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A geochemical reconnaissance of the Alid volcaniccenter and geothermal system, Danakil depression, Eritrea
Abstract
Geological and geochemical studies indicate that a high-temperature geothermalsystem underlies the Alid volcanic center in the northern Danakil depression of Eritrea Alid is avery late-Pleistocene structural dome formed by shallow intrusion of rhyolitic magma some of which vented as lavas and pyroclastic flows Fumaroles and boiling pools distributed widelyover an area of ∼10 km2 on the northern half of Alid suggest that an activehydrothermal system underlies much of that part of the mountain Geothermometers indicate thatthe fumarolic gases are derived from a geothermal system with temperatures >225°C Theisotopic composition of condensed fumarolic steam is consistent with these temperatures andimplies that the source water is derived primarily from either lowland meteoric waters or fossilRed Sea water or both Some gases vented from the system (CO2 H2Sand He) are largely magmatic in origin Permeability beneath the volcanic center may be high given the amount of intrusion-related deformation and the active normal faulting within theDanakil depression
... The sedimentary sequence overlying the Precambrian basement includes hyaloclastite, submarine debris flow, shallow marine sediments and tephra. The domal structure is caused due to the intrusion of rhyolitic magma within the crust (in to the Precambrian basement) (Duffield et al. 1997;Lowenstern et al. 1999) (Fig. 5). This intrusion and subsequent doming of the Precambrian shield induced (tensional) fractures at the contact between the rhyolitic body and the Precambrian granites. ...
... These fractures became the loci of circulating fluids that eventually became the geothermal reservoir (Fig. 5). Since the rhyolitic body temperature is 870°C , the estimated reservoir temperature of 250 to 350°C is quite reasonable (Duffield et al. 1997;Lowenstern et al. 1999). ...
... However, the reservoir temperature estimated from gas thermometers gave values between 250 and 325°C (Duffield et al. 1997;Lowenstern et al. 1999). Such reservoir temperatures are expected since, the Danakil depression lies over an active hot spot with active volcanoes located along the axis of the depression (Erta Ale, Alid, Nabro), with Nabro volcano recording eruption in 2011 (Beyene 2012). ...
Eritrea is a country with rich gold, silver and base-metal deposits and geothermal energy resources associated with all the five volcanoes located within the Danakil graben. Due to low rainfall, the country has to depend on imported food and food imports have crossed > 46% in the recent years. Although the cultivable land is about 16,000 km², only 5030 km² land is being cultivated due to insufficient water resources. The per capita water requirement is projected to fall below 1300 m³/year from the present 1470 m³/year. The country’s GDP has fallen from 1.3% in 2013 to 0.3% in 2015. Each geothermal province associated with the active volcanoes can support to generate 445 million m³ of desalinated water from the Red Sea. Providing basic needs like water and energy will boost the country’s economy and lift the socio-economic status of 6 million people in the country.
... The line between the Arabian and Nubian shield is the future Red Sea rift axis that initiated around 31 Ma (adapted from Chandrasekharam et al. (2016a)). plume below the present day Afar (at about 31-26 Ma, Wolfenden et al., 2005;Bosworth et al., 2005;Lowensterna et al., 1999;Chandrasekharam et al., 2015a), that represent initial phase of the formation of the Red Sea, gave rise to landmasses with similar structural, geological and geomorphologic features on either side of the Red Sea. Initial magmatic activity started with the intrusion of dikes swarms over the Arabian shield and outpouring of basaltic lava over Saudi Arabia, Yemen, Djibouti, Eritrea and Egypt (Fig. 2). ...
... This sedimentary sequence constitutes the top shallow aquifer. The reservoir temperature estimated using gas thermometers varies between 250 and 350°C (Duffield et al., 1997;Lowensterna et al., 1999). Besides the rhyolitic intrusion, the Danakil, being located over an active hot spot (Nebro volcano recorded an eruption in 2011, Beyene, 2012, Fig. 1) with young ocean floor in the making, hosts several high temperature geothermal systems. ...
