Isotopic Evolution of the Mantle in Numerical Convection Models

To investigate dynamical mechanisms that have been proposed to explain geochemical observations, a model of mantle convection that combines a treatment of major and trace element geochemical evolution with a dynamically-consistent mantle convection-plate tectonics is presented. Melting is simulated using a realistic solidus and is responsible for the generation of heterogeneity including the partitioning of trace elements between oceanic crust and residue; the subsequent hydrothermal alteration of element concentrations in the crust is also included. Trace elements studied are the U-Th-Pb and Sm-Nd isotope systems, helium and argon. Both olivine and pyroxene-garnet system phase transformations are included, with the relative density profiles of basalt, pyrolite, and harzburgite following those of Ringwood (1990) and Ono et al (2001) up to 800km depth, but varied in the deeper mantle to reflect present uncertainties. A suite of numerical experiments has been run to systematically investigate the sensitivity of the results to uncertain physical properties such as the density of subducted crust in the deepest mantle and elemental partition coefficients. Results indicate that the system can self-consistently evolve regions that have a HIMU-like signature (by segregation of subducted crust at the CMB) and regions with high ^3}He/{4He. Low ^3}He/{4He ratios evolve in crustal material even though He may be more incompatible that U, due to outgassing of He to the atmosphere. Some parameter combinations simultaneously lead to Earth-like distributions of ^3}He/{4He ratios, Pb-Pb and Sm-Nd ratio plots for erupted material, and ~ 50% outgassing of radiogenic ⁴⁰Ar consistent with geochemical constraints. In the preferred model, helium is highly incompatible and crust is dense near the CMB. The Sm-Nd age is 1-2 billion years. Earth-like ^3}He/{4He histograms are produced (in erupted material) when sufficient subducted crust mixes back into the shallow mantle to bring its He ratio down to the MORB-like range.