Neon measurements of individual Greenland sediment particles: proof of an extraterrestrial origin and comparison with EDX and morphological analyses

The work reported here confirms the existence of (largely) unmelted micrometeorites in sediments from seasonal lakes on the Greenland ice sheet. Approximately1/3 of the dark and irregularly shaped particles selected from disaggregated sediments had sufficient contents of Ne to permit measurement of isotopic compositions. The sediments consisted of a mixture of biologically derived contaminants and minor amounts of terrestrial minerals and extraterrestrial fragments. Although terrestrial mineral contamination varied by a factor of 20 for the two different sediments, a yield of ∼ 1000 cosmic particles/kg of wet cryoconite was found for both samples. The isotopic compositions of the Ne in both rounded and irregular (25% of total) particles that contained enough gas to measure were non-terrestrial, plotting in a field defined by solar wind (SW), solar energetic particle (SEP) and spallation. Exposure ages ranged from < 0.5 Ma to 20 Ma, consistent with previous estimates from 10Be and 26Al measurements of larger spherules removed from similar sediments. Comparison of the Ne data with compositional-morphological data demonstrates the general validity of criteria previously used to identify extraterrestrial particles; however, two particles identified as of probable terrestrial origin by three independent, experienced meteorite petrologists were shown by the Ne data to be extraterrestrial and probably extraterrestrial, respectively. Future studies of Greenland micrometeorites are outlined and improved collection techniques are discussed.