... Bore-hole temperatures recorded at depth of 1000 m and modelled temperatures for the Lake Asal site at similar depths gave values within the range of 200-250°C (Houssein and Axelsson, 2010). While the Lake Asal geothermal system Table 1 Representative geochemical analyses of thermal springs from the Sub-Saharan geothermal provinces (n: data not available: data source 1. Chandrasekharam et al. (2016a); 2. Zerai (1999); 3. Kamondo (1988), Mutonga (2015); 4. Bosch et al. (1977); 5. Houssein et al. (2013); 6. Allen et al. (1989); 7. Lowensterna et al. (1999), 8. Sturchio et al. (1996), 9. Meseret and Beyene (2001 is strongly influenced by the Red Sea, the Lake Abhe thermal waters are less saline but do exhibit incidence of high Na, Cl and SO 4 ions ( Table 1, Fig. 10) relative to the Awash River water, the main recharging source, and move away from the Cl apex in Fig. 11. Presence of several linear travertine mounds (Fig. 7) in this province (Houssein et al., 2013) suggest that CO 2 -SO 4 rich Na-Cl fluids of magmatic origin together with meteoric waters interact with Ca-HCO 3 waters of the Lake Abhe, precipitating travertine deposits (Fig. 7). ...
The geothermal provinces of the western Red Sea coast and East African countries have evolved synchronous with the evolution of the landmasses around the Red Sea. The Red Sea opening was initiated subsequent to the opening of the Aden Gulf that was triggered by the Afar plume. During the Late Oligocene, the Red Sea rift started extending from south towards north culminating into a failed arm near the Suez Gulf. The geothermal systems evolved due to shallow mantle below the Danakil depression and circulating seawater. Besides the volcanic flows, the regional sedimentary aquifers like the Wajid Sandstone and Amron limestone, that occur in Ethiopia, Egypt, Eritrea and Djibouti, that were deposited prior to the younger tectonic events that started at 31 Ma are the main geothermal reservoirs in these countries. In contrast, rain surface and evaporated lake water are the main sources for the Kenyan geothermal systems. Although these geothermal fluids show variation in their chemical components due to different recharging sources, all the geothermal systems are of high temperature, capable of generating electricity.
... The Traps series (Lowenstern, et al., 1999). ...
... The third series of volcanoes is divided into three units ( (Lowenstern, ET, 1999). ...
... Eritrea is likely to be rich in steam power because it high-temperature reservoirs (Zerai,1996;Lowenstern, et al., 1999). ...
This book is an introduction to the country's natural and human geography. Geographically, It helps readers to understand the various development change, and culture in Eritrea since independence.
... Most of these latter deposits are sourced from the eastern escarpment of the Ethiopian Plateau, and locally appear interbedded with basaltic lava flows. In the southern reach of the belt are located the Jalua and the Alid volcanic centers (Marini, 1938;Duffield et al., 1997;Lowenstern et al., 1997Lowenstern et al., , 1999Lowenstern et al., , 2006 that are aligned with the Erta-Ale volcanic system located~160 km to the south in Afar (Figs. 1 and 2). Differently from the Jalua volcano, the geology of which is almost unknown (Yohannes, 2014), the Alid volcanic center was the goal of several studies (Duffield et al., 1997;Lowenstern et al., 1997Lowenstern et al., , 1999Lowenstern et al., , 2006. ...
... In the southern reach of the belt are located the Jalua and the Alid volcanic centers (Marini, 1938;Duffield et al., 1997;Lowenstern et al., 1997Lowenstern et al., , 1999Lowenstern et al., , 2006 that are aligned with the Erta-Ale volcanic system located~160 km to the south in Afar (Figs. 1 and 2). Differently from the Jalua volcano, the geology of which is almost unknown (Yohannes, 2014), the Alid volcanic center was the goal of several studies (Duffield et al., 1997;Lowenstern et al., 1997Lowenstern et al., , 1999Lowenstern et al., , 2006. The Alid elliptical dome is elongated in an ENE-WSW direction and formed by a shallow (2-5 km: Duffield et al., 1997) rhyolitic magma intrusion. ...
... The oldest products consist of basalts dated at 212 ± 19 ka (Duffield et al., 1997) that are interbedded with shallow marine sediments. Above these rocks is the 'lava shell', composed of basalts, pyroclastic deposits, and amphibole-bearing rhyolites (Duffield et al., 1997;Lowenstern et al., 1997Lowenstern et al., , 1999Lowenstern et al., , 2006, the latter ranging in age from 34.5 ± 6.2 to 14.5 ± 3.7 ka (Duffield et al., 1997). All these rocks rest on the latest products of the Stratoïd Series of Afar, whose top has been referred the Middle Pleistocene (0.39-0.37 Ma: Barberi et al., 1972). ...
