Melting and refertilization of the Arctic mantle from the ultra-slow spreading Gakkel Ridge

Mid-Ocean Ridge basalts (MORB) from the slowest spreading zone of the ultra-slow spreading Gakkel Ridge (the eastern volcanic zone or EVZ), comprise ~100 km long segments showing coherent geochemical and isotopic affinities, created by melt depletion and refertilization of the sub-ridge mantle. The EVZ lies to the east of a "sparsely magmatic zone" where peridotite outcrops in the ridge axis. Basalts just to the east of the sparsely magmatic zone (at 29°E - 40°E, segment EVZ1) show some of the highest MORB Hf and Nd isotope ratios on Earth (with ɛHf values up to nearly +26). They plot significantly above the Nd-Hf mantle-crust array. Basalts further to the east (segments EVZ2 and EVZ3), where spreading rate is even lower, show lower ɛNd and ɛHf values and plot closer to the mantle-crust array. Basalts from EVZ1 also show higher Lu/Hf and lower Dy/Yb ratios than basalts from EVZ2 and EVZ3. Comparing the differences in Hf-Nd isotope ratios for these segments, and accounting for the Lu/Hf-Sm/Nd ratios, the "depletion age" for the EVZ1 segment can be estimated, and the data are consistent with significant ancient melt removal (~9%) in the garnet stability field from the EVZ1 mantle source ca. 200Ma ago. In contrast to their high Hf-Nd isotope ratios (indicating severe incompatible element depletion), basalts from EVZ1 segment show strong enrichments in highly incompatible elements (for example, K2O/TiO2 up to 0.37 and (La/Sm)N up to 1.4). These geochemical signatures are consistent with recent melt refertilization of the sub-ridge mantle. In contrast to basalts from segment EVZ1, with enriched trace elements but Hf-Nd isotopes indicating long-term incompatible depletion, a group of samples from segment EVZ2, further to the east, show the opposite characteristics, that is, depleted incompatible element signatures (K2O/TiO2 down to 0.05 and low Dy/Yb), but long-term trace element enrichment (low ɛNd and ɛHf). These basalts may represent a second-round of melting of an enriched mantle component that had already lost a melt in the garnet stability field shortly before re-melting under the ridge axis to generate the Gakkel basalts. The basalts of the Gakkel Ridge EVZ thus appear to record melt depletion-enrichment events that would likely be lost or "blurred" in basalts from faster spreading ridges, where melt pooling and mixing is more prominent under the ridge axis.