Seismic Evidence for Partial Melt Below Tectonic Plates

Abstract

The seismic low velocity zone (LVZ) of the upper mantle is generally associated with a low-viscosity asthenosphere that plays a key role for the dynamics of plate tectonics 1 . However, its origin remains enigmatic, some authors attributing the reduction in seismic velocity to a small amount of partial melt 2,3 , others invoking solid-state mechanisms near the solidus [4][5][6] , or the effect of volatile contents 6 . Observations of shear attenuation provide additional constraints to unravel the origin of the LVZ 7 . Here, we report the discovery of partial melt within the LVZ from the simultaneous interpretation of global 3D shear attenuation and velocity models. We observe that partial melting down to 150-200 km depth beneath mid-ocean ridges, major hotspots and back-arc regions feeds the asthenosphere. A small part of this melt (<0.30%) remains trapped within the oceanic LVZ. The amount of melt is related to plate velocities and increases significantly between 3 and 5 cm yr -1 , similar to previous observations of mantle crystal alignement underneath tectonic plates 8 . Our observations suggest that by reducing viscosity 9 , melt facilitates plate motion and large-scale crystal alignment in the asthenosphere. Melt is absent under most of the continents.Our finding results from the simultaneous analysis of two upper mantle tomographic models of shear wave velocity (Vs) and attenuation (parameterised

Hydrous silicate melts and the deep mantle H2O cycle

“…Low seismic velocity regions above 410 km and beginning at depths near 350 km have been observed both regionally and globally and have been interpreted as arising from partial melting and potentially indicating neutrally or negatively buoyant melt (Revenaugh and Sipkin, 1994;Song et al, 2004;Tauzin et al, 2010). Conversely, global three-dimensional models coupling shear attenuation and seismic velocity do not show evidence of a global partial melt at depths of 350 km (Debayle et al, 2020). Recent numerical models indicate that a hydrous transition zone containing at least 0.2-0.3 wt% H 2 O can explain regions of low seismic velocity observed above and below the Pacific slab by pockets of hydrous melt that are in turn responsible for intraplate and petitspot volcanism in northeast China and offshore Japan (Yang and Faccenda, 2020).…”