.-Alkali metals prove to be very sensitive to alteration process even if this process is moderate. It is very difficult to use these elements to trace magmatic processes since their concentration may Vary within an homogeneous "unaltered" unit. From all the basaltic units investigated, a classification of "low partition coefficient" elements is presented (Sc, Ti, V, Y, Zr, Nb, La-Tb , Hf, Ta, Th) . The incompatible elements ratios (Y/Tb, Zr/Hf, Nb/Ta) are constant in the samples studied and are nearly equal to the same ratios in chondrites. This is consistent with the existence of an homogeneous primordial mantle at the scale of the North Atlantic, compatible with a chondritic composition of the earth. [NOT CONTROLLED OCR]