The major element, alkali metal, alkaline earth, rare earth, and nickel content of mid-ocean-ridge rocks from 30° North on the mid-Atlantic ridge, the Gorda rise, the Juan de Fuca ridge, and the equatorial East Pacific rise are presented. The mid-ocean ridge volcanic rocks are characterized by olivine tholeiite normative compositions with variable Al₂O₃ and total iron contents. Chondritic rare earth element (REE) relative-abundance patterns, with some depletion in La and Ba, are found in almost all samples. Negative europium anomalies are found to correlate with high total rare earth (RE) contents. The observed variations in chemical composition are ascribed to shallow differentiation dominated by plagioclase and olivine. The major characteristics described above are ascribed to extensive partial melting (∼30%) at shallow depths (15–25 km) in mantle material being intruded under the axial valley of the mid-ocean ridges. The extent of partial melting may be controlled by the total water content of the mantle.