Evolution of mid-plate hotspot swells, mantle plumes, and Hawaiian basalts.

Abstract

Studies of the evolution of hotspot swells, mantle plumes, and Hawaiian basalts are presented in three parts in this dissertation. In part 1, the evolution of mid-plate hotspot swells are simulated numerically as an oceanic plate rides over a hot, upwelling mantle plume. The transient heat transfer equations, with time- and space-dependent boundary conditions, are solved in cylindrical coordinates. Geophysical data are used to constrain the models. Formation of the Hawaiian swell requires a mechanism of convective thinning of the lithosphere. The models constrain the Hawaiian heat source to have a maximum anomalous temperature of 250-300°C, and a perturbing heat flux 5-6 times the background value. On the other hand, the Bermuda swell is likely produced by heat conduction due to weakness of the heat source. In part 2, an analytic model of axisymmetric mantle plumes is presented. Plume parameters beneath the lithosphere, which are constrained from the swell models, are used to infer the plume source regions. The Hawaiian plume likely originates near the core-mantle boundary, but other hotspots may have shallower sources. Chemical plumes are much narrower than thermal plumes because of low chemical diffusivity in the mantle. For mantle plumes driven by combined thermal-chemical diffusion, the chemical signature of the source regions may only be observed near plume centers. Finally, melt generation and extraction along the Hawaiian volcanic chain are discussed in part 3. As a part of the plate moves over the heat source, melting largely takes place in the region where the lithospheric material is engulfed and swept away by the flow of the heat source. At least three mantle components must be involved in the melt generation: the plume material, the asthenosphere, and the engulfed lithospheric material. Significant amount of melts may also come from direct melting of the upwelling plume at depths below the initial plate-plume boundary. Melt extracts continuously from an active partial melting zone of 10-20 km thick, which moves outward as heating and compaction proceed. The models explain quantitatively the general characteristics of Hawaiian volcanism as the result of plume-plate interaction.