Metamorphism, metasomatism, retrogression, and aqueous alteration are based on the same underlying mechanism at the atomic scale; their different names depend on the large-scale context. They all require recrystallization, which can be viewed as nano-scale dissolution/reprecipitation, mediated by an aqueous fluid . What drives compositional or isotopic modification? Even if 19th century chemistry emphasized “diffusion”, mass balance arguments made it quicky clear that changes in stoichiometry need a chemically open system, i.e. advection, rather than diffusion. Aqueous fluids are the main control on the formation of metamorphic parageneses , and on isotope exchange in minerals . The reason is that the rate constants for fluid-mediated isotope transport are orders of magnitude larger, and activation energies much smaller, than those for diffusion. Recrystallisation is energetically less costly at almost any temperature than diffusive reequilibration . In a companion abstract , it is argued that stepwise release and spatially resolved analyses are a decisive tool in understanding the petrologic processes controlling isotope exchange. However, unambiguous constraints can also derive from petrology alone, provided one knows what to look for. Diffusion is detectable against a background of faster transport only when water was absent and P-T-A-X calculations give an “asterisk” (an overdetermined set of independent reaction equilibria, all intersecting in one point) as proof of retrogression-free rocks. The observations demonstrate that only in rare cases diffusion is the sole promoter of isotope resetting. Further, the observations require a major shift in perspective on the significance of mineral ages. Just as the “diffusionist” view that zircon discordance is due to thermal disturbances  was superseded by the petrological understanding that it is due to recrystallization , interpretations of intra-mineral age variations in terms of a purely thermal history neglecting the microchemical-petrogenetic context is no longer tenable. Because fluid-mediated dissolution/reprecipitation depends mainly on water activity and only very loosely on temperature, isotope data provide a geohygrometric but not an unambiguous geothermometric datum.  Putnis A (2009) Rev Mineral Geochem 70, 87-124.  Lasaga A (1986) Mineral Mag 50, 359-373.  Cole DR et al (1983) Geochim Cosmochim Acta 47, 1681-1693.  Villa IM (2012) this meeting, Theme 17.  Steiger RH, Wasserburg GJ (1969) Geochim Cosmochim Acta 33, 1213-1232.  Mezger K, Krogstadt EJ (1997) J Metam Geol 15, 127-140.
Villa, I.M., & Williams, M.L. (2012). Geochronology and hygrochronology of metamorphic and metasomatic rocks. In Goldschmidt Montréal 2012. Montréal : Geochemical Society and the European Association of Geochemistry.
|Citazione:||Villa, I.M., & Williams, M.L. (2012). Geochronology and hygrochronology of metamorphic and metasomatic rocks. In Goldschmidt Montréal 2012. Montréal : Geochemical Society and the European Association of Geochemistry.|
|Carattere della pubblicazione:||Scientifica|
|Presenza di un coautore afferente ad Istituzioni straniere:||No|
|Titolo:||Geochronology and hygrochronology of metamorphic and metasomatic rocks|
|Autori:||Villa, IM; Williams, ML|
|Data di pubblicazione:||giu-2012|
|Nome del convegno:||2012 Goldschmidt Conference|
|Appare nelle tipologie:||02 - Intervento a convegno|