The interface geochemistry of mineral-fluid assemblages is of fundamental importance, both in the crust and deep within the mantle.

Top Priority, Theme III: Fluid Pressure and Composition in the Crust

Understanding of the three-dimensional distribution of fluids in the crust, including their pressure and compositional variations.

The three-dimensional distribution of fluid pressure and fluid composition in the crust can best be addressed by an integrated approach that uses information obtained in a variety of ways. Direct observation of fluids within the crust and of fluids that have been extracted from the crust is essential. This is most feasible for groundwater, oil, and natural gas, but hydrothermal fluids are now being sampled. How to sample magmas at shallow depth is a current challenge. Measurements of the chemical and isotopic composition of these fluids is required. Continental drilling in selected environments is needed; the operation of instruments in drill holes for geochemical and geophysical measurements is desirable on short- and long-term bases.

There is a long and successful tradition of inferring the properties of fluids that have flowed through rocks from the physical and chemical properties of rocks accessible at the surface or in drill holes. Inferences about the subsurface distribution of fluids have long been made from geophysical observations at the surface and in boreholes. A current challenge is to develop reliable techniques for the direct remote sensing of subsurface fluids and their permeability.

There are well-defined problems related to the distribution of fluid in the oceanic crust. At spreading centers both hydrothermal and magmatic fluids are accessible, and the RIDGE initiative includes plans for ocean drilling and submersible study. An important step has been taken in the first dedicated ODP drilling of a sedimented spreading center. Fluid fluxes through accretionary prisms at convergent plate boundaries are recognized as representing a critical element in geochemical cycling but are proving very difficult to quantify. An integrated approach, using ocean drilling with a variety of other geophysical, geological, and geochemical techniques, is likely to be needed.

With sufficient observational data, scientific understanding, and insight, the problem of the role of fluid-driven mass (and heat) transport in rocks can be defined in a manner amenable to a mathematical solution that simulates coupled flow problems and involves physical transport and chemical reactions and kinetics. Much future research will be directed toward placing confidence bands about the output of analytical models, including predictions of future system behavior.

Other High Priorities

Of the many other research opportunities cited in Chapters 3, 4, and 5, two are listed in Table 7.8 for Theme III. The first is a most important part of the top-priority selection—to model the fluid flow in sedimentary basins. The subject of sedimentary basins as an integrative theme has recurred throughout this volume. The second focuses on the details of the chemical interaction at surfaces between minerals and fluids, particularly the role of bacteria. The same topic, applied specifically to organic wastes, is the top-priority recommendation for Priority Theme D.

Programs and Infrastructure

For Theme III Table 7.8 summarizes the relevant programs along with the industry involvement and facilities required to accomplish the research. The preeminent need is for experimental facilities, for hydrothermal systems and high-temperature, large-volume experiments. Access to data-handling and high-speed computational facilities for modeling is required, as are adequate core-storage facilities.

Priority Theme IV: Crustal Dynamics—Ocean and Continent

The principal aim of this priority theme is to understand how the Earth's crust originates and evolves, including the nature and history of the deformations and mass transfer processes responsible for building and modifying the continents, mountain belts, island arcs, and ocean basins.

New and continuing studies regarding the origin, structure, mass transfer processes, and history of continents and continental building blocks promise excellent research returns, as discussed in Chapters 2, 3, and 4. The shaping of the land surface into landforms is treated in Chapters 3 and 5. Plate tectonics provides a basic framework for understanding how the crust is deformed. In oceanic regions the deformation is relatively simple, with creation of new crust at spreading centers, destruction or reorganization of crust at oceanic trenches, and lateral movement of crustal blocks on transform faults. Studies of these fundamental processes are

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