Susan E. Humphris,2 Jeffrey C. Alt,3 Damon A.H. Teagle,3 and Jose J. Honnorez4


Major, trace, and rare earth element (REE) analyses of 57 samples of altered and relatively fresh basalts from four different areas on the active Trans-Atlantic Geotraverse (TAG) hydrothermal mound have been completed to determine the geochemistry of alteration of the shallow oceanic crust beneath the mound and to calculate the magnitudes and directions of elemental exchanges between seawater and the oceanic crust during each step of the alteration sequence.

Early high-temperature water-rock reactions resulted in the initial conversion of fresh basalt to chlorite ± quartz ± pyrite by reactions between basalt and a hydrothermal fluid–seawater mixture. Fluid-rock reactions resulted in uptake of Al, Fe, Mg, H2O+, S, V, and Co. In addition, essentially all of the Ca, Na, and Sr were lost from the rock during alteration of plagioclase. The trace metals Cu, Ni, and Zn were leached from the rock, with almost all of the Cu being removed. Changes in Si were variable in direction and in general quite small. This was followed by replacement of the chlorite-rich assemblage by paragonite ± quartz ± pyrite during reactions with a hydrothermal fluid enriched in alkalis. This resulted in additional uptake of Si, Fe, S, and Co, as well as small amounts of Na, K, Sr, Ba, and Zn. Other components were lost from the rock, including Mg, H2O+, and V, and small amounts of Al. Further silicification of the paragonite-rich assemblage resulted in continuing and complete loss of Mg and H2O+, and almost complete loss of V. This stage of the alteration sequence is also responsible for the largest increases in the Si, Fe, S, and Co contents of the altered rocks, some of which show gains in Cu and Zn, presumably in association with the sulfides.

Chloritization within basalts from the edges of the mound attests to reactions between hydrothermal fluids and rocks at high temperatures extending in the subsurface at least to the periphery of the mound. Distinct geochemical and mineralogical differences during alteration on different parts of the mound are indicative of fluids with varying proportions of hydrothermal fluids and seawater.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. shumphris@whoi.edu
3Department of Geological Sciences, The University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109, U.S.A.
4Institut de Géologie, Université Louis Pasteur, 1 rue Blessig, 67084 Strasbourg Cedex, France.