Rates of flow and patterns of fluid circulation

The rates and patterns of fluid circulation in seafloor hydrothermal systems control the efficiency of lithospheric heat extraction, the nature of fluid–rock interaction, and the extent of seafloor and sub-seafloor biospheres supported by fluid, energy, and solute fluxes. It has been challenging to determine the patterns and rates of fluid flow within seafloor hydrothermal systems for several reasons. Active systems are remote and measurements are often difficult to make. Although newly developed tools have led to important advances in in situ characterization (Chapter 8), considerable challenges remain in determining where and how quickly fluids move through the sub-seafloor environment. Fossil systems provide an integrated record of fluid flow (Chapter 9), but system heterogeneity and overprinting of multiple generations of activity make quantitative interpretation difficult. Exposures are limited within both active and fossil systems, with the former often restricted to widely spaced or isolated boreholes and seafloor outcrops, and the latter comprising mainly ophiolites and rare seafloor exposures (for example, along fracture zones or rifts, e.g. Karson, 2002). Fluid flow within seafloor hydrothermal systems is transient on a range of time scales, and some of these systems operate over lateral and vertical distances of kilometers or more. Additional complexities arise due to the scaling of rock properties within heterogeneous, fractured crust (Chapter 7). A more subtle difficulty is the influence of assumptions and ideas that became embedded in the literature at an early stage of research. Even after some of these ideas have been found to conflict with observations, they continue to influence interpretations. In this chapter, rates of fluid flow in hydrothermal systems, the lateral extent of circulation, preferred directions of fluid flow, and the “shapes” of fluid pathways are discussed. Techniques applied to resolve these parameters include use of thermal and chemical tracers, seafloor and borehole observations, and coupled modeling. Considerably more is known about apparent rates of fluid flow than is known about overall patterns of fluid circulation, but even estimates of rates should be viewed with caution because estimates are generally based on fundamental assumptions regarding the primary direction of flow, nature of