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Journal ArticleDOI

A LREE-depleted component in the Afar plume; further evidence from Quaternary Djibouti basalts

01 Feb 2010-Lithos (Elsevier)-Vol. 114, Iss: 3, pp 327-336

AbstractMajor, trace element and isotopic (Sr, Nd, Pb) data and unspiked K–Ar ages are presented for Quaternary (0.90–0.95 Ma old) basalts from the Hayyabley volcano, Djibouti. These basalts are LREE-depleted (Lan/Smn = 0.76–0.83), with 87Sr/86Sr ratios ranging from 0.70369 to 0.70376, and rather homogeneous 143Nd/144Nd (eNd = + 5.9–+ 7.3) and Pb isotopic compositions (206Pb/204Pb = 18.47–18.55, 207Pb/204Pb = 15.52–15.57, 208Pb/204Pb = 38.62–38.77). They are very different from the underlying enriched Tadjoura Gulf basalts, and from the N-MORB erupted from the nascent oceanic ridges of the Red Sea and Gulf of Aden. Their compositions closely resemble those of (1) depleted Quaternary Manda Hararo basalts from the Afar depression in Ethiopia and (2) one Oligocene basalt from the Ethiopian Plateau trap series. Their trace element and Sr, Nd, Pb isotope systematics suggest the involvement of a discrete but minor LREE-depleted component, which is probably an intrinsic part of the Afar plume

Topics: Basalt (51%), Trace element (51%)

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Journal ArticleDOI
Abstract: The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher 87Sr/86Sr and lower 206Pb/204Pb ratios than the southern Fatu Hiva group at any given 143Nd/144Nd value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or "plumelets" that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other "weak" Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Reunion Island, and the higher degrees of melting blur the isotopic variability of the plume source.

51 citations


Cites background from "A LREE-depleted component in the Af..."

  • ...…Thirlwall et al., 2004; Kokfelt et al., 2006], the Galapagos [White et al., 1993;Hoernle et al., 2000; Blichert-Toft and White, 2001; Saal et al., 2007], Afar [Furman et al., 2006; Beccaluva et al., 2009; Daoud et al., 2010] and Hawaii [Yang et al., 2003; Frey et al., 2005; Fekiacova et al., 2007]....

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Journal ArticleDOI
Abstract: The Tadjoura rift forms the westernmost edge of the westerly-propagating Sheba ridge, between Arabia a nd Somalia, as it enters into the Afar depression. Fro m structural and remote sensing dataset, the Tadjou ra rift is interpreted as an asymmetrical south-facing half-gr aben, about 40 km-wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Ma-old Gulf Basalts which onlapped the older S omali Basalts along its shallower southern flexural margin. The m ajor and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basa ltic types, namely the Gulf, Somali, Goumarre and H ayyabley Basalts. These results, together with radiometric a ge data, lead us to propose a revised volcano-strat igraphic sketch of the two exposed Tadjoura rift margins, an d to discriminate and date several distinct fault n etworks of this oblique rift. Morphological and statistical an alyses of onshore extensional fault populations sho w marked changes in structural styles along-strike, in a dir ection parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagati on against pre-existing discontinuities in the NS-o riented Arta transverse zone. According to our model, the sinist ral jump of rifting into the Asal-Ghoubbet rift seg ment results from structural inheritance, in contrast with the en echelon or transform mechanism of propagation that prevail ed along the entire length of the Gulf of Aden extensi onal system.

