Plume scar in the mantle lithosphere beneath the Seychelles revealed by seismic imaging

Continental flood basalts are commonly thought to form by direct melting of lithospheric mantle, either during rifting episodes or when a mantle plume impinges on the base of the lithosphere. If correct, then significant alteration of the chemical and physical properties of the lithospheric mantle should be observable beneath such provinces. To date, however, this hypothesis remains unproved through lack of direct evidence. Here we present the results from a seismic experiment conducted in the Seychelles, a region that separated from India at the time of the eruption of the Deccan Traps. The analysis shows a stratified upper mantle with exceptionally low S-wave velocities at depths between 50 and 190 km. It is difficult to explain these anomalies without invoking the presence of melt. However, the depth extent of the low velocities rules out the presence of silicate melt. We argue that the most probable interpretation is that these layers contain sulphide melt trapped from earlier interactions of the lithosphere with episodes of continental-flood basalt generation. The presence of this melt over a depth interval of ∼ 130 km suggests that it is stagnant, and has remained coupled with the Seychelles lithosphere.