Talc-rich rocks covered by Fe-Mn coatings were recovered from the St. Paul F.Z. (00°37'S-25°34'W, equatorial Atlantic) and Conrad F.Z. (55°29'S-02°05'W, American-Antarctic Ridge). In both occurrences, the talc-rich rocks are associated with serpentinized peridotites, gabbroic rocks and minor basalts. The two rocks have very similar trace element, particularly rare earth element, distributions. The St. Paul F.Z. samples are breccias consisting of angular clasts of botryoidal/colloform talc in a subordinate foraminiferal ooze sediment. These breccias probably formed by the collapse of fragile structures formed by the precipitation of talc at hydrothermal vents. Talc formed when seawater mixed with hydrothermal fluids from a mafic-ultramafic reaction zone. The talc-rich hydrothermal rock found at the Conrad F.Z. shows evidence of a replacement origin. We suggest the protolith 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, apparently minor, products collected by dredging could be used to reveal the presence of hydrothermal systems in such impervious settings.

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