Cosmogenic ⁴¹Ca in diogenites: Production rates, pre-atmospheric size and terrestrial ages

We measured concentrations of cosmogenic ⁴¹Ca in 37 diogenites, including 8 falls and 29 Antarctic finds, using accelerator mass spectrometry (AMS). The measured ⁴¹Ca concentrations in falls range from 2.8 to 9.3 dpm/kg, those in Antarctic finds range from 1.6 to 28 dpm/kg. The ⁴¹Ca concentrations in four of the falls agree within 10% with estimated saturation values based on spallation from Ti, Cr, Mn and the main target element, Fe. Four other diogenite falls and several Antarctic finds show elevated ⁴¹Ca concentrations, which are attributed to capture of thermal neutrons on ⁴⁰Ca. The neutron-capture ⁴¹Ca contributions range from 0.1 to 1.5 dpm/g Ca. Monte Carlo based model calculations show that contributions of neutron-capture ⁴¹Ca are negligible (<0.05 dpm/g Ca) in diogenites with pre-atmospheric radii <20 cm, but become dominant in diogenites larger than 30 cm in radius and reach a maximum value of ∼5.6 dpm/g Ca in the center of objects with radii of 80-100 cm. The contributions of neutron-capture ⁴¹Ca in diogenites were used to constrain their pre-atmospheric radii. Many Antarctic diogenites show low ⁴¹Ca concentrations, up to 60% below the saturation value determined for falls. From these low ⁴¹Ca concentrations we derive terrestrial ages up to ∼135 kyr. We compare these terrestrial ages with those of other Antarctic achondrites and Antarctic chondrites and discuss the role of terrestrial weathering in limiting the lifetime of meteorites falling in different environments, i.e. Antarctic versus hot desert conditions.