Density variation amongst mid-ocean ridge basalts: Implications for magma mixing and the scarcity of primitive lavas

Densities calculated from glass compositions of observed mid-ocean ridge basalts show that the more primitive melts are likely to be buoyant in more evolved melts. Consideration of this and other physical properties indicates that convective mixing between most basaltic magmas occurs under intermittently turbulent to turbulent conditions (transitional Reynolds Numbers) accounting for the widespread occurrence of hybrid lavas. Hypothetical picritic melts inferred to be parental to mid-ocean ridge basalts are by contrast denser than most basalts erupted on the sea floor. The most primitive basalts observed to erupt occupy a density minimum when compared to more primitive and more fractionated melts. The density minimum occurs approximately at those compositions where plagioclase and/or pyroxene join olivine as major fractionating phases. Picritic basalts are rarely erupted, because they stratify at the base of magma reservoirs.