Evolution of fluid pressure and fracture propagation during contact metamorphism

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Abstract Rock fracture enhances permeability and provides pathways through which fluids migrate. During contact metamorphism, fluids contained in isolated pores and fractures expand in response to temperature increases caused by the dissipation of heat from magmas. Heat transport calculations and thermomechanical properties of water‐rich fluids demonstrate (1) that thermal energy is a viable mechanism to produce and maintain pore fluid pressure (Pf) in a contact metamorphic aureole; (2) that the magnitude of Pf generated is sufficient to propagate fractures during the prograde thermal history (cause hydrofracture) and enhance permeability; and (3) that Pf‐driven fracture propagation is episodic with time‐scales ranging from years to thousands of years. Because Pf dissipation is orders of magnitude faster than P, f buildup, Pf oscillations and cyclical behaviour are generated as thermal heating continues. The Pf cycle amplitude depends on the initial fracture length, geometry and the rock's resistance to failure whereas the frequency of fracture depends on the rate of heating. Consequently, oscillation frequency also varies spatially with distance from the heat source. Time series of fluid pressures caused by this process suggest that cyclical fracture events are restricted to an early time period of the prograde thermal event near the intrusive contact. In the far field, however, individual fracture events have a lower frequency but continue to occur over a longer time interval. Numerous fracture cycles are possible within a single thermal event. This provides a provisional explanation for multiple generations of veins observed in outcrop. P f cycling and oscillations may explain several petrological features. If pore fluids are trapped at various positions along a pressure cycle, the large amplitude of Pf variations for small fractures may account for different pressures recorded by fluid inclusions analysed from a single sample. Pf oscillations, during a single thermal episode, also drive chemical reactions which can produce complex mineral textures and assemblages for discontinuous reactions and/or zoning patterns for continuous reactions. These can mimic polymetamorphic or disequilibrium features. Temporal aspects of fracture propagation and permeability enhancement also constrain the likely timing of fluid flow and fluid‐mineral interactions. These data suggest that fluid flow and fluid‐mineral reactions are likely to be restricted to an early period in the prograde thermal history, characterized by high Pf coincident with relatively high temperatures, fracture propagation and consequent increases in permeability. This early prograde hydration event is followed by diffusional peak metamorphic reactions. This relationship is evident in the complex mineralogical textures common in some metamorphosed rocks. Copyright © 1995, Wiley Blackwell. All rights reserved

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Journal of Metamorphic Geology

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