MANTLE REFLECTIVITY STRUCTURE BENEATH THE HAWAIIAN HOTSPOT
Beneath Hawaii, we see evidence for topography on the mantle transition zone discontinuities, with a localized downwarping of the 410 and upwarping of the 660 km discontinuities, both consistent with the presence of a high temperature mantle plume. The impedance contrasts present at the 410 and 660 are significantly perturbed from PREM expected values, with the 410 1-2% weaker than predictions from pyrolitic mantle, and the 660 2-3% stronger than predicted for pyrolite. In addition, there are upper mantle discontinuities that are enhanced in the their reflectivity in the vicinity of Hawaii. As geochemical and dynamical constraints indicate that upwellings may carry both chemical and thermal heterogeneity, we favor an interpretation in which lateral changes in mantle composition (both in basalt enrichment and hydrous content) are responsible for the observed seismic structures at Hawaii. Altogether, there appears to be evidence for significant lateral heterogeneity associated with the Hawaiian plume that may be related to phase changes outside the dominant olivine system.