Noble gases in individual L chondritic micrometeorites preserved in an Ordovician limestone

We have measured the He and Ne concentrations and isotopic ratios of individual sediment-dispersed extraterrestrial chromite grains (63–180 μm in diameter) from an Ordovician limestone in southern Sweden. In the same sediment, many fossil meteorites were found and have been attributed to the L chondrite parent body breakup event ∼ 470 Ma ago. In this analysis of 37 individual extraterrestrial chromite grains of L chondritic major element composition, at least 35 (∼ 95%) contain surface-implanted helium and neon of fractionated solar wind composition, implying that these grains are (relict parts of) fossil micrometeorites of asteroidal origin. Similar to what has been observed in recent micrometeorites collected in the polar regions, significant amounts of cosmogenic 21Ne were found in several grains, resulting in cosmic ray exposure (CRE) ages of up to ∼ 77 Myr. These ages exceed both dynamical lifetimes for asteroidal micrometeorites of this size as well as CRE ages observed in chromites from fossil meteorites from the same sediment beds. Significant contributions from terrestrial sources, like nucleogenic and cosmic-ray muon induced 21Ne can be excluded in the extraterrestrial chromites, since 11 terrestrial chrome spinel grains from the same sediment beds did not contain any measurable 21Ne excesses. Having found micrometeorites of undisputed asteroidal origin with cosmic ray exposure ages of several 107 years implies that high cosmic ray exposure ages alone are not a good indicator of cometary origin, in opposition to what has been suggested for recent micrometeorites and IDPs. We propose instead that these grains collected their cosmogenic 21Ne while residing in the regolith layer of their parent body.