A DYNAMIC-MODEL FOR GENERATING SMALL-SCALE HETEROGENEITIES IN OCEAN-FLOOR BASALTS

Small-scale distribution of basaltic compositions along the mid-ocean ridge crests suggests that the magmatic processes are sensitive to spatial and temporal fluctuations related to proximity of ridge hot spots, changes in spreading rate, thermal boundary effects associated with ridge offsets, off-axis volcanoes and ridge propagators, and variations in magma supply through time. A detailed sampling of the East Pacific Rise (EPR) axis, near 13 degrees N, revealed that complex basalt compositional variations unrelated to morphology and structure occur both on the ridge axis and on nearby off-axis seamounts. This small-scale heterogeneity is attributed to successive magmatic cycles separated by periods of quiescence (amagmatic phases) where each cycle involves several melting stages of a composite mantle source, interrupted by extraction and a rapid migration of the melt toward the upper levels of the lithosphere. For a given composition of the mantle source, this process will produce primitive melts which are progressively depleted in incompatible elements. The final contrast in composition between the various extruded magmas depends on the opposing effects of mixing and fractionation during transport toward the ocean floor. A multistage melt extraction model for trace elements, based on nonmodal near-fractional melting (1% increments) with three steps of accumulation (<2.5-5%) and extraction above the melting region, gives reliable results for less than 8-13% total melting of a spinel-lherzolite (olivine 49-55%, orthopyroxene 25-29%, clinopyroxene 18-21%, spinel 1-2%). This partial melting model is only partly constrained on the basis of available chemical and physical data, as well as laboratory experiments, and it has several implications for the dynamics of the upper mantle and the lower crust which are not taken into account by present-day physical models. The major consequences of this model are the introduction of discontinuities in the melting regime and the cyclicity of magma production.