Seismic signature of small melt fraction atop the transition zone

Abstract

This article explores the combined effect of thermal, chemical, and melting anomalies of seismic velocities above the transition zone. While thermal and chemical effects influence the seismic velocities at subsolidus temperatures, the velocity structures are greatly modified in the presence of partial melting. We model the impedance contrast atop a low velocity layer at a depth of 350 km beneath the south Pacific, based on seismic observations of ScS wave reflectivity in the region. A compositionally distinct layer with varying basalt fraction fails to produce the observed average shear impedance contrast of -2.7% , for a range of potential temperatures between 1500 and 1700 K. A partially molten layer containing approximately 1 vol.% melt, explains the observed shear impedance. The melt fraction necessary to explain the observed shear impedance also trades off with the dihedral angle of the aggregate. For dihedral angles between 25 o and 30 o, between 1 to 1.1 vol.% melting is necessary to explain the observed impedance contrast. For such small volume fraction of melting, a near neutrally-buoyant melt can be redistributed by surface tension over the observed layer thickness of ∼ 70 km. Due to the high frictional resistance and strong surface tensions prevalent at such small melt fractions, density-driven melt drainage will likely be inefficient.