Ethiopian Flood Basalt Research Articles

Controls on sediment distribution in a volcanically-affected basin: insights from the Ethiopian Flood Basalt Province

Abstract This study provides a comprehensive examination of the Oligocene–Miocene Ethiopian Flood Basalt Province (EFBP) within the Blue Nile (Abay) Basin (BNB), Ethiopia. Extensive logging through 29 new sections has enabled an updated stratigraphic account of the BNB to be elucidated and reveals the significance of intravolcanic sedimentary systems within the EFBP. Five sub- and intra-volcanic surfaces were modelled using spot heights to generate isopach maps that were used to constrain the thickness variations within the stratigraphy. Additionally, lineaments, Strahler stream segments and volcanic cones were mapped out from digital elevation models (SRTM DEM) and satellite data (ASTER and LANDSAT 8 imagery). These data were used in conjunction with the isopach maps to better understand the basinal controls on the distribution of intravolcanic sedimentary systems within the BNB. The study reveals how NNE and ESE/SE structural trends have influenced the distribution of volcanoes within the basin, which has in turn disrupted the distribution and lithofacies architecture of the volcanic sequences. The presence of intrabasinal ridges parallel to the major structural trends are also revealed, which formed relatively short-lived drainage divides. The implications of these intrabasinal highs on the composition and the reservoir potential of intravolcanic sedimentary successions are discussed.

Stratigraphic framework of the northeastern part of the Ethiopian flood basalt province

Stratigraphic framework of the northeastern part of the Ethiopian flood basalt province

Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics

The isotopic composition of the detrital sediment record harbours a valuable proxy for estimating the composition of the erodible upper crust since the Archaean. Refractory elements such as titanium (Ti) and zirconium (Zr) can display systematic variations in their isotopic composition as a result of magmatic differentiation. Hence, the isotopic composition of such elements in detrital sediments could potentially be used to infer the average composition (e.g., SiO2 content) of their source region, even when elemental systematics are obfuscated by weathering and diagenetic processes. A key premise of this approach is that the isotopic composition of sediments remains unbiased relative to their protolith. To what extent isotopic fractionation can occur during sedimentary processes, notably the hydrodynamic sorting of heavy mineral assemblages with contrasting isotopic compositions, remains poorly understood. We investigate the effects of such processes on the Ti and Zr isotope composition of a suite of detrital sediments from the Eastern Mediterranean Sea (EMS). These sediments are binary mixtures of two main provenance components, Saharan dust and Nile sediment, with strongly contrasting mineralogical and geochemical signatures.The EMS sediments display clear evidence for hydrodynamic sorting of zircon, expressed as a large variation in Zr/Al2O3 and deviation of εHf relative to the terrestrial εNd-εHf array. Our new data, however, do not show pronounced Zr isotope variation resulting from either hydrodynamic sorting of zircon or sediment provenance. Although this agrees with theoretical models that predict negligible equilibrium zircon-melt Zr isotope fractionation, it contrasts with recent observations suggesting that kinetic Zr isotope fractionation might be a common feature in igneous rocks. For the EMS sediments, the negligible shift in Zr isotope composition through hydrodynamic sorting means that fine-grained samples accurately reflect the composition of their source. The nearly overlapping Zr isotope compositions of Sahara- and Nile-derived sediment, however, mean that Zr isotopes, in this case, have insufficient resolution to be a useful provenance proxy.Titanium behaves differently. A small but resolvable, systematic difference in Ti isotope composition is observed between the Sahara and Nile provenance components. Samples with a strong Saharan dust signature show some Ti isotope evidence for hydrodynamic sorting of oxides in tandem with zircon, but a much stronger effect is inferred for Nile sediment. Regression of the EMS sediment samples shows that the Ti isotope composition of the Nile-derived component is strongly fractionated compared to its protolith, the Ethiopian flood basalts. Whereas Ti in Nile sediment is carried in essentially unmodified concentration, and by inference isotope composition, from its sources to the delta, large-scale hydrodynamic sorting of Fe-Ti oxides occurs in the littoral cell. This process causes a decrease in TiO2/Al2O3 of the residual fine-grained sediment fraction and shifts its Ti isotope composition to heavier compositions. The potential of such an “oxide effect” in detrital sediments has implications for crustal evolution models that use Ti isotopes as a proxy for the proportion of felsic crust and can account for the observed scatter in the shale record.

