40Ar/39Ar dating of microgram feldspar grains from the paired feldspathic achondrites GRA 06128 and 06129

40Ar/39Ar ages of single feldspar grains from the paired meteorites Graves Nunatak 06128 (GRA8; 8 grains) and 06129 (GRA9; 26 grains) are presented. Plateau ages (⩾70% of the 39Ar released) ranged from 4000 to 4600Ma with an average 1-σ uncertainty of ±90Ma. The most precise ages obtained were 4267±17Ma for a grain from GRA8 and 4437±19Ma and 4321±18Ma for two grains from GRA9. Isotope correlation diagrams yield less precise ages ranging from 3800 to 5200Ma with an average 1-σ uncertainty of 250Ma; they indicate a negligible trapped component. Plateau ages, integrated total fusion ages, and isochron ages are internally concordant at the 95% confidence level. The distribution of the plateau ages for GRA9 is bimodal with peaks at 4400 and 4300Ma. In contrast, the plateau age distribution for GRA8 peaks at about 4260Ma with broad wings extending toward younger and older ages. To explain the distributions of grain ages we prefer a scenario that includes a major post-formation event about 4400Ma ago and a later melt intrusion event that heated GRA8 more than some parts of GRA9.