Disequilibrium textures versus equilibrium modelling: geochronology at the crossroads

Abstract

Submicroscopic microchemical disequilibrium in minerals is extremely widespread. 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 re-equilibration.Radiogenic isotopes (except 4 He) never diffusively re-equilibrate faster than major elements forming the mineral structure. Isotopic inheritance tied to relicts was demonstrated for zircon, monazite, amphibole, K-feldspar, biotite and muscovite.The mechanism for resetting the isotope record in nature depends more on the availability of recrystallization-enhancing water than on reaching a preset temperature. Laboratory diffusion experiments on hydrous minerals were plagued, to a variable but always large extent, by dissolution-reprecipitation. Mineral geochronometers should be viewed as 'geohygrometers' that essentially date the fluid circulation episodes.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.

Petrology and geochronology of ‘muscovite age standard’ B4M

“…As a consequence of petrologic disequilibrium, the Sr and Ar isotopic systems preserve an isotopic inheritance (cf. Villa 1998Villa , 2010. This supports the conclusions by Allaz et al (2011) that the 'closure temperature' at which the K -Ar system in white mica is reset during regional metamorphism (and ensuing mediumslow exhumation and cooling) exceeds 500 8C.…”

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…(1) Growth or partial or complete recrystallization of mica (Willigers et al 2004;Glodny et al 2008;Villa 2010). Of the samples presented in this study, all micas and feldspars show evidence of dynamic recrystallization, and are therefore interpreted as recording metamorphic or deformational ages unless otherwise stated.…”

“…The 87 Rb/ 86 Sr uncertainty used in age calculations was therefore taken as 0.3% (2σ), although this is likely to be an overestimate. Ages and uncertainties were calculated in Isoplot version 4 (Ludwig, 2003), using the decay constant for 87 Rb recommended by Villa et al (2015). Using this decay constant, ages are 1.6% older than Rb-Sr ages calculated using the decay constant recommended by Steiger & Jäger (1977), which has been pervasive throughout the last 40 years.…”

Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

“…In order to minimize problems with Sr-isotopic inheritance and disequilibria, we preferentially selected strongly mylonitized samples showing no visible texturally relic porphyroclasts because textural equilibration is a reliable indicator for achievement of isotopic re-equilibration (e.g. Villa, 2010). Isotopic data were generated at GFZ Potsdam on a Thermo Scientific TRITON thermal-ionization mass spectrometer.…”