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U-Pb and Pb-Pb apatite ages for Antarctic achondrite Graves Nunataks 06129
Authors:Qin Zhou  Qing-Zhu Yin  Charles K Shearer  Xian-Hua Li  Qiu-Li Li  Yu Liu  Guo-Qiang Tang  Chun-Lai Li
Institution:1. Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012 China;2. Department of Earth and Planetary Sciences, University of California Davis, One Shields Avenue, Davis, California, 95616 USA;3. Department of Earth and Planetary Sciences, Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA;4. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
Abstract:The Antarctic achondrite Graves Nunataks 06128 (GRA 06128) and Graves Nunataks 06129 (GRA 06129) represent a unique high-temperature, nonbasaltic magmatism in the early solar system. These objects have been interpreted as products of low-degree partial melting of volatile-rich chondritic material, which may have been the asteroid parent bodies of brachinite. Previous studies have investigated their crystallization and metamorphic history with various isotope systematics. Here, we report the U-Pb intercept age of 4466 ± 29 Ma and the weighted-average 207Pb-206Pb age of 4460 ± 30 Ma for the Cl-apatite grains from GRA 06129. Our apatite ages are obviously younger than that of the 26Al-26Mg model age (4565.9 ± 0.3 Ma; Shearer et al. 2010a ), but are the same as the 40Ar-39Ar age obtained via step-heating of the bulk rock (4460 ± 28 Ma; Fernandes and Shearer 2010 ; Shearer et al. 2010a ). Based on petrographic observations, merrillites are usually rimmed by apatite and exist as inclusions in apatite. Therefore, the apatite U-Pb age from GRA 06129 probably records a metamorphic event of replacing merrillite with apatite, caused by Cl-rich melts or fluids on their parent body. A collisional event has provided the impact heating for this metamorphic event. Increasing amounts of geochronologic evidence show that the giant impact of the Moon-forming event has affected the asteroid belt at 4450–4470 Ma (Bogard and Garrison 2009 ; Popova et al. 2013 ; Yin et al. 2014 ; Zhang et al. 2016 ). Considering the contemporary metamorphic events for GRA 06129 (4460 ± 30 Ma), it is likely that the asteroid parent body of GRA 06129 was also affected by the same giant impact as the Moon-forming event.
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