Determining the flow of magma and solid mantle beneath midocean ridges is crucial for understanding the dynamics of plate spreading and the formation of new oceanic crust. Theoretical models suggest a range of possible now regimes-from passive, plate-driven flows(1,2) to 'active' buoyantly driven solid convection(3-7)-and have spurred an ambitious field programme to attempt to distinguish these flow fields using geophysical techniques(8). Models that explore the geochemical consequences of melt transport(9), however, suggest that these different flow fields can also have distinctive geochemical signatures. Here we compare model predictions to the chemistry of well located and closely sampled basalts from across the ridge-crest of the fast-spreading East Pacific Rise at 12 degrees N (refs 10-12). These data show features that are not explained by traditional geochemical models of ocean-ridge magma generation, yet are consistent with the geochemical consequences of the new transport models that have passive mantle flow and convergent lateral melt migration. These results are also consistent with those of the seismic MELT experiment(8), but add new information about the relative flow of melt and solid in the mantle which is probably unmeasurable by geophysical techniques.
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