Where are U and He?

There is no plausible mechanism for creating a uniform distribution of U, Th and K in the mantle, or for having the mantle cool uniformly or for retaining undegassed reservoirs. Accretion of planets is an energetic affair that results in melting, degassing and differentiation rather than homogenization. A planet is stratified by volatility, composition, radioactivity and density by radial zone refining. Volatiles, including noble gases and heat producing elements, were sweated out of the materials that are now in the deep interior and concentrated into the outer shells. Delaminated and tectonically eroded crust and carbonatites from the slab are overlooked mantle components that account for missing U and Th and heat-flow; these oversights have promoted models of deep enriched, hidden and accessible, reservoirs. About half the continental crust is recirculating in the mantle at any given time and carbonatites contain upwards of a thousand times the concentrations of incompatible elements of primitive mantle. These components are also required to balance the concentrations in MORB and the surface crust. Most of the radioactivity that is not in the crust is in these enriched components in the upper mantle. There is little U, Th and K in the deep mantle. The concentration of heat production in the shallow mantle has not been allowed for in any model investigated by isotope geochemists or convection modelers although it was widely discussed by Birch, Armstrong and others prior to the widespread use of over- simplified geochemical box and reservoir, and whole mantle convection models. Stratification of the mantle is largely irreversible but the eclogite and carbonatite cycles are reversible on a short time scale because of the drastic lowering of the melting point. The upper mantle is mainly depleted and infertile harzburgite, populated by fertile blobs that contain essentially all of the heat producing elements. These blobs rise and fall, driven by phase changes (basalt-eclogite, eclogite-melt) and act as heat exchangers. Every part of the chain that starts out with K, U and Th and ends up with Ar, He, Ne, Pb, continental crust, heat and short-lived isotopes is plagued with paradoxes and conundrums. This indicates that the underlying assumptions, models and dogmas are wrong.