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 reequilibration. 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. 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.
Villa, I. (2010). Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads. In M.I. Spalla, A.M. Marotta, G. Gosso (a cura di), Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling (pp. 1-15). london : geological Society of London [10.1144/SP332.1].
Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads
VILLA, IGOR MARIA
2010
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 reequilibration. 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. 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.