I have used the global heat flow data set to better constrain the amount of hydrothermal heat loss and water circulation, and find several important results [Stein and Stein, 1992; Stein et al., 1995].
First, about 2/3 of the hydrothermal heat loss occurs by off-ridge and presumably low-temperature flow in crust older than 1 Myr. Because the water temperature for near-ridge flow exceeds that for off-ridge flow, the near-ridge water flow should be even a smaller fraction of the total water flow. Hence in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux, or underestimate the water flux for an assumed heat flux. Second, the ratio of observed to predicted heat flow increases with age until about 65 Myr, but shows little dependence on the sediment thickness at the sites. In particular, contrary to earlier suggestions, about 100-200 m of sediment is generally neither necessary nor sufficient to bring heat flow to the value predicted by lithospheric cooling models. The most straightforward inference from these observations is that the fraction of heat transported by hydrothermal flow varies primarily with crustal age and at most secondarily with the sediment thickness. Third, although heat flow in young lithosphere is highly variable, due presumably to local hydrologic complexities, the average heat flow decreases approximately linearly from near the axis to about 20 Myr and then is roughly constant to about 50 Myr.
Top: Cumulative predicted, observed, and inferred hydrothermal heat fluxes
as a function of age. The lines connect
the points whose values were computed. For clarity, the 1 Ma point
is not plotted, and the observed values are offset.
Error bars are one standard deviation of the data.
Bottom: Cumulative inferred hydrothermal heat flux for 0-65 Ma.
Recently Hofmeister and Criss  have suggested that the total global heat flow is about 30 TW, about 25% less than previously estimated by Pollack et al. . The main difference between the two estimates is whether oceanic heat flow values are based on the predicted from conductive lithospheric cooling models or the lower measured values, reflecting the additional near-surface heat transfer by hydrothermal circulation. While hydrothermal circulation is spectacularly displayed at hot springs at midocean ridges (see photographs at the top of this web page) detailed heat flow measurements and pore water chemistry do show that significant amounts of water flow near the top of the basaltic oceanic crust and transfer heat within off-axis regions.
Davis, E., D. Chapman, M. Mottl, W. Bentkowski, K. Dadey, C. Forster, R. Harris, S. Nagihara, K. Rohr, G. Wheat, and M. Whiticar, FlankFlux: nature of hydrothermal circulation in young oceanic crust, Can. J. Earth Sci., 29, 925-952, 1992.
Green, K. E., R. P. Von Herzen, and D. L. Williams, The Galapagos Spreading Center at 86 degrees W: A detailed geothermal field study, J. Geophys. Res., 86, 979-986, 1981.
Hofmeister, A. M., and R. E. Criss, Earth's heat flux revised and linked to chemistry, Tectonophysics, 395, 159-177, 2005.
Pollack, H. N., S. J. Hurter, and J. R. Johnston, Heat loss from the earth's interior: analysis of the global data set, Rev. Geophys., 31, 267-280, 1993.
Stein, C. A., and S. Stein, Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow, J. Geophys. Res., 99, 3081-3095, 1994. For pdf click here
Stein, C., and S. Stein, Estimation of lateral hydrothermal flow distance from spatial variations in oceanic heat flow, Geophys. Res. Lett., 24, 2323-2326, 1997. For pdf click here
Stein, C., S. Stein, and A. Pelayo, Heat flow and hydrothermal circulation, in: Physical, chemical, biological and geological interactions within hydrothermal systems, AGU Mono., edited by Humphris, S., L. Mullineaux, R. Zierenberg and R. Thomson, Am. Geophys. Un., Washington, D.C., 425-445, 1995. For pdf click here
Von Herzen, D. P., E. E. Davis, A. Fisher, C. A. Stein, and H. N. Pollack, Comments on "Earth's heat flux revised and linked to chemistry" by A. M. Hofmeister and R. E. Criss, Tectonophysics, 409, 193-198, 2005. For pdf click here