42 citations


Journal ArticleDOI

39 citations


Journal ArticleDOI
01 Feb 2014-Lithos
Abstract: This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta 'Ale complexes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene–Oligocene. The earlier NW–SE direction of extension of the Rift, which generated NE–SW trending faults, rotated around E–W in Quaternary times, and produced the still active N to N–NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100 km SE of Addis Ababa. It is characterized by a wide central caldera, about 8 km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta 'Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterized by a summit caldera of which the diameter is about 4 km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice-fallout deposits. The only mafic unit is represented by a basaltic eruption that occurred in 1870 AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta 'Ale in this part of the Main Ethiopian Rift. New geochemical and Sr–Nd–Pb isotope data on representative samples from Gedemsa and Fanta 'Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic–felsic bi-modal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift as well as on the plateau. Geochemical data and modeling suggest that magmas evolved mainly through fractional crystallization processes, accounting for the entire mafic–felsic compositional variation. However, Sr–Nd–Pb isotope data reveal also open-system evolution processes. The most differentiated, Sr-poor rhyolites suffered important low temperature contamination by shallow fluids of hydrothermal and/or meteoric origin. This affected mostly the Sr isotopic composition of whole-rocks, and much less that of separated feldspars that provide more reliable 87Sr/86Sr values. Mafic rocks, as well as the least contaminated felsic rocks, provide evidence for two components involved in the genesis and evolution of mafic magmas: a mantle component, carrying the isotopic composition of the Afar plume, and a crustal component, likely Pan-African sialic lower crust, that might have been added in small amounts, about 2%, to mafic magmas. The origin of the primary magmas is inferred to have occurred by 7% partial melting of a mixed source region including both depleted and enriched mantle components.

34 citations


Journal ArticleDOI
Abstract: Major-element, lithophile trace element, and Sm^Nd and U^Pb zircon isotopic data are presented for Palaeoproterozoic mid-ocean ridge basalt (MORB)-type tholeiitic dikes ranging in age from 2140 3 to 2126 5 Ma studied at six localities within three terranes in the Karelian Craton, eastern Fennoscandian Shield. All the studied dikes have remarkably uniform geochemical and isotope characteristics.They are tholeiitic basalts with low contents of large ion lithophile elements, high field strength elements, and rare earth elements (REE), nearly flat chondrite-normalized REE patterns [(La/Sm)n1⁄4 0·9^1·2, (Gd/Yb)n1⁄41·0^1·2], and positive Ti, Nb, and Zr anomalies in the primitive mantle-normalized diagrams. The dikes also show relatively uniform initial Nd isotope compositions, with eNd values ranging fromþ1·4 toþ3·0, despite the occurrence of these dikes within Archaean terranes with different crustal history. According to the results of U^Pb (zircon) and Sm^Nd internal isochron dating the crystallization age of the dikes is constrained to be c. 2·14 Ga. The studied MORB-type tholeiitic dikes are probably comagmatic with Palaeoproterozoic MORB-type basalts that have previously been recognized in the Karelian Craton, and might represent relicts of their magma feeder system.The uniformity of ages and geochemical and isotope characteristics of the MORB-type dikes and volcanic rocks suggest that they are probably related to a common magmatic event.This event was nearcontemporaneous with the eruption of high-Ti plume-related basalts and intrusion of dikes in the c. 2·1 Ga Jatulian continental flood basalt province. Geochemical modelling indicates that the chemical and isotopic compositions of the dikes are best explained by derivation of their parental magmas by partial melting of a uniformly depleted mantle source in the spinel peridotite stability field, followed by fractional crystallization and minor (56%) assimilation of continental crustal material. This suggests that magma-storage processes in upper crustal chambers were very short-lived; this could be the

31 citations


Cites background from "A LREE-depleted component in the Af..."

  • ...Analogues of the Karelian Palaeoproterozoic continental MORB-type tholeiities have been recognized in the North Atlantic and Afar continental flood basalt provinces, where depleted MORB-type tholeiitic basalts are shown to have formed simultaneously with high-Ti basalts (Barrat et al., 2003; Daoud et al., 2010; S ager & Holm, 2011;Waight & Baker, 2012)....

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  • ..., 2012), and dikes (this study) compared with low-Ti tholeiites from the Faroe Islands (S ager & Holm, 2011), depleted basalts from Central East Greenland (Waight & Baker, 2012), LREE-depleted basalts from Central Afar (Barrat et al., 2003; Daoud et al., 2010), Kolbeinsey Ridge and Reykjanes Ridge basalts (PetDB), and global N-MORB (PetDB)....

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  • ...MORB-type continental tholeiitic basalts are thought to have formed as a result of melting of depleted mantle components in mantle-plume source regions (Daoud et al., 2010; S ager & Holm, 2011; Waight & Baker, 2012)....

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  • ..., 1990; Perttunen & Hanski, 2003); Fe^Ti basalts and low-Ti basalts, Faroe Islands (S ager & Holm, 2011); Fe^Ti basalts and depleted basalts, Central East Greenland (Waight & Baker, 2012); LREE-depleted basalts, Central Afar (Barrat et al., 2003; Daoud et al., 2010)....

