The Origin of Alkaline Lavas

摘要

Alkaline lavas-mantle-derived magmas rich in alkali metals such as potassium and sodium-are commonly found in the interiors of tectonic plates, both on continents and on islands in ocean basins. Melting of metasomatic materials in the mantle lithosphere has long been conjectured to be the main source of these magmas (1,2), but this has not been successfully simulated experimentally. On page 916 of this issue, Pilet et al. (3) report experiments that explain the properties of alkaline magmas in a simple and elegant way. Oceanic crust forms at mid-ocean ridges, where tectonic plates move apart. As it moves further away from the ridge, the lithosphere thickens, reaching its full thickness after about 70 million years (see the figure, left panel). It has long been widely accepted that the oceanic crust-after recycling into the deep mantle through subduc-tion-becomes the source of alkali-rich melts needed to explain the compositions of ocean island basalts (4), but this explanation has been criticized and remains debated (5-7). One of the problems is that melting of the oceanic crust would lead to silica-rich rather than alkaline melts.