Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

We report high-precision K isotopes, apatite U-Pb ages, whole-rock elements, and Sr-Nd isotopes of Saima nepheline syenite in the North China Craton. Trace element and Sr-Nd-Hf-O isotope data indicate the presence of subducting sediments in source region, while K isotopic compositions showed a narrow range between –0.54 ‰ and –0.31 ‰, with an average of –0.41 ± 0.06 ‰, identical to the value of asthenosphere. The nearly identical K isotopic compositions are low probability events compared with K isotopic compositions of the island arc lavas reported previously (–1.55 ‰ to +0.2 ‰). Although crustal contamination is consistent with Sr-Nd-K isotopic data, alternatively we propose the isotopic data also reconcile to the interaction between cratonic roots and underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatized lithospheric mantle root, the source of the Saima, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a metasomatized lithospheric mantle by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion in a long-time interval.