Recent experiments1 have shown that, at a range of upper-mantle temperatures and pressures, a carbonatite (carbonate-rich) melt occurs as a very small melt fraction in equilibrium with pargasite lherzolite. This melt has low water content and low TiO2 content, and will be extremely effective in transporting large-ion lithophile elements. Here we show that such melts will alter the chemical composition of (‘metasomatize’) the mantle through which they pass, producing distinctive mineralogical and geochemical signatures. The predicted signatures can be recognized in peridotite xenolith suites from Australia, suggesting that metasomatism by carbonatite melts is an effective process for redistributing certain elements in the lithosphere, particularly in continental regions. (This article was corrected on 21 October 2015.)