Extremely light Li in orogenic eclogites:: The role of isotope fractionation during dehydration in subducted oceanic crust

Eclogites from Trescolmen, Switzerland, derive from basaltic protoliths that experienced variable degrees of low-T seafloor alteration prior to high pressure metamorphism. delta(7)Li of the eclogites (-11 to + 5parts per thousand) ranges to dramatically lower values than observed in fresh MORB, or altered MORB (+4.5, to +14parts per thousand for low-T and -2 to +8parts per thousand for high-T altered MORB). These low values cannot be explained by fluid interaction with surrounding garnet mica schist, which is generally isotopically heavier (delta(7)Li of +2.4 to +3.8parts per thousand). The low delta(7)Li 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 yeleased into the forearc mantle wedge in subduction zones, while an isotopically light component is suibducted deeply, and may form a distinct mantle reservoir that could be sampled by plume-related magmas. (C) 2003 Elsevier Science B.V. All rights reserved.