The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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2. Pore Fluid Pressure Near Magma Chambers
2. Pore Fluid Pressure Near Magma Chambers
Pages 42-49
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From page 42... ...
Pore fluids typically found in the host rocks have large positive values of the isochoric coefficient of thermal pressure, whereas those in the magma have large negative values. Therefore, fluid pressure increases during the dissipation of thermal en 42 orgy from magmas are a natural consequence of the cooling process, where temperature increases in the host and concurrently decreases in the magma.
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From page 43... ...
To ox yy zz demonstrate the role of fluid pressure in deformation, we will assume that the stress field is homogeneous. Pressure conditions actually encountered in the subsurface are unlikely to correlate closely with those values derived from the integrations using extreme values of (>m in Eq.
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From page 44... ...
They also found that only for thermal gradients of less than 10°C/km will the pressure within the constant-volume pore remain less than the mean confining pressure, Pm, as subsidence occurs; for larger thermal gradients the fluid pressure increases to values much greater than the confining stress. This overpressure occurs at depths that depend primarily on the magnitude of the thermal gradient.
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From page 45... ...
They are therefore delicate indicators of fluid pressure conditions because the mean confining pressure is relatively constant over long times, whereas the fluid within the fractures is sensitive to local changes in temperature. FAILURE CRITERIA The small finite strength of the pore wall implies that for the initial condition of Pf = Pm only a small pressure increase is necessary to reach the yield strength of the wall.
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From page 46... ...
Because fluid flows away from the breached pore wall a relatively short distance and for a limited time, viscose dissipation of energy is likely to be small. The rate of volume change in a fluid subjected to differential DENTS L
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From page 47... ...
The fluid flow is driven by the pervasive buoyancy force field, which is an inevitable consequence of changes in heat flux, particularly those changes caused by the infiltration of magma. Therefore, when the synchronous propagation of fractures produces an interconnected network of flow channels, fluid motion ensues and augments the flux of heat and chemical components.
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From page 48... ...
In the situation discussed in this chapter, highly nonlinear properties of H2O-rich fluids have long been recognized as exerting primary control on the evolution of temperature, pressure, and fluid velocity (Norton and Knight, 1977~. Independent lines of evidence-one from transport theory and the other from the geometric properties of veins and fracture systems point to anomalous fluid pressure as the force that not only generates systematic fractures and causes them to interconnect and form percolation networks but that also retards flow through mineral deposition.
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From page 49... ...
Differential thermal expansion of pore fluids: Fracture propagation and microearthquake production in hot pluton environments, Journal of Geophysical Research 82, 2515-2522. Knapp, R
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