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We report on the rare earth and Nd‐Sr‐Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800‐km‐long Nd‐Sr‐Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil‐Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd‐Sr‐Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end‐members which can be interpreted as Pan‐African continental lithosphere along the Gulf of Tadjoura (a hybrid EM‐l‐EM‐2), a mantle plume (relatively young HIMU‐like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL‐like). Combined data from the Gulf of Aden‐Red Sea‐Afar‐Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical‐thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan‐African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU‐like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient st...

We report on the petrology and geochemistry of basalts dredged at 40–50 km intervals along the Galapagos Spreading Center, between 83°W and 101°W (40 stations). Emphasis is on spatial variations of ‘whole rock’ major elements, rare earths, trace metals of the first transition series, and the nature of phenocryst assemblages and their abundances. These results provide new constraints on the nature and scale of mantle source heterogeneities, melting conditions, thermal field, and dynamics of crustal formation of the region. We suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading. Basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements, which appears of worldwide extent. We reject Vogt and DeBoer's [1976] model invoking damming at fracture zones of subaxial astenosphere flow of crystal slushes and increasing fractional crystallization down the flow line, because this model would not explain the gradients in REE observed about the Galapagos Platform. Our preferred model combines the mantle‐plume binary mixing model of Schilling [1973] with the concept of recurring rift propagation proposed by Hey [1977a]. We further propose that pulsating mantle plume flux, perhaps in the form of a chain of blobs, may initiate the development of new rifts and their propagation. The present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow. A two million year period is suggested for the last 4 m.y. from Wilson and Hey's [1979] information. Rigorous testing of our preferred model is possible.

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[1] We report on 45 Pb, Hf, Nd, and Sr isotope ratios in basalt glasses from the Galápagos Spreading Center (GSC) from 101°W to 83°W, along with related parent and daughter element concentrations. The purpose is to delineate the effect of the Galápagos mantle plume on this NE migrating spreading ridge and the nature of the plume dispersion in the region. Two 1000 km-long Pb-Hf-Nd-Sr isotope mixing gradients symmetrically distributed about 91.5°W are observed along the GSC axis. The gradients are nearly radially distributed about the center of the Galápagos plume, located presumably beneath Fernandina Island (91.5°W, 0.4°S) on the Nazca plate. A simple model computation taking into account spreading rate variation suggests that the along-ridge integrated plume flux is some 15% greater west than east of the 91.5°W point of symmetry. This result is counter-intuitive considering that (1) eastward plate drag is imposed by the moving Nazca plate relative to the plume, and (2) at equal radial distance from the plume center, the GSC axis east of the 91°W FZ is offset 110 km closer to the plume and is systematically some 500 m more elevated as a result of thermal effects. The plume flow toward the east is systematically more diluted than toward the west, probably due to greater entrainment of depleted upper mantle material during the ascent and bending of the plume conduit, such as suggested by Richards and Griffiths [1989] and White et al. [1993]. The lack of clear lithosphere damming effects on the isotopic gradients at transform faults suggests that entrainment of depleted upper mantle material has occurred prior to reaching the melting zone underlying the GSC axis. The decoupled Pb-Hf-Nd-Sr isotope patterns observed along the GSC relative to those previously reported over the Galápagos platform on a smaller geographic scale further indicates a non-steady state ascent and dispersion of the Galápagos mantle plume. GSC MORB compositions in Pb, Hf, Nd, and Sr isotope space suggest the presence of two distinct components in the Galápagos mantle plume, a HIMU type and an EM1-like type, most likely representing, respectively, a recycled mix of oceanic crust -lower mantle material and recycled continental-derived material. Only 0.05 to 0.5% of the EM1 component is required if the continental-derived material is terrigenous sediment, and an order of magnitude lower if it is recycled pelagic sediment. The EM1 component in the dispersing plume along the GSC is a factor of 1.5 to 1.75 richer west of 91.5°W than east of it. The possible presence of distinct protolith units in the Galápagos mantle plume could not be ascertained in the absence of any detectable ridge-transform intersection (RTI) thermal effects on the Pb-Hf-Nd-Sr isotope variations along the GSC, and the lack of correlation of the Pb-Hf-Nd-Sr isotope ratios with major elements. If present, the protolith units must be smaller than the scale of RTI thermal effects on the ridge, and may have been eradicated by mixing during the melting, melt segregation, and storage ...