Gabbroic veins enclosed in mantle peridotite from oceanic core complexes next to oceanic transform faults demonstrate sub-crustal crystallization of silicate minerals from a mid-oceanic ridge basalt (MORB)–like melt. Cooler lithosphere there may also affect sulfide crystallization and the metal budget of the lower and upper crust, but the related sulfide behavior is poorly understood. Here, we use chalcophile elements to trace sulfide crystallization in a suite of MORBs erupted at the Kane Megamullion south of the Kane Fracture Zone along the Mid-Atlantic Ridge. Cool lithosphere there is inferred from a low magma supply and lithostratigraphic evidence for thin crust with abundant mantle rock exposed to the seafloor. We show that the concentrations of Cu, Zn, As, Ga, Pb, Sb, and Tl in the Kane Megamullion MORBs rise linearly with melt differentiation expressed by decreasing MgO and Ni content. The low-pressure fractional crystallization within the crust thus occurs at sulfide-undersaturated conditions. Sulfide-undersaturated MORBs are unusual. At the Kane Megamullion, however, the thin crust allows melt to more extensively interact with the shallow and serpentinized mantle. We argue that sulfur and chalcophile elements have been lost from the melt due to sulfide crystallization during melt-rock reaction in the shallow mantle.