Geochronology of metamorphic rocks

Metamorphic minerals are formed at moderate to high depth in the Earth's interior. Those that have been subsequently exhumed to the Earth's surface are those that survived metastably outside their stability fields. The majority of studied metamorphic rocks did not survive unscathed, and contains diachronic mineral assemblages that record a plurality of P-T-A-X conditions along the exhumation path, rather than a single P-T-A-X point that includes all minerals in that rock. Such mixed assemblages are in mutual disequilibrium. Dating a metamorphic event therefore requires great interpretive caution. While an apparent age always results from mathematical equations, translating a number into a geological interpretation can be ambiguous and requires establishing a context among different isotope systems, and even more importantly, between the isotope record and mineralogy, microtextures, and microchemistry. Electron microscopy reveals that many minerals consist of mixtures at the sub-µm scale, as they are intergrown with their retrogression products, and/or contain unequilibrated relics (e.g. zircons, monazites, amphiboles, micas). Lack of chemical homogeneity means that diffusivity under anhydrous conditions is extremely low. Modern, direct determinations of dry diffusion in minerals yield rates that are orders of magnitude slower than what was thought twenty years ago. If recrystallization is incomplete, petrographic relics ensure isotopic inheritance. By contrast, water accelerates recrystallization and so enhances element and isotope reequilibration, both in experiments and in natural systems. Availability of water, not temperature, is the principal factor controlling isotope transport. The isotope record of a mineral is thus a geohygrometer and can be used, but only in connection with petrological constraints, to reconstruct a rock's petrogenetic path