Abstract
Summary The results of an oxygen-isotope investigation of the Sarmiento ophiolite complex show that the extrusive rocks are isotopically enriched in 18 O compared to fresh basalts as a consequence of low-temperature interaction with seawater. The basaltic sheeted dykes, dykes cross-cutting plutonic rocks, and the plutonic rocks themselves are depleted in 18 O compared to fresh basalts as a consequence of high-temperature interaction with seawater. K 2 O contents of pillow lavas indicate minimum water/rock ratios of 15 to 90. Within the sheeted dyke complex and the plutonic section, the inferred water/rock ratios become <0.6. Modelling of basalt-seawater interaction indicates that hydrothermal fluids in equilibrium with actinolite-facies assemblages (525–700°C) have δ 18 O=+5 to +7‰, whereas hydrothermal fluids in equilibrium with greenschist-facies assemblages (230–525°C) have δ 18 O=0 to +5‰. It appears that an inverse correlation exists between the temperature of metamorphism and the water/rock ratio. This would be anticipated from the cooling effect that circulating seawater has on the oceanic crust.
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