Clay mineral formation in DSDP Leg 34 basalt

A blue-green smectite (iron-rich saponite) and green mica (celadonite) are the dominant sheet silicates in veins within the 10.5 m of basalt cored during DSDP Leg 34, Site 321, in the Nazca plate. Oxygen isotopic analyses of these clays, and associated calcite, indicate a formation temperature of ≤25°C. Celadonite contains appreciable Fe₂O₃, K₂O and SiO₂, intermediate MgO, and very little Al₂O₃. Celadonite is commonly associated with goethite and hematite, which suggests that this phase formed by precipitation within a dominantly oxygenated environment of components leached from basalt and provided by seawater. A mass balance estimate indicates that celadonite formation can remove no more than 15% of the K annually transported to the oceans by rivers. In contrast, iron-rich saponite containing significant Al₂O₃ appears to have precipitated from a non-oxidizing, distinctly alkaline fluid containing a high Na/K ratio relative to unmodified seawater. Seawater-basalt interaction at low temperatures, resulting in the formation of celadonite and smectite may explain chemical gradients observed in interstitial waters of sediments overlying basalts.