ABSTRACT

The classical concept of mantle plumes describes a thermally buoyant upwelling that rises through the entire mantle to feed a thin (~100 km) ?pancake? of hot material ponding beneath the lithosphere and spawn hotspot volcanism. While this theory is elegant in its simplicity and its ability to explain a variety of observations, recent discoveries suggest that this idealization may no longer be tenable for all hotspots. At the archetypal Hawaiian hotspot, for example, the PLUME seismic tomography results shows compelling evidence for a plume-like body originating in lower-mantle, however, they also reveal a low-velocity body in the upper mantle that appears far too thick and asymmetric to be consistent with a classical thermal pancake. In the South Pacific, a cluster of hotspots populating the broad South Pacific Superswell each tend to be short-lived, show inconsistent age progressions, and are not connected to a large igneous province. Consequently, the classical plume theory has all but been discarded for these hotspots, giving way to the hypothesis that relatively small, short-lived ?plumelets? rising from the roof of a giant ?superplume? that is stagnating in the mid mantle. For many ocean islands, including those in the South Pacific and Hawaii, geochemical evidence reveals that mafic materials in the mantle source?not only excess temperature?contribute to volcanism. In the researchers? recent numerical simulations, mantle upwellings that are thermally buoyant but compositionally (partially eclogite) dense show irregular and time-dependent forms with potential for explaining many of above observations. Indeed, ?thermochemical? mantle convection is topic of vigorous research but very little work has been done to quantitatively explore the dynamical processes, melting behavior, geophysical manifestations, and geochemical consequences of thermochemical plumes interacting with mantle phase changes and a moving lithospheric plate.

The project has 3 main objectives. (1) Explore the physics of thermochemical plumes in the mantle transition zone and upper mantle and characterize the different forms of upwellings as a function of properties such as plume radius (e.g. superplume, Hawaiian-type plume), excess temperature, and eclogite content. (2) Establish relationships between the above properties and observables that can apply generally to hotspots world-wide such as the distribution, volume, and mafic content of magmatism, swell geometry, and mantle seismic structure. (3) Test the thermochemical plume hypothesis for hotspots in Hawaii, and the South Pacific by comparing model predictions with geochemical and geophysical constraints, especially the PLUME body wave tomography for Hawaii. This study will help develop a new class of plume concepts that is both motivated by, and can be tested against geophysical and geochemical data sets of ever increasing quality. Thermochemical convection displays a such rich diversity of shapes and dynamic regimes, and therefore this high-resolution modeling study has excellent potential for discovering yet unrecognized behaviors that are relevant hotspots and other upper mantle processes.

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