Abstract

Zeolites, clays, carbonates, palagonite, and other secondary phases in basalts and basaltic breccias from holes 332A, 332B, 333A, and 335 provide information about The alteration of the volcanic layer of the oceanic crust. Microprobe analyses of fresh and altered glasses from all holes show that palagonitization involves addition of H2O, K2O, TiO2 and total Fe, and loss of SiO2, Al2O3, CaO, MgO, and Na2O. Fresh glass is tholeiitic in composition; however, glass microinclusions in plagioclase phenocrysts are enriched in FeO and MgO and have normal CaO contents, but are strongly depleted in Al2O3. Secondary minerals include phillipsite, calcite, siderite, saponite, and Fe. K-rich clay (celadonite?), and hydrated Fe-oxides. Phillipsite, found in cavities, vesicles, and veins, is present in all holes and is K2O-rich and Al2O3-poor, showing deviations from its ideal composition. TheCaCO3 content of siderite (~15 mol%) is surprisingly high, suggesting a formation temperature in excess of 400 °C. All other indications, however, are that the secondary minerals were formed at low temperatures by interaction of seawater with basalts of oceanic layer 2. The incorporation of substantial amounts of K into palagonite, zeolites, and some clays during seawater–basalt interaction may represent one of the major components in the K-budget of the ocean.

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