The survival of mantle geochemical heterogeneities

Abstract

The survival of mantle geochemical heterogeneities depends on the relative values of the residence times of individual elements in the mantle and of the scale-dependent mixing rate imposed by convection. Major and trace element data on fresh glasses from the PETDB data base were compiled to extract statistics on MORB compositions. It was found that the log-normal distribution provides an adequate description of the data. New means, modes, and standard deviations have been calculated after correction for crystal fractionation to 8% MgO. The residence times of the incompatible lithophile elements in the mantle were calculated as ratios of the mean mantle (Bulk Silicate Earth minus the continental crust) composition to fluxes of elements into the oceanic crust. They vary within a rather narrow range (4-9 Gy) and the larger uncertainties on U, Th, Ba, K, and Rb come from those on the continental crust composition. The mantle is therefore not at geochemical steady-state and the effect of its primordial composition on models of modern mantle geochemistry is still strong. Up to 50% of incompatible lithophile elements may never have been extracted into the oceanic crust, which expands a conclusion reached previously for 40 Ar. The residence time of heat in the mantle is indistinguishable from those of the incompatible lithophile elements implying that 50% of the Earth's primordial heat may still be buried at depth. This simple observation relieves the heat/helium paradox, which was used as an argument in favor of a conductive boundary layer at intermediate depth in the mantle. The balance between buoyancy flux and dissipation provides frame-independent estimates of the rates of mixing by mantle convection: Primordial geochemical anomalies with initial length scales comparable to mantle depths of plate lengths may be marginally visible at the scale of mantle melting underneath mid-ocean ridges (50 km), but may show strongly in hot spot basalts and even more so in melt inclusions. A theory of residence time distributions of tracers in a convecting mantle with recycling of the oceanic crust is presented. It demonstrates that up to 50% primordial material may be present in the mantle, but scattered throughout as small (<10 km) domains, strongly sheared and refolded, and interbedded with younger recycled material.