Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11 deg 20 min N

摘要

The East Pacific Rise (EPR) at 11 deg 20 min 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, ~(87)Sr/~(86)Sr = 0.702508-0.702822, and ~(143)Nd/~(144)Nd = 0.513053-0.513215). The ~(87)Sr/~(86)Sr and ~(143)Nd/~(144)Nd 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 (TiO_2, Al_2O_3, FeO, CaO, Na_2O, and CaO/Al_2O_3) 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 deg 20 min N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high ~(87)Sr/~(86)Sr and low ~(206)Pb/~(204)Pb and ~(143)Nd/~(144)ND. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11 deg 20 min 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 Na_8-Si_8/Fe_8 and negative Ca_8/Al_8-Si_8/Fe_8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.