Sr isotopic constraints on hydrothermal alteration of ultramafic rocks in two oceanic fracture zones from the South Atlantic Ocean

Abstract

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.