Magnesian-suite volcanism and ancient crust building on the Moon 4.25 billion years ago

• We identify the first definitive extrusive sample of the lunar Mg suite. • Apollo 17 basalt 73002,1017C with a Pb-Pb age of 4246 ± 4 million years. • Oldest basaltic rock among returned lunar samples with precise age determination by about 43 Ma. • Bulk composition after olivine subtraction matches that of a glass bead from same core. • Models support past Mg-suite petrogenesis hypothesis of LMO source interactions. We present a 4.25-billion-year-old volcanic rock petrogenetically linked to the Mg suite, together with a related glass bead from the same Apollo 17 core 73002, which was unsealed as part of NASA’s ANGSA program. This basalt, with porphyritic texture and magnesian-suite composition, has a crystallization age of 4246 ± 4 million years. Phase equilibrium modeling indicates that this picritic basalt is related to glass in 73002 and formed when a mantle melt interacted with anorthositic crust before eruption. The presence of extrusive, olivine-rich Mg-suite samples indicates that high-temperature lunar magmas had sufficient energy or buoyancy to breach the early crust. Subsequent events, namely the late heavy bombardment and mare volcanism, possibly obscured evidence of ancient volcanism, or the early Moon was dominated by intrusive rather than extrusive magmatism. These findings also offer insight into magmatic processes on early Earth before the onset of plate tectonics, where such samples have been destroyed by geologic processes and where most ancient igneous ages have come from individual zircon grains rather than whole rocks. Our results support a dynamically evolving early Moon, expanding our understanding of primordial crust formation and thermal evolution on early Earth and other rocky planets.