87Sr/ 86Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 21°N

⁸⁷Sr/ ⁸⁶Sr ratios of three hydrothermal waters collected on the East Pacific Rise at 21°N define a mixing line between seawater and a hydrothermal end-member at 0.7030 which is derived by seawater-basalt interaction at ca. 350°C and water/rock ratio of about 1.5. Sr concentrations are not affected in the process while Mg uptake from seawater is almost complete. Up to2/3 of this hydrothermal component is involved in anhydrite precipitation while the Sr isotopic ratio in sulfides (chalcopyrite + sphalerite) cannot be distinguished from that of sulfate. It is estimated that ca. 1 × 10 ¹⁰ moles of strontium are yearly cycled in the hydrothermal systems of mid-oceanic ridges, thereby affecting the ⁸⁷Sr/ ⁸⁶Sr budget of seawater. Mass balance between river runoff, limestone precipitation and ridge basalt alteration suggests that the ⁸⁷Sr/ ⁸⁶Sr ratios of the river runoff are in the range 0.7097-0.7113, and are largely dominated by limestone alteration.