A systematic search for evidence of high-temperature hydrous alteration within the gabbros of the Samail ophiolite (Oman) shows that most of the gabbros have been affected by successive stages of alteration, starting above 975°C and ending below 500°C. Sr and O isotopic analyses provide constraints on the nature and origin of the fluids associated with these alteration events. Massive gabbros, dykes and veins and their associated minerals depart from mid-ocean ridge basalt (MORB)-source magma signatures (87Sr/86Sr >0·7032 and depleted δ18O <6‰). These isotopic characteristics identify seawater as the most likely hydrothermal contaminant. Samples affected by low-temperature alteration during greenschist-facies metamorphism are characterized by high water/rock ratios (>10). A second group of samples, including massive gabbros and high-temperature dykes and veins, is characterized by lower water/rock ratios, in the range of 3–5. These samples display a high-temperature hydrothermal alteration related to seawater ingression. The main fluid channels may be sub-millimetre-sized microcracks with a dominantly vertical attitude, which constitute the recharge hydrothermal system, whereas the dykes and veins represent the discharge part. This model requires that these dykes have been generated by hydration of the crystallizing gabbros via seawater penetration, near the internal wall of the magma chamber.