Cooling History of Chang'e‐5 Basalts Recorded by Diffusion‐Driven Mg and Fe Isotope Fractionation

Abstract The basalt clasts returned by the Chang'e‐5 (CE‐5) mission recorded a spatial diversity of cooling histories, with various cooling rates and timescales reported for different clasts. However, the temporal evolution of thermal conditions during cooling has received limited attention. Here, we report ∼0.3‰ and ∼0.8‰ in δ 56 Fe and δ 26 Mg variability at the mineral‐grain scale in a CE‐5 basalt clast, which are negatively correlated, unambiguously pinpointing kinetic isotope fractionation driven by inter‐diffusion in the minerals. Combined with systematic diffusion modeling and grain size analysis, the Mg–Fe isotopic variability reveals a substantial decrease in the cooling rate from 30–100°C/day to 1–10°C/day during the crystallization and cooling of the clast. The change in thermal conditions during eruption or magma overlaying caused the cooling rate to decrease. Our results highlight the importance of temporal evolution of thermal conditions in the crystallization and cooling history of magmatic systems.