High-resolution palaeomagnetic results of Ethiopian trap series from Lima Limo section: implications for the Oligocene geomagnetic field behaviour and timing of volcanism

SUMMARY We present high-resolution palaeomagnetic and rock magnetic results from 92 successive volcanic units on a 2 km tall Lima Limo (LL) section of the Oligocene Ethiopian flood basalts (OEFB). A total of 85 well-defined flow-mean characteristic remanent magnetization directions representing seven stable magnetic polarity zones with six transitional/excursional directions were determined. These results indicate the identification of four more polarity reversals than previous palaeomagnetic studies; the resulting magnetostratigraphy has, therefore, more polarity zonation. Comparison of this result with the most recently revised geomagnetic polarity time scale (GPTS, hereafter GPTS2016), by using available radiometric age data, could not find a unique correlation. Instead, it strongly suggests the recording of at least one very brief polarity chron or putative geomagnetic excursion. Our magnetostratigraphic interpretation infers that the relative timing between the OEFB eruptions and the Oi-2 global cooling estimated two possible scenarios: one where the lowermost reverse-polarity (R1) interval for the LL section is correlated to Chron C11r of GPTS2016, same as the interpretations of earlier studies, would make a causal linkage between the OEFB eruptions and the Oi-2 cooling less possible; and, the eruption timing estimated in another scenario correlating the R1 to Chron C12r might preserve the probability of its causal influence in the Oi-2 cooling. The resulting overall mean pole for the LL section of this study, 78.2° N in latitude and 213.0° E in longitude (A95 = 3.4°, N = 79), is closer to other co-eval palaeomagnetic poles from different regions and the reference pole of Africa. Our analysis of geomagnetic palaeosecular variation (PSV) shows that, compared to the Plio-Pleistocene, the Early Oligocene has higher degrees of PSV but its slightly weaker latitude dependence.

Constraining the Magmatic Plumbing System in a Zoned Continental Flood Basalt Province

AbstractThe geographic heterogeneities in lava composition observed in continental flood basalt provinces could provide a probe of material upwelling from the deep mantle and their length scales, but their utility is limited by uncertainties in the locus of magmatism. We examine the magma plumbing system for the Oligocene Ethiopian flood basalts. The province, which exhibits domains defined by the eruption of low‐Ti (LT) and high‐Ti (HT) lavas, requires a magmatic plumbing system that facilitates the transit of compositionally distinct magmas through the crust without mixing. Here we present a geochemical and geochronological study of a suite of 43 dikes from western Ethiopia. We find that the dikes were dominantly emplaced contemporaneously with the Oligocene flood basalt phase of activity. The composition of the dikes is overwhelmingly LT in character, typified by an overall flat rare earth element pattern (median value of La/LuCN = 2.6), and a lack of enrichment in incompatible trace elements in comparison to the HT lavas. These observations confirm the western Ethiopian dike swarm as a source for the LT flood basalts in the Ethiopian flood basalt province. We also present tentative evidence for an eastward migration in the LT dike system over time. These observations are consistent with the terminal stages of the LT magmatism being centered on the Simien shield volcano. We conclude that the apparent separation of ~400 km between the LT and HT magma plumbing systems allowed for the development of a strongly geochemically zoned continental flood basalt province.

Melting the lithosphere: Metasomes as a source for mantle-derived magmas

Peridotite constitutes most of the Earth's upper mantle, and it is therefore unsurprising that most mantle-derived magmas exhibit evidence of past equilibrium with an olivine-dominated source. Although there is mounting evidence for the role of pyroxenite in magma generation within upwelling mantle plumes, a less documented non-peridotite source of melts are metasomatic veins (metasomes) within the lithospheric mantle. Here we present major and trace element analyses of 66 lavas erupted from a small Miocene shield volcano located within the Ethiopian flood basalt province. Erupted lavas are intercalated with lahars and pyroclastic horizons that are overlain by a later stage of activity manifested in small cinder cones and flows. The lavas form two distinctive petrographic and geochemical groups: (A) an olivine–phyric, low Ti group (1.7–2.7 wt.% TiO2; 4.0–13.6 wt.% MgO), which geochemically resembles most of the basalts in the region. These low Ti lavas are the only geochemical units identified in the later cinder cones and associated lava flows; (B) a clinopyroxene–phyric high Ti group (3.1–6.5 wt.% TiO2; 2.8–9.2 wt.% MgO), which resembles the Oligocene HT-2 flood basalts. This unit is found intercalated with low Ti lavas within the Miocene shield. In comparison to the low Ti group, the high Ti lavas exhibit a profound depletion in Ni, Cr, Al, and Si, and significant enrichment in Ca, Fe, V, and the most incompatible trace elements. A characteristic negative K anomaly in primitive-mantle normalized diagrams, and Na2O>K2O, suggests a source rich in amphibole, devoid of olivine, and perhaps containing some carbonate and magnetite. While melt generation during rift development in Ethiopia is strongly correlated with the thermo-chemical anomalies associated with the African Superplume, thermobaric destabilization and melting of mantle metasomes may also contribute to lithospheric thinning. In regions impacted by mantle plumes, such melts may be critical to weakening of the continental lithosphere and the development of rifts.

