Rare meteorites common in the Ordovician periodPhilipp R. Heck, Birger Schmitz, William F. Bottke, Surya S. Rout, Noriko T. Kita, Anders Cronholm, Celine Defouilloy, Andrei Dronov & Fredrik TerfeltNature Astronomy 1, Article number: 0035 (2017)doi:10.1038/s41550-016-0035Asteroids, comets and Kuiper belt MeteoriticsReceived: 27 July 2016 Accepted: 14 December 2016 Published online: 23 January 2017Source/Fonte: Lund UniversityAbstractMost meteorites that fall today are H and L type ordinary chondrites, yet the main belt asteroids best positioned to deliver meteorites are LL chondrites 1,2 . This suggests that the current meteorite flux is dominated by fragments from recent asteroid breakup events 3,4 and therefore is not representative over longer (100-Myr) timescales. Here we present the first reconstruction of the composition of the background meteorite flux to Earth on such timescales. From limestone that formed about one million years before the breakup of the L-chondrite parent body 466 Myr ago, we have recovered relict minerals from coarse micrometeorites. By elemental and oxygen-isotopic analyses, we show that before 466 Myr ago, achondrites from different asteroidal sources had similar or higher abundances than ordinary chondrites. The primitive achondrites, such as lodranites and acapulcoites, together with related ungrouped achondrites, made up ~1534% of the flux compared with only ~0.45% today. Another group of abundant achondrites may be linked to a 500-km cratering event on (4) Vesta that filled the inner main belt with basaltic fragments a billion years ago 5 . Our data show that the meteorite flux has varied over geological time as asteroid disruptions create new fragment populations that then slowly fade away from collisional and dynamical evolution. The current flux favours disruption events that are larger, younger and/or highly efficient at delivering material to Earth.To investigate the past meteorite flux, we searched for relict chrome-spinel grains of coarse micrometeorites in condensed marine sediments in northwestern Russia, in a time window of ~10100 kyr in the geological epoch of the Middle Ordovician period, which ranges from 470 to 458 Myr ago (Fig. 1; see Methods). Chrome spinels are the only minerals of meteorites and coarse micrometeorites that survived diagenesis in Ordovician limestone 6 . They retained their elemental and oxygen isotopic composition, enabling reliable classification based on single-grain microanalysis 7,8 . We also dissolved 32 meteorites of different types in HF or HCl acid to quantify their content of chrome-spinel grains. The sediment sample that we studied is about a million years older than the ~466-Myr-old sediments that contain the first collisional fragments from the L-chondrite parent body breakup (LCPB), the largest known asteroid disruption event in the past three billion years. The sampling level was chosen to exclude the extreme flux enhancement (more than two orders of magnitude 6,7 ) of L-chondritic fragments after the LCPB that obscures the background flux for more than 1 Myr (refs 7,8,9 ). The low, 50- to 100-kyr, cosmic-ray exposure ages of the oldest recovered fossil L chondrites 9 imply that any fragments from the LCPB that might have arrived on Earth before should have even shorter exposure ages. This indicates that a sample separation of one million years before the strata