Mg, Zn and Fe isotope evidence for derivation of K-rich basalts from the mantle hybridized by recycled carbonate-bearing silicate sediments

Abstract

Potassium-rich basalts provide valuable information about mantle heterogeneity and crust-mantle interaction because of their distinctive geochemical features. One of the long-standing issues regarding the origin of K-rich basalts is the nature of the K-rich mantle component required to generate such lavas. To provide further constraints on this issue, we present combined analyses of Mg, Zn and Fe isotopic compositions in the Cenozoic intraplate K-rich basalts from northeast China. The samples include both potassic and ultrapotassic basalts with K₂O/Na₂O ratios up to 3.42 and have typical Enriched Mantle-I Sr-Nd isotopic signatures. The ultrapotassic basalts have lighter Mg isotopic compositions than those of mid-ocean ridge basalts (MORB), but their Zn isotopic compositions are similar to or slightly higher than those of MORB, defining a strong Mg-Zn isotopic decoupling that suggests a dominant role of recycled carbonate-poor siliciclastic sediments in their sources. These rocks have Fe isotopic compositions (δ⁵⁶Fe) identical with those of MORB, indicating a peridotite-dominant source that has not been oxidized by the recycled sediments. Along with the high Fo (molar Mg²⁺/(Mg²⁺+Fe²⁺) × 100) of olivine phenocrysts, we suggest that the ultrapotassic basalts originated from the metasomatized (K-rich) sub-continental lithospheric mantle. Compared with ultrapotassic basalts, the potassic basalts have heavier Zn isotopic compositions at similar δ²⁶Mg, defining a weaker Mg-Zn isotopic decoupling that suggests a large proportion of carbonates in the recycled siliciclastic sediments (i.e., carbonate-rich sediments). In addition, the potassic basalts have higher δ⁵⁶Fe and lower Fo of olivine phenocrysts, pointing to a deeper, fertile mantle source that may have been oxidized by recycled carbonates, most likely at the mantle transition zone (MTZ). These results collectively suggest that both the potassic and ultrapotassic lavas in northeast China were formed via partial melting of the mantle hybridized by recycled siliciclastic sediments containing different proportions of carbonates. Therefore, the K-rich mantle component required for generating the extreme K enrichment in some intraplate lavas was primarily produced by recycled carbonate-bearing siliciclastic sediments in different mantle domains.