Along-axis variations in magma plumbing system of the ultraslow-spreading Gakkel Ridge

Abstract:

The Gakkel Ridge (Arctic Ocean) is the world’s slowest-spreading ridge, and is therefore important in informing the processes that control lithosphere formation at ultraslow spreading rates. Here, we examine along-axis variations in the Gakkel Ridge magma plumbing system, using a combination of bathymetry, basalt geochemistry, mineral compositions of crystal cargo and melt inclusion data. Although basalts erupted along the Gakkel Ridge are overall more primitive than MORB from slow- and fast-spreading ridges, suggesting an overall less well-developed magma plumbing system, there are significant variations in the degree of differentiation and hence maturity of the plumbing system. Basalts from the Western Volcanic Zone have the lowest and least variable Mg# (average ~60), coupled with relatively low Na₈ and relatively shallow crystallisation depth as indicated by melt inclusions (~0-11 kmbsf). Coupled with its relatively shallow axial depth (3-4 km) and characteristic axial volcanic ridge morphology, this indicates that accretion along this segment is akin to ‘Atlantic-style’ magmatic accretion. In contrast, the Sparsely Magmatic Zone and Eastern Volcanic Zone contain prominent ridge-perpendicular basement ridges rising from deep (~ 4-5 kmbsl) seafloor. These basement ridges (e.g., 31°E) erupt moderately high Na₈ lavas that have differentiated significantly, and whose crystal cargo records crystallisation from the deep lithospheric mantle (16 kmbsf) to shallow magma reservoirs. This suggests significant volumes of low-degree melts are focused in these areas, likely along the base of the lithosphere, enabling well-established, vertically extensive plumbing systems to develop. Other areas (e.g., 48°E) are characterised by a deep ridge axis (~5 kmbsl) with small point source volcanoes, and erupt exclusively primitive (Mg# ~65), but high-sodium, basalts carrying forsteritic olivine ± spinel only, indicative of limited differentiation of low-degree melts in a poorly developed magma plumbing system. On the basis of the combined data, we conclude that the Gakkel Ridge comprises variably developed trans-lithospheric magma plumbing systems, and is controlled by both degree of mantle melting and focused melt delivery to the lithosphere.