Abstract-Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ~480 Myr ago. An association with the L-chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40 Ar- 39 Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L-chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.

available online at Abstract-Isheyevo is a metal-rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7-90 vol%). The metal-rich lithologies with 50-60 vol% of Fe,Ni metal are dominant. The metal-rich and metal-poor lithologies are most similar to the CB b and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components-Fe,Ni metal, chondrules, refractory inclusions (Ca, Al-rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts-but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non-porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15 N (δ 15 N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non-magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CB b chondrites, ~20% of Fe,Ni-metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni-rich (>20 wt% Ni). In contrast to CB b and CH chondrites, most metal grains are thermally decomposed into Ni-rich and Ni-poor phases. Similar to CH chondrites, chondrules have porphyritic and non-porphyritic textures and ferromagnesian (type I and II), silica-rich, and aluminum-rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite-, grossite-, and melilite-rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico-chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine-grained dust. Such a scenario is consistent with the presence of solar wind-implanted noble gases in Isheyevo and with its comparatively old K-Ar age. We cannot exclude that the K-Ar system was affected by a later collisional event. The cosmic-ray exposure (CRE) age of Isheyevo determined by cosmogenic 38 Ar is ~34 Ma, similar to that of the Bencubbin (CB a) meteorite.

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