REE abundances in melts produced by partial melting of a hornblende‐bearing eclogite were examined using a dynamic disequilibrium‐melting model and following the P‐T path of a young subducting slab. Model calculations show that melts produced by small degrees of slab melting at 700∼900°C are more likely affected by sluggish kinetics, and have lower La/Yb ratios, higher Li2O and Na2O abundances than melts produced by large degrees of melting at higher temperatures. The extent of disequilibrium depends on deformation and melting histories of the slab, and effective grain sizes and transport properties of the minerals. The LREE abundances in the large‐degree melts are broadly similar to those in adakites. If kinetics is important in slab melting, the degrees of melting required to generate adakites may be higher than values predicted by equilibrium partial melting models and REE abundances in the melts.