Seismic evidence favoring depletion of Precambrian lithosphere and partial melt at the base of tectonic plates

Most global tomographic studies of the upper mantle and their thermochemical interpretations have focused on shear velocity (Vs). Shear attenuation has a different sensitivity to temperature, composition and melt content and therefore provides complementary constraints on the origin of seismic heterogeneities. In the upper mantle, shear attenuation is negligibly dependent on major element chemistry and exponentially dependent on temperature.

Here, we first simultaneously interpret two recent global Vs and Qs models, which are obtained from the same Rayleigh-wave dataset, at the same resolution and using the same modelling approach. Comparison with mineralogical data suggests that partial melt occurs within the LVZ and down to 150–200 km beneath mid-ocean ridges, major hotspots and back-arc regions. A small part of this melt (less than 0.3%) remains trapped within the oceanic LVZ.

Melt is mostly absent under continental regions. In these regions, we observe high seismic velocity keels extending to depths that often exceed 200 km. The thermochemical interpretation of our global shear velocity models requires mineralogical depletion and a decrease of compositional density beneath Precambrian cratons. These conditions ensure their preservation for billions of years in a convective mantle, in agreement with mantle xenoliths suggesting that high viscous keels formed early in the history of cratons.