The Ethiopian subcontinental mantle domains: geochemical evidence from Cenozoic mafic lavas

Summary Since the Cenozoic, Ethiopia was affected by a widespread volcanic activity related to the geodynamic evolution of the Afar triple junction. The plateau building phase was followed by the formation of the Main Ethiopian Rift (MER) accompanied by a bimodal volcanic activity in both the inner parts of the rift and its shoulders. Outside the rift, a concurrent volcanic activity occurred mainly along transversal tectonic lineaments, the most important of which is the Yerer-Tullu Wellel Volcano-Tectonic Lineament (YTVL) developing for ∼500 km westward of Addis Abeba. Scattered Pliocene – Quaternary volcanoes are reported also inside the plateau such as those out cropping nearby Lake Tana. Here we present the result of a study on carefully screened mafic lavas outcropping in two sectors located off-axis the MER, namely, the YTVL and the southern part of Lake Tana; and in one sector located in the southern tip of the MER close to Megado, in the Sidamo region. The screened samples are petrographically fresh and have SiO₂<52 wt.% and MgO>4 wt.%, to minimise crystal fractionation effects. Most of the samples belong to the Late Miocene – Quaternary volcanic activity of the East African Rift System (EARS), although a number of samples along the YTVL are representative of the Late Eocene – Early Miocene Ethiopian Volcanic Plateau flood basalts. The selected mafic lavas offer the opportunity to assess the geochemical diversity, if any, of the subcontinental mantle domains along the MER (Megado and the easternmost part of the YTVL) and in sectors far away from the MER (YTVL and Lake Tana). The samples have a wide compositional range: from basanite to alkali basalt, hy-normative basalt, qz-normative basalt, basaltic andesite, hawaiite, trachybasalt, and trachyandesite. The major and trace element characteristics of the mafic lavas demonstrate an origin from a relatively fertile and trace element enriched lithospheric mantle at pressure variable from ∼2.0 to 3.5 GPa. Moreover, systematic variations in K/Nb, Ba/Nb, and Ba/Rb demand for the contribution of trace amounts of phlogopite to melt production. The geochemical signature coupled with the geographical distribution of the Late Miocene – Quaternary samples along the YTVL (∼500 km) and the Lake Tana and Megado sectors set constraints on a relatively homogenous lateral continuity of the deeper lithospheric mantle domains (∼2–3.5 GPa). On the other hand, the trace element characteristics of the Ethiopian Volcanic Plateau samples along the YTVL, demand for a chromatographic process en route to the surface and indicate a shallower lithospheric mantle domain (<2 GPa) with a different geochemical signature. Overall, the selected mafic lavas provide evidence for vertically zoned lithospheric mantle domains: the shallower domain (<2 GPa) consists of an enriched mantle component with a geochemical signature similar to continental crust material (EM II), whilst the deeper domain (∼2–3.5 GPa) consists of an enriched component similar to the average composition of the subcontinental lithospheric mantle (SCLM). Supplementary material to this paper is available in electronic form at Appendix available as electronic supplementary material     

Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift

The lithospheric and sublithospheric processes associated with the transition from continental to oceanic magmatism during continental rifting are poorly understood, but may be investigated in the central Main Ethiopian Rift (MER) using Quaternary xenolith-bearing basalts. Explosive eruptions in the Debre Zeyit (Bishoftu) and Butajira regions, offset 20 km to the west of the contemporaneous main rift axis, host Al-augite, norite and lherzolite xenoliths, xenocrysts and megacrysts. Al-augite xenoliths and megacrysts derived from pressures up to 10 kb are the dominant inclusion in these recent basalts, which were generated as small degree partial melts of fertile peridotite between 15 and 25 kb. Neither the xenoliths nor the host basalts exhibit signs of carbonatitic or hydrous (amphibole + phlogopite) metasomatism, suggesting that infiltration of silicate melts resulting in pervasive Al-augite dyking/veining dominates the regional lithospheric mantle. Recent geophysical evidence has indicated that such veining/dyking is pervasive and segmented, supporting the connection of these Al-augite dykes/veins to the formation of a proto ridge axis. Al-augite xenoliths and megacrysts have been reported in other continental rift settings, suggesting that silicate melt metasomatism resulting in Al-augite dykes/veins is a fundamental processes attendant to continental rift development.     

