LIMITS ON THE MASS FLUX BETWEEN LOWER AND UPPER-MANTLE AND STABILITY OF LAYERING

Geochemical arguments for a layered mantle based upon both lithophile and rare gas element abundances are well established. The negative Cla yperon slope, associated with phase changes at 670 km, is expected to impede circulation between the upper and lower mantle and induce layer ing at this depth. Consideration of estimates for the He and Ar isotop ic composition of the upper and lower mantle, together with Ar-36 and Ar-40 abundances in the atmosphere, place limits on the mass flux betw een the lower and upper mantle when combined with estimates for K, U a nd Th. Using the assumption that rare gases have steady-state abundanc es in the upper mantle (with residence time tau = 1 Ga), then the time -averaged mass flux from the lower mantle is less than or equal to 2 . 10(13) kg a(-1). The estimated mass flux is at least 50 times less th an the present-day slab flux of similar to 10(15) kg a(-1). The result imposes restriction on the amount of upper mantle material and the fr equency with which possible avalanches into the lower mantle might occ ur and further suggests that models of episodic crustal growth related to such effect are unlikely to be correct.