Mafic deep crustal and shallow upper mantle xenoliths hosted in the Chino Valley and Camp Creek latites of central Arizona have 187Os/188Os (0.216 to 1.51) ratios dramatically higher than values typical of mantle peridotite (<0.13). Re-Os model ages for the xenoliths range from 0.57 to 2.3 Ga with a mean of 1.1 ± 0.6 Ga consistent with the possibility that the xenoliths sample Proterozoic lower crust and upper mantle underplated to the ca. 1.6–1.8 Ga basement. Combined Os and Sr, Nd, Pb data indicate that these xenoliths were not formed by crustal contamination of Cenozoic asthenospheric melts, but allow for recent interaction between old mafic materials in the lithosphere and high 87Sr/86Sr, low 143Nd/144Nd melts of Proterozoic lithospheric mantle. The potassic latite hosts to the xenoliths also have radiogenic Os, indicating that they are melts of a mafic rather than a peridotitic source in the old lithosphere. These data demonstrate the utility of the Re-Os system in defining mantle separation ages, or the initial time of melt extraction from a peridotitic mantle, even in cases where the lithospheric mantle is remelted long after its formation.