41Ca 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% the41Ca (1/2 = 104 ky) contents of extraterrestrial samples weighing approximately 100 mg. In essence, these advances have elevated41Ca to the role of a new and potent cosmogenic radioisotope with wide-ranging applications. We present here the results from the first phase of our41Ca cosmogenic studies program, aimed at establishing baseline concentrations and trends in selected meteorites and the use of41Ca in estimating exposure ages and pre-atmospheric meteorite radii. The average41Ca saturation activity recorded in four small iron falls is 24 ± 1 dpm/kg. This result, together with measurements at the center and surface of the large iron Grant, indicates that production of41Ca from spallation on iron is weakly dependent on shielding to depths as large as 250 g/cm2. We estimate the41K—41Ca exposure age of Grant to be 330 ± 50 My, and an upper limit to its terrestrial age of 43 ± 15 ky. A comparison of the41Ca 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 the41Ca 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.