MANTLE REFLECTIVITY STRUCTURE BENEATH THE HAWAIIAN HOTSPOT

Seismic discontinuities arise from solid-to-solid phase changes in mantle minerals; it is well established that the 410 km discontinuity arises from the transition of olivine to wadsleyite, and the 660 km discontinuity from the dissociation of ringwoodite into bridgmanite and ferropericlase. The impedance, sharpness, and depth of each discontinuity is dependent upon the thermochemical state of the mantle, thus making the upper mantle discontinuities a sensitive probe of mantle temperature, composition, and underlying dynamics. Here, we study the amplitudes of underside reflections of P- and S-wave energy from the 410 and 660 km discontinuities beneath Hawaii, along with other seismic discontinuities near 200-400 km depth and 800-1200 km below the transition zone, using a large seismic dataset of broadband SS and PP precursors. The amplitudes of these seismic phases are sensitive to the changes in density, shear, and compressional velocities at each boundary.

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.