CA-41 IN IRON FALLS, GRANT AND ESTHERVILLE - PRODUCTION-RATES AND RELATED EXPOSURE AGE CALCULATIONS

Recent technical developments in accelerator mass spectrometry have enabled us to measure routinely and with a precision of 5-7% the Ca-41 (T1/2 = 104 ky) contents of extraterrestrial samples weighing approximately 100 mg. In essence, these advances have elevated Ca-41 to the role of a new and potent cosmogenic radioisotope with wide-ranging applications. We present here the results from the first phase of our Ca-41 cosmogenic studies program, aimed at establishing baseline concentrations and trends in selected meteorites and the use of Ca-41 in estimating exposure ages and pre-atmospheric meteorite radii. The average Ca-41 saturation activity recorded in four small iron falls is 24 +/- 1 dmp/kg. This result, together with measurements at the center and surface of the large iron Grant, indicates that production of Ca-41 from spallation on iron is weakly dependent on shielding to depths as large as 250 g/cm2. We estimate the K-41-Ca-41 exposure age of Grant to be 330 +/- 50 My, and an upper limit to its terrestrial age of 43 +/- 15 ky. A comparison of the Ca-41 contents of stony and metallic material separated from the mesosiderite Estherville identifies low-energy neutron capture on native Ca as a second important channel of production. We find that the Ca-41 signal in the stone phase from three meteorites correlates with their size, and that the inferred low-energy neutron fluxes vary by a factor of at least 20.