Formal education and disaster response of children: evidence from coastal villages in Indonesia

Natural Hazards - Although children are exposed to a high mortality risk during disasters, what determines their disaster response, especially during earthquakes, remains largely unexplored. The...     

Remittances economy,remittances landscape: an analysis of the economic and socioecological implications of remittances to households in South Eastern Nigeria

This study evaluated the level of household dependence on remittances, its uses and linkages with landscape/environmental conditions in South Eastern Nigeria. The study area is a landscape unit defined by the Idemili River Basin of Anambra State. The concept of socio-ecological systems provided the basis for an integrated assessment of remittances environment linkages using primary data from a survey of 840 households in 25 localities in the basin and landscape data from a Nigerias at 32 m resolution image dataset. Based on this, linear and non linear regression modelling of locality level relationships between four indicators of household remittances (HR) and a key landscape variable (vegetation cover density-VCD) was implemented. About 45 % of the households had access to remittances; 10 % of receiving households depended on it as their main income source; and over 87 % used the receipts mainly to meet basic household needs. Only 4 % of the households used it in activities that directly affected the vegetation cover. The linear and non linear relationships between the indicators of HR and VCD were generally weak (R² < 0.250). However, the pattern of relationship was different for various indicators of HR. It was inverse for locality aggregate (total) and average HR and positive for level of household access to remittances/locality. This illustrates the complex relationship between remittances and landscape variables in the basin. The study shows that the impact of remittances on households and the environment in the region is both positive (through the alleviation of poverty and land pressure) and negative (through reduction of vegetation cover density).     

Interplay between shoreline migration paths, architecture and pinchout distance for siliciclastic shoreline tongues: evidence from the rock record

Facies, geometry and key internal stratigraphic surfaces from eight Cretaceous and Eocene clastic shoreline tongues have been documented. The regressive parts of all the studied tongues represent storm‐wave influenced strandplains, deltas or fan‐deltas, and the regressive shoreline trajectories varied from descending to ascending. The transgressive parts of the tongues are dominated by either estuarine or coastal‐plain deposits. The distance from the coeval, up‐dip non‐marine deposits to the basinward pinchout of amalgamated shoreface sandstones, measured along depositional dip, is here termed the sand pinchout distance. The study shows that the angle of regressive‐to‐transgressive turnaround (defined by the angle between the regressive and subsequent transgressive shoreline trajectories) and the process regime during turnaround largely control the sand‐pinchout distance. The amount of transgressive erosion can also partly control the pinchout distance, but this parameter was comparable for the different examples presented here. If the type of depositional system at turnaround and the depth of transgressive erosion are constant, small angles of turnaround are associated with large pinchout distances, whereas larger angles of turnaround result in smaller pinchout distances. The model developed allows sand‐pinchout distance to be predicted, using data for the landward parts of shoreline tongues. The dataset also shows that steeply rising (aggrading) shoreline trajectories tend to produce more heterolithic sandstone tongues than those formed by lower‐angle trajectories.     

松辽盆地钱家店砂岩型铀矿床油气运移与铀成矿的关系:来自流体包裹体的证据

为探究油气运移与铀成矿的关系,笔者以松辽盆地钱家店砂岩型铀矿床为研究对象,系统开展了含矿砂岩薄片鉴定、电子探针研究以及流体包裹体岩相学和显微测温研究.结果表明:①研究区强矿化段砂岩硅质胶结物含量较多,为隐晶质玉髓胶结物;成岩演化序列为,泥晶方解石、自生黏土膜,石英加大边、石英加大边外围黏土膜、颗粒间压实的隐晶质玉髓胶结...     

Ridge axis deformation and coeval melt migration within layer 3 gabbros: evidence from the Lizard Complex,U.K.

