ROCK-FLUID INTERACTIONS

Traditionally, the science of geology concerned rocks and minerals—the solid part of the natural environment. But over the past two centuries, geologists have become increasingly aware that divisions between solid, liquid, and gaseous environments can be unnecessarily restrictive. The phases are interactive and interdependent; they are not distinct. The nature of the crust cannot be understood separately from the atmospheric and oceanic systems. Most recently, earth scientists have been surprised to detect evidence of organic forms thriving in the depths of the crust and in the heights of the atmosphere. Rock, water, air, and biota interchange molecules throughout a layer of skin covering the planet. The list of interdependent processes lengthens every day as new discoveries are made, and it reflects much geological research—as it is currently defined.

Understanding of rock-fluid interactions began with the study of groundwater flow. Empirical laws were developed in the 1800s to define the rate and intensity of the underground flow feeding springs and wells. Further studies have revealed characteristics of fluid systems that are determined by permeability of the rock, composition and volume of the fluid, and pressure and temperature gradients within the system. These characteristics can be determined close to the surface through wells, but rock-fluid interaction occurring at great depth must be deduced from examination of outcropping rocks that formerly lay far below the surface. Outcrops generally provide evidence of the effects left by deeply circulating fluids, only rarely offering samples of the fluid itself.

Indications from wells and outcrops suggest that fluids may be present in significant amounts at most crustal levels, although they are locally sporadic, and that the surrounding rocks undergo chemical modification as a result of contact with migrating fluids. These conclusions are consistent with observations of the pivotal role played by fluids in the genesis of both mineral and fossil fuel concentrations. Recent geophysical studies have added to the insights about fluids within the crust. Electrical conductivity inferred for great depths suggests a persistent aqueous fluid phase, while seismic reflections from possible low-velocity zones imply abnormally high fluid pressures. This information supports the opinion that extensive fractures and large volumes of aqueous fluids permeate the deep crust.

FIGURE 4.1 Vein/joint abundance map of the Sierrita system, Arizona. The total number of fractures per centimeter increase as the pluton (patterned area) is approached. Figure from S. R. Titley, NRC, 1990, The Role of Fluids in Crustal Processes.

Researchers testing the behavior of crustal rock have concluded that fluid proportion decreases with depth, but they do not understand the transition between the extremes of saturation and scant traces. At every level water acts to redistribute heat in hydrothermal systems. Because an increase in temperature raises fluid pressure, any substantial introduction of magma into the crust will initiate a convecting system of groundwater around the magma body (Figure 4.1). The convecting water transports heat from the magma, controlling its cooling rate, which affects the crystallization rate of the rock and thus some of its subsequent identifying characteristics as a solid.

The convecting water also redistributes chemical components from the intrusive body, and from the surrounding host rock, to more remote locations. Even if the surrounding rocks are initially impermeable, the heat generates enough pore-fluid pressure to open hydraulic fractures, creating permeability. Subsequent episodes repeat the whole process until the fractures fill with minerals, permeability decreases, and fluid flow abates. At the same time, the magma must release its influence along a different front to shut down the intricately integrated feedback system.

Horizontal and vertical movement—convection and advection—of fluid through rock disperses chemical components and supports chemical reactions between the minerals and fluids. Dissolution, precipitation, ion exchange, and sorption continue as the fluids migrate through the matrix. Material that does not dissolve may be forced into migration



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