The oldest rocks on Earth occur within the continents are of Archean age (~3.0-4.0 Ga), and are comprised of granitoid ("TTG") rocks that form the physical nucleus, or craton, of these continents. The cratons are underlain by mantle peridotites of the continental lithosphere, which possess unique chemical and physical properties that are responsible for their long-term stability since the Archean. Amongst the terrestrial planets of our solar system, the Earth appears to be unique in possessing extensive, highly-differentiated continental crust (or cratons). This study considers the origins of the oldest continental crust, and addresses how crustal growth in the Archean was related to the chemical evolution of the cratonic lithosphere. Laboratory experiments, to be performed at pressures and temperatures relevant to those of the subcratonic lithosphere, will consider to what extent and by what chemical mechanisms Archean granitoid (TTG) magmas interacted with peridotite rocks in evolving cratonic roots. These experiments will produce variable proportions of melt ("magma") and crystalline (mineral) phases as a function of pressure, temperature and other variables, and the detailed chemical features of the experimental melts and minerals will be compared with appropriate natural samples (respectively, Archean granitoids, and samples of the subcratonic lithosphere brought to the surface in kimberlite eruptions, in the form of peridotite "xenoliths, or as minute mineral "inclusions" in diamonds). Geometrically, modern subduction zones consist of a "wedge" of mantle lithosphere (peridotite) overlying a "slab" of subducting oceanic lithosphere. Questions for Archean crustal evolution, which this project will address, include whether or not a conventional mantle wedge-subducting slab geometry is appropriate for the early Earth, or if some unique style of tectonism was operative, and at what point in time did modern-style, plate tectonic processes take over? A better understanding of the genetic relationship between granitoid magmatism on the early Earth and the development of deep roots or "keels" for the cratons in the underlying mantle is an important prerequisite for on-site studies of crusts on the other terrestrial planets (e.g., Mars and Venus), and for the search for Earth-like planets beyond our solar system.
