From nanometer to megameter: Isotopes, atomic-scale processes, and continent-scale tectonic models

Earth sciences deal with physical entities spanning 15 orders of magnitude, from nanometer (crystal lattice unit cells) to megameter (faults and orogens). The understanding of megameter-scale processes is greatly improved by the study of the nanometer-scale processes: the growth of crystals, their deformation under shear, and the transport of isotopes used for dating. From a physical point of view, the transport of atoms across a crystal with high potential barriers and no interatomic voids is not accurately described by Fickʹs equations, which were developed for dilute solutions with unhindered particle mobility. A variety of more general descriptions have been recently proposed, all of which share features such as complex temperature dependence as opposed to linear Arrhenian behaviour. From a petrological point of view, the ubiquitous recognition of microstructures in chemical and isotopic disequilibrium underscores the fact that diffusive reequilibration is a much slower process than creating the structures themselves. Diffusion in an inert matrix (“Fickian” diffusion) is observed to be negligible with respect to the growth of discrete mineral generations. As the latter frequently involves aqueous fluids, isotope geoscientists should follow petrologists and stress the geohygrometric properties of minerals (dissolution/reprecipitation). The fact that mineral reactions and microstructure development do not solely depend on temperature may explain why some “thermochronological” models from the literature have made predictions whose field tests have failed. The demise of one class of models might initially seem unfortunate; actually it is a great benefit because it shifts our attention to the reality of petrogenesis and to the sequence of events that any mineral assemblage records.