THE ROLE OF DISSOLUTION-REPRECIPITATION IN MINERAL EQUILIBRIA DURING HIGH-GRADE METAMORPHISM

In nature and experimentally, dissolution-reprecipitation has been shown to be a process by which, in the presence of a reactive fluid acting as a flux, a mineral phase is replaced either by a new or altered composition of that phase or else is entirely replaced by another mineral phase as a pseudomorph (Putnis 2002). The dissolution-reprecipitation process operates essentially as a chemical reaction, involving a Gibb's free energy, in which this replacement represents a lower, more stable energy state than that of the original mineral phase under the then current P-T-X conditions. Dissolution-reprecipitation is characterized by a pervasive fluid-filled porosity throughout the metasomatised region and a sharp compositional boundary between the metasomatised and original mineral. This fluid-filled porosity allows for rapid mass transfer to and from the reaction front at a rate at least 10 orders of magnitude higher than simple volume diffusion through the crystal lattice. It also allows for communication with the surrounding outside fluid. Dissolution-reprecipitation has also been shown to be a process by which one mineral may nucleate and grow inside another and, in so doing, exhibit all the properties of a mineral growing in a fluid-rich environment (Harlov et al. 2005).

The implication then is that, given sufficient fluid resources and time, total recrystallisation of mineral phases, via dissolution-reprecipitation, could represent one means by which minerals re-equilibrate in a rock during metamorphism. More to the point, dissolution-reprecipitation would allow for increased fluid penetration along grain boundaries, which could help to promote mass transfer along the rock column as material is brought in and removed. In that respect, fluid-rock interaction, on either the local or more regional scale, would then not just be a simple matter of fluid flow along grain boundaries, but fluid flow through grain boundaries or even through the minerals themselves (e.g. as a “ porosity wave “ along the rock column, cf. Connolly, 1997) thereby allowing for both mineral equilibrium and mass transfer on a scale far grander than has so far been conceptualized.

Connolly JAD (1997) J Geophys Res 102, 18149–18173

Harlov DE, Wirth R, and Förster H-J (2005) Contrib Mineral Petrol 150, 268–286

Putnis A (2002) Mineral Mag 66, 689–708