Distribution of extraterrestrial chromite in Middle Ordovician Komstad Limestone in the Killeröd quarry, Scania, Sweden

Abstract

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.