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References
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01 Jan 1989
Abstract: Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

17,505 citations


"A LREE-depleted component in the Af..." refers background in this paper

  • ...The primitive mantle values are from Sun and McDonough (1989)....

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  • ...Major oxides in wt%, trace elements in ppm. n denotes 849 ratios normalized to the primitive mantle composition from Sun and McDonough (1989)....

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  • ...Major oxides in wt%, trace elements in ppm. n denotes 4 ratios normalized to the primitive mantle composition from Sun and McDonough (1989).5 6 207Pb/204Pb and 208Pb/204Pb ratios with respect to the Northern Hemisphere Reference 3 Table 4 Click here to download Table: Hayy_Table 4nb.doc...

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  • ...Moreover, they show a limited range of Ce/Pb ratios from 24 to 350 28, similar to values measured in oceanic basalts (e.g., Sun and McDonough, 1989)....

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Journal ArticleDOI
Abstract: On August 24, 1976 the IUGS Subcommission on Geochronology (FOOTNOTE 4) met in Sydney, Australia, during the 25th International Geological Congress. They unanimously agreed to recommend the adoption of a standard set of decay constants and isotopic abundances in isotope geology. Values have been selected, based on current information and usage, to provide for uniform international use in published communications. The Subcommission urges that all isotopic data be reported using the recommended values (see appendix). The recommendation represents a convention for the sole purpose of achieving interlaboratory standardization. The Subcommission does not intend to endorse specific methods of investigation or to specifically select the works of individual authors, institutions, or publications. All selected values are open to and should be the subjects of continuing critical scrutinizing and laboratory investigation. Recommendations will be reviewed by the Subcommission from time to time to bring the adopted conventional values in line with significant new research data.

9,109 citations


"A LREE-depleted component in the Af..." refers methods in this paper

  • ...87 Ages were calculated using the constants recommended by Steiger and Jäger (1977)....

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Journal ArticleDOI
28 Jun 1984-Nature
Abstract: Basalts from many Southern Hemisphere regions have anomalous Sr and Pb isotopic characteristics. This article shows that the isotopic mantle anomaly is globe-encircling in extent, centred on latitude 30° S. Arguments suggesting that this mantle anomaly has been in existence for billions of years place severe constraints on mantle convection models.

2,139 citations


"A LREE-depleted component in the Af..." refers background in this paper

  • ...Δ7/4 and Δ 8/4 denote the deviation (in 853 0 /00) of 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios with respect to the Northern Hemisphere Reference 854 Line (NHRL: Hart, 1984, 1988)....

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  • ...1600 km east of Hayyabley volcano (Hart, 1984) but do extend away from the Indian Ocean MORB toward a more HIMU composition....

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  • ...The Hayyabley basalts display almost uniform Pb isotopic 285 compositions ( 206 Pb/ 204 Pb= 18.47-18.55, 207 Pb/ 204 Pb= 15.52-15.57, 208 Pb/ 204 Pb= 38.62-38.77) 286 well above the NHRL (Hart, 1984, 1988; see Table 4)....

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  • ...These submarine basalts do not have the unradiogenic Pb isotopes of the 390 Carslberg Ridge ca. 1600 km east of Hayyabley volcano (Hart, 1984) but do extend away 391 from the Indian Ocean MORB toward a more HIMU composition....

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Journal ArticleDOI
Abstract: Fifteen chondrites, including eight carbonaceous chondrites, were analyzed for rare earth element abundances by isotope dilution. Examination of REE for a large number of individual chondrites shows that only a small proportion of the analyses have flat unfractionated REE patterns within experimental error. While some of the remaining analyses are consistent with magmatic fractionation, many patterns, in particular those with positive Ce anomalies, can not be explained by known magmatic processes. Elemental abundance anomalies are found in all major chondrite classes. The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibrium or were introduced at a late stage of chondrite formation. Large-scale segregation of gas and condensate is implied, and bulk variations in REE abundances between planetary bodies is possible.

1,291 citations


Additional excerpts

  • ...The 805 reference chondrite is from Evensen et al. (1978)....

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