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The evolution and chemical variation observed in basalts from the East Pacific Rise (EPR), OCP Ridge and Siqueiros fracture zone are discussed in terms of major oxide, trace and rare-earth element (REE) data. All the samples are tholeiitic, and in general are Mgpoor and Ti-rich compared with basalts from the Mid-Atlantic Ridge. Their chondrite-normalized REE patterns are moderately depleted in light REE, and show Eu anomalies ranging from negligible (OCP Ridge) to small and negative (EPR and Siqueiros fracture zone). Closed-system, fractional crystallization of plagioclase and ferromagnesian minerals is the dominant process in the evolution of these magmas to their erupted compositions. Enrichment factors of Ce, Nd, Tb, Yb, Y, and Hf indicate that the extent of solidification required to produce the evolved form from the primitive end member is just under 50 per cent. However, evidence for local mantle heterogeneity is also provided by the ultra-incompatible elements — La, Nb, Ta, and Th — and by the range of TiO2 concentrations seen at fixed values of FeOVMgO. The origin of the OCP Ridge magmas is discussed, and an hypothesis is advanced for evolution beneath the EPR and migration laterally within the oceanic crust to the OCP Ridge site immediately before eruption.

Comparison of Geochemistry of Basalts from the East Pacific Rise, OCP Ridge, and Siqueiros Fracture Zone, Deep Sea Drilling Project Leg 54