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 fro m 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-4 1, corrected for a high-energy spallogenic component and a terrestrial age of similar to 50 ka, give average neutron-capture production rate s 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, respect ively. If sustained for the similar to 65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron flue nces 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 impac t melt shows Ar-36/Ar-38 ratios as large as similar to 9. The correlat ion of high Ar-36/Ar-38 With high Cl/Ca phases in neutron-irradiated C hico indicates that the excess Ar-36 above that expected from spallati on is due to decay of neutron-produced Cl-36. Excess Ar-36 in Chico re quires 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 incre ases in Sm-150/Sm-152 for three samples of Chico, and one of the Torin o H-chondrite. The 0.08% decrease in Sm-149/Sm-152 shown by Chico corr esponds to a neutron fluence of 1.23x10(16) n/cm(2). This fluence deri ved from Sm considers capture of epithermal neutrons and effects of ch emical composition on the neutron energy distribution. Excess Ar-36 id entified in the Arapahoe, Bruderheim, and Torino chondrites and the Sh allowater 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 lun ar soils. Consideration of exposure ages suggests that the neutron flu x in Chico may have been greater than that in many lunar soils. Neutro n-capture effects, although seldom reported, may be common for large m eteorites and could affect calculation of exposure ages based on cosmo genic Ar. Combining measurements of radioactive and stable species pro duced from neutron-capture has the potential for identifying large met eorites with complex exposure histories.