LITHOSPHERIC THINNING BY MIDPLATE MANTLE PLUMES AND THE THERMAL HISTORY OF HOT PLUME MATERIAL PONDED AT SUBLITHOSPHERIC DEPTHS

Mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotsp ots. The fate of this material and the thermal structure of the lithos phere depend on the vigor of secondary convection through the steep th ermal gradient above the ponded plume material. It has been proposed e ither that (1) secondary convection is so vigorous that material hotte r than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger litho sphere or (2) that the plume material retains its excess temperature f or a significant time and the lithosphere thins slowly. A parameterize d method of modeling transient secondary convection is developed to ex amine this question. Calculations indicate that secondary convection i s likely to be sluggish unless partial melting greatly lowers viscosit y. In addition, vigorous secondary convection would greatly retard ups tream asthenospheric flow of plume material beneath the nose of the Ha waiian swell and asthenospheric flow of hot material away from hotspot s on slowly moving plates like Cape Verde. This conclusion is compatib le with limited seismic evidence that indicates that the lithosphere i s Dot detectably thinned beneath the Hawaiian swell.