 Ridge-parallel extensional shear zones within the layer 3 gabbros of the Lizard ophiolite have recorded varied and numerous chemical and mineralogical changes under a variety of P-T conditions. These changes accompany tectonic exhumation of lower crustal levels by a listric fault mechanism during amagmatic extension at a slow-spreading centre. Deformation within the mid-crustal level of layer three resulted in the migration and impregnation of an evolved iron-titanium-rich silica-poor melt along the shear zones. This syntectonic infiltration occurred along the lowermost portions of listric faults which rooted in a partially molten zone beneath the ridge axis. Residual melt was tapped from this underlying zone and tectonically redistributed about deforming gabbros situated immediately above the partially molten zone. The occurrence of sub-solidus synkinematic assemblages within the shear zones, which were generated at various temperatures and water : rock ratios, is explained in terms of their simultaneous development on listric fault planes in the presence of a hydrothermal fluid phase. The sequential overprinting of pre-existing assemblages arises from repeated block rotation during amagmatic episodes of extension within an oceanic slow-spreading environment. Received: 8 February 1994/Accepted: 4 April 1995     

A comparative analysis of trace metal pollution parity between sandy and shaly soils: evidence from two mechanic villages in the Imo River basin

Heavy metal enrichment parity in sandy and shaly mechanic village (MV) soils was studied in order to explain the technical and economic imperatives of setting an environmentally friendly MV on either soil. Okigwe (shaly) and Nekede (sandy) mechanic village soils in the Imo River basin Nigeria were tested and compared. Spectroscopic analysis of the soil samples collected from three surface layers (L), where L1: 0–10 cm; L2: 10–20 cm and L3: 90–100 cm shows that Pb > Cu > Mn in the Okigwe MV, and Mn > Pb > Cu in the Nekede MV. Mean concentration of metals (mg kg⁻¹) in Okigwe MV are Fe: 51,291 ± 18,148, Ni: 22 ± 4, Cd: 20 ± 3, Pb: 500 ± 513, Cu: 616 ± 369, Cr: 16 ± 9, and Mn: 378 ± 207. Similarly, Nekede MV has 22,101 ± 7,273 of Fe; 8 ± 0.8 of Ni; 11 ± 4 of Cd; 320 ± 122 of Pb; 265 ± 145 of Cu; 11 ± 2 of Cr; and 350 ± 191 of Mn. Pollution factor (Pf): Okigwe MV has 0.77, and Nekede has 0.68. Pollution degree: Okigwe MV is greater in L1 and L2, while Nekede is greater in L3, with greater potentials for Pb, and Mn mobility than the Okigwe. Both have similar trends of metal distribution, and significant correlation with their background values. Low clay-silt content in Nekede MV soil suggests low sorption capacity, whereas the high clay-silt content (47–64%) of the Okigwe soil suggests lower bioavailability. Infiltration basin is not recommended in a MV on sandy soil if water table is near surface. In such case, the MV must be moved to a location where water table is >37 m, or have clay-shale material transported to the site to form impervious layer base for detention basin. For groundwater safety and sustainability, shaly soils are most recommended for MVs so that detention basin could be economically used for storm water treatment.     

The agonies of displacement: Ethiopian women refugees in Khartoum,Sudan

Conclusion For the overwhelming majority of the Ethiopian refugees in urban Sudan, there seems to be no end to their plight. Their lives are in limbo. When they left their homeland most thought thei exile would be temporary. They sought temporary refuge only until the conditions at home improved and they might return in safety. Unfortunately, the conditions for their repatriation have never developed. Armed conflicts in Ethiopia still continue to rage unabated. the regime in power also continues to abuse fundamental human rights.Many of the refugees also thought they would have peace and security and, perhaps, a decent livelihood in their asylum home. These have not materialized either. The Sudan, faced with numerous problems, including civil war, drought, famine, huge external debt (in excess of $12 billion), open unemployment, and pervasive underemployment, is not even able to cope adequately with the basic needs of its own citizens, let along that of a refugee population of over a million. Worse, refugees are blamed for every problem from bread and water shortages to skyrocketing rents and have become convenient targets for harassment by the frustrated public.Others hoped for resettlement in another country. However, for the vast majority of refugees, resettlement opportunities elsewhere are virtually impossible to attin. The industrial countried are unwilling to grant asylum to large numbers of Third World refugees. These countries content that most Third World refugees are not victims of political persecution but simply migrants in search of better economoc opportunities. The few refugees to whom these countries are willing to grant asylum are those with the best qualifications: the young, the educated, and the highly skilled.As long as the current status quo in Ethiopia remains unaltered, voluntary repatriation is highly unlikelt. On the contrary, one sees more and more evidence in the country of continued uprooting of masses of people from their homes and land. It is sad that, goven the immense contradictions in the country, the possibility of negotiated solutions to the wars and a return to democracy is nowhere in sight. The plight of hundred of thousands of innocent women and men will not be solved anytime in the immediate future of them to return home in safety, peace, and dignity. For most of these refugees the future is uncertain, probably unpleasant, possibly bleak.     

