correlate with increases in Sm-150/Sm-152 for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in Sm-149/Sm-152 shown by Chico corresponds to a neutron fluence of 1.23x10(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 Ar-36 identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of similar to 0.2-0.6x10(16) n/cm(2). Depletion of Sm-149 in Torino and the LEW86010 angrite suggest neutron fluences of 0.8x10(16) n/cm(2) and 0.25x10(16) n/cm(2), respectively. Neutron fluences of similar to 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.