The upper mantle in the Eifel Plume region
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New Pb, Sr, Nd, Hf, and He isotope data for Quaternary basalts, erupted from Debre Zeyit, Butajira, and the Wonji Fault Belt of the Main Ethiopian Rift, show systematic mixing relationships involving three distinct mantle sources. The Pb, Sr, Nd, and Hf isotopic arrays converge in a specific region of isotopic multi-space where they define the composition of the Afar mantle plume (centered about 206Pb/204Pb = 19·5, 87Sr/86Sr = 0·7035, εNd = +4·6, εHf = +9·3, 3He/4He > 15 RA). This plume end-member has an identical composition to that observed previously in oceanic basalts. The distinct isotopic arrays for the various volcanic areas in the Main Ethiopian Rift vary spatially in a systematic manner, and may be viewed as pseudo-binary mixing arrays. This further suggests that the Afar mantle plume interacts with the local continental lithosphere and upper mantle asthenosphere (mid-ocean ridge basalt-like source) through an ordered sequence of mixing events. Simple mixing models require that the mass proportions of continental lithosphere and upper mantle involved in magma generation must be nearly constant within each volcanic area, but that the proportion of plume material decreases regularly with distance southwestward along the Main Ethiopian Rift, away from the central axis of the plume. This systematic behavior means that continental lithosphere can become detached and mixed into the shallow mantle prior to the flow of upwelling plume material beneath the developing rift system. Detachment and mixing into the asthenosphere during continental rift evolution is an important process for producing the range of ambient upper mantle compositions sampled by mid-ocean ridge volcanism away from island hotspots.
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Abstract Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south‐western Arabian Peninsula. Very high 3 He/ 4 He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial 3 He/ 4 He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high 3 He/ 4 He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous‐like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.
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The N-Atlantic plate boundary has a WNW motion relative to the Iceland mantle plume. Evolution of the rift system in Iceland during the Quaternary is characterized by eastwards readjustment of spreading centers and fracture structures above the mantle plume. The overall effect is a stepwise eastwards motion of the plate boundary towards the plume. As inferred from volcano-tectonics, volcanic productivity and from the intensity of a regional 3He anomaly (Condomines et al., 1983) the mantle plume has its production maximum beneath a 130 km long segment of axial rift. This short rift segment (Figure 1) defines the Southern termination of the ERZ and grades into a propagating rift towards the South West (SEZ in Figure 1) away from the axial rift (Oskarsson et al., 1982). We report the composition of 375 basalts from the plume related axial rift segment including the first chemical analysis of basalts from Bhrtarbunga which is the most productive basalt volcano above the mantle plume. Three distinct compositional trends are encountered within the axial rift segment; a) Low potassium, high magnesia tholeiites at its westen (Veitiv6tnBhrtarbunga) and northern margins, b) a broad range of tholeiites with high titania and alkalic affinities on the eastern margin and c) evolved oltholeiites to qz-tholeiites at the junction of the propagating rift to the South. The compositional grouping of the basalts is related to the kinematics of the plate tectonic processes demanding interaction between the mantle derived magma and the rift zone crust and masking the chemical signature of the mantle source to a different degree. We argue, however, that one of the three compositional groups represent a plume melt with minimal contamination.
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Pleistocene to historic basalts in the northern part of the eastern volcanic zone in Iceland may have formed by partial melting over an extended depth range in the centre of the assumed Icelandic mantle plume. Practically all basalt types found on the ocean ridges are represented in this volcanic rift zone. Volume relations are, however, in favour of low degree partial melting products. The basalts differ from ocean ridge basalts in being undepleted in large trace ions, indicating a primary mantle source. The prevalence of low degree partial melting products in Iceland may explain the depleted nature of the astenosphere flowing away from the plume and along the northern part of the Mid-Atlantic Ridge. Volumes of lavas are found to correlate with degree of partial melting. Exceptions from this correlation are found locally and may be explained on the basis of volcano-tectonic implications. A simple model of thermal structure in the mantle plume-ocean ridge system is suggested which may explain some aspects of the compositional variations in basalts within the system.
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