A major dyke swarm in the Ogaden region south of Afar and the early evolution of the Afar triple junction

Abstract Geological mapping in the Ogaden region of SE Ethiopia, integrated with aeromagnetic data, has revealed a large dyke swarm extending SSE more than 600 km from the southern Afar margin across the Somali Plate to the Ethiopia–Somalia border. 40 Ar/ 39 Ar age dating shows that emplacement occurred at 24–27 Ma, contemporaneous with early rifting and dyking in the Red Sea. Slab-pull forces generated at the Zagros subduction zone strained the lithosphere in the Afro-Arabian plate, and dyking began to extend south from the Red Sea at c. 27 Ma, extending across Afar, fed by a plume-related magma source, rather than the rift-related source prevailing along the Red Sea. Immediately south of the Afar margin, the dyke system was emplaced along the Precambrian Marda Fault Zone, and the continuation across the Ogaden may have been controlled by lithospheric weakness associated with a splay of the Marda Fault. We suggest that the Ogaden Dyke Swarm is a zone of crustal dilation continuing the Red Sea trend across the Horn of Africa and constituting the original third ‘arm’ of the Afar triple junction. Geochemical and geochronological analyses indicate that basaltic outpourings from the Ogaden Dyke Swarm flowed at least as far east as the Ethiopia–Somalia border and emanated from the same magma source as the Ethiopian flood basalts, which had erupted earlier at c. 30 Ma. Dykes are emergent only occasionally and are marked at the surface by linear sand-filled troughs varying from 2 to 20 m deep caused by tensional collapse above the dyke tip. Magnetic anomalies associated with the dykes vary in width up to 1 km and likely identify dyke zones. Supplementary material: Geological map of the Marda Fault Zone and surroundings; argon dating analytical protocol and isotopic data corrected from baseline; petrographic description of representative samples of the Ogaden Dyke Swarm and analytical protocol used for geochemistry are available at http://www.geolsoc.org.uk/SUP18829 .

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ∼31Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31Ma and 22Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22Ma to 12Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

Possible correlation of Oligocene climate changes with Ethiopian Oligocene ignimbrite eruptions

The Oligocene Ethiopian continental flood basalt province (ca. 29–31 Ma) contains significant silicic pyroclastic rocks (>60,000 km3 constituting up to 20% of the volcanic stratigraphy). Rhyolitic tephras, synchronous with the Ethiopian silicic pyroclastic rocks, are found in Indian Ocean ODP holes 711A. They are geochemically akin to the Ethiopian silicic pyroclastic rocks. This suggests that the Indian Ocean tephras originated from Ethiopian silicic eruptions and represents more distal fallout of this volcanism. The temporal coincidence of the Ethiopian flood volcanism with the Oligocene global cooling event (Oi2 ∼ 30.3 Ma) and the emplacement of the Ethiopian silicic pyroclastic eruptions on a near-global scale strongly suggest that the Ethiopian continental flood basalt province may have contributed or at least accelerated the climate change that was already underway.

Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)

Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects. 87Sr/ 86Sr and 143Nd/ 144Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. 87Sr/ 86Sr and 143Nd/ 144Nd, ranging 0.705–0.709 and 0.5124–0.5130 for Blue Nile tributaries, are 0.704–0.705 and 0.5127–0.5128 for largely volcaniclastic sediments of River Tekeze–Atbara, and 0.705–0.706 and 0.5126–0.5127 for main Nile sediments upstream Lake Nasser. Model mantle derivation ages ( t DM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4–3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4–2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6–1.3 Ga (White Nile upstream Khartoum). Instead, t DM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. T DM ages are younger for sediments shed by Ethiopian (1.2–0.7 Ga) and Eritrean basements (1.5–1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3–0.2 Ga). Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and t DM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand. The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.

