Geochemical measurements from the MESSENGER mission indicate distinct geochemical terranes on the surface of Mercury. We report chemical compositions and derived mineralogy for four geochemical terranes, as well as Mercury’s average surface composition. The geochemical terranes share higher Mg and S, and lower Al, Ca, and Fe, than terrestrial oceanic basalts. The low Fe and high S concentrations suggest that all terranes formed under highly reducing conditions. All terranes are enriched in plagioclase. Heating melted the silicate shell of Mercury and produced a global magma ocean in which stratification developed during crystallization, with basal ultramafic material grading to incompatible-element-enriched material near the surface. Later differentiation began with partial melting as result of mantle convection and heating from the decay of radioactive elements. These high-Mg, high-temperature partial melts were exceptionally fluid and produced thin, laterally extensive flows. The largest impacts excavated into the upper layers of the mantle and deposited distinctive material, including remnants of a graphite-rich flotation crust from the magma ocean, at the top of the crust. Smooth plains deposits originated as laterally extensive flood basalts that efficiently covered pre-existing layers. Distinct source compositions suggest that convection was insufficient to homogenize the mantle at ~3.8–3.9 Ga.