The Twannberg (Switzerland) IIG iron meteorites: Mineralogy, chemistry, and CRE ages

The original mass (15915 g) of the Twannberg IIG (low Ni-, high P) iron was found in 1984. Five additional masses (12 to 2488 g) were recovered between 2000 and 2007 in the area. The different masses show identical mineralogy consisting of kamacite single crystals with inclusions of three types of schreibersite crystals: cm-sized skeletal (10.5% Ni), lamellar (17.2% Ni), and 1-3 x 10 mu m-sized microprismatic (23.9% Ni). Masses 1 and 11 were compared in detail and have virtually identical microstructure, hardness, chemical composition, cosmic-ray exposure (CRE) ages, and (10)Be and (26)Al activities. Bulk concentrations of 5.2% Ni and 2.0% P were calculated. The preatmospheric mass is estimated to have been at least 11,000 kg. The average CRE age for the different Twannberg samples is 230 +/- 50 Ma. Detrital terrestrial mineral grains in the oxide rinds of the three larger masses indicate that they oxidized while they were incorporated in a glacial till deposited by the Rhone glacier during the last glaciation (Wurm). The find location of mass I is located at the limit of glaciation where the meteorite may have deposited after transport by the glacier over considerable distance. All evidence indicates pairing of the six masses, which may be part of a larger shower as is indicated by the large inferred pre-atmospheric mass.