Ultramafic rocks from the Islas Orcadas and Shaka Fracture Zones have been progressively hydrated, starting at temperatures greater than 700°C and continuing to less than 250°C. The hydration is manifest as a sequence of alteration minerals, including (in order of decreasing temperature) amphibole, talc, talc plus secondary olivine, chlorite, secondary clinopyroxene, prehnite and serpentine. Early, high-temperature minerals display lower 87Sr/ 86Sr ratios and higher K, Rb and Sr than later, lower-temperature minerals. One explanation for the change in 87Sr/ 86Sr ratios is that early, high-temperature reactions are the result of hydration by fluids characterized by a relatively strong basaltic geochemical signature, and the later, lower-temperature alteration is the result of hydration by fluids characterized by a relatively weaker basaltic signature and relatively stronger seawater signature.