Low seismic velocities below mid‐ocean ridges: Attenuation versus melt retention

Abstract

The first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low‐velocity zone between 40 and 100 km depth below the East Pacific Rise (EPR) that has been attributed to substantial retained melt fractions of 0.3–2%. Such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry. Here, we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions. We start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure, accounting for temperature‐, pressure‐, composition‐, phase‐, and melt‐dependent anharmonicity, and temperature‐, pressure‐, frequency‐ and hydration‐dependent anelasticity. Our models predict a double low‐velocity zone: a shallow—approximately triangular—region due to dry melting, and a low‐velocity channel between 60 and 150 km depth dominantly controlled by solid state high‐temperature seismic attenuation in a damp mantle, with only a minor contribution of (<0.1%) melt. We test how tomographic inversion influences the imaging of our modeled shear velocity features. The EPR experiment revealed a double low‐velocity zone, but most tomographic studies would only resolve the deeper velocity minimum. Experimentally constrained anelasticity formulations produce VSas low as observed and can explain lateral variations in near‐ridge asthenospheric VS with ±100 K temperature variations and/or zero to high water content. Furthermore, such QS formulations also reproduce low asthenospheric VS below older oceans and continents from basic lithospheric cooling models. Although these structures are compatible with global QS images, they are more attenuating than permitted by EPR data.