Multi-stage melting of enriched mantle components along the eastern Gakkel Ridge

The global endmember ultra-slow spreading Arctic Gakkel ridge is an ideal place to study mantle melting and the contributions of different mantle components to ridge volcanism. We carried out a high-resolution geochemical study of basalts from a seemingly normal section of the ultraslow-spreading Arctic Gakkel Ridge between 40 degrees E and 60 degrees E, which we refer to as EVZ2. While the majority of volcanics sampled from EVZ2 could be characterized as normal Mid-Ocean Ridge Basalts (NMORB), we identified a group of Isotopically Enriched, Incompatible Element Depleted MORB (IED-MORB) with low MREE/HREE ratios (i.e., "ghost garnet" signature). EVZ2 IED-MORB are mostly found near the rift valley walls away from axial volcanic centers. We propose that IEDMORB are the products of two stages of melting. They are shallow secondary melts of incompatible trace element enriched low-solidus mantle components that had lost some initial melt in the presence of residual garnet at depth. While the deep initial melts are more likely to be focused toward axial volcanic centers and subsequently diluted by normal peridotite melts, some of the shallow secondary melts could erupt as IEDMORB via pre-existing crustal weaknesses, such as deep-rooted high-angle normal faults that are ubiquitous along ultra-slow spreading ridges, thereby preserving their enriched isotopic compositions. While most dredges that sampled IEDMORB also recovered other types of MORB, all the samples from Dredge 55 are IEDMORB with distinctive arc-type trace element signatures, including relative depletion in Nb (and Ta), as well as enrichment in Th and fluid mobile elements (e.g., high Th/Nb, La/Nb, Pb/Ce, and H2O/Ce). These signatures suggest that they sampled recycled metasomatized arc-mantle wedge material. Other EVZ2 IEDMORB also show relative enrichment in fluid mobile elements and depletion in Nb (e.g., high La/Nb), but lack enrichment in Th. As Th has extremely low mobility in aqueous fluids, Th enrichment requires metasomatism involving silicate melts. Thus, we propose that the high Th/Nb, high La/Nb IEDMORB contain silicate-melt metasomatized arc mantle wedge material while the low-Th/Nb, high La/Nb IEDMORB only contain aqueous-fluid metasomatized arc mantle wedge material. Globally, IEDMORB with ghost garnet signatures are mostly found along ridges near mantle plumes where low-solidus, incompatible element and isotopically enriched mantle components that suffered initial melt loss at depth could be entrained in the upwelling subridge mantle and undergo further melting. However, most near-plume IEDMORB do not show arc-type geochemical signatures. Therefore, the discovery of IEDMORB from 20% of dredges along EVZ2, where most ridge volcanics are NMORB with depleted isotopic compositions, reflects the sporadic distribution of recycled arc mantle wedge material in the Arctic mantle and the prevalence of preexisting crustal conduits, such as high-angle normal faults, along ultra-slow spreading ridges that facilitate melt migration with limited melt pooling and mixing.