Dating deformation: the role of atomic-scale processes

Abstract

Dating deformation is difficult, as textures and petrogenesis of deformed rocks are complex. Moreover, geochronometer categories are pursued by communities that often do not communicate. Hygrochronology dates the retrograde metasomatic/metamorphic reactions caused by aqueous fluid circulation events. Thermochronology models time-temperature histories by assuming that mineral ages can be uniquely assigned to a ‘closure temperature Tc’, the only process occurring in rocks being Fick’s Law diffusion. Diffusion by definition produces a bell-shaped concentration profile. In contrast, patchy intra-grain isotope concentration profiles denounce aqueous retrogression, whose rate is orders of magnitude faster than diffusion. Petrochronology is based on opposite assumptions, as the mobility of structure-forming major cations is higher than that of radiogenic Pb, Ar, and Sr. Whenever the formation of a mineral occurs at T < Tc, its apparent age dates its formation. Nanochronology analyses samples at the nanometre-scale. These analyses illuminate atomic-scale processes, e.g. open-system transport of soluble ions along self-sealing networks of nanopores. The key to dating deformation and producing correct, regional-sized (up to hundreds of kilometres) tectonic models is the realization that minerals consist of atoms, whose behaviour is only firmly constrained by nanometre-scale analyses.