[1] Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These "fore-arc basalts" (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.

Linear arrays in lead isotope space for mid-ocean ridge basalts (MORBs) converge on a single end-member component that has intermediate lead, strontium, and neodymium isotope ratios compared with the total database for oceanic island basalts (OIBs) and MORBs. The MORB data are consistent with the presence of a common mantle source region for OIBs that is sampled by mantle plumes. 3He/4He ratios for MORBs show both positive and negative correlation with the 206Pb/204Pb ratios, depending on the MORB suite. These data suggest that the common mantle source is located in the transition zone region. This region contains recycled, oceanic crustal protoliths that incorporated some continental lead before their subduction during the past 300 to 2000 million years.

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...

[1] The geochemical variations in basalts from Iceland and the Reykjanes Ridge have for a long time been attributed to mixing between the depleted mid-ocean ridge basalt (MORB) asthenosphere source and the enriched Iceland mantle plume (Schilling, 1973;Hart et al., 1973;. In contrast, the occurrence of some Iceland basalts with relatively depleted incompatible trace element ratios and low Pb and Sr isotope ratios compared to normal MORB (NMORB) have been interpreted to indicate that the Iceland mantle plume transports both geochemically enriched and depleted material from the lower mantle into the upper mantle and that the depleted MORB asthenosphere source does not contribute significantly to the generation of the Iceland basalts (Thirlwall et al., 1994;Thirlwall, 1995;Hards et al., 1995;Kerr, 1995;Kerr et al., 1995;Fitton et al., 1997;Kempton et al., , 1999Nowell et al., 1998). As a follow-up to this controversy, we show on the basis of new Nb, Zr, Y, La, and Sm elemental abundances and Hf isotope ratios reported here, and published trace element and isotope data for Iceland and the North Atlantic MOR (50°-79°N), that the apparent distinction between Iceland basalts and MORB can readily be accounted for by a three-component mixing model involving two incompatible trace element enriched components, one with relatively high, the other with low 206 Pb/ 204 Pb ratios, and the usual surrounding depleted MORB mantle source as the third component. The radiogenic 206 Pb-rich component represents the hot Iceland mantle plume source, while the enriched but low-206 Pb/ 204 Pb EM1-like component most likely represents entrained subcontinental lithospheric material embedded in the North Atlantic depleted MORB source. The incompatible element and isotope ratio variations do not require a depleted Iceland plume (DIP) component, nor do they exclude the presence of a MORB depleted asthenosphere source component for some of the Iceland basalts, particularly in recent time.

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