The Three Bares are small volcanoes that host the only-known hydrothermal vents active in oceanic crust older than 2 Ma. Major element data for Baby and Mama Bare lavas indicate formation by low extents of partial melting of a depleted mantle source, with limited cooling and crystallization prior to eruption. These lavas are compositionally unlike all lavas produced at the adjacent spreading center but resemble a subset of lavas from small seamounts nearby. We conclude that the Three Bares represent off-axis volcanism rather than normal crustal accretion. Such off-axis volcanism increases basement roughness locally, and thus plays an important role in hydrothermal circulation in sedimented ridge flank settings. ¿ 1998 American Geophysical Union