Temporal evolution of mantle sources during continental rifting: The volcanism of Djibouti (Afar)
TL;DR: In this article, the authors investigated the evolution of the mantle sources through time in the Republic of Djibouti and found that three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (87Sr/86Sr ≈ 0.706, 206Pb/204Pb ≈ 17.9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (high U/Pb ratio)-type reservoir, and (3) depleted mantle.
Abstract: Magmatism occurred almost continuously over the past 25 m.y. in the Republic of Djibouti. Lavas are mainly basic to intermediate with some rhyolites. Large chemical and isotopic variations among the volcanic series are interpreted in terms of mantle source heterogeneity. Crustal contribution is only evidenced in the oldest rhyolites emplaced during the initial stages of rifting. Excluding these old rhyolites, a clear evolution through time of the mantle sources is observed in relation to rifting. Three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (87Sr/86Sr ≈ 0.706, 206Pb/204Pb ≈ 17.9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (high U/Pb ratio)-type reservoir, and (3) a depleted mantle. As rifting goes on, there is an increasing contribution of an HIMU-type mantle source. It is attributed to the influence of a mantle diapir (Afar plume) thermally eroding the subcontinental lithosphere. The geochemical characteristics of 9 to 1 Ma old lavas, erupted after the strong increase of spreading rate in Afar, reflect this evolution of mantle sources. The influence of the mantle plume is most prominent in the northern youngest lavas (<1 Ma), particularly Manda, characterized by the strongest HIMU signature (87Sr/86Sr ≈ 0.7035, 206Pb/204Pb ≈ l9.2). The contribution of the depleted mantle component originating from the asthenosphere is best recognized in the young (<4 Ma) lavas, particularly Tadjoura and Asal lavas (3 to 1 Ma). The evolution of Djibouti lava sources through time may be accounted for by the recent models developed for plume structure.
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TL;DR: Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkal magmas and the significance of the Daly Gap occurring at local and regional scales as mentioned in this paper.
Abstract: Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr 0 70406---0 70719; Nd/Nd 0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.
393 citations
TL;DR: The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities as discussed by the authors, and the three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/εb = 4.2], moderate in the alkali basalts (La7 = 24, La/β = 9.2), and very high in the magnesian alkaline magmas (
Abstract: The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguflu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Υb = 4.2], moderate in the alkali basalts (La7 = 24, La/Υb = 9.2), and very high in the magnesian alkaline magmas (La7 = 43, La/Υb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of ∼17·9-18·6 in the tholeiites and ∼19·0-19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes. © Oxford University Press 2004; all rights reserved.
380 citations
Cites background or result from "Temporal evolution of mantle source..."
...Although the post-trap volcanism in the regions of active rifting around Addis Ababa and Djibouti has been the subject of numerous publications (e.g. Justin Visentin et al., 1974; Zanettin et al., 1978; Barrat et al., 1990; Deniel et al., 1994), little attention has been paid to the shield volcanoes....
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...…the post-trap volcanism in the regions of active rifting around Addis Ababa and Djibouti has been the subject of numerous publications (e.g. Justin Visentin et al., 1974; Zanettin et al., 1978; Barrat et al., 1990; Deniel et al., 1994), little attention has been paid to the shield volcanoes....
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...This combination corresponds quantitatively to that of the HIMU mantle component, which is reported to be present both in lavas from the Afar depression (Deniel et al., 1994) and in the mantle lithosphere adjacent to the Red Sea (Chazot & Bertrand, 1993)....
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...This combination corresponds quantitatively to that of the HIMU mantle component, which is reported to be present both in lavas from the Afar depression (Deniel et al., 1994) and in the mantle lithosphere adjacent to the Red Sea (Chazot & Bertrand, 1993). The 45---30 Ma Getra-Kele basalts in southern Ethiopia (Figs 14 and 15) have a similar composition, which was attributed by Stewart & Rogers (1996) and George & Rogers (2002) to be a characteristic of their source in the mantle lithosphere....
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TL;DR: In this article, trace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of prerift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up.
280 citations
TL;DR: In this paper, a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing 40 Ar/39 Ar age data along 6 transects is presented.
Abstract: The process of strain localization as rifting proceeds to continental breakup is readily observed along the Oligocene-Recent southern Red Sea rift, yet much of the Red Sea margin in Ethiopia remains unmapped. Rifting initiated above or near a mantle plume, which is marked by the Eo-Oligocene Ethiopia-Yemen fl ood basalt province. Objectives of this fi eld, remote sensing, and geochronology study are to establish a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing 40 Ar/ 39 Ar age data along 6 transects. We present new sketch geological maps and cross sections to document the timing of extension in relation to magmatism and its variation along strike. These new data are integrated with plate kinematic, geological, and geophysical data to present a model for evolution of the southern Red Sea margin. Faults commonly marked by eruptive centers initiated between 29 and 26 Ma, coincident with rifting in the Gulf of Aden. The Red Sea rift terminated at 10°N until linkage of the Main Ethiopian rift and southern Red Sea occurred at ca. 11 Ma. Rifting progressed in three distinct stages; each new phase saw a marked change in the style of volcanism and a narrowing of the locus of extension. Stage 1 rhyolites were emplaced from 29 to 26 Ma in basins bounded by a steep border fault system. Between 25 and 20 Ma, strain localized to narrow zones of basaltic fi ssural eruptions and minor faulting. Stage 2 faults and eruptive centers are located ~50 km to the east of the border faults, and they comprise fl ows spanning at least 16‐7 Ma. After ca. 7 Ma, the locus of strain again migrated eastward (Stage 3). Strain in Stage 3 was largely accommodated by dike injection. Plate reconstructions predict high stretching factors (β ~3) in the southern Red Sea, suggesting that Stages 2 and 3 mark the onset of formation of crust transitional between oceanic and continental.
241 citations
TL;DR: In this paper, the authors used trace element data to identify distinct geochemical groups and evaluate the role of differentiation processes in the discovery of low-Ti basalts in the Afro-Arabian CFB province.
230 citations
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5,637 citations
TL;DR: In this paper, the authors show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle.
Abstract: When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.
2,821 citations
TL;DR: The concept of crustal plate motion over mantle hotspots has been advanced to explain the origin of the Hawaiian and other island chains and the origin the Walvis, Iceland-Farroe and other aseismic ridges as discussed by the authors.
Abstract: THE concept of crustal plate motion over mantle hotspots has been advanced1 to explain the origin of the Hawaiian and other island chains and the origin of the Walvis, Iceland-Farroe and other aseismic ridges. More recently the pattern of the aseismic ridges has been used in formulating continental reconstructions2. I have shown3 that the Hawaiian-Emperor, Tuamotu-Line and Austral-Gilbert-Marshall island chains can be generated by the motion of a rigid Pacific plate rotating over three fixed hotspots. The motion deduced for the Pacific plate agrees with the palaeomagnetic studies of seamounts4. It has also been found that the relative plate motions deduced from fault strikes and spreading rates agree with the concept of rigid plates moving over fixed hotspots. Fig. 1 shows the absolute motion of the plates over the mantle, a synthesis which satisfies the relative motion data and quite accurately predicts the trends of the island chains and aseismic ridges away from hotspots.
2,277 citations
TL;DR: In this paper, the exact determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique.
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