Records of the Moon‐forming impact and the 470 Ma disruption of the L chondrite parent body in the asteroid belt from U‐Pb apatite ages of Novato (L6) |
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| Authors: | Qing‐Zhu Yin Qin Zhou Qiu‐Li Li Xian‐Hua Li Yu Liu Guo‐Qiang Tang Alexander N Krot Peter Jenniskens |
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| Institution: | 1. Department of Earth and Planetary Sciences, University of California Davis, , Davis, California, 95616 USA;2. Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, , Beijing, 100012 China;3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, , Beijing, 100029 China;4. Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Manoa, , Honolulu, Hawai‘i, 96822 USA;5. SETI Institute, , Mountain View, California, 94043 USA;6. NASA Ames Research Center, , Moffett Field, California, 94035 USA |
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| Abstract: | Novato, a newly observed fall in the San Francisco Bay area, is a shocked and brecciated L6 ordinary chondrite containing dark and light lithologies. We have investigated the U‐Pb isotope systematics of coarse Cl‐apatite grains of metamorphic origin in Novato with a large geometry ion microprobe. The U‐Pb systematics of Novato apatite reveals an upper intercept age of 4472 ± 31 Ma and lower intercept age of 473 ± 38 Ma. The upper intercept age is within error identical to the U‐Pb apatite age of 4452 ± 21 Ma measured in the Chelyabinsk LL5 chondrite. This age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the Moon‐forming giant impact on Earth. The lower intercept age is consistent with the most precisely dated Ar‐Ar ages of 470 ± 6 Ma of shocked L chondrites, and the fossil meteorites and extraterrestrial chromite relicts found in Ordovician limestones with an age of 467.3 ± 1.6 Ma in Sweden and China. The lower intercept age reflects a major disturbance related to the catastrophic disruption of the L chondrite parent body most likely associated with the Gefion asteroid family, which produced an initially intense meteorite bombardment of the Earth in Ordovician period and reset and degassed at least approximately 35% of the L chondrite falls today. We predict that the 470 Ma impact event is likely to be found on the Moon and Mars, if not Mercury. |
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