Source fertility modulates crustal thickness variation along the Gakkel Ridge

• Basalts at 100°E were sampled for the first time using a ROV at Arctic Gakkel Ridge. • Parental melts are produced by high-Al fertile lherzolites from the deep asthenosphere. • Source potential temperature and final melting depth at 31–100°E are all similar. • The proportion of fertile mantle controls crust thickness variations along the axis. The ultraslow-spreading Gakkel Ridge shows marked variation in crustal thickness at 31–100°E, decreasing from ∼7.5–8.9 km in the east to ∼2 km in the west, with peridotite exposures, but the causes of the crustal thickness variations remain unclear. This is explored using elemental and isotopic systematics of glasses and basalts from two segments at 85°E and 100°E (eastern Gakkel Ridge) combined with literature data from 31 to 85°E. Geochemical evidence suggests that the primary endmember composition of the samples had high Al 2 O 3 , N-MORB-like Sr-Nd-Hf-Pb isotopic systematics and slightly LREE-enriched patterns. The mantle source is probably an aluminum-rich fertile lherzolite related to deeper asthenospheric domains propagating south across the central Arctic at ∼150 km depth. Thermodynamic modelling shows that the source potential temperatures and final depths of melting of the basalts at 31–100°E volcanic centres are all similar and thus cannot account for the variations in crustal thickness along the ridge. Integrating literature data that suggest the existence of prior refractory mantle residues along the Gakkel ridge, we propose that the sub-ridge uppermost mantle is a mixture of fertile and refractory mantle domains and the lateral variations in the proportion of fertile mantle domains modulate the change in crustal thickness along the axis.