Refertilized roots of ophiolites record slab-melt addition to the mantle during subduction infancy

Basal lherzolite tectonites of supra-subduction zone ophiolites are thin (~10–100s m) fertile layers commonly interpreted to be inherited from oceanic spreading. Yet, their structural and chemical similarities between ophiolites may suggest that they record processes generic to subduction infancy. Here, we document a systematic compositional, thermal, and strain gradient through the basal mantle of the Mont Albert ophiolite (~450 Ma, Québec). Peridotites up to ~600 m above the contact with underthrust lithologies exhibit major, trace and rare earth element (REE) enrichments. Fertility increases toward the contact and coincides with cooling from >900°C to ~750°C, syn-kinematic decrease in spinel Cr# and mylonitization. We interpret the coupled changes to reflect addition of slab-derived hydrous melts to a cooling, deforming mantle wedge during subduction infancy. We propose that basal lherzolites are fossil reaction fronts that capture the earliest chemo-mechanical development of subduction interfaces; refertilization thus postdates original lithosphere formation but predates forearc spreading to form ophiolite crust. Reports of basal lherzolites show that they thin with time over the Phanerozoic; this may reflect a waning role of slab-melts in nascent subduction processes.