Ages determined for extraterrestial samples by the Sm-Nd and Rb-Sr techniques are commonly assumed to record igneous crystallization events, because in solid silicates, Nd and Sr diffuse at exceedingly slow rates. However, it is found that for coarse-grained igneous cumulate rocks from the Moon or from a large, thoroughly brecciated asteroid, this assumption may not be reliable. The Moon and at least one asteroid (the parent body of the eucrite, diogenite, and howardite meteorites) appear to have been largely molten at or about the time they formed. Global cooling of the Moon and large (R = 40-250 km) asteroids, has been modelled starting at or near the solidus. A crucial factor in determining the prevailing interval (Ic) of cooling between igneous crystallization and isotopic closure, for any given depth in the crust, is the extent to which the body is insulated by a regolith/megaregolith layer of porous, fragmental impact debris. Results indicate that deep-crustal regions tend to remain above the Nd and Sr isotopic closure temperature for intervals that are long in comparison to the precision of modern Nd- and Sr-based age measurements, and in comparison to suggested chronologic scenarios of global differentiation. Ic intervals of as long as 100 m.y. may be common among available samples of primordial, deep-crustal cumulates from both bodies. -from Authors