PETROLOGY OF BABY BARE AND MAMA BARE LAVAS

The Three Bares are small volcanoes that host the only-known hydrother mal vents active in oceanic crust older than 2 Ma. Major element data for Baby and Mama Bare lavas indicate formation by low extents of part ial melting of a depleted mantle source, with limited cooling and crys tallization prior to eruption. These lavas are compositionally unlike all lavas produced at the adjacent spreading center, but resemble a su bset of lavas from small seamounts nearby. We conclude that the Three Bares represent off-axis volcanism rather than normal crustal accretio n. Such off-axis volcanism increases basement roughness locally, and t hus plays an important role in hydrothermal circulation in sedimented ridge flank settings.