Geophysical Constraints on the Volume of Hydrothermal Flow at Ridge Axes
Hydrothermal circulation at the ridge axis removes heat from the oceanic crust more rapidly than would conduction alone. The top of the axial magma chamber is thus deeper and possibly wider than the theoretical shape computed from conductive thermal models. At 9°N on the East Pacific Rise seismic reflection indicates that the roof of the magma chamber is relatively flat, 2 km deep, and extends 4 km from the axis. This is about a kilometer deeper than predicted by a purely conductive model.
We believe that the magma chamber is mostly filled with mush at ridges with both fast and slow spreading rates. At fast rates the mush is formed by crystallization at the top of a magma chamber that is wide and flat topped. At slow rates a narrow magma chamber is probably an anastomosing complex of partially molten dikes and associated cumulate layers. Thermal modeling indicates that the hydrothermal heat flux is between 0.7×108 and 1.5×108 cal/cm2, or less than 1/10 of the total missing heat flux (the difference between obsereved and theoretical heat flow) at the ridge axis. By using the observation that Mg is totally depleted from exiting axial fluids, we find that the minimum amount of crust which reacts with axial hydrothermal flow is equivalent to a 80 m thick section of crust. A minimum thickness of 200 m is obtained from K which is leached from the basalt into the hydrothermal fluid. These estimates indicate that there is no requirement that the bulk of the oceanic crust react strongly with the axial hydrothermal fluid.
KeywordsOceanic Crust Magma Chamber Oceanic Lithosphere Seafloor Spreading Ridge Axis
Unable to display preview. Download preview PDF.
- Bibbee, L. D., Dorman, L. M., Johnson, S. H., and Orcutt, J. A., 1983, Crustal. structure of the East Pacific Rise at 10°S, J. Geophys. Res., 87 (in press), (unseen).Google Scholar
- Cann, J. R., 1974, A model for oceanic crustal structure developed, Geophys. J. R. astron. Soc., 3.9 169–187.Google Scholar
- Casey, J. F., Dewey, J. F., Fox, P. J., and Karson, J. A., 1981, Heterogeneous nature of oceanic crust and upper mantle: A perspective from the Bay of Islans ophiolite complex, in: “The Oceanic Lithosphere, The Sea, Vol. 7,” C. Emiliani, ed., John Wiley, New York, 305–338.Google Scholar
- Converse, D. R, Holland, H. D., and Edmond, J. M., 1982, Hydrothermal flow rates at 21°N, EOS, Trans. Amer. Geophys. Union, 63: abstract V551–4, 472.Google Scholar
- Dreyer, J. I., 1974, The magnesium problem, in: “Marine Chemistry, The Sea, Vol. 5,” E. D. Goldberg, ed., Wiley-Interscience, New York, 337–357.Google Scholar
- Fox, P. J., and Stroup, J. B., 1981, Geological and geophysical properties of the lower oceanic crust, in: “The Oceanic Lithosphere, The Sea, Vol. 7,” C. Emiliani, ed., John Wiley, New York, 119–216.Google Scholar
- Gregory, R. T., and Taylor, H. P., 1981, An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail ophiolite, Oman: Evidence for d1130 buffering of the oceans by deep (5 km) seawater-hydrothermal circulation at mid-ocean ridges, J. Geophys. Res., 86: 2737–2755.CrossRefGoogle Scholar
- Hart, R. A., 1973, A model for chemical exchange in the basalt-sea water system of oceanic layer II, Can. J. Earth Sci., 10: 801–816.Google Scholar
- Hess, H. H., 1965, Mid-oceanic ridges and the tectonics of the sea-floor, in: “Submarine Geology and Geophysics,” W. F. Whittard and R. Bradshaw, eds., Butterworths, London, 317–333.Google Scholar
- Macdonald, K. C., 1982, Mid-ocean ridges: Fine scale tectonic, volcanic and hydrothermal processes within the plate boundary zone, Ann. Rev. Earth Planet Sci., 10: 155–190.Google Scholar
- Macdonald, K. C., Becker, K., Spiess, F. N., and Ballard, R. D., 1980, Hydrothermal heat flux of the “black smoker” vents on the East Pacific Rise, Earth Planet. Sci. Lett., 48: 1–7.Google Scholar
- Matti, M. J., 1976, Chemical exchange between sea water and basalt during hydrothermal alteration of the oceanic crust, Ph. D. thesis, Harvard University, Cambridge, Mass.Google Scholar
- Sclater, J. G., and Francheteau, J., 1970, The implications of terrestrial heat flow observations on current tectonic and geothermal models of the crust and upper mantle of the earth, Geophys. J. R. astron. Soc., 2a 509–542.Google Scholar
- Wolery, T. J., 1978, Some chemical aspects of hydrothermal processes at mid-oceanic ridges - A theoretical study. I. Basalt-sea water reaction and chemical cycling between the oceanic crust and the oceans. II. Calculation of chemical equilibrium between aqueous solutions and minerals. Ph. D. thesis, Northwestern University, Evanston, Ill., 263 pp.Google Scholar
- Wolery, T. J., and Sleep, N. H., 1983, Interactions between ex:ogenic cycles and the mantle, in, “Chemical Cycles and the Evolution of the Earth,” R. M. Garrels, C. B. Gregor, and F. T. Mackenzie, eds., (in press).Google Scholar