Compositional mixing trends in eruptive mid-ocean ridge basalt (MORB) seem to be confined to short time–space intervals, possibly corresponding to single fracture-zone-bounded segments of a spreading ridge axis. Binary mixing is discernible from the variation in Ta, Hf, Rb, Sr and rare earth elements (REEs), and may be interpreted as reflecting dual mantle sources for locally-defined units of the axis or associated transform fracture zones. These sources reflect contrasting degrees and types of incompatible (or ‘low-KD’) element enrichment relative to a ‘normal’ MORB source characterized by uniform depletion in these elements. Enrichment in low-KD elements, radiogenic isotopes and normative nepheline has long been recognized in some fracture zone magmas1–4 and it is important to speculate on the possible influence of fracture zones on the chemical character of spreading axes. Consideration of chemical and isotopic variation at a single Atlantic spreading segment (36–37 °N) in a regional context suggests that transform fracture zones may be instrumental in both generating compositional heterogeneity in the mantle and permitting its expression in eruptive crust. It is proposed here that hybridization of ‘enriched’ fracture zone-derived melt and less-enriched or depleted rift-derived melt occurs in sub-axial magma supply systems during an active spreading phase.