A comparative study on 28Al and 53Mn in eighteen chondrites

An investigation on the low energy cosmic ray products ²⁶Al and ⁵³Mn in 18 chondrites encompassed several side aspects in meteoritics. The ratio of observed to calculated specific activity of ²⁶Al in 13 chondrites with recovered masses ranging from ~ 3 to 2000 kg remains constant within ±15 per cent of a mean of 0.93. This is consistent with Fuse and Anders' observation of invariance of the ²⁶Al-production rate with depth and size. A plot of the ⁵³Mn specific production rate (dpm/kg Fe) versus recovered meteorite mass revealed that the compensation by the secondaries for the absorption of the primaries is also effective in the case of ⁵³Mn. Thus, the ⁵³Mn production rate remains rather constant within ±15 per cent of a mean of 520 dpm/kg Fe for most of the stones. Applying this information to two chondrites, Ladder Creek and Scurry, of known relatively short exposure and terrestrial ages, the ⁵³Mn half-life (T₅₃), computed from their ⁵³Mn_obs/²⁶Al_obs ratios, is found to be 3.85 ± 0.4 m.y. which agrees very well with the currently accepted value of T₅₃ = 3.8 ± 0.6 m.y. From the observed ⁵³Mn/⁵⁴Mn activity ratio in Allende, relative to the production rate ratio of 0.98, an exposure age of t_R = 4.2 m.y. is derived, in concordance with literature data. The result strongly suggests that this isotope pair holds promise as an index of absolute radiation ages of fresh falls.