Osmium isotopes and Fe/Mn ratios in Ti-rich picritic basalts from the Ethiopian flood basalt province: No evidence for core contribution to the Afar plume

Variations in the Fe/Mn ratio and Osmium isotopes in magnesium-rich mafic rocks from plume-related volcanic provinces have been exploited to imply the entrainment of core material in mantle plumes and the involvement of ancient recycled oceanic lithosphere. Here we present new major and trace element, 187Os/ 188Os ratios and precise Fe/Mn ratios on a suite of MgO-rich basalts, picritic basalts and ankaramites from the 30 Ma Ethiopian flood basalt province that shed new light on these arguments. The lavas show a range of compositions with MgO varying from 5 to 20 wt.% although the primary magma is inferred to have an MgO content of 15–16 wt.%. The lavas are also characterised by low Al 2O 3 contents (7–9 wt.% at 15 wt.% MgO), implying an origin from ∼ 150 km depth at a temperature in excess of 1600 °C, consistent with an origin in the early phases of Afar mantle plume activity. Osmium isotopes in samples with > 10 wt.% MgO are unradiogenic with 187Os/ 188Os < 0.127 while those with lower MgO define a positive correlation with Re/Os and appear to have been contaminated by crustal material. Fe/Mn ratios determined by ICP-MS vary from 65.4 to 78.5 in rocks with > 10 wt.% MgO and show greater variation to both higher and lower values in less magnesian samples. These values are high compared with MORB and Icelandic basalts, comparable with the Fe/Mn ratios of Hawaiian and other selected ocean island basalts, and are a characteristic of the primary magma. There is no evidence to suggest that Fe/Mn is fractionated during peridotite melting at low pressures less than 5 GPa, implying in agreement with previous studies that high Fe/Mn ratios are a compositional feature of the magma source region. The lack of association of radiogenic 187Os/ 188Os with the high Fe/Mn ratios of the Ethiopian picritic basalts calls into question the link to possible entrainment of core material in the source of the Afar mantle plume. Similarly, the unradiogenic 187Os/ 188Os ratios preclude a significant contribution from ancient recycled oceanic lithosphere. An alternative model is suggested in which melts generated at high pressures (> 7 GPa) during the initial turbulent ascent of the Afar plume head form pyroxene rich veins with high Fe/Mn ratios and high incompatible element contents in a peridotite matrix. These highly fertile source regions contribute significantly to melt compositions during the early phases of plume emplacement.

Geological and geochemical variations in Mid-Tertiary Ethiopian Flood Basalt Province, Maychew, Tigray Region, Ethiopia

