Ultraslow Spreading and Volcanism at the Eastern End of Gakkel Ridge, Arctic Ocean

Ultraslow spreading ridges are poorly understood plate boundaries consisting of magmatic and amagmatic segments that expose mostly mantle peridotite and only traces of basalt and gabbro. The slowest part of the global spreading system is represented by the eastern Gakkel Ridge in the Central Arctic Ocean, where crustal accretion is characterized by extreme focusing of melt to discrete magmatic centers. Close to its eastern tip lies the unusual 5,310-m-deep Gakkel Ridge Deep (GRD) with limited sediment infill, which is in strong contrast to the broader sediment-filled rift valleys to the east and west. Here, we report an 40Ar/39Ar age of 3.65 +/- 0.01 Ma for a pillow basalt from a seamount located on the rim the GRD, confirming ultraslow spreading rates of similar to 7 mm/year close to the Laptev Sea as suggested from aeromagnetic data. Its geochemistry points to an alkaline lava, attributed to partial melting of a source that underwent prior geochemical enrichment. We note that the GRD extracts compositionally similar melts as the sparsely magmatic zone further west but at much slower spreading velocities of only similar to 6-7 mm/year, indicating the widespread occurrence of similarly fertile mantle in the High Arctic. This enriched source differs from subcontinental lithospheric mantle that influences magmatism along the Western Volcanic Zone (Goldstein et al., 2008, ) and is similar to metasomatized mantle, shown to influence melt genesis along the Eastern Volcanic Zone.