EPSS Colloquium fall-2017

A deep thermal plume in the mantle beneath Hawaii?
Constraints from tomography and reflection seismology

The topic of today’s lecture is mantle convection in broad sense and, specifically, the question as to whether or not a hot thermal plume exists in the deep mantle beneath Hawaii. Mantle transition zone (MTZ) discontinuities due to phase transitions in silicate minerals (e.g., olivine, garnet) near 410 and 660 km depth play an important role in modulating mantle flow. Convection is foremost a thermally driven system and most MTZ studies use discontinuity topography to estimate in situ temperature anomalies. Compositional heterogeneity is also expected but direct observational evidence is scarce and often qualitative. We study the base of the MTZ with a joint seismological and mineral physics analysis of the amplitudes of so-called SS precursors (S waves that bounce off MTZ discontinuities). The study area includes Hawaii, where a hot upwelling has long been proposed. The data are not consistent with a simple thermal plume but provide evidence for lateral variation in composition at the base of the MTZ as expected in high temperature, low viscosity environments near lower mantle upwellings. Combined with tomographic models the results suggest that the MTZ acts as a partial barrier for mantle flow.