The paper presents the results of a comprehensive major element, trace element, and Sr-Nd-Pb-Hf isotopic study of Mid-Tertiary volcanic sequences from the northwestern flood basalt province in Ethiopia. The volcanic rocks studied range in composition from basanites, alkaline basalts, and ankaramites, which form the 1st three sequences at the base associated with basaltic agglomerate (sequence 1, 2 and 3) to transitional and tholeiitic basalts and picrites confined to the upper three sequences (sequences 4, 5 and 6). Sequence 5 is bimodal with intermediate-felsic pyroclastic rocks intercalating the transitional-tholeiitic basalts. There is a good correlation of sequences with geochemical enrichments, such as an increase La/LuN with TiO2, and decrease in Al2O3 and SiO2 towards the base. The smooth increase of La/LuN ratios in lower sequences reflect the general decrease of degree of partial melting that sampled heterogeneous packages of mantle plume materials. In the Sr-Nd isotopic diagram two clusters are formed one with restricted and low- Sr- (0.70356-0.70345) and Nd- (0.51290-0.51284) isotopic compositions, defined by sequence 1, and the other cluster with relatively higher 87Sr/86Sr (0.7052-0.7036) and 143Nd/144Nd (0.51296-0.5127) isotopic compositions defined by samples from sequence 2, and most other sequences with little scatter. The Pb-isotopic compositions vary systematically within each sequence (from base to top), and each sequence clustered systematically in a different Pb–Pb– isotopic space with the highest 206Pb/204Pb (19.10-19.30) and 207Pb/204Pb (15.60-15.65) measured in sequence 1, quite differently from other sequences and from previously reported for northwestern Ethiopian flood basalt province, and the lowest 206Pb/204Pb (18.20-18.56) and 207Pb/204Pb (15.51-15.55) measured in sequence 4. Sequence 2 and 3 lavas display a similar 206Pb/204Pb and 207Pb/204Pb ranges falling between sequence 1 and 4. In contrast, sequence 6 samples displayed towards higher 206Pb/204Pb than sequence 2, 3, and 4, but with lower and higher 207Pb/204Pb than sequence 1 and 5 respectively. The lavas of sequence 1 and 4 have relatively less radiogenic 176Hf/177Hf than other sequences with slight scatter. The systematic geochemical variations in lavas are remarkable and reflects three mantle end component mixing with minimal crustal in put as a fourth component. The enriched sequence 1 has very similar geochemistry to HIMU-type ocean island basalts (OIBs), and this end member ascribed to be the most enriched Afar plume component 1, which is the most enriched Afar Plume component during initial continental break. The second enriched component is defined by sequence 2 and 3 samples and may reflect the second enriched component in Ethiopian flood basalt, previously reported for the high-Ti2 basalts assumed to be the enriched Afar Plume component, whereas the third component is the depleted component defined by sequence 4 partly overlap the range previously reported for low-Ti basalts. Keywords: Flood basalts, Geochemistry, Sr-Nd, Pb-Pb, Hf-Hf isotopes, Afar Plume, Partial melting, Crustal input

Improved method for Hf separation from silicate rocks for isotopic analysis using Ln-spec resin column

An improved chemical separation method for Hf isotope ratio measurement using both the thermal ionization mass spectrometer (TIMS) and the multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) is presented in this paper. In the first column (2.5-ml Eichrom Ln-spec resin), Hf cut was collected with 2M HCl-0.2M HF after washing the major elements, HREE, Ti, Nb, and Zr. For further Hf purification, the second column (1-ml Ln-spec resin) chemistry was conducted in a manner similar to that of the first column. The first column is designed for treatment with a 0.5-g silicate rock sample for TIMS analysis. Thus, because the Hf amount required for MC-ICP-MS analysis is much lesser than that required for TIMS analysis, the column chemistry for MC-ICP-MS analysis can be scaled down, depending on the amount of digested sample. Although there is a need to improve the TIMS technique, the TIMS Hf data obtained for geological reference rocks and Ethiopian flood basalts after the application of the proposed separation methods are consistent, within analytical error, with the previously reported data obtained using the MC-ICP-MS. The advantages of the proposed method include a reduction in the amount of reagents used (hence, a consequent reduction in the blank contribution), reduction in the time required, and a simplified preparation requiring a fewer number of acids.

Permanent groundwater storage in basaltic dyke fractures and termite mound viability

Many basaltic dykes of the Ethiopian flood basalt province are observed in the northwestern Ethiopian lowlands. In this area, the termites preferentially build their epigeous mounds on the top of dolerite dykes. The relationship between termite mounds and dykes is investigated from the analysis of their distribution along one of these dykes, of thickness 2–5 m, that we could follow over 2000 m. Termite mounds are periodically spaced (mean distance 63 m, R 2 = 0.995), and located exclusively where the topographic relief of the dyke is not more than 2 m above the surrounding area. From these observations and from the geological context, a hydrological circuit model is proposed in which (1) dykes are preferential conduits for groundwater drainage during the rainy season due to pervasive jointing, (2) during the dry season, the portion of the dyke forming a local topographic relief area dries up more quickly than the surroundings, the elevation difference between the dyke summit and the surroundings being a factor restricting termite mound development. For dyke topographic relief >2 m, drying is an obstacle for maintaining the appropriate humidity for the termite colony life. Periodic termite mound spacing is unlikely to be related to dyke or other geological properties. It is more likely related to termite population behaviour, perhaps to clay shortage, which restricts termite population growth by limiting the quantity of building material available for mound extension, and triggers exploration for a new colonization site that will be located along the dyke at a distance from the former colony that may be controlled by the extent of the zone covered by its trail pheromones. This work brings out the importance of dykes in channelling and storing groundwater in semiarid regions, and shows that dykes can store groundwater permanently in such settings even though the dry season is half the year long. It contributes also to shedding light on water supply conditions tolerated by termite populations, and factors governing termite mound distribution.

