Detection of melting atop the mantle transition zone

We explore the combined effect of thermal, chemical, and melting anomalies on shear wave velocities above the transition zone. While thermal and chemical heterogeneities influence the seismic velocities at subsolidus temperatures to some extent, the velocity structures are greatly modified in the presence of partial melting. We interpret two sets of data; ScS reflectivity beneath Coral Sea in the Southwest Pacific and receiver function analysis of P-to-S conversions beneath the Hawaiian islands. Both datasets indicate the presence of a Low Velocity Layer (LVL) at an average depth of 350 km. Temperature and subsolidus composition fail to fully explain the observed impedance contrast atop the LVL. Beneath the Coral Sea, an average melt volume fraction of 1% is necessary to explain the seismic observations, while beneath Hawaiia range of melt fractions describe the seismic signature. Melt fraction appears to increase from Northwest to Southeast along the island chain and trades off with dihedral angle.Map of calculated melt volume fraction at the Low Velocity layer (LVL) located approximately 350 km below the surface of Hawaii. The melt fraction is calculated for variations in the subsolidus basaltic component (X) and reference potential temperature (T).