The nature of metasomatizing fluids and melts in the mantle are of interest for understanding the chemical evolution of the Earth’s interior(1-3). The study of noble-gas isotopes in appropriate mantle-derived samples has the potential to provide valuable insight into this question, by constraining the origin of the fluids and the timing of metasomatic events. Here we report the application of neon-isotope systematics to investigate the metasomatic history of apatite grains in spinel-lherzolite xenoliths from the Australian lithospheric mantle. We find that the apatite has a neon-isotope signature similar to that associated with plume-related volcanism, as is found in Hawaii, whereas coexisting mineral phases (olivine and amphibole) and non-apatite-bearing Iherzolites have isotope signatures more typical of mid-ocean-ridge basalts. The occurrence of plume-like neon in the apatite implicates deep plume-like material beneath southeastern Australia as the source of the metasomatizing agent.