PROTEROZOIC LITHOSPHERIC MANTLE SOURCE FOR THE PRAIRIE-CREEK LAMPROITES - RE-OS AND SM-ND ISOTOPIC EVIDENCE

Geochemical and isotopic studies of the Cretaceous (106 Ma) Prairie Cr eek olivine lamproites in southwestern Arkansas indicate that these di amond bearing ultrapotassic magmas were derived from metasomatized Pro terozoic subcontinental lithospheric mantle. Re-Os isotopic data yield a nonradiogenic initial gamma(Os) = -3.2 to -3.6 (percent deviation i n Os-187/Os-188 from chondritic mantle) and mantle model ages (T-MA) o f 0.9 to 1.2 Ga. Sm-Nd isotopic data yield a nonradiogenic initial eps ilon(Nd) = -10 and a depleted mantle model age (T-DM) of 1.2 Ga. These data are consistent,vith initial stabilization of low-density lithosp heric mantle at similar to 1.2 Ga, followed closely in time by reenric hment in incompatible trace elements. This Middle Proterozoic lithosph eric mantle source is younger than overlying Early Proterozoic crust o f 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 cr ust and imparted the necessary slab-derived chemical components for th e growth of diamonds.