Abundant extraterrestrial chromite grains from decomposed meteorites occur in middle Ordovician (480 million years ago) marine limestone over an area of approximately 250,000 square kilometers in southern Sweden. The chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to Earth during the mid-Ordovician, as previously indicated by fossil meteorites. Extraterrestrial chromite grains in mid-Ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar-system history.

All appendix tables for this article are available online at http://meteoritics.org.Abstract-The distribution of sediment-dispersed extraterrestrial chromite grains and other Cr-rich spinels (>63 µm) has been studied in Middle Ordovician Orthoceratite Limestone from two quarries at Kinnekulle, southern Sweden. In the Thorsberg quarry, an ∼3.2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment-dispersed extraterrestrial chromite grains. Typically, 1-3 grains are found per kilogram of limestone. In the nearby H‰llekis quarry, the same beds show similarly high concentrations of extraterrestrial chromite grains, but in samples representing the 9 m downward continuation of the section exposed at this site, only 5 such grains were found in a total of 379 kg of limestone. The extraterrestrial (equilibrated ordinary chondritic) chromite grains can be readily distinguished by a homogeneous and characteristic major element chemistry, including 2.0-3.5 wt% TiO 2 and stable V 2 O 3 concentrations close to 0.7 wt%. Terrestrial Cr-rich spinels have a wide compositional range and co-exist with extraterrestrial chromite in some beds. These grains may be derived, for example, from mafic dykes exposed and weathered at the sea floor. Considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied. Instead, the difference may be primarily related to an increase in the ancient flux of extraterrestrial matter to Earth in connection with the disruption of the L chondrite parent body in the asteroid belt at about this time. The coexistence in some beds of high concentrations of chondritic chromite and terrestrial Cr-rich spinels, however, indicates that redistribution of heavy minerals on the sea floor, related to changes in sea level and sea-floor erosion and currents, must also be considered.

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Sediment-dispersed extraterrestrial chromite grains (63–355 µm) have been recovered in a sectionacross the Arenig-Llanvirn transition in the Killeröd quarry in southeastern Scania. Previous studies of the same stratigraphic interval in the Orthoceratite Limestone at Kinnekulle, ca. 350 km to the north in Västergötland, have shown a two orders of magnitude increase in extraterrestrial chromite beginning close to the Arenig-Llanvirn boundary. The stratigraphic distribution and abundance trends of extraterrestrial chromite are essentially identical at Killeröd and Kinnekulle. In the Killeröd section extraterrestrial chromite grains are extremely rare (2 grains/125 kg of rock) in the Arenig lower 8 m of the section (Megistaspis simon to Asaphus expansus trilobite zones) and abundant (326 grains/162 kg of rock) in the succeeding ca. 3 m of uppermost Arenig and lower Llanvirn (upper A. expansus and A. raniceps zones) section. The extraterrestrial chromite grains at Killeröd and Kinnekulle are very similar in chemical composition, including characteristic elevated values and narrow ranges of V2O3 (0.6–0.9 wt%) and TiO2 (2.0–3.5 wt%). At Killeröd there is a small group (<4%) of chromite grains with relatively low TiO2 (1.5–2.0 wt%), but otherwise typical extraterrestrial compositions. These grains may reflect a somewhat more aggressive diagenetic environment at Killeröd than at Kinnekulle. As in previous studies, rare chrome spinel grains with terrestrial or uncertain origin have also been found in the limestone. The results of this study support previous suggestions that after the disruption of the L chondrite parent body in the asteroid belt at ca. 470 Ma, the flux of extraterrestrial matter to Earth was enhanced by up to two orders of magnitude compared to the present. This is supported by finds in Sweden of abundant fossil meteorites in uppermost Arenig and lower Llanvirn sediments. Whether these conclusions can be put into a global context awaits further study of Middle Ordovician limestones from other continents.