Seamounts can provide conduits for the entry and exit of hydrothermal fluids in ocean basins. However, only a few ridge flank hydrothermal systems that discharge through seamounts have been discovered, all located on relatively young crust. We have retrieved samples from 126 m.y. old Henry Seamount, an extinct volcano near the youngest Canary island of El Hierro, that provide evidence for Holocene low-temperature hydrothermal fluid discharge. This is the first documented finding of such activity at the Canary archipelago. The samples include shells from vesicomyid clams <18.6 k.y. old, massive barite, and trachytes that are pervasively barite metasomatized. Sulfur, oxygen, and strontium isotope ratios of barite indicate that the fluid contained residual sulfate from microbial reduction at the recharge site and reacted with basement rocks. Recharge probably occurred at basement outcrops of El Hierro's submarine flank at 25–30 km distance, the driving force for hydrothermal circulation through old crust being provided by increased basal heat flow from Canary magmatism. The data show that island flanks may provide important recharge sites for seawater circulation and that even old and small seamounts can contribute to heat and mass exchange between ocean crust and seawater.