Mica and apatite are widely used geo/thermochronometers, yet what mechanisms dominate their isotopic records remains unclear. This thesis evaluates how fluids-reactions, temperature-dependent volume diffusion and deformation control chemical and isotopic (re)equilibration in white mica and apatite. Spatially resolved petrochronology from the Black Hills reveals ubiquitous intragrain disequilibrium resulting from pseudomorphic replacement and interface-coupled-dissolution–(re)precipitation. Laboratory heating experiments show that white mica dehydroxylates above 700 °C, modifying the structure, redistributing Na and Ar and invalidating diffusion parameters and plateau dates derived after dehydroxylation. Deformed meta-granites from the Gotthard nappe and Yarrabubba impact crater demonstrate that increasing strain does not increase mineral reactivity and isotopic resetting; requiring direct links between fluid-reactions and deformation. Consequently, chemical mapping is essential for linking isotopic dates to fluid-reactions and geological processes. The kinetics of volume diffusion and deformation are subordinate to fluid-reactions in driving isotopic resetting. Therefore, thermochronological interpretation must demonstrate the absence of fluid-driven reactions.