[S-IT31] [EE] Revisit Bullen's layer C - Mantle transition zone and beyond

[SIT31-04] A sporadic low-velocity layer atop the 410-km discontinuity beneath the Pacific Ocean

*Songqiao Shawn Wei¹, Peter M Shearer¹ (1.Scripps Institution of Oceanography, UC San Diego)

Keywords:410-km discontinuity, SS precursor, Partial melting

The seismic discontinuity at 410 km depth is usually attributed to an isochemical phase transformation from olivine to wadsleyite. In addition to this globally observed feature, a low-velocity layer immediately above it has been observed regionally in many places, mainly under continents and continental margins. This low-velocity layer is thought to represent partial melting due to dehydration of ascending mantle across the 410-km discontinuity. Here we present seismic observations of a sporadic low-velocity layer atop the 410-km discontinuity beneath the Pacific Ocean by stacking and analysing long-period seismic body waves. The lateral variations of this low-velocity layer show no geographical correlation with 410-km discontinuity topography or tomographic models of seismic velocity, suggesting that it is not caused by regional thermal anomalies. If this low-velocity layer indeed indicates dehydration melting across the 410-km discontinuity, its strong lateral heterogeneity needs to be taken into account in future geodynamic models of mantle convection and the deep water cycle.