Download Continuous and Episodic Fluid Flow in Regional Metamorphism

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.