Ar-39-Ar-40 Ages of Eucrites and the Thermal History of Asteroid 4-Vesta

New Ar-Ar ages of several cumulate and unbrecciated eucrites cluster at approx.4.48 Gyr. We suggest that this age cluster was produced by interruption of eucrite metamorphism within the Vesta parent asteroid by a giant impact, which excavated and rapidly cooled these meteorites. We see no evidence for younger formation ages of the cumulate eucrites, as previously proposed.