Kinetic MgFe isotope fractionation in natural basalts during melt-peridotite interaction

Abstract

Although Mg and Fe isotopes are frequently used to trace the source heterogeneity of basalts (e.g., recycled carbonated materials or eclogite in the mantle), they can also be fractionated by geological processes during magma migration from the source to the surface. Recently, the light Mg and heavy Fe isotopic compositions observed in high-Ti lunar basalts have been attributed to the kinetic fractionation that occurs during interaction between the melt and ambient harzburgite cumulates. Due to the thick lithospheric mantle, the melt-peridotite interaction is frequently invoked to explain the chemical variations observed in continental basalts on Earth. However, the isotope fractionation behavior of MgFe isotopes during this process has not been well addressed. Here we present MgFe isotopic compositions of alkaline basalts from the Nanjing basaltic field in eastern China, alongside the peridotite xenoliths hosted within these basalts, in order to assess the potential isotopic fractionation under this specific condition. Previous petrographic evidence suggests that these peridotite xenoliths had undergone significant interaction with the hosting basalts. Peridotite xenoliths exhibit homogeneous, normal-mantle-like δ²⁶Mg (from −0.243 ‰ to −0.281 ‰), but they have highly variable, low δ⁵⁷Fe (from 0.156 ‰ to −0.057 ‰), indicating that melting reaction has not altered the Mg isotopic compositions but has modified the Fe isotopic compositions of these xenoliths. The hosting basaltic rocks exhibit heavy Fe isotopic compositions (δ⁵⁷Fe = 0.218 ‰–0.366 ‰) and extremely light Mg isotopic compositions (δ²⁶Mg = −0.641 ‰−−0.389 ‰). The δ²⁶Mg values of these xenolith-rich basalts are well correlated with their δ⁵⁷Fe values, Hf isotopes, and certain elemental ratios (e.g., Lu/Hf, La/Sm, Ca/Al, and Na/Ti). These correlations can be explained by mixing between the host basaltic magma and a type of melt derived from peridotite xenolith. These xenolith-derived melts have significantly lighter Mg and heavier Fe isotopic compositions than the normal mantle. Given their extremely low Mg/Fe ratios resulting from near-solidus melting, they are not in Mg or Fe equilibrium with the residual solid. Consequently, kinetic MgFe isotope fractionation is inferred to occur during the interaction between the melt and peridotite. Therefore, our results serve as a representative sample for characterizing MgFe isotope fractionation via chemical diffusion in natural systems, and they highlight the significant influence of melt-peridotite interaction on the heterogeneity of MgFe isotopes in those xenolith-rich basalts (characterized by low melt/olivine ratios).