The concept of plumes within the mantle of the earth has played a central role in linking deep mantle convection to surface expressions of volcanism, topographic uplift, and geochemical signatures. However, the concept continues to remain incomplete. Two issues remain to be resolved to complete our understanding of mantle plumes and their role in the dynamical and geochemical evolution of the planet. The first, and the subject of vigorous controversy, is the very existence and survival of plumes in an internally heated, convecting mantle dominated by the large-scale flow associated with plate motions. The second relates to the degree of geochemical variation revealed by the isotopic signatures of ocean island basalts, which we identify as the surface expression of mantle plumes. In order to address both issues, PIs Lithgow-Bertelloni, Cotel, Whitehead and Hart are conducting laboratory experiments on single thermal and compositional plumes in homogeneous and stratified fluids. The primary goals of the present experiments are first, to accurately measure and characterize the buoyancy flux throughout an evolving plume as a function of the initial heat flux, and fluid composition, and second, to analyze the effects of shearing induced by the large-scale circulation on plume breakup and on stirring within the conduit. The emphasis lies in the use of new non-intrusive optical techniques (Particle Image Velocimetry, Laser-Induced Fluorescence, light deflection, and thermochromic liquid crystals for Digital Particle Image Thermometry). These can directly measure the three-dimensional flow structure in addition to giving the simultaneous measurements of composition and temperature. Over the next three years the PIs intend to provide rigorous scaling relationships to illuminate - The conditions that lead to steady plumes and steady-state conduits or plume breakup upon ascent - Spatial and temporal variations in plume structure as it relates to seismic imaging - The scales at which entrained material survives and interacts in thermal and thermochemical plumes - The link with the isotopic geochemical variability of ocean island basalt. The funded project involves close collaboration between four PIs with widely varying scientific and technical expertise ranging from geophysical fluid dynamics to geochemistry. Hence, the results of these experiments are likely to be of interest to diverse segments of the Earth Sciences community. All four PIs are involved in the experimental design and interpretation of the results.
