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Proterozoic lithospheric mantle source for the Prairie Creek lamproites: Re-Os and Sm-Nd isotopic evidence

¹ Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia
² Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, D.C. 20015
³ ARCO Oil and Gas Company, 2300 Plano Parkway, Plano, Texas 75075

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 _Os = –3.2 to –3.6 (percent deviation in ¹⁸⁷Os/¹⁸⁸Os from chondritic mantle) and mantle model ages (T_MA) of 0.9 to 1.2 Ga. Sm-Nd isotopic data yield a nonradiogenic initial _Nd = –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.

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