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

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 +/- 29Ma and the weighted-average Pb-207-Pb-206 age of 4460 +/- 30Ma for the Cl-apatite grains from GRA 06129. Our apatite ages are obviously younger than that of the Al-26-Mg-26 model age (4565.9 +/- 0.3Ma; Shearer etal. ), but are the same as the 40Ar-39Ar age obtained via step-heating of the bulk rock (4460 +/- 28Ma; Fernandes and Shearer ; Shearer etal. ). 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 4450-4470Ma (Bogard and Garrison ; Popova etal. ; Yin etal. ; Zhang etal. ). Considering the contemporary metamorphic events for GRA 06129 (4460 +/- 30Ma), it is likely that the asteroid parent body of GRA 06129 was also affected by the same giant impact as the Moon-forming event.