Stable isotope evidence for regional-scale fluid migration in a Barrovian metamorphic terrane,Vermont, USA

We measured the C- and O-isotopic composition of carbonate minerals in the Waits River Formation, eastern Vermont, to determine the extent of fluid infiltration during regional metamorphism over an approx. 2000 km² area in a deep-seated (>25 km) Barrovian terrane. From a petrologic study of this terrane, Ferry proposed the existence of a large regional metamorphic hydrothermal system with two first-order features: (1) on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent; (2) on a smaller scale (about 100 km²) flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms. We find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons. The evidence for hydrothermal activity around the plutons is large heavy isotope depletions, up to 6–9 in ¹⁸O_carb and ¹³C_carb, in diopside zone rocks adjacent to the plutons. These isotopic shifts are greater than can be explained solely by prograde metamorphic reactions. We find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms. First, ¹⁸O_carb and ¹³C_carb, within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes, indicating that the rocks equilibrated with a permeating fluid. Second, there are depletions in ¹⁸O near the margins of the Waits River Formation which can be interpreted as a dispersed, advective infiltration front displaced toward the antiform axes. These fronts were modelled using Eq. 13 of Bickle and Baker and imply time-integrated fluid of 10⁵–10⁶ cm³/cm², which are consistent with values derived by Ferry from measured progress of prograde devolatilization reactions.     

Obsidian from the northern sector of the Main Ethiopian Rift: implications for archeology

Obsidian is abundant in the Main Ethiopian Rift (MER). Petrological and geochemical features of obsidian from four volcanic centers in the MER, namely Birenti, Dofen, Fentale and Kone, are presented. Compositional and petrological variability is noted among the Dofen and Fentale obsidian, but not in those from Kone and Birenti where each have separate but uniform elemental composition. The Fentale and Kone obsidian were source materials for the artifacts of a number of Middle Stone Age and Later Stone Age/Neolithic sites in the region. We have yet to determine whether Dofen and Birenti were sources for archeological artifacts. The study also shows that volcanic episodes from a single center do not necessarily result in compositional variability.     

Subarctic Front migration at the Reykjanes Ridge during the mid‐ to late Holocene: evidence from planktic foraminifera

Expansion of fresh and sea‐ice loaded surface waters from the Arctic Ocean into the sub‐polar North Atlantic is suggested to modulate the northward heat transport within the North Atlantic Current (NAC). The Reykjanes Ridge south of Iceland is a suitable area to reconstruct changes in the mid‐ to late Holocene fresh and sea‐ice loaded surface water expansion, which is marked by the Subarctic Front (SAF). Here, shifts in the location of the SAF result from the interaction of freshwater expansion and inflow of warmer and saline (NAC) waters to the Ridge. Using planktic foraminiferal assemblage and concentration data from a marine sediment core on the eastern Reykjanes Ridge elucidates SAF location changes and thus, changes in the water‐mass composition (upper ˜200 m) during the last c. 5.8 ka BP. Our foraminifer data highlight a late Holocene shift (at c. 3.0 ka BP) in water‐mass composition at the Reykjanes Ridge, which reflects the occurrence of cooler and fresher surface waters when compared to the mid‐Holocene. We document two phases of SAF presence at the study site: from (i) c. 5.5 to 5.0 ka BP and (ii) c. 2.7 to 1.5 ka BP. Both phases are characterized by marked increases in the planktic foraminiferal concentration, which coincides with freshwater expansions and warm subsurface water conditions within the sub‐polar North Atlantic. We link the SAF changes, from c. 2.7 to 1.5 ka BP, to a strengthening of the East Greenland Current and a warming in the NAC, as identified by various studies underlying these two currents. From c. 1.5 ka BP onwards, we record a prominent subsurface cooling and continued occurrence of fresh and sea‐ice loaded surface waters at the study site. This implies that the SAF migrated to the southeast of our core site during the last millennium.     

