Nd Isotopic Studies: Some new perspectives on Earth structure and evolution

Geochronology is generally regarded as a branch of geochemistry that focuses on the determination of the ages of rocks. Although the reliable determination of ages of rocks is no trivial task, the above definition of geochronology is overly restrictive because the long-lived radioactive nuclides that can reveal the age of rocks can also yield a unique type of information about the internal structure of planets and the processes by which that structure is formed. In the case of the earth and the terrestrial planets, the internal structure can be described grossly by the division into core, mantle, crust, and an atmosphere-hydrosphere system where present. The present state of the earth (moon, Mars, etc.) can be studied by a number of methods, including photogeologic, petrologic, and chemical characterization of crustal rocks, and geophysical studies of the interior through the use of seismicity, gravity, heat flow, and other methods. But when and how this structure formed cannot generally be ascertained by those methods except by inference from theoretical models. The time dimension is uniquely accessible through the study of variations in the natural abundances of radioactive elements and their decay products, i.e., through geochronology. By using information about the geochemical properties of the elements, coupled with the measured isotopic variations, the nature of planetary differentiation processes and the associated time scales can be inferred. Such information is critical for linking what is known about the present structure of the earth to models of its evolution over the past 4.5 billion years. There is, however, an important restriction to the applicability of isotopic measurements, and that is that only if the geochemical properties of the particular elements are well understood can isotope abundance variations caused by radioactive decay be translated into information on planetary evolution.