Origin of enriched ocean ridge basalts and implications for mantle dynamics

Abstract

The Mid-Atlantic Ridge (MAR) south of the Kane Fracture Zone at ∼23°N (the MARK area) is distant from hot spots and a type area for “normal” mid-ocean ridge basalt (N-MORB) depleted in highly incompatible elements. High-density sampling reveals that a small proportion of basalt are enriched in incompatible elements (enriched mid-ocean ridge basalts, E-MORB) from the MARK area. It is apparent that enriched magma sources, not associated with hot spots, are widespread in the upper mantle and are a common occurrence on both fast- and slow-spreading ridges. Evaluation of the trace-element systematics shows that E-MORB generation requires two stages. Low-degree melts metasomatise the upper mantle to create an enriched source, which later undergoes large extents of melting. A significant time lapse between the two events is required by differences in radiogenic isotope ratios. Atlantic, Pacific, and Indian ocean ridges that are far from hot spots show “mantle isochron” ages of ∼300 Ma for the Sm–Nd, Rb–Sr, and 238U– 206Pb systems after corrections for melting, but these ages need not be indicative of a specific event. Instead, they can result from continuous processes of formation and destruction of enriched mantle sources by melting and convective mixing. A two-box model describing these processes illuminates relationships between mantle isochron ages and upper mantle dynamics. If formation–destruction of enriched mantle is at steady state, constant “mantle isochron” ages are maintained and depend on the residence time of enriched mantle sources, the half-life of the radioactive system, and the daughter element behavior during mantle melting. The common ages of the Sr, Nd, and Pb systems reflects their long half-lives and similar melting behavior. In contrast, 207Pb/ 204Pb– 206Pb/ 204Pb ages are approximately twice as old due to the short half-life of 235U relative to the age of the Earth. For the long-lived systems, the mantle isochron ages approximate the residence time of the enriched reservoir, if its mass is a few percent of the system. We propose that the first stage of melting occurs at depth in subduction zones where the mantle wedge is enriched by the addition of low-degree melts of subducted crust. The second stage of greater extents of melting occurs beneath ocean ridges. The model results suggest that the mantle is currently in quasi-steady state and that the size of the system (N-MORB plus E-MORB sources) is similar to the upper mantle. The time scale of ∼300 Ma for survival of E-MORB sources indicates rapid convective stirring and efficient reprocessing of the upper mantle by plate tectonics.