Abstract:
Basalts exhibit alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in superimposed alteration zones within individual basalt fragments. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained units, at high temperatures during initial cooling: No chemical changes. 2) Mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims: Gains of K, Rb, H (sub 2) O, increase of Fe (super 3+) /Fe (super T) , and some losses of Ca and Mg. 3a) Extensive circulation of oxygenated seawater resulted in the formation of smectites, K-feldspar, and Fe-hydroxides alteration zones around formerly exposed surfaces: K, Rb, H (sub 2) O, and P were added to the rocks, Fe (super 3+) /Fe (super T) increased, and Ca, Mg, Si, Al and Na were lost. 3b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed: Only slight increases in Fe (super 3+) /Fe (super T) and H (sub 2) O. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt: Increases of Ca, Na, H (sub 2) O, and CO (sub 2) . 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks: No chemical changes.