Tadpole-Shaped Nanoparticles in Impact Melt and Implication of High Temperature Chemical Garden in Lunar Soil

Nanoparticles within lunar soil grains are a primary product of space weathering. The microstructural and chemical characteristics of the nanoparticles are diverse and their formation mechanisms are still under debate. In this paper, for the first time, tadpole-shaped nanoparticles (with Fe-Ni(-S) head and Fe-Ti-O tail) were found in the impact melt glass spherule of an agglutinate in the returned Chang'e-5 lunar soil, and their possible formation mechanisms were discussed. In terms of the Fe-Ni(-S) “head” formation mechanisms, they probably produced by shock-induced dissemination. Another possibility is that the Fe-Ni(-S) heads were derived from the impact glass due to liquid immiscibility. The S degassing of FeS was contributed to nanophase Fe-Ni metal. For the Fe-Ti-O tails, they are devitrified ilmenites, nucleated as a result of the passage of the Fe nanoparticles through the melt. These nanoparticles formed though impact-induced nonequilibrium growth and recorded the movement and migration of the Fe-Ni-S nanoparticles within the melt. The tadpole-shape nanoparticles provide a new example of viscous fingering in impact melts and the associated ilmenite dendrites point to the formation of high-temperature chemical gardens in lunar impact melt.