Coupled zinc and iron isotope fractionation in mantle xenoliths: records of carbonated melt–lithosphere interaction

Coupled zinc and iron isotopic variations in basaltic magmas are potential indices of recycled crustal materials that have modified their mantle sources. Nevertheless, how these isotopes fractionate in response to various types of mantle metasomatism is not well constrained as of yet. Here we present high-precision Zn (δ66ZnJMC-Lyon) and Fe (δ57FeIRMM-014) isotope data and in situ chemistry for direct mantle samples (peridotites, pyroxenites, and their mineral separates), occurring as xenoliths within Cenozoic alkaline basalts from northeast China. Based on mineralogical and chemical compositions, these xenoliths are classified into four groups. Group I peridotites are almost non-metasomatized, whereas Group II peridotites exhibit typical characteristics of carbonated silicate melt metasomatism (e.g., high La/Yb, low Ti/Eu). Distinct from the normal mantle-like δ66Zn (∼0.18 ‰) and δ57Fe (∼0.03 ‰) values of Group I peridotites, Group II peridotites have highly variable and positively correlated δ66Zn (−0.04 ‰ to 0.29 ‰) and δ57Fe (−0.38 ‰ to 0.07 ‰; N = 6). The generally light Zn and Fe isotopic compositions of Group II peridotites are attributed to kinetic isotope fractionation during carbonated melt infiltration. Low-Mg# pyroxenites, characterized by Mg# (100 × atomic ratio of Mg/(Mg + Fe)) of <89 and positive Eu anomalies in clinopyroxenes, have extremely high and positively correlated δ66Zn (0.27 ‰–0.62 ‰) and δ57Fe (0.19 ‰–0.84 ‰; N = 5). These pyroxenites are interpreted as cumulates from carbonated melts, akin to the host basalts originating from partial melting of the asthenospheric mantle containing recycled carbonate. By comparison, high-Mg# pyroxenites (Mg# > 90) display gradual modal variations in the composite xenoliths, consistent with being the product of silicate melt-peridotite reaction. These pyroxenites have higher δ57Fe (0.02 ‰–0.24 ‰) but lower δ66Zn (−0.05 ‰ to 0.03 ‰; N = 5) than those of the normal mantle. The negatively correlated Zn and Fe isotopic ratios were caused by diffusion-induced fractionation during silicate melt metasomatism. Collectively, these observations demonstrate that asthenosphere-derived, carbonated and silicate melts could induce substantial and coupled Zn and Fe isotopic variations in the lithospheric mantle through reaction and diffusion during mantle metasomatism. Therefore, coupled variations of Zn and Fe isotopes in mantle-derived rocks or magmas provide valuable approaches for tracing carbonated melt–lithospheric mantle interaction.