Submicroscopic iron-rich grains throughout impact glasses in Chang'E-5 regolith

Abstract

In lunar regolith returned by Chang'E-5, Apollo and Luna missions, submicron-sized iron-rich grains that are dominated by reduced iron and with sizes of about 10 s to 100 s nanometers are widespread on surfaces of impact glasses, such as glass spherules and glass that welds agglutinates. With flat or mound-like tops, such submicroscopic iron-rich grains typically occur in chains, nets and/or mists, and they were interpreted to be surface adherents that are mainly composed of native iron. Processes such as cooling of iron melt veneer, condensation of iron vapor, and in situ thermal reduction of regolith particles have been separately invoked to explain their origin. Meanwhile, some iron-rich grains have lower hemispheres being embedded in their host glasses, and spherical iron-rich grains with similar sizes, compositions and spatial distributions are also widespread in interiors of their host glasses. It is possible that both the surface and interior iron-rich grains were formed simultaneously during cooling of the host impact melt. However, the precise composition and crystallography of the surface and interior iron-rich grains have not been adequately constrained. In this study, we prepared focused ion beam (FIB) sections for chains of iron-rich mounds that occur on surfaces of impact glasses in the Chang'E-5 lunar regolith. Several FIB sections also exposed interior iron-rich grains that exhibit similar sizes and regular distribution patterns. High-resolution elemental mapping shows that both the surface and interior iron-rich grains are mainly composed of iron and sulfur, and crystallographic inspections reveal that the grains are similarly composed of α-Fe and troilite with varying volume proportions. As the glass spherules and agglutinate glasses are mainly melt products of local regolith materials, both the surface and interior iron-rich grains are most consistent with being formed due to liquid immiscibility between FeS and silicate melt. Troilite preferentially occurs along margins of the iron-rich grains due to heterogeneous distribution of S in the iron-rich melt and the relatively greater compatibility between troilite and silicate melt. The varying morphology of the surface iron-rich grains (e.g., flat tops, mounds) is likely caused by different surface tensions of FeS melt droplets and different wettability between the solidifying Fe-dominating melt and FeS melt with silicate melt. This interpretation predicts that iron-rich grains formed during cooling of impact melt may be dispersed in melt strips and even detached from the melt body during ejecta motion, and they could be subsequently adhered to surfaces of other lunar regolith particles. For reflectance spectra of lunar regolith at visible to near-infrared wavelength, the long-noticed darkening effect caused by submicron-sized iron may be caused by such two-phase iron-rich grains (i.e., α-Fe plus troilite).