Iron isotopes of Chang'e-5 soil and mineral components: Implications for post-eruption processes on lunar surface

Abstract

Due to rapid magma cooling and extensive space weathering, significant disequilibrium crystallization and secondary modification widely occur in lunar mare basalt after its eruption on the lunar surface. In this study, we conducted bulk and in-situ Fe isotope analyses to investigate the post-eruption processes on Chang'e-5 (CE-5) samples. The CE-5 soil shows a minor elevation of δ⁵⁶Fe value (∼0.05 ‰) relative to the CE-5 basalt clasts. Correlations between Ni and Cu contents with δ⁵⁶Fe values suggest that the minor increase in the δ⁵⁶Fe from the CE-5 basalt to soil primarily occurred during evaporation caused by meteorite impacts. Such isotopic variation between CE-5 basalt and soils is notably lower than what is observed for Apollo samples and reflects the low maturity of CE-5 soils. This is consistent with the low Is/FeO value constrained by magnetic approaches. Therefore, measuring the δ⁵⁶Fe values of lunar soil is suitable to evaluate the degrees of maturity for lunar soils due to space weathering. In-situ analyses of δ⁵⁶Fe reveal significant variations in different grains of olivine (δ⁵⁶Fe: −0.57 to −0.17 ‰) and ilmenite (−0.06 to +0.42 ‰) and also in their interior (mainly for olivine). These δ⁵⁶Fe variations in minerals can be ascribed to the disequilibrium crystallization of lava flow and fast cooling, which is consistent with conclusions based on petrologic observations such as its extensive differentiation and silicate liquid immiscibility. Therefore, the post-eruption processes on the lunar surface could lead to significant variations in isotopic compositions at different scales of basalts, which in turn record the history of late-stage magma evolution and space weathering on the lunar surface.