A geochemical survey of spring water from the main Ethiopian rift valley, southern Ethiopia: implications for well-head protection

The report discusses the stable isotope values and major solute compositions of 16 springs and river-water samples along a topographic gradient in the main rift valley of southern Ethiopia. Most of the springs used for drinking water supplies discharge from local flow systems at scales of only tens of meters. The δ¹⁸O and δD values of waters unaffected by shallow evaporation form a local meteoric water line of δD=8.1δ¹⁸O+19.0, almost identical to that for the eastern Mediterranean area. The δ¹⁸O values show an altitude effect of –0.5‰ per 100-m elevation rise. Total dissolved nitrogen concentrations locally exceed 6 mg/L (as N), and phosphate concentrations were elevated above background levels in some springs, exceeding 0.2 mg/L PO₄, probably due to agricultural practices upgradient of the springs. Modest well-head protection should be considered to protect public health from the effects of pollution by agricultural waste, given the very local scale of the flow systems providing spring water to Ethiopian villages. Electronic Publication     

Calcite-dolomite thermometry: Negative evidence from the Marmorilik Formation,West Greenland

Calcite grains from 19 composite dolomite-calcite marble samples from the Marmorilik Formation, West Greenland, have been analysed for Ca, Mg, and Fe with the microprobe in order to estimate metamorphic temperatures using the calcite-dolomite solvus geothermometer. The results show that there are large variations within single samples, and, insofar as any thermal gradient can be detected, it is the opposite of that indicated by mineral parageneses. Current practice in the application of the method is therefore questioned.     

Risk,perception and adaptation to climate change: evidence from arid region,India

The paper contributes to the growing literature highlighting the significance of assessing risk and vulnerability, micro-level perceptions and adaptation decision-making in building resilience of farm communities to climate change in dryland region of India. To select a region for grassroots enquiry, spatial differential in risk to climate change was assessed in Rajasthan, using IPCC AR5 framework. Among the highly vulnerable and risky districts, Bikaner district was selected for elicitation of micro-level imperatives. Rising atmospheric temperature, inter-seasonal displacements of rainfall and recurrence of extreme events were perceived by the farmers resulting in resource degradation, production risks and erosion of households’ socio-economic dynamics. As risk preventive measures, suitable adjustment in agricultural practices, natural resource management, shift to off-farm activities and other relief measures were adopted by the farmers. Farmer’s choice of adaptation was influenced by several climatic, socio-economic and infrastructural & institutional factors in varying degree. Moreover, several financial, economic, infrastructural and informational bottlenecks to adaptations were reported during household survey and FGDs. The results suggest that capturing grassroots evidence is crucial for directing locally tailored adaptation strategies, along with the improving deficiencies in the developmental pathways for climate-resilient agriculture.

     

Together apart: Migration,integration and spatialised identities in South African border villages

This article explores spatialised identity construction as part of the process of refugee and migrant integration. It uses an empirical case study – of villages in a rural border area of South Africa – to argue that identity groups can be constructed in relation to micro-spaces within a single village, refer to identity characteristics which are largely independent of cross-border mobility or territorial origin, and be negotiated through micro-mobilities within different segments of a ‘local’ space. This stands in contrast to debates opposing sedentary ‘roots’ or transnational or transient ‘routes’ as identity forming spaces. Establishing the relevant spatial aspects of identity construction is an empirical matter, rather than an ideological one.     

Diamondiferous eclogites from Yakutia,Siberia: evidence for a diversity of protoliths

Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Neal 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are inconsistent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application to Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However, reconstructed REE compositions using various garnet: clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (either directly or through partial melting) as normal or Type-I ocean floor basalt. Others, however, require material of eclogitic or pyroxenitic affinities to undergo partial melting; this facilitates the depletion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of anomalous or Type II ocean floor basalt. This fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification.