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There have been many studies of the geochemical effects associated with the low-temperature alteration of oceanic crust by seawater (see for instance, R. A. Hart, 1970,1976a). Much of this work has been concerned with typical mid-ocean ridge basalts (MORB), low in incompatible elements, such as K, Rb, Ba, Sr, U, and Th, and where the effects of any significant addition of these elements through interaction with seawater is usually obvious. Leg 49, where many of the holes penetrated basaltic basement with "transi t ional" or moderately large-ion-lithophile (LIL)-enriched characteristics, provided an opportunity to study the effects of seawater interaction on basalts with generally higher primary levels of some incompatible elements. Our approach was to analyze basalt samples for water, carbon dioxide, and ferrous/ferric ratios, to determine the degree of alteration in terms of hydration, carbonation, and oxidation, respectively. We also looked for any depthor age-related trends or characteristics that might be significant with respect to these general alteration parameters and the majorand trace-element data of Tarney et al. (this volume) on the same samples.

First-Order Alteration Chemistry of Leg 49 Basement Rocks