A mid miocene flood basalt event observed in the osmium isotope record of seawater

Two Fe–Mn crusts from the central Pacific and one from the central Atlantic were dated by osmium (Os) isotope stratigraphy. All three crusts show a pronounced Os isotope minimum around 12 Ma. The Os/Os decreased from 0.8 at 15 Ma to 0.7 at 12 Ma and reached a value of 0.8 again at 9 Ma indicating symmetrical shoulders of this excursion. In the modern ocean, the concentration of dissolved Os as well as the Os/Os of 1.06 are essentially homogenous, which is accurately reflected by recent marine sediments in several depositional settings. The observed negative Os isotope excursion at 12 Ma, which has not been observed in earlier studies of pelagic and metalliferous sediments, thus must have been caused by an event that changed the osmium isotope signature of global seawater. Possible interpretations for the Mid-Miocene decrease include changes in continental weathering intensity i.e. a reduction of the radiogenic Os flux, or an increase in the unradiogenic input fluxes, such as intensified weathering of ultramafic rocks, exhalations derived from large igneous flood basalts or their weathering, a meteorite impact, or a combination of several of these reasons. In the Mid-Miocene there was at least one larger meteorite impact, the Nordlinger Ries Crater and at the same time the large Columbia River igneous flood basalt province formed. Application of a simple mass balance model RSW = Rr (fr) + Ru (1 fr) where fr is the ratio of riverine flux of Os to the total flux of Os, and were subscripts sw, r and u refer to seawater, riverine and unradiogenic, respectively, shows that exhalations from a large igneous flood basalt are the most likely cause of the observed minimum. Therefore the most viable explanation of the unradiogenic Os is the eruption of the Columbia River flood basalts. Such an interpretation is also possible for the two other more pronounced minima in the Cenozoic Os isotope record of seawater that have previously been reported for the Cretaceous/Tertiary and Eocene/Oligocene boundaries. The exhalations of the Deccan Traps and the Ethiopian Flood Basalts were large enough to release sufficient amounts of unradiogenic Os into the ocean to cause the observed minima.

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-μ Volcanism in Africa

The East African Rift System is important to understanding plume-initiated rifting as manifest in the geochemistry of mafic lavas erupted along the rift throughout its evolution. We present new data from high-MgO Tertiary lavas from Turkana, northern Kenya, to investigate regional melt source components, to identify the depths and degrees of melting, and to characterize spatially and temporally the chemical structure of the underlying mantle. The Turkana area is a region of high lithospheric extension that sits between two topographic uplifts thought to be surface expressions of one or more upwelling mantle plumes. Thinning of local crust is believed to be accompanied by widespread removal of the mantle lithosphere, causing the asthenosphere to be in close contact with the overlying crust. New geochemical data on basanites, picrites and basalts (MgO >7 wt %) tightly constrain the primary melt source regions of Tertiary volcanism. Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283, 87Sr/86Sr = 0·7031–0·7036) and trace element abundances (Ce/Pb ∼30, La/Nb = 0·6–0·8 and Ba/Nb = 3–10) in these lavas are consistent with derivation from sub-lithospheric sources. Basalts and picrites erupted between ∼23 and 20 Ma have Sr–Nd–Pb–He isotopic characteristics indicative of high-μ influence, record high depths and degrees of partial melting, and are associated with rift propagation to the north and south. Accordingly, these lavas sample a source region that is geochemically distinct from that reflected both in Oligocene Ethiopian flood basalts and in the modern Afar region. The geochemical data support numerical and theoretical models as well as tomographic results providing for a complex thermal structure in the mantle beneath East Africa and are interpreted to reflect isotopically distinct plume heads beneath Tanzania and Afar that are derived from the chemically heterogeneous South African superplume.

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas 206Pb/204Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.

