Geochemical and isotopic studies of the Cretaceous (106 Ma) Prairie Creek olivine lamproites in southwestern Arkansas indicate that these diamond-bearing ultrapotassic magmas were derived from metasomatized Proterozoic subcontinental lithospheric mantle. Re-Os isotopic data yield a nonradiogenic initial γOs = –3.2 to –3.6 (percent deviation in 187Os/188Os from chondritic mantle) and mantle model ages (TMA) of 0.9 to 1.2 Ga. Sm-Nd isotopic data yield a nonradiogenic initial ϵNd = –10 and a depleted mantle model age (T(DM)) of 1.2 Ga. These data are consistent with initial stabilization of low-density lithospheric mantle at ∼1.2 Ga, followed closely in time by reenrichment in incompatible trace elements. This Middle Proterozoic lithospheric mantle source is younger than overlying Early Proterozoic crust of the southern U.S. midcontinent. Pre-Grenville subduction of oceanic lithosphere may have juxtaposed 1.2 Ga mantle beneath 1.6 to 2.0 Ga crust and imparted the necessary slab-derived chemical components for the growth of diamonds.