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.

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