Differences in composition and phenocryst assemblages between dikes and lavas collected from 1-m.y.-old oceanic crust exposed at the Hess Deep Rift have important implications for density filtering of magmas ascending through the upper crust in mid-ocean ridge settings. The majority of crystalline lavas collected have lower FeOT and MgO, and elevated Al2O3 and CaO concentrations compared to crystalline samples of dikes. These chemical differences, as well as associated modal variations, can be attributed to the accumulation of plagioclase in the magmas that ultimately erupt as lavas on the seafloor. The accumulation of plagioclase combined with the fractionation of mafic phases lowers the magma density by more than 0.04 g/cm3 relative to most magmas represented by the dikes. We suggest that these lower-density magmas are preferentially erupted because of their increased buoyancy, resulting in the predominance of this magma type as lavas. Conversely, the majority of dikes are crystallized from the higher-density magma type, which is rarely represented by lavas. These relationships suggest that most dikes never reach the surface and erupt lava. Examination of phenocryst contents and modal abundances in mid-ocean ridge basalts worldwide indicates that magma density variations and crustal heterogeneity may influence magma eruptibility across a range of spreading rates. Specifically, lavas from slow spreading ridges have a wider range of modal phenocryst proportions compared to lavas from intermediate or fast spreading ridges. Presumably, these lavas will have a greater range of densities and their eruption on the seafloor may reflect the heterogeneous upper crust of slow spreading ridges compared to that of fast spreading ridges.