Double layering of a thermochemical plume in the upper mantle beneath Hawaii

•We simulate thermochemical plume dynamics in the upper mantle in 3D geodynamic models.•The interaction of eclogitic material with phase changes complicates plume dynamics.•An eclogite-bearing plume is therefore expected to pool in the mid upper mantle.•This behavior can explain seismic observations from regional tomography in Hawaii.•Pulsations of eclogite-bearing plumes can also produce variations in hotspot activity.

According to classical plume theory, purely thermal upwellings rise through the mantle, pond in a thin layer beneath the lithosphere, and generate hotspot volcanism. Neglected by this theory, however, are the dynamical effects of compositional heterogeneity carried by mantle plumes even though this heterogeneity has been commonly identified in sources of hotspot magmas. Numerical models predict that a hot, compositionally heterogeneous mantle plume containing a denser eclogite component tends to pool at ∼300-410 km depth before rising to feed a shallower sublithospheric layer. This double-layered structure of a thermochemical plume is more consistent with seismic tomographic images at Hawaii than the classical plume model. The thermochemical structure as well as time dependence of plume material rising from the deeper into the shallower layer can further account for long-term fluctuations in volcanic activity and asymmetry in bathymetry, seismic structure, and magma chemistry across the hotspot track, as are observed.