Abstract

E. H. Rutter writes: I noted during this meeting many references to the role of pressure solution in diagenesis, including a statement expressing some incredulity that pressure solution features should develop over a wide range of diagenetic environments. As a structural geologist, I feel it pertinent to point out that pressure solution is of special interest because it is one of the most important mechanisms by which rocks are deformed in low grade metamorphic environments.

If one considers a simple theoretical model of the process, it comes as no surprise that pressure solution should be important in natural processes over the full temperature range from 20°C to c. 350°C (beginning of greenschist-facies metamorphism). Pressure solution may be considered to be the sum of three sequential processes:

Transfer of material from the solid phase into an activated (solution) state in a supposed in-tergranular fluid film;

Diffusive mass transfer through the stressed grain boundary region in response to a chemical potential gradient;

Precipitation in intergranular void space, or removal from the system altogether.

In sequential processes the slowest component controls the overall rate. In this case the slowest component is likely to be diffusion through the intergranular fluid film (note that diffusion in the relatively wider pore spaces will be about 105 faster).

On the assumption that stressed grain boundary diffusion is rate-controlling, it can be shown (Rutter 1976) that the displacement rate, ù, at which two grains interpenetrate is given by

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