New chemical analyses and a review of published data show that there is a compositional diversity between volcanics of basaltic composition found in the M.A.R. rift valley, M.A.R. transform faults and aseismic ridges. The basaltic rocks from, the M.A.R. transform faults are less mafic (depleted in olivine content) than those from the M.A.R. rift valley. The transform fault basalts have a higher range of TiO2 content (1-4%), of Fe2O3 + FeO content (8-14%) and a lower range of Cr content (50-500 ppm) and Ni content (50-300 ppm). The volcanics from aseismic ridges around the world are considered to be the more felsic types of the two provinces. They have a higher range of variation for their TiO2, and Fe203 + Fe0 (1.6-5% ; 9-15% ; respectively) and a lower range of variability for their Cr and Ni (250 and 100 ppm respectively) than both the M.A.R. rift valley and transform fault volcanics. It is suggested that transform faults have, by faulting, exposed more fractionated types of basaltic rocks (may be as intrusives) than the rift valley volcanics. While aseismic ridges have undergone a greater degree of differentiation than both transform faults and rift valley volcanics. [NOT CONTROLLED OCR]