Explosive Alkaline Volcanism on the SW Indian Ridge

Abstract We report explosive alkaline volcanism at a segment of the ultraslow spreading (14 mm/yr) Southwest Indian Ridge (SWIR). While gentle effusive tholeiitic (MORB) volcanism typifies the global ridge system, little evidence for violent alkaline volcanism producing debris cones, lava bombs and scoria as at ocean islands has been reported. However, morphologic evidence in the form of craters exists 1, and alkali basalts have been found on the SWIR and Lena Trough e.g., 2,3,4. Ours are the first definitive samples of pyroclastic debris sampled from a cratered terrain on an ocean ridge. Critically, they are also the result of shallow mixing of an alkaline magma with entrained MORB crystal mush from beneath the flank of a rift valley. This suggests a major paradigm shift for ocean ridge volcanism. The alkali basalt represents an early volatile-rich low-viscosity melt fraction erupted from near the base of the melting column violently to the seafloor — the product of deep low-degree melting of local enriched heterogeneities in an otherwise depleted mantle. The MORB mush represents a later volatile-poor viscous melt erupted effusively from beneath the axial valley onto the seafloor — the product of high-degree melting due to penetrative flow and melt-rock reaction through the remaining ascending depleted mantle to the base of the lithosphere. Mixing of small volumes of the former with the latter in the melt storage region beneath the ridge then explains the local major element uniformity of MORB and its notorious isotopic and trace element diversity. We suggest this may apply globally as at magmatically more robust ridges the role of alkali basalt is likely masked by the far more voluminous MORB.