Archean, highly unradiogenic lead in shallow cratonic mantle

Here, we present coupled geochemical and Sr-Nd-Pb-S isotopic data of Early Cretaceous primitive gabbros from the North China craton. Strikingly, these rocks have highly unradiogenic lead compositions (²⁰⁶Pb/²⁰⁴Pb = 16.58 ± 0.24) and anchor one extreme end member (low ²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd) in the global array of oceanic-island volcanics. Our study shows that they originated from an Archean fluid-metasomatized refractory peridotite source, in which highly unradiogenic lead was preferentially released with subducted Archean seawater and sequestered into recrystallized sulfides at shallow mantle depths. Sulfide/silicate partition coefficients for lead show a negative pressure dependence: Lead is more enriched in sulfide with decreasing pressure. Sulfide-bearing and iron-poor harzburgite as well as dunite residues at shallow mantle are expected to develop low U/Pb (and thereby low time-integrated ²⁰⁶Pb/²⁰⁴Pb) relative to a deeper upper-mantle source. Our preferred interpretation is that an Archean, highly unradiogenic lead reservoir may be stored in the spinel-facies refractory cratonic mantle.