Disequilibrium Textures vs Equilibrium Modelling in Geochronology

Observations made by electron microscopy show the processes affecting minerals at the atomic scale. Submicroscopic microchemical disequilibrium in minerals is extremely widespread. A classic example are overgrowths of one mineral generation by a secondary one, which may be recognized on textural grounds. Disequilibrium recrystallization is promoted by water in metamorphic terranes and near granites, contact aureoles, and faults. Recrystallization is energetically less costly at almost any temperature than diffusive reequilibration. First principle arguments and experimental data of the last decade concordantly show that radiogenic isotopes (except 4He) never diffusively reequilibrate faster than major elements forming the mineral structure. Isotopic inheritance tied to relics was demonstrated for zircon, monazite, amphibole, K-feldspar, biotite, and muscovite. End-member ages of mineral mixtures can be unravelled if the petrogenesis is understood. The mechanism for resetting the isotope record in nature depends more on the availability of recrystallization-enhancing water than on reaching a preset temperature. Mineral geochronometers should be viewed as “geohygrometers” that essentially date the fluid circulation episodes. The determination of isotope mobility in the laboratory is fraught with difficulties. All long-term experiments on hydrous minerals were plagued, to a variable but always large extent, by dissolution-reprecipitation. Short-term stepheating experiments have been assumed to purely reflect Fick’s Law of volume diffusion. However, intercomparison of laboratory release of Ar and Xe in Ba-fsp and Ba-mica require the same in vacuo degassing mechanism for hydrous and anhydrous silicates. This is not Fick’s Law diffusion, and stems instead from a discrete structural rearrangement. Extrapolation of diffusion rates from stepheating experiments to natural systems is demonstrably a mirage, giving testably false predictions. In a parallel universe, diffusive equilibration is the basis for isotopic models providing numbers to be input into tectonic models. Is this the role of geochronology? Instead, thanks to submicroscopic petrology, isotopic disequilibria can be put into context with petrogenetic disequilibria. Analytical advances allow the successful dating of each mineral generation. This has opened up a much richer wealth of data on the P-T-A-X-d history of rocks, which in the long run will also improve our ability to develop credible numeric models.