Abstract The breakup of the L-chondrite parent body (LCPB) in the mid-Ordovician is the largest documented asteroid breakup event during the past 3 Gyr. It affected Earth by a dramatic increase in the flux of L-chondritic material and left prominent traces in both meteorite and sedimentary records. A precise constraint on the timing of the LCPB breakup is important when evaluating the terrestrial biotic and climatic effects of the event, as well as for global stratigraphic correlations. Direct dating using heavily shocked L chondrites is hampered by both incomplete initial K-Ar degassing and isotopic resetting by later impact events. In order to better constrain the absolute age of this event we carried out high-precision U–Pb dating of zircons from three limestone beds recording discrete volcanic ash fallouts within mid-Ordovician strata in southern Sweden. These strata are rich in fossilized L-chondritic meteorites (1-20 cm large) that arrived on Earth shortly after the breakup event. Zircons from the ash-bearing layers provide stratigraphically consistent depositional ages that range from 464.22 ± 0.37 Ma to 465.01 ± 0.26 Ma. Combined with recently published 3He profiles that pinpoint the arrival on Earth of the first dust from the breakup, and sedimentation rates constrained by cosmogenic 21Ne in the fossil meteorites, the LCPB breakup is estimated to have occurred at 465.76 ± 0.30 Ma. This provides the presently most precise absolute dating of the LCPB breakup, enabling a robust global stratigraphic correlation of bounding strata. Based on our new U–Pb data for the ash-bearing beds, the absolute ages for the boundaries of biozones and Dapingian–Floian stages overlap within error with those given by the 2012 Geological Timescale and require no modification.

中文翻译：

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奥陶系石灰岩中 L 型球粒陨石母体破裂的绝对年代学与高精度 U-Pb 锆石年代学

摘要 中奥陶世 L-球粒陨石母体 (LCPB) 的破裂是过去 3 Gyr 中记录的最大的小行星破裂事件。它通过 L 型球粒陨石物质通量的急剧增加影响了地球，并在陨石和沉积记录中留下了显着的痕迹。在评估该事件的陆地生物和气候影响以及全球地层相关性时，对 LCPB 分裂时间的精确限制非常重要。使用受到严重冲击的 L 球粒陨石进行直接测年受到不完整的初始 K-Ar 脱气和后来撞击事件的同位素重置的阻碍。为了更好地限制这一事件的绝对年龄，我们对来自三个石灰岩床的锆石进行了高精度 U-Pb 测年，记录了瑞典南部中奥陶世地层中离散的火山灰沉降。这些地层富含化石化的 L 型球粒陨石（1-20 厘米大），它们在破裂事件发生后不久抵达地球。来自含灰层的锆石提供地层一致的沉积年龄，范围从 464.22 ± 0.37 Ma 到 465.01 ± 0.26 Ma。结合最近发表的 3He 剖面图，这些剖面图确定了第一批尘埃到达地球的时间，以及化石陨石中受宇宙成因 21Ne 约束的沉积速率，估计 LCPB 破裂发生在 465.76 ± 0.30 Ma。这提供了目前最精确的 LCPB 解体绝对年代，使边界地层具有强大的全球地层相关性。根据我们新的含灰层 U-Pb 数据，生物区边界和大坪阶-弗洛亚阶的绝对年龄与 2012 年地质年表给出的那些在误差范围内重叠，不需要修改。