Petrology of Baby Bare and Mama Bare lavas

Abstract

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.