Magnesium isotopic constraints on the lithospheric mantle metasomatism beneath the craton margin

Abstract

The magnesium (Mg) isotopic distinction between surficial carbonate and mantle rock makes the Mg isotopes a potential tracer for recycled crustal materials (especially the carbonate-bearing materials) within the mantle. However, whether or to what extent recycled carbonate-bearing crustal materials could influence the Mg isotopes of the lithospheric mantle is still in debate. Here, we report the Mg isotope data for three well-characterized alkaline complexes from the northern margin of the North China Craton, i.e., the Hekanzi, Yaojiazhuang and Saima alkaline complexes. These alkaline rocks were all derived from enriched lithospheric mantle sources beneath the craton margin that were variably metasomatized by carbonate-bearing recycled crustal materials. However, they have relatively homogeneous and normal mantle-like δ26Mg values, i.e., −0.23 to −0.25‰ for the Hekanzi, −0.20 to −0.30‰ for the Yaojiazhuang and − 0.25 to −0.30‰ for the Saima alkaline rocks, respectively. These Mg isotopic characteristics were unrelated to shallow crustal processes (i.e., surface alteration, fractional crystallization and crustal assimilation) but inherited from the mantle sources. The mantle-like Mg isotopes suggest that the recycled carbonate-bearing crustal materials did not change the Mg isotopic compositions of the lithospheric mantle overlying the subduction zone, which mainly ascribes to the large contrast of MgO contents between the recycled crustal materials and the lithospheric mantle peridotite. The MgO contents of calcite-rich carbonates in the metasomatic agents released from the subducted crustal materials are too low to induce Mg isotopic anomaly in the MgO-rich peridotite. Our work reveals that Mg isotopes might not be a sensitive tracer of carbonate melt/fluid metasomatism in the lithospheric mantle beneath the craton margins.