The hydrothermal alteration of oceanic basalts by seawater

Abstract

Considerable geological and geophysical evidence now exists to support the hypothesis that seawater circulates through freshly intruded basalt at the mid-ocean ridges. As a consequence of this process, reactions between basalt and seawater take place at elevated temperatures. The mineralogy and chemistry of hydrothermally altered pillow basalts dredged from the Mid-Atlantic Ridge, and belonging to the greenschist facies, have been studied in order to determine the mineralogical changes that result from hyrdrothennal alteration, and to assess the chemical fluxes that result from these reactions in terms of their possible significance in elemental geochemical budgets as potential sources and sinks for elements in seawater. Where possible, pillow basalts were studied that showed varíous degrees of a1teration within a single rock. Such samples provide the best evidence that they have been affected by hydrothermal alteration, rather than regional burial metamorphism, and provide the most useful information for elemental flux calculatìons. During hydrothermal alteration, plagioclase is generally albitised, sometimes with the formation of epidote, and albite may be subsequently a1tered to chlorite. Plagioclase, in association with skeletal clinopyroxene, alters to chlorite and epidote. Olivine is pseudomorphed by chlorite, and clinopyroxene alters to actinolite. The glassy matrix alters to an intergrowth of actinolite and chlorite. Vein minerals irclude chlorite, actinolite, epidote, quartz, and sulphides. On the basis of their minaralogy, the samples may be subdivided into chlorite-rich (>15% chlorite and <15% epidote) and epidote-rich (>15% epidote and <15% chlorite) assemblages. The chlorite-rich assemblages lose CaO and gain MgO, while the epidote-rich samples show very little change in composition compared with their basalt precursor. The epidote-rich samples are more oxidised than their precursors, while the chlorite-rich rocks can be further suhdivided into those that maintain the same proportions of fetrous and ferric iron, and those that show an increase in ferrous iron due to the precipitation of pyrite. The major chemical changes that occur during hydrothermal alteration of pillow basalts are uptake of MgO and H2O, and loss of SiO2 and CaO. The concentrations of Na2O and K2O are apparently not greatly changed, although. they do show some variations in the core-to rim analyses. Consideration of the elemental fluxes in terms of steady-state geochemical mass balances indicates that hydrothermal alteration provides a sink for Mg, which is extremely important in solving the problem of apparent excess magnesium input to the oceans. The amount of calcium that is leached from the rock may be of significance in the geochemical budget of calcium. The concentration of silica in the circulating fluid is probably controlled by the solubility of quartz, and considerable redistribution of silica takes place within the basaltic pile. The changes in the redox conditions during hydrothermal alteration do not affect the present-day oxidation states of the atmosphere and hydrosphere. Trace element analyses indicate that copper and strontium are leached out of the rock and migrate in the circulating fluid, with local precipitation of Cu as sulphides in veins. Li, B, Mn, Ba, Ni and Co show sufficient variation in concentration and location within the altered basalts to indicate that some leaching does take place, and hence hydrothermal alteration of basalts could produce a metal-enriched solution, which may be important in the formation of metalliferous sediments at active mid-ocean ridges.