A mechanism for magmatic accretion under spreading centres

The most active volcanic zone on Earth occurs along the global mid-ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km Myr −1 . This volcanism is segmented, and is concentrated along the axial valleys and highs of slow- and fast-spreading ridges respectively, with transform fault zones separating, and generally offsetting, each active ridge segment. Volcanic activity may be nearly absent at the ridge transform intersection where a slow-spreading ridge is offset, and greatly reduced where a fast-spreading ridge is offset, implying thinner crust in these regions 1 . The source of magmas erupted along ocean ridges is the underlying mantle, which undergoes decompression melting when rising to fill the gap between the spreading plates. The resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock. The melt is believed to pool in crustal magma chambers—whence it periodically erupts to the surface. Typically, the formation, ascent and aggregation of magmas along the mid-ocean ridges is regarded two-dimensionally and modelled in a section through the crust and mantle across the strike of the ridges (see ref. 2). Here, however, we consider the problem with a three-dimensional model for which we provide supporting geological evidence, in which the magmas rise as a result of a gravitational instability (modified Rayleigh–Taylor) in the underlying partial melt zone.