Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for ⁵³Mn (t_1.2 = 3.7Myr), ^26Al(t_1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for ²⁶Al and ⁵³Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for ²⁶Al and ⁵³Mn to deduce the respective saturation values ²⁶Al₀ and ⁵³Mn₀ and to obtain ²¹Ne production rates (²¹P).²⁶Al and ⁵³Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of ²¹p based on ⁵³Mn and ²⁶Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity.