Dating deformation: the role of atomic-scale processes

Villa, Igor M. (2022). Dating deformation: the role of atomic-scale processes. Journal of the Geological Society, 179(5), pp. 1-7. Geological Society of London 10.1144/jgs2021-098

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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 analyzes samples at the nm-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 100s of km) tectonic models is the realization that minerals consist of atoms, whose behavior is only firmly constrained by nm-scale analyses.

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