Along Axis Variability in the Sources of Eastern Gakkel Ridge Basalts

The Gakkel Ridge is the ultra-slow global end-member among ocean ridges, with spreading rate decreasing progressively from ~15 mm/y (full rate) in the west near Greenland to ~6 mm/y at the Siberian margin in the east. Michael et al. (Nature 2003) divided the western 850km of the Gakkel Ridge (sampled by AMORE 2001) into three distinct regions: the magmatically robust Western and Eastern Volcanic Zones (WVZ and EVZ) and the magma-starved Sparsely Magmatic Zone. The EVZ, comprising the eastern ~350 km, displays the lowest spreading rate (decreasing eastward from 12.7 to 11.3 mm/y) in the sampled portion of the global ocean ridge system. There is no significant transform offsets along the Gakkel that could complicate interpretations of source or process variability along axis. Thus, effects on magma compositions derived from progressive changes in melting under ultra-slow conditions might be emphasized in the EVZ. Michael et al. (Nature 2003) showed that compared to the WVZ, EVZ lavas have higher Na8.0, indicating lower extents of melting, and low SiO2 at high Fe8.0, indicating greater segregation depths and consistent with a thicker lithospheric lid. Here we focus on along axis chemical variability in the EVZ. Nd and Hf isotope ratios decrease progressively eastward in on-axis lavas, signaling greater time-integrated incompatible element enrichment in the mantle magma sources. Off-axis lavas follow the same decreasing trends, although with more variability. In addition, eastward increasing Tb/Yb and Dy/Yb suggest an increasing effect of melting with residual garnet. The range in Tb/Yb and Dy/Yb also increases eastward, with the easternmost region of the EVZ (66°E to 85°E) showing the largest garnet signature and the greatest variability. Incompatible element ratios also show progressive changes, with eastward decreasing Ba/La, Ba/Ti, K/Ti, Th/La, La/Sm, Nb/La indicating greater present-day depletions in trace elements toward the east. Taken together, the Nd-Hf isotopes along with the HREE are consistent with increasing contributions from veins (isotopically enriched) that accompany decreasing seafloor-spreading rate and thicker lithospheric lid eastward along the EVZ. At the same time the trace elements indicate greater present-day trace element depletion in the magma sources. Thus the data either contradict the hypothesis of a veined mantle, or indicate that the veins that are preferentially melting beneath the EVZ have experienced a previous low-degree melting event whereby they became depleted in incompatible elements but retained their enriched isotopic signature and remained fertile.