Inter-comparison of cosmogenic in-situ 3He, 21Ne and 36Cl at low latitude along an altitude transect on the SE slope of Kilimanjaro volcano (3°S, Tanzania)
Résumé
Because the intensity and energy spectrum of the cosmic ray ux are affected by atmospheric depth and geomagnetic-eld strength, cosmogenic nuclide production rates increase considerably with altitude and to a lesser degree with latitude. The scaling methods used to account for spatial variability in production rates assume that all cosmogenic nuclides have the same altitude dependence. In this study we evaluate whether the production rates of cosmogenic 36 Cl, 3 He and 21 Ne change differently with altitude, which is plausible due to the different threshold energies of their production reactions. If so, nuclide-specic scaling factors would be required. Concentrations of the three cosmogenic nuclides were determined in mac phenocrysts over an altitude transect between 1000 and 4300 m at Kilimanjaro volcano (3 S). Altitude dependence of relative production rates was assessed in two ways: by determination of concentration ratios and by calculation of apparent exposure age ratios for all nuclide pairs. The latter accounts for characteristics of 36 Cl that the stable nuclides 3 He and 21 Ne do not possess (radioactive decay, high sensitivity to mineral composition and signicant contributions from production reactions other than spallation). All ratios overlap within error over the entire transect, and altitudinal variation in relative production rates is not therefore evident. This suggests that nuclide-specic scaling factors are not required for the studied nuclides at this low-latitude location. However, because previous studies have documented anomalous altitude-dependent variations in 3 He production at mid-latitude sites, the effect of latitude on cross-calibrations should be further evaluated. We determined cosmogenic 21 Ne/ 3 He concentration ratios of 0.1864 AE 0.0085 in pyroxenes and 0.377 AE 0.018 in olivines, agreeing with those reported in previous studies. Despite the absence of independently determined ages for the studied lava surfaces, the consistency in the dataset should enable progress to be made in the determination of the production rates of all three nuclides as soon as the production rate of one of the nuclides has been accurately dened. To our knowledge this is the rst time that 36 Cl has been measured in pyroxene. The Cl extraction method was validated by measuring 36 Cl in co-existing plagioclase phenocrysts in one of the samples.
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