The origin of the dunitic transition zone (DTZ) between the Earth mantle and the oceanic crust is poorly constrained. This work was dedicated to the petrological, geochemical and structural study of 20 cross-sections (600 samples) sampled in the Sumail massif (Oman ophiolite). The development of an analytical procedure allowed the determination of trace element compositions in dunites. The DTZ is made of pure dunites (olivine and chromite) and of impregnated dunites, containing a variable amount of interstitial minerals that crystallized from a percolating melt and including, in addition to those that clearly crystallized from a MORB (clinopyroxene and plagioclase), orthopyroxene, amphibole, garnet, and diopsides, witnesses of a hybridizati...