Observations and Models

Geophysical observations and models provide important constraints on the properties of seafloor hydrothermal systems and associated fluxes. These studies indicate how the upper lithosphere is constructed and evolves, characteristics that are related to hydrothermal systems through the mechanical state of the lithosphere and the availability of heat to drive fluid flow. Geophysical studies illustrate correlations between the rate of seafloor spreading and the depth of an axial melt lens reflection, the occurrence and intensity of near-ridge seismicity, and advective heat output from hydrothermal systems. The flux of latent heat released when new crust is formed is sufficient to drive many high-temperature systems on a short-term basis, but circulating fluids also extract heat associated with lithospheric cooling. Hydrothermal rebound following the end of ridge-crest circulation could extend to tens of kilometers off axis and, if neglected, may lead to overestimates of ridge-flank hydrothermal fluxes within young seafloor, but also might provide a way to evaluate the depth of advective cooling near the ridge. Fluid residence times within high-temperature hydrothermal systems based on geophysical considerations are on the order of years, consistent with geochemical estimates. Borehole observations suggest that basement permeabilities are very high locally within both ridge-crest and ridge-flank hydrothermal systems (10–9 to 10–10 m2), but several lines of reasoning indicate the likely importance of anisotropy and heterogeneity in restricting most fluid flow to occur within a small fraction of rock. Future advances based on geophysical methods and modeling will result from in situ experiments and observations and interdisciplinary approaches that focus on the nature of fluid pathways, the size of hydrothermal reservoirs, and the coupling among physical, chemical, and geological dynamics. Energy and Mass Transfer in Marine Hydrothermal Systems Edited by P.E. Halbach, V. Tunnicliffe, and J.R. Hein © 2003 Dahlem University Press ISBN 3-934504-12-4 To order book, contact: [email protected]