Continuous and Episodic Fluid Flow in Regional Metamorphism
The overall pattern of fluid behaviour in regional metamorphism is controlled by large scale factors such as rate of heat input/loss, rock rheology and original lithological mix, and so cannot be considered to be independently variable, but locally anomalous behaviour can occur. Is it able to produce regional changes to how metamorphism progresses? The original mix of sediments and/or igneous material in a sequence that subsequently undergoes regional metamorphism is itself of considerable significance. It dictates both the amount of water and other fluid species available to be released subsequently, and controls the salt content of the sequence, which will influence whether or not fluid immiscibility extends significantly into the metamorphic realm. Continental shelf sequences with abundant carbonates and evaporites will experience very different fluid evolution patterns from forearc volcaniclastic sequences.
The rate of fluid release during progressive metamorphism is controlled overall by the rate of heat supply, due to the strongly endothermic character of devolatilisation reactions, and this leads to a steady average release of fluid. Nevertheless, local anomalies may be possible. It is certainly the case that metasomatic interactions between marbles and hydrous, quartz-bearing sediments can lead to positive feedbacks as breakdown of carbonates enhances permeability and hence promotes further infiltration of water, leading to marked overstepping of decarbonation equilibria. As a result of such enhanced volatile production, the local temperature will drop. Dehydration reactions are much less likely to become overstepped in this way, and so comparable episodic enhanced release of water is unlikely, but structural and lithological heterogeneities may nevertheless lead to localised and focussed water flows. Rocks undergoing progressive metamorphism normally have low permeabilities, thus accounting for the high fluid pressures that permit opening of fluid-filled cracks and growth of veins. During heating, fluid pressure rises to the point where the permeability is sufficient to permit fluid to escape at the rate at which it is generated. Enhanced permeability pathways can only be sustained briefly because of the effectiveness of creep collapse and mineral precipitation in reducing permeability at high temperatures; it has been argued that the vertical extent of an open fracture is unlikely to exceed c.100m under amphibolite facies conditions because of the differences that exist between fluid pressure and lithostatic pressure at top and bottom of the crack. Because of these self-sealing properties, it is unlikely that a significant volume of metamorphosing rocks can be depressurised, except in the vicinity of a decarbonating marble where porosity is continuously regenerated. It is, regrettably, difficult to escape the conclusion that the progressive loss of fluid from metasediments, once they have experienced initial compaction, is a slow and pervasive process.