Petrochronology and hygrochronology of tectono-metamorphic events

International audience ; U-Th-Pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic Pb is negligiblysmall relative to retrograde reaction rates. Multi-element maps demonstrate that patchy textures tightly correspondto (U + Th)-Pb age variations, requiring that fluid-induced dissolution/ reprecipitation is the principalcause of Pb mobility. Attempts to model intracrystalline core-rim Pb zonations as diffusive transport are not legitimateunless genuine bell-shaped diffusion profiles in minerals can be documented, which happens only exceptionally.Monazite and zircon intra-grain age maps confirm that coupled dissolution-reprecipitation andretrogression reactions assisted by fluids control (Th+U)-Pb ages, not temperature. The chemical zonations observedin many (Th+ U)-bearing mineral chronometers (e.g. monazite, allanite, xenotime, zircon) provide petrologicalconstraints. Linking petrology with textures and the isotope record allows reconstructing entiresegments of the P-T-A-X-D-t history of a rock and its geodynamic environment.The dearth of mathematically sound diffusion profiles equally applies to the isotope records of micas and feldspars.The tight link between petrology, microtextures, chemical composition and geochronology also pertainsto Rb-Sr and K-Ar. Overdetermined multi-mineral Rb-Sr isochrons with excess scatter, and spatially resolved/stepwise release 39Ar-40Ar results, demonstrate ubiquitous correspondence between relict phases and isotopicinheritance. Many rock-forming minerals are highly retentive of Sr and Ar, unless they are obliterated by retrogradereactions. The rates of dissolution in fluid-controlled reactions are several orders of magnitude faster atupper and mid-crustal levels than diffusive reequilibration rates. Thus, as a rule Rb-Sr and K-Ar chronometersdate their own formation.Accurately establishing P-T paths of monometamorphic rocks requires assessing petrologic equilibrium usingmultivariate thermodynamic software. Dating complex parageneses of ...