Abstract We present a detailed mineralogical and petrological description of the low-temperature alteration patterns in basalts from four new sites drilled during ODP Leg 192 on the Early Cretaceous Ontong Java Plateau. Three main alteration types have been identified: pervasively altered dark grey basalt; black or dusky green halos; and brown halos. Dark grey basalts are the most common and represent the least intensive, but most pervasive, alteration phase. Early interaction of the basalts with low-temperature sea-water-derived hydrothermal fluids lead to the development of black and dusky green halos characterized by the replacement of groundmass and olivine phenocrysts by celadonitic phyllosilicates and smectite. Later interaction of basalts with cold oxidizing sea water produced brown halos characterized by replacement of primary phases and mesostasis by smectite and iron oxyhydroxides. Secondary minerals in order of decreasing abundance include phyllosilicates, calcite, iron oxyhydroxides, pyrite, chalcedony, quartz and zeolites. Veins, resulting from symmetrical infilling of open cracks, commonly contain phyllosilicates, iron oxyhydroxide or pyrite, and late calcite. Carbonate veins cross-cut all other alteration features and stable isotope analyses of vein carbonates indicate formation from marine bicarbonate below about 40°C. A positive correlation between vein density and overall degree of alteration is observed resulting in pervasive development of brown alteration halos in highly fractured rocks. Overall, alteration of basalts from the Ontong Java Plateau is similar to that observed from other DSDP/ODP sites throughout the oceans.

Abstract Fragments of the Ordovician sea floor preserved in the Solund-Stavfjord Ophiolite Complex in Western Norway serve as proxies for the δ 18 O of Ordovician seawater. The pillow basalt sections at Oldra and Strand are both enriched in 18 O, recording their alteration by seawater at low temperature on the sea floor. In contrast, the sheeted dykes and gabbros generally are depleted of 18 O, reflecting the modal proportion of secondary, low- 18 O chlorite and epidote formed from seawater at high temperature. These isotopic contrasts simply reflect the high water to rock ratio of sea-floor alteration and the temperature dependence of the 18 O partitioning between minerals and water. Superposition of high-δ 18 O pillows on low-δ 18 O dykes and gabbros is a necessary consequence of alteration at both low and high temperatures by a fluid near 0‰ and is easily recognized in well-preserved ophiolites. Also, the δ 18 O of seawater can be independently calculated from 18 O fractionations among secondary minerals. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Suites of diamond-bearing nodules from kimberlites still have contrasting high- and low-δ 18 O eclogites, proving that even subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of the sea-floor, ophiolite and eclogite data reveals no secular trend in δ 18 O, indicating that the δ 18 O of seawater has not changed with geological age. Because the δ 18 O of seawater itself is fixed by sea-floor-seawater exchange, the constancy of δ 18 O of seawater implies that the scale and style of sea-floor-seawater interactions has not changed over time.