Cosmic ray tracks in chondritic material with focus on silicate mineral inclusions in chromite

Sammanfattning: It has been suggested that the cosmic ray flux varies as Earth moves relative to the spiral arms in the galaxy. However, no satisfying record over the cosmic ray flux over long time scales has been presented. This work aims to investigate the possibility to use cosmic ray tracks in silicate mineral inclusions in sediment-dispersed extraterrestrial chromite (SEC) grains as such a record. Chromite is the only common meteoritic mineral that survives on Earth for hundreds of millions of years. SEC grains, interpreted as parts of micrometeorites, have successfully been used as a proxy for the flux of chondritic material to Earth. Chromite is slightly conductive and does not record nuclear tracks. However, two-thirds of chondritic chromite grains contain inclusions of silicate minerals which do register tracks. To test etching techniques and learn more about tracks and their appearance, terrestrial apatite in thin sections of granite were etched for fission tracks, and olivine, pyroxene and merrillite in thin sections of meteorites were etched for cosmic ray tracks. Large (~40 μm) inclusions of pyroxene in chromite from a recent meteorite (Kernouvé, H6) were etched for cosmic ray tracks and such tracks were found in one of the inclusions. Hence, it is here shown that cosmic rays can penetrate chromite and register tracks in inclusions in chromite grains. Inclusions of olivine and plagioclase in four SEC grains from mid-Ordovician (470 Ma) sediments were etched for tracks; however, no tracks were found. It is concluded that it is likely that cosmic ray tracks could be found in inclusions in fossil SEC grains, but that several factors have to work together for such tracks to be found. The track studies in mineral grains in recent meteorites and inclusions of pyroxene in chromite from recent meteorites showed that inclusions have to be large (preferably ≥ 30 μm) and largely unaffected by other defects if cosmic ray tracks are to be found. Crucial is also that the inclusions have been subjected to cosmic rays for enough time to make track recording probable and that tracks were not annealed during atmospheric entry or later during their history on Earth.