Geochronology and hygrochronology of metamorphic and metasomatic rocks

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 [1]. 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 [2], and on isotope exchange in minerals [3]. 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 [3]. In a companion abstract [4], 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 Just as the “diffusionist” view that zircon discordance is due to thermal disturbances [5] was superseded by the petrological understanding that it is due to recrystallization [6], interpretations of intra-mineral age variations in terms of a purely thermal history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. [1] Putnis A (2009) Rev Mineral Geochem 70, 87-124. [2] Lasaga A (1986) Mineral Mag 50, 359-373. [3] Cole DR et al (1983) Geochim Cosmochim Acta 47, 1681-1693. [4] Villa IM (2012) this meeting, Theme 17. [5] Steiger RH, Wasserburg GJ (1969) Geochim Cosmochim Acta 33, 1213-1232. [6] Mezger K, Krogstadt EJ (1997) J Metam Geol 15, 127-140.