Oxygen-Deficient Magnetite in Chang’e-6 Sample as Evidence for Impact Induced Deoxygenation of the Moon

Abstract

The Moon’s highly reduced nature contrasts with recent findings of ferric iron (Fe³⁺)-bearing phases, suggesting an active redox cycle on the Moon. While solar wind proton reduction and impact-vaporization oxygen loss from minerals are proposed drivers for lunar reduction, direct nanometer-scale evidence for impact-driven deoxygenation has been lacking. Here we report the discovery of oxygen-deficient magnetite within impact melt glass in Chang’e-6 lunar farside samples from the South Pole-Aitken basin. This subhedral magnetite grain (Length 2.5 μm; width 1.5 μm.) exhibits a non-stoichiometric composition and crystal chemical formula [(Fe²⁺_1.158(7)Mg_0.070(5))_∑1.228(Fe³⁺_1.534(9)Cr³⁺_0.137(14)Al_0.101(8))_∑1.772]_∑3.000(O_3.886□_0.114)_∑4.000, confirming significant oxygen vacancies (□). Magnetite heating experiments (1000°C, in Ar atmosphere) replicated these features, demonstrating thermal deoxygenation generates oxygen vacancies through O₂ evolution and concurrent Fe³⁺ reduction. Our findings provide the first mineral structural evidence that post-shock heating drives deoxygenation in oxygen-bearing minerals, establishing impact-induced oxygen removal as a fundamental mechanism making the localized lunar surface chemically reduced.