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)

Autor: Samuel Niedermann, Robert C. Finkel, Lucilla Benedetti, Pete Burnard, Irene Schimmelpfennig, B.S.H. Schneider, Alice Williams, Raphaël Pik
Přispěvatelé: Centre européen de recherche et d'enseignement des géosciences de l'environnement (CEREGE), Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Collège de France (CdF)-Institut national des sciences de l'Univers (INSU - CNRS)-Aix Marseille Université (AMU)-Institut National de la Recherche Agronomique (INRA), Centre de Recherches Pétrographiques et Géochimiques (CRPG), Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), GeoForschungsZentrum - Helmholtz-Zentrum Potsdam (GFZ), Vrije Universiteit, Department of Isotope Geochemistry, Amsterdam, Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Collège de France (CdF (institution))-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut National de la Recherche Agronomique (INRA), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Vrije Universiteit Amsterdam [Amsterdam] (VU), Institut de Recherche pour le Développement (IRD)-Institut National de la Recherche Agronomique (INRA)-Aix Marseille Université (AMU)-Collège de France (CdF (institution))-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de Lorraine (UL)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)
Jazyk: angličtina
Rok vydání: 2011
Předmět:
Zdroj: Quaternary Geochronology
Quaternary Geochronology, Elsevier, 2011, 6 (5), pp.425-436. ⟨10.1016/j.quageo.2011.05.002⟩
Quaternary Geochronology, 2011, 6 (5), pp.425-436. ⟨10.1016/j.quageo.2011.05.002⟩
ISSN: 1871-1014
DOI: 10.1016/j.quageo.2011.05.002⟩
Popis: Because the intensity and energy spectrum of the cosmic ray flux are affected by atmospheric depth and geomagnetic-field 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-specific scaling factors would be required. Concentrations of the three cosmogenic nuclides were determined in mafic 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 significant 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-specific 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 ± 0.0085 in pyroxenes and 0.377 ± 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 defined. To our knowledge this is the first 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.
Databáze: OpenAIRE
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