U‐Pb and Pb‐Pb apatite ages for Antarctic achondrite Graves Nunataks 06129

Abstract

AbstractThe 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. ), but are the same as the 40Ar‐39Ar age obtained via step‐heating of the bulk rock (4460 ± 28 Ma; Fernandes and Shearer ; Shearer et al. ). 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 ; Popova et al. ; Yin et al. ; Zhang et al. ). 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.