Remote sensing of the Earth’s interior using seismic waves and other geophysical observations reveals a complex, three-dimensional structure. This structure is thought to be a consequence of mantle convection, driven by slowly cooling of the planet from a hot initial state. Supporting evidence for internal complexity comes from geochemical analysis of melts that erupt on to the surface. Systematic variations in chemistry and isotopic composition are related to the physical structure, but the connection between the physical structure and the geochemical heterogeneity is a long-standing question that holds the key to unraveling the geological evolution of our planet.
The goal of linking geophysical and geochemical heterogeneity is motivated by the prospect of adding the dimension of time. Local changes in chemical composition are facilitated by melting and mixing. Fractionation of major constituents is often accompanied by changes in minor radiogenic isotopes, which accumulate distinctive decay products over time. Short-lived isotopes (such as 182Hf, 129I and 146Sm) were present at the time of the Earth’s formation and their decay products now reflect events that happen in the first few hundred million years of evolution. The fact that these isotopic fingerprints have survived to the present time means that mixing due to convection has not homogenized the planet after billions of years. Complementary insights from long-lived isotopes, like Uranium and Thorium, offer a time-integrated perspective of processes in the interior.Return to CIDER Summer 2018 Main Page