Models of Crust-Mantle Evolution

Abstract

The Sm-Nd isotopic data have been used as a basis for testing models of the chemical evolution of the earth which lend themselves to mathematical description. The Sm-Nd system offers a particularly powerful constraint because of the ability to predict the Sm/Nd ratio of the total earth. This characteristic also applies to the Lu-Hf system. The models that have been considered are relatively crude, considering the large number of complex processes that they must attempt to represent. By comparison, for instance, to stellar evolution models, they are simple back-of-an-envelope calculations as opposed to hour-long computer calculations. The geochemical earth models have treated only mass transport, and only in a quasi-equilibrium manner. Various parts of the earth’s interior are considered to be “boxes”, each generally homogeneous within, and only the transfer of material between these boxes has been considered. There has been little attempt to concurrently model the other aspects of the physical evolution of the reservoirs, such as heat transfer and the fluid dynamic problems associated with the retention of “homogeneity” in the boxes. Instead, there have been attempts to describe the mass transport in sufficiently general terms to allow some relatively robust first-order conclusions to be drawn.