Eclogites from Trescolmen, Switzerland, derive from basaltic protoliths that experienced variable degrees of low-T seafloor alteration prior to high pressure metamorphism. δ7Li of the eclogites (−11 to +5‰) ranges to dramatically lower values than observed in fresh MORB, or altered MORB (+4.5 to +14‰ for low-T and −2 to +8‰ for high-T altered MORB). These low values cannot be explained by fluid interaction with surrounding garnet mica schist, which is generally isotopically heavier (δ7Li of +2.4 to +3.8‰). The low δ7Li values were likely produced by isotope fractionation through Rayleigh distillation during dehydration of clays and/or chlorite at early stages of metamorphism. These data are consistent with isotopically heavy Li being released into the forearc mantle wedge in subduction zones, while an isotopically light component is subducted deeply, and may form a distinct mantle reservoir that could be sampled by plume-related magmas.