Abstract
AbstractWe measured specific activities of the long‐lived cosmogenic radionuclides 60Fe in 28 iron meteorites and 53Mn in 41 iron meteorites. Accelerator mass spectrometry was applied at the 14 MV Heavy Ion Accelerator Facility at ANU Canberra for all samples except for two which were measured at the Maier‐Leibnitz Laboratory, Munich. For the large iron meteorite Twannberg (IIG), we measured six samples for 53Mn. This work doubles the number of existing individual 60Fe data and quadruples the number of iron meteorites studied for 60Fe. We also significantly extended the entire 53Mn database for iron meteorites. The 53Mn data for the iron meteorite Twannberg vary by more than a factor of 30, indicating a significant shielding dependency. In addition, we performed new model calculations for the production of 60Fe and 53Mn in iron meteorites. While the new model is based on the same particle spectra as the earlier model, we no longer use experimental cross sections but instead use cross sections that were calculated using the latest version of the nuclear model code INCL. The new model predictions differ substantially from results obtained with the previous model. Predictions for the 60Fe activity concentrations are about a factor of 2 higher, for 53Mn, they are ~30% lower, compared to the earlier model, which gives now a better agreement with the experimental data.
Highlights
Most meteorites are routinely measured by accelerator mass spectrometry (AMS) for the radionuclides 10Be, 26Al, 36Cl, and 41Ca, which, if combined with concentrations for cosmogenic noble gases, provide information on cosmic ray exposure (CRE) histories, that is, CRE ages, terrestrial ages, pre-atmospheric sizes, and shielding depths
For the large iron meteorite Twannberg, we studied six samples
Note that even the longest terrestrial age of 285 ka for Puentel del Zacate reduces the 53Mn concentration by less than 5%, which is below typical uncertainties for the AMS measurements
Summary
Most meteorites are routinely measured by accelerator mass spectrometry (AMS) for the radionuclides 10Be, 26Al, 36Cl, and (more rarely) 41Ca, which, if combined with concentrations for cosmogenic noble gases, provide information on cosmic ray exposure (CRE) histories, that is, CRE ages, terrestrial ages, pre-atmospheric sizes, and shielding depths. The Gebel Kamil data were used together with data from the aforementioned AMS studies to constrain the activity ratio of 60Fe to 53Mn of (2.68 Æ 0.35)910À3 (dpm kgÀ1[Ni]/dpm kgÀ1[Fe]) This ratio allowed workers to disentangle the cosmogenically produced 60Fe from interstellar 60Fe in lunar and terrestrial material (Fimiani et al 2016; Koll et al 2019a, 2019b). We studied six samples from the large Twannberg iron meteorite (for the corresponding lighter radionuclides, see Smith et al 2017)
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