Insights into the nature of plume–asthenosphere interaction from central Pacific geophysical anomalies

The central Pacific is one of the type localities where the influence of a plume on the properties of the asthenosphere can be investigated based on the geophysical observations. Recent seismological studies showed enigmatic structures of the upper mantle in the central Pacific. The asthenosphere (~ 100–200 km depth) in this region has unusually strong VSH/VSV > 1 anisotropy associated with relatively weak azimuthal anisotropy. Although less well-constrained, there are suggestions of higher than normal viscosity and lower than normal electrical conductivity in this region. Previous models for the plume–asthenosphere interaction do not easily explain these observations. I propose that these observations are the consequence of a single process: depletion of water by deep partial melting in the plume column. The plume-fed materials in the asthenosphere will be dominated by the olivine A-type fabric whereas in the surrounding normal asthenosphere olivine E-type fabric dominates. This contrast in olivine fabrics provides an explanation for the observed anomalous seismic anisotropy in the central Pacific. Water depletion will also cause high viscosity and low electrical conductivity. The present model implies that plumes supply depleted materials (rather than enriched materials) to the asthenosphere.