Thorium-uranium systematics require layered mantle convection

Two of the most important sources of information on the style of mantle con vection come from geochemical constraints and mantle tomography. Constraint s imposed by uranium-thorium systematics are examined in this paper. Observ ations require that the mid-ocean ridge basalt (MORB) source region be near ly homogeneous with a thorium-uranium ratio kappa = 2.5 +/- 0.1. Simple mas s balance considerations utilizing limits on the value of K for the contine ntal crust (kappa < 6) and a bulk earth value Ic = 4.0 +/- 0.2 with heat pr oduction constraints show that the whole mantle has a thorium-uranium ratio greater than <kappa> = 3.35 +/- 0.1. This leads to the conclusion that a s ubstantial fraction of the mantle has a much more primitive value of kappa (closer to 4.0) than the MORE source region. Our model assumes a near-unifo rm, depleted upper mantle that is the source region for MORE and has kappa approximate to 2.5 and a near-primitive lower mantle with kappa approximate to 4.0. Our results indicate that it is very difficult to have the upper m antle reservoir larger than 65% of the whole mantle and our preferred value is 45%. Three alternative hypotheses can be proposed: (1) A vertical strat ification of kappa through the mantle (since the role of whole mantle conve ction is to extract heat, the resulting vertical transport in the mantle wo uld be expected to eliminate the vertical stratification of kappa), (2) a m antle permeated with blobs (since the blobs would have to constitute some 6 5 +/- 10% of the mantle and be absent from the MORE source region, it is di fficult to support this hypothesis), and (3) layered mantle convection with a compositional barrier to convection in the lower mantle. The results giv en here strongly favor this layered mantle hypothesis as recently set forth by Kellogg et al. [1999] and van der Hilst and Karason [1999].