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Basalts cored on legs 2 and 3 of the Deep‐Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr. Although many of the basalts are highly altered, fresh glass is usually present. Except for site 2–10 the fresh glasses are petrographically and geochemically similar to mid‐Atlantic ridge (MAR) axial basalts. There are no systematic compositional differences as a function of distance from the MAR axis. Two sites contain basalts with olivine (Fo90) phenocrysts, high Mg/Mg + ΣFe, high Ni and Cr abundances, and very low large ion lithophile (LIL) element abundances. These basalts are the best candidates for primary magma recovered from the sea floor; fractional crystallization of such basalt may yield the more evolved basalts typical of the MAR. More fractionated basalts with clinopyroxene phenocrysts occur at twp other sites, but they retain low LIL element abundances. Site 2‐10 contains titaniferous augite and is relatively enriched in LIL elements. It is unlikely that this basalt was derived by fractional crystallization from LIL element depleted tholeiites; instead, the site 2‐10 basalt requires a different mantle source. These results imply that the upper Atlantic Ocean basement is dominantly LIL element depleted tholeiite.

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Petrographic and geochemical studies of basalt glass and related rocks from the Deep‐Sea Drilling Project (DSDP) provide a random sampling of magmas which have crystallized to form sea floor basement rocks. The samples range in age from 155 m.y. to less than 4 m.y. and represent a variety of geologic settings within the three major ocean basins. The majority show the mineralogy, textures, and large‐ion lithophile (LIL) element‐depleted characteristics of modern basalts from mid‐ocean spreading ridges (group I). A smaller but compositionally diverse group shows distinctive mineralogy and LIL enrichment (group II). Group II magmas from spreading ridges are very similar to group I magmas in terms of major element variations but are more enriched in LIL elements. Group II magma from seamounts and off‐ridge centers tends to be olivine normative and enriched in normative plagioclase, while that from aseismic ridges tends to be iron‐enriched and quartz normative. The diversity among DSDP rocks is only slightly greater than that shown at a single well‐sampled mid‐Atlantic ridge (MAR) site (FAMOUS). There are a few unusual Fe‐ or Ti‐rich samples which so far are unique to the Pacific and Indian oceans. Compositional diversity in the Indian Ocean is especially striking. Preliminary petrogenetic modeling suggests that the samples as a whole are not products of a single high‐level fractionation sequence. Glasses which project on a pseudobinary cotectic in the Di‐An‐Fo system may differ significantly in FeO*/MgO, TiO2, and LIL element enrichment at comparable points on the co‐tectic. These liquids probably represent several compositionally distinct mantle‐derived partial melts and their fractionation products. Within the data set, individual fractionation sequences can be defined, in which most major elements are adequately explained by simple crystal‐melt equilibria. However, even in these sequences, inferred residual liquids tend to show excess LIL element enrichment which is not yet understood. Most DSDP basement rocks can be related back to a spreading ridge enviroment if due account is taken of apparent ‘plume‐related‘ geochemical variations along modern ridges. Unusual LIL element‐enriched sea floor basalts may be related to areas such as the one at 45°N on the MAR where there is little evidence of structural or bathymetric anomalies associated with a ‘plume’. We see no evidence for geochemical or mineralogical features diagnostic of variations in spreading rate, nor is there any obvious relation between composition and age.

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