Magmas trapped at the continental lithosphere-asthenosphere boundary

•Density and structure of molten alkali basalt measured in situ up to 6.25 GPa.•Alkali basalts are gravitationally stable at the conditions of the continental LAB.•This provides an explanation for the longevity of cratonic keels.

The lithosphere-asthenosphere boundary (LAB) beneath the continents is a key interface in plate tectonics, yet its nature remains elusive. A partial melt layer has been advocated to explain its geophysical characteristics. However, the main counter-argument is that such a layer cannot be stable as melts should rise through the lithosphere. Density measurements of volatile-containing alkali basalts taken as a proxy for LAB melts show that they are neutrally buoyant at the pressure (P)-temperature (T) conditions of the LAB under continents. Complementary X-ray diffraction and Raman data provide structural insights on melt compaction mechanisms. Basalts generated below the lithosphere may thus be gravitationally trapped and accumulate over time. Their presence provides answers to key questions on continental lithosphere geodynamics, and in particular on cratonic keels stability. This buoyancy trap would cease to exist at higher mantle T such as those relevant of the Archean, and as such, could be linked to the onset of plate tectonics.