Observations made by electron microscopy show the processes affecting minerals at the atomic scale. The majority of reported analyses demonstrate chemical disequilibrium. A classic example are overgrowths of one mineral generation by a secondary one, which may be recognized on textural grounds. Disequilibrium recrystallization is promoted by water, which is everywhere on this planet (granites, contact aureoles, regional metamorphism, faults). It is mostly easier and energetically less costly to recrystallize a mineral at any temperature than to induce genuine volume diffusion in it. However, these observations are only relevant to geochronologists if chemical disequilibria are also accompanied by isotopic disequilibria. If a mineral mixture gives a mixed isotope record, then the interpretation of ages does not come cheap. If, on the contrary, diffusive reequilibration of the isotopic record is faster than that of chemical heterogeneities, then the petrology and microchemistry of a mineral could be ignored and its apparent age termed a ¿cooling age¿. First principle arguments and experimental data of the last decade concordantly show that the diffusivity of radiogenic isotopes is never higher than that of major elements forming the mineral structure. And indeed, end-member ages of mineral mixtures can be unravelled if the petrogenesis is understood. This was first shown by CL images of zircon grains (Gebauer et al, Schweiz Min Pet Mitt 68 (1988) 485-490). Similar progress was reported on monazite (Williams et al, Ann Rev Earth Planet Sci 35 (2007) 137-175), amphibole (Belluso et al, Eur J Mineral 12 (2000) 45-62), K-feldspar (Nyfeler et al, Schweiz Min Pet Mitt 78 (1998) 11-21), biotite (Villa et al, Water Rock Interaction 10 (2001) 1589-92). The mechanism for resetting the isotope record in nature thus seems more dependent on the availability of water to enhance disequilibrium recrystallization than on reaching a preset temperature. Intercomparison of laboratory release of Ar and Xe in Ba-fsp and Ba-mica require the same in vacuo degassing mechanism for hydrous and anhydrous silicates. This is not volume diffusion. Instead, it was observed to stem from a discrete structural rearrangement; fortunately, successive muscovite overgrowths do degas diathermally, so that successful dating of each mica generation is possible (Hetherington & Villa, GCA 71 (2007) 3336-47). In a parallel universe, diffusive equilibration is the basis for isotopic models providing numbers to be input into tectonic models. Is this the role of geochronology? With the coming of age of submicroscopic petrology, isotopic disequilibria can be put into context with petrogenetic disequilibria. 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 be beneficial to those who now just ask for numbers.
Villa, I. (2007). Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads. Intervento presentato a: American Geophysical Union Fall Meeting, San Francisco.
Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads
VILLA, IGOR MARIA
2007
Abstract
Observations made by electron microscopy show the processes affecting minerals at the atomic scale. The majority of reported analyses demonstrate chemical disequilibrium. A classic example are overgrowths of one mineral generation by a secondary one, which may be recognized on textural grounds. Disequilibrium recrystallization is promoted by water, which is everywhere on this planet (granites, contact aureoles, regional metamorphism, faults). It is mostly easier and energetically less costly to recrystallize a mineral at any temperature than to induce genuine volume diffusion in it. However, these observations are only relevant to geochronologists if chemical disequilibria are also accompanied by isotopic disequilibria. If a mineral mixture gives a mixed isotope record, then the interpretation of ages does not come cheap. If, on the contrary, diffusive reequilibration of the isotopic record is faster than that of chemical heterogeneities, then the petrology and microchemistry of a mineral could be ignored and its apparent age termed a ¿cooling age¿. First principle arguments and experimental data of the last decade concordantly show that the diffusivity of radiogenic isotopes is never higher than that of major elements forming the mineral structure. And indeed, end-member ages of mineral mixtures can be unravelled if the petrogenesis is understood. This was first shown by CL images of zircon grains (Gebauer et al, Schweiz Min Pet Mitt 68 (1988) 485-490). Similar progress was reported on monazite (Williams et al, Ann Rev Earth Planet Sci 35 (2007) 137-175), amphibole (Belluso et al, Eur J Mineral 12 (2000) 45-62), K-feldspar (Nyfeler et al, Schweiz Min Pet Mitt 78 (1998) 11-21), biotite (Villa et al, Water Rock Interaction 10 (2001) 1589-92). The mechanism for resetting the isotope record in nature thus seems more dependent on the availability of water to enhance disequilibrium recrystallization than on reaching a preset temperature. Intercomparison of laboratory release of Ar and Xe in Ba-fsp and Ba-mica require the same in vacuo degassing mechanism for hydrous and anhydrous silicates. This is not volume diffusion. Instead, it was observed to stem from a discrete structural rearrangement; fortunately, successive muscovite overgrowths do degas diathermally, so that successful dating of each mica generation is possible (Hetherington & Villa, GCA 71 (2007) 3336-47). In a parallel universe, diffusive equilibration is the basis for isotopic models providing numbers to be input into tectonic models. Is this the role of geochronology? With the coming of age of submicroscopic petrology, isotopic disequilibria can be put into context with petrogenetic disequilibria. 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 be beneficial to those who now just ask for numbers.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
