What is the threshold plume flux for a persistent plume-fed asthenosphere?

Chao Shi and Jason Phipps Morgan, EAS Dept., Snee Hall, Cornell Univ., Ithaca, NY 14853 In past years we have presented observation evidence that in Earth's mantle convection there exists a buoyant asthenosphere layer fed by upwelling in mantle plumes, and consumed by accretion and transformation into overlying lithosphere by ridge upwelling and melt-extraction (which creates a ~60km-thick layer of compositional lithosphere at mid-ocean ridges), by plate cooling (which accretes a further ~40km of asthenosphere after 100 Ma of near-surface cooling), and by dragdown by subducting slabs (which drags a further ~20km sheet of buoyant asthenosphere on either side of the subducting slab). This scenario has been recently reviewed in Yamamoto et al (GSA Vol. 431). We believe that the reason this mode of mantle convection has not yet been seen in numerical models of mantle convection is due to the inability of current models to model the correct magnitudes of upwelling in focused lower- viscosity plumes (which they currently underpredict) and to correctly model the magnitude of downdragging of a more buoyant but lower viscosity asthenosphere layer (which they currently overpredict, cf. Phipps Morgan et al., Terra Nova, 2007). Here we present results from a suite of 2-D and 3-D calculations that include the effects of ridge accretion, plate cooling and well-resolved asthenosphere dragdown by subducting slabs. In these experiments we do not let mantle plumes spontaneously form at the hot base of the mantle. Instead we extract mantle at a prescribed rate from a single region near the bottom of the mantle (the 'base of the plume stem') and inject this hot material into the uppermost mantle. The point is to bypass the correct treatment of plume upwelling, in order to explore what upwelling flux is needed to form a persistent plume-fed asthenosphere. We will present results on the plume-flux needed to create a global sub-oceanic plume-fed asthenosphere as a function of: (1) the rate of lithosphere creation and subduction; (2) the buoyancy contrast between asthenosphere and underlying mantle; and (3) the asthenosphere viscosity.