A Regolith Pre-exposure Signature in Fossil Micrometeorites from an Asteroid Collision 470 Million Years Ago

Introduction: More than 80 fossil meteorites have been found in a ~4 m quarry layer of mid-Ordovician limestone in Sweden [1, 2]. Their cosmic-ray exposure (CRE) ages correlate inversely with sediment deposi-tion age and indicate very rapid (<1.2 Myrs) transport from the asteroid belt [3]. The same layer contains up to 3 sediment-dispersed, extraterrestrial chromite (SEC) grains (>63 µm) per kg of sediment, around two orders of magnitude more than sediments immediately below the meteorite-rich strata [2]. Both fossil meteorite chromite and SEC have L chondritic composition in major elements [2] and oxygen isotope ratios [4, 5]. They have both been linked to the L chondrite parent body break-up event ~470 Myr ago [6]. Since the highly resistant SEC kept their extraterrestrial noble gas inventory , and since SEC are much more abundant than fossil meteorites (which are mere chance finds), SEC offer the potential to study the Ordovician asteroid collision and its terrestrial effects on a broader basis. SEC could become a tracer for as yet unknown showers of