Small amount of CO2-H2O rich melts in the lithosphere-asthenosphere boundary (Invited)

A low viscosity layer at the Lithosphere-Asthenosphere boundary (LAB) is a requirement for plate tectonics. The petrological nature of the LAB as deduced from its seismic and electrical properties is attributed either to water-related defects in minerals or to partial melting. If partially molten, the seismic and electromagnetic signature of the LAB is sometimes considered to be caused by elevated melt fractions. High melt mobility at such melt volume fractions, however, can lead to gravitational segregation. Furthermore, the average depth of the LAB does not vary much with crustal age under the ocean, demanding the unlikely presence of elevated melt fractions even at lower temperatures. Here we show that the presence of small melt fractions in the LAB, triggered by small amounts of CO2 and H2O, can reconcile most geophysical and petrological observations. We measure electrical conductivities of CO2-rich melts that are an order of magnitude larger than that of hydrated basalts. Numerical simulations, varying temperature and both fractions and volatile contents of melt, show that the electrical signature of the LAB can be explained by 0.1 to 0.3 vol% melt (300 ppm H2O and 300 ppm CO2 in bulk rock composition) in peridotite matrix. The seismic discontinuities atop the LAB and their constant depths can also be explained by such small amounts of CO2-rich melts wetting the grain boundaries in peridotite. Mantle metasomatism must then broadly prevail in the LAB and may well play a role in the processes that allows plate tectonics on Earth.