Study of Iron Meteorites

Iron meteorite Toluca showing widmanstätten patterns.

Measuring noble gases in iron meteorites is not straightforward and only very few laboratories have been able to measure iron meteorites routinely. Therefore, in contrast to stony meteorites, not many noble gas studies on iron meteorites exist. However, such studies are essential for the understanding of cosmogenic nuclides and their production rates, e.g., to investigate the dynamics in the asteroid belt or to determine a possible long term variation of the galactic cosmic radiation (GCR), provided by the longer exposure time of iron meteorites compared to their stony counterparts. To establish a physical model to determine the production rates is therefore the main goal of this project, which was carried out during the last three years mainly by Dr. Katja Ammon as the topic of her PhD.

Close-up photograph showing the iron meteorite Buenaventura.

The new physical model is based on the current best knowledge of the particle spectra and cross sections for the relevant nuclear reactions. In order to validate the new model, depth profiles of He, Ne, and Ar have been measured so far in the two iron meteorites Grant and Carbo. The results reveal a significant influence of sulphur and phosphorus on the production of cosmogenic Ne. These depth profiles are accompanied by measurements of artificially irradiated Fe and Ni targets as the base for a complete and consistent data base for the relevant cross sections of cosmogenic nuclides. Finally, the model includes calculations of production rates combined with Monte Carlo simulations for the depth and size dependent primary and secondary particle spectra of 3,4He, 21,22Ne, 36,38Ar, 10Be, 26Al, 36Cl, 39,40,41K, 41Ca, 53Mn, and 60Fe for iron meteoroids with radii from 5 – 1000 cm.
The present model works fairly well for Grant and Carbo and allows determination of their exposure age, radii, sample depths, and minor element chemistry.