Talc-rich hydrothermal rocks from the St. Paul and the Conrad fracture zones in the Atlantic Ocean.

Talc-rich rocks covered by Fe-Mn coatings were recovered from the St. Paul F.Z. (00°37’S-25°34’W, equatorialAtlantic) and Conrad F.Z. (55°29’S-02°05’W, American-Antarctic Ridge). In both occurrences, the talc-rich rocks are associatedwith serpentinized peridotites, gabbroic rocks and minor basalts. The two rocks have very similar trace element, particularly rareearth element, distributions. The St. Paul F.Z. samples are breccias consisting of angular clasts of botryoidal/colloform talc in asubordinate foraminiferal ooze sediment. These breccias probably formed by the collapse of fragile structures formed by theprecipitation of talc at hydrothermal vents. Talc formed when seawater mixed with hydrothermal fluids from a mafic-ultramaficreaction zone. The talc-rich hydrothermal rock found at the Conrad F.Z. shows evidence of a replacement origin. We suggest theprotolith was a gabbroic rock that underwent multi-stage hydrothermal alteration, possibly in a shear zone.These two occurrences represent an evidence of off-axis ocean floor hydrothermal activity, and the study of similar, apparentlyminor, products collected by dredging could be used to reveal the presence of hydrothermal systems in such impervious settings.