Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden, and new evidence for the L-chondrite parent body breakup event

摘要

We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from similar to 470 Myr old sediments from two quarries (Hallekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhogen quarry, all located ill Southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhogen 001 were generated in the L-chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during similar to 470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment-dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into all orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhogen 001 has a low cosmic-ray exposure age of similar to 0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.