The role of mantle heterogeneity in generating intraplate volcanism

Plate tectonics and mantle plumes explain most volcanism on earth, but there are numerous actively forming linear volcanicchains in the middle of tectonic plates that are not explained by these theories. Many intraplate volcanic chains are also associated with free airgravity lineaments that have been previously interpreted as being due to thermo-elastic cracking of the lithosphere or small-scale convection. Using the multidisciplinary geophysical dataset of the MELT and GLIMPSE experiments, we show that associated with 3 volcanic chains west of the East Pacific Rise there are low seismic velocities and densities in the asthenosphere that extend to the East Pacific Rise spreading center. Analogous to the Hawaiian swell, the low-density anomalies produce swells beneath the volcanoes on young seafloor. None of the available models (small scale convection or lithospheric cracking) completely explain the data. In order to explain the formation of the volcanic chains and associated density anomalies we have developed a 3-D geodynamic model of a heterogeneous viscosity/chemistry mantle subjected to an asthenospheric return flow, to simulate the potential channelized return flow from the Pacific Superswell to the East Pacific Rise. The viscosity contrast createsshear-driven upwelling, which is able to sustain significant decompression melting and volcanism that progresses towards the MOR, spreads laterally close to the axis, and weakly continues on the opposite plate. These predictions can explain the anomalously fast eastward progression of volcanism, and its spatial distribution near the SEPR. Finally, for a heterogeneous mantle source, predicted systematics in the share of fertile materials in the melt are consistent with geochemical trends along Pukapuka, and the enriched anomaly of SEPR MOR-basalt at 16ºS-20.5ºS11.