Thermochemical anomalies in the upper mantle control Gakkel Ridge accretion.

Despite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use Ar/Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle.