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 spa tial 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 variation s in magma supply through time. A detailed sampling of the East Pacifi c Rise (EPR) axis, near 13 degrees N, revealed that complex basalt com positional 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 se veral melting stages of a composite mantle source, interrupted by extr action and a rapid migration of the melt toward the upper levels of th e lithosphere. For a given composition of the mantle source, this proc ess will produce primitive melts which are progressively depleted in i ncompatible elements. The final contrast in composition between the va rious extruded magmas depends on the opposing effects of mixing and fr actionation during transport toward the ocean floor. A multistage melt extraction model for trace elements, based on nonmodal near-fractiona l melting (1% increments) with three steps of accumulation (<2.5-5%) a nd extraction above the melting region, gives reliable results for les s than 8-13% total melting of a spinel-lherzolite (olivine 49-55%, ort hopyroxene 25-29%, clinopyroxene 18-21%, spinel 1-2%). This partial me lting model is only partly constrained on the basis of available chemi cal and physical data, as well as laboratory experiments, and it has s everal 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 discont inuities in the melting regime and the cyclicity of magma production.