Seismic detection of sublithospheric plume head residue beneath the Pitcairn hot-spot chain

We study array seismograms of French nuclear explosions from two islands in French Polynesia and use these to constrain the structure in the upper mantle beneath the islands. Seismograms of nuclear explosions on the hot-spot-related Fangataufa atoll show discrete, large-amplitude P-coda phases which are not observed in recordings of explosions on Mururoa, an atoll ∼40 km to the north. The source for these P-coda phases is located beneath the Fangataufa atoll, indicating that structural heterogeneities are present in the oceanic upper mantle in this region on very small scale lengths. Synthetic seismograms for models of the upper mantle beneath Fangataufa require a layer with P-wave velocity elevated by ∼10% between depths of 51 and 85 km with a sharp termination in the north, possibly at the Austral Fracture Zone, to match these P-coda phases. Few mineralogic scenarios exist that can explain this structure, and the properties of this layer imply that extensive enrichment in garnet occurs in this depth range. The garnet-enriched layer is likely of similar origin to the well-known xenolithic ‘garnet megacryst’ suite found in kimberlitic regions. We propose a model for the formation of the Fangataufa and Mururoa atolls involving garnet-enriched zones being generated at depth through magmatic processes at the Pitcairn plume head. Thus, the initiation of hot-spots could produce complex geochemical and structural heterogeneities at depth in the suboceanic mantle.