New paleomagnetic result from the Ethiopian flood basalts in the Abbay (Blue Nile) and Kessem gorges

New paleomagnetic investigations on the Ethiopian trap series have been undertaken at the Abbay and Kessem gorges in an attempt to better constrain the 30 Ma paleomagnetic pole of Africa. We sampled six thick massive basaltic lava flows, totaling 230 m, from Abbay Gorge and 10 lava flows, 180 m in thickness, from Kessem Gorge. Detailed paleomagnetic analyses disclosed that the carriers of the characteristic remanent magnetization (ChRM) are different in different lava flows. These are mostly titanomagnetites, titanomaghemites, and magnetite minerals with a broad range of coercive force and blocking temperatures. The heating and cooling susceptibility vs. temperature curves, many of which are irreversible, may indicate chemical remagnetization, notably low temperature maghemitization. Only one flow (KS04) with a clear 580°C Curie temperature was apparently unaffected by chemical remagnetization. The ChRM direction of this flow is identical to that in other flows, which suggests that if and when remagnetization occurred, this was shortly after emplacement of the lava flows. All of the flows sampled have normal polarity. However, a reversed component of low to medium coercive force and low to medium unblocking temperature occurs in flow KS01 at Kessem Gorge. The ChRM directions for the 16 sites are D=3.1°, I=5.8° (α95=12.7°). The paleomagnetic pole obtained from these is at λ=83.0°N, φ=193.3°E (A95=9.0°). Comparison with three previous studies of the traps shows remarkable consistency and a number of means are derived and discussed. Two final preferred poles for the traps are at λ=79.0°N, φ=196.9°E (A95=2.8°) when all 112 published flows are used, and λ=78.7°N, φ=209.4°E (A95=3.4°) when only the 76 flows from the four more recently analyzed sections are included. Both are compatible with the recent reference synthetic pole for Africa of Courtillot and Besse [J. Geophys. Res. (2002) in press]. In that sense, the Ethiopian trap pole is not anomalous and does not require more of a non-dipolar contribution than indicated by analyses of the global paleomagnetic data base covering the last few million years.

Isotopic and trace element signatures of Ethiopian flood basalts: evidence for plume–lithosphere interactions

Trace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau. Our aim was to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) basalts display a strong and variably developed lithospheric signature characterized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and Pb compared to oceanic basalts. The High-Ti magma type basalts (HT2) display much more homogeneous compositions and have ocean island basalt-like trace element signatures, whereas HT1 basalts exhibit intermediate compositions between those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variations (87Sr/86Sr = 0.70304–0.70429; 143Nd/144Nd = 0.51271–0.51298; 206Pb/204Pb = 18.00–18.86). Correlations among isotopic and trace element ratios provide evidence for the involvement of various mantle and crustal components in the petrogenesis of these flood basalts. Two distinct mantle components are involved in the genesis of the LT and HT2 extreme magma types. The HT2 basalts were derived from an ocean island basalt-like mantle component (87Sr/86Sr ∼ 0.704; 143Nd/144Nd ∼ 0.51295; 206Pb/204Pb ∼ 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a more depleted mantle component (87Sr/86Sr ∼ 0.7033; 143Nd/144Nd ∼ 0.5130; 206Pb/204Pb ∼ 18.6), either intrinsic to the plume itself or entrained in the Afar plume head during its ascent. Correlations of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature of the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. The effects of crustal contamination are much more evident in the LT basalts because of their much less enriched initial characteristics.

Palaeointensity results from Ethiopian basalts: implications for the Oligocene geomagnetic field strength

From a large collection of Ethiopian flood basalts (~30 Myr old) sampled for magnetostratigraphy, 40Ar/39Ar geochronology and geochemical investigations, 47 samples were selected in order to test their suitability for Thellier palaeointensity experiments. Only 17 samples from eight individual flows yielded reliable palaeointensity estimates, with flow-mean virtual dipole moments ranging from 3.0 to 10.5×1022 Am2. A critical review of the Oligocene palaeointensity data set, including these new Ethiopian data, indicates an Oligocene mean virtual dipole moment of 5.1±2.5×1022 Am2 for the complete data set. After applying mild selection criteria, the reduced data set yields a mean value of only 4.6±1.9×1022 Am2. This value is significantly lower than the present-day field strength but is higher than the Mesozoic dipole low mean field. This low Oligocene field might be in agreement with the high palaeosecular variation and rather high non-dipole field invoked around 30 Ma. However, the Oligocene data set is largely dependent on the palaeointensity determinations from Armenia, obtained mainly from baked contacts, which show amazingly low dispersion at both flow and between-flow levels. More data are needed to reduce the weight of these determinations on the mean value and avoid a possible bias.