The cosmic-ray exposure history of the Twannberg iron meteorite (IIG)

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its similar to 570 recently recovered specimens was a large object with a preatmospheric radius in the range of similar to 2m, which corresponds to similar to 250x10(3)kg. The cosmic-ray exposure age for Twannberg is 182 +/- 45Ma. The most surprising result is the long terrestrial age of T-terr=202(-34)(+33) ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east-northeast during the second last ice age (185-130ka ago) from an original position west of Mt. Sujet.