Origin of enriched‐type mid‐ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N

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Abstract
The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid‐ocean ridge basalts (E‐MORB) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two‐component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched domains, whereas the recycled mantle/peridotitic residues are the most depleted matrix. On Pb‐Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume‐ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume‐ridge interactions” seen along the Mid‐Atlantic Ridge. The so‐called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic lithologies. The positive Na8‐Si8/Fe8 and negative Ca8/Al8‐Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.

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