CSEDI: Reconciling Geophysical and Geochemical Observations of Mantle Structure

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White and Turcotte 0079975 This study will critically evaluate models of mantle structure in light of geochemical observations. Any successful model of mantle structure and dynamics must simultaneously explain both geochemical and geophysical observations. The model of mantle structure proposed by Kellogg et al. [Science, 283: 1881-1884, 1999] appears promising in this respect. A major objective of this study will be to determine whether this model is actually consistent with geochemical observation. Specifically, the investigators will carry out the following studies: (1) Mass Balance and Heat Flow. For refractory lithophile elements, such as Sr, Ce, Sm, Nd, Lu, Hf, U, and Th, the crust plus mantle should sum to give chondritic relative abundances. Mantle heat flow is largely due to decay of U, Th, and K and hence provides a further constraint on the abundances of these elements. Since both the heat flow and incompatible element and isotopic composition of the crust and depleted mantle are known within limits, the composition and size of other mantle reservoirs can be constrained. The investigators will evaluate crust-mantle mass balance using refined estimates of crustal and depleted mantle composition and incorporating the constraint of heat flow. (2) Isotope evolution models. This work will examine the evolution of radiogenic isotopes, particularly the U-Th-Pb-He system, in a multi-reservoir system where residence times of elements are short. Alternative configurations of reservoirs will be considered with both steady-state and episodic recycling of material between reservoirs. The objective will be to constrain the sizes of and fluxes between mantle reservoirs. (3) Models of convective mixing. The investigators will model mixing and homogenization within reservoirs using parameterizations of mixing times versus temperature and viscosity based on available numerical simulations. The analyses will be divided into two parts. First, variable rates of interchange, including strongly time-dependent interchange, between major terrestrial reservoirs will be considered to determine whether a self-consistent model can be obtained that satisfies the geochemical constraints. The second part will consider isotopic evolution within a reservoir.