The Afar region is a triangular area located at the triple junction between the African, Somalia, and Arabian plates, which are currently diverging at different rates. Currently, the extension vector is roughly oriented in a NE-SW direction in the Afar, Red Sea and Gulf of Aden, in respect to Arabia plate, whereas the Nubian–Somalian divergence, evidenced by the Main Ethiopian Rift (MER), is approximately WNW-ESE (N95-100°E). This study focuses on the tectono-sedimentary evolution of a sector from Massawa to the north up to the continental Early-Middle Pleistocene Dandiero Basin to the south. This basin is filled with approximately 500 m thick fluvial-lacustrine deposits and includes six formations. Sedimentation occurred mainly along the basin axis and allowed accumulation of sand and mud deposits with subordinate gravels close to the basin margin. The age of the basin infill succession is well constrained through integration between paleomagnetic and paleontological data and ranges between ~ 1.2 and 0.75 Ma. The Dandiero Basin is controlled by two main roughly NNW-SSE trending, east dipping normal faults. The westernmost fault delimits the basins from the plateau, whereas the easternmost marks the limit between the basin succession and the Late Pleistocene Samoti Plain. We infer that the NNW-trending faults were progressively activated as a consequence of the Danakil Block counter clockwise rotation and were superimposed to the N-S trending faults that delimited the basin at the time of its inception as a marginal graben roughly aligned to the Eritrean-Ethiopian plateau. The timing of deformation (1.2 Ma up to Present) is well constrained by the age of syntectonic sediments of the Dandiero Basin and volcanic products of the Alid Volcano. These relations allowed us to refine the timing and evolution of this sector of Afar and giving some insights on the geodynamics of the area.
... Subsequent to the rift, the region was covered by sediments (Oligocene to Pliocene; marine sandstone, siltstone, gypsum beds and fossiliferous limestone of Pleistocene age. The Quaternary basalt and rhyolite flows erupted due to the intense activity of the Danakil depression covered large part of the landmass around the Red Sea and the volcanic activity continues to be active (Bosworth et al. 2005;Lowensterna et al. 1999). The Alid volcanic centre in Eritrea is a structural dome formed due to intrusion of rhyolitic magma at shallower levels after the main Pleistocene sedimentation. ...
... The geothermal activity around the Alid volcanic centre is represented by thermal springs, fumaroles and steaming ground. Both meteoric and Red Sea water flowing through the fractures and faults (Fig. 2) appear to be the main feeders to the circulating geothermal fluids (Yohannes 2007;Lowensterna et al. 1999;Duffeld et al. 1997). Since the annual rain fall is very low (500-700 mm, Duffeld et al. 1997), a major component in the circulating fluids is the sea water. ...
The current thermal regime around the red Sea was initiated by a mantle plume beneath Ethiopia, Eretria, Yemen and SW of Saudi Arabia at about 31 Ma before the present. The evolution of the geothermal systems around the Red Sea is coeval to the initial onset of volcanism over the separated land masses (Eritrea, Ethiopia, Djibouti, Yemen, Saudi Arabia and Egypt at 14 Ma. The entire tectono-magmatic activities around the Red Sea gave rise to several geothermal provinces over the continents surrounding the Red Sea, represented by thermal springs and fumaroles at several locations in Eretria, Djibouti, Ethiopia, Yemen and Saudi Arabia. These countries have the potential to develop these resources, like Ethiopia, to mitigate CO2 emissions in countries like Saudi Arabia and to enhance the GDP (gross domestic product) of the economically backward countries. The geothermal resources have the potential to make these countries energy independent in future.
... Future research could specifically focus on groundwater salinity in this country, as some authors have reported problem areas (e.g. Lowenstern et al., 1999;Zerai, 1996). ...
Background:
Changes in climate and anthropogenic activities have made water salinization a significant threat worldwide, affecting biodiversity, crop productivity and contributing to water insecurity. The Horn of Africa, which includes eastern Ethiopia, northeast Kenya, Eritrea, Djibouti, and Somalia, has natural characteristics that favor high groundwater salinity. Excess salinity has been linked to infrastructure and health problems, including increased infant mortality. This region has suffered successive droughts that have limited the availability of safe drinking water resources, leading to a humanitarian crisis for which little spatially explicit information about groundwater salinity is available.
Methods:
Machine learning (random forest) is used to make spatial predictions of salinity levels at three electrical conductivity (EC) thresholds using data from 8646 boreholes and wells along with environmental predictor variables. Attention is paid to understanding the input data, balancing classes, performing many iterations, specifying cut-off values, employing spatial cross-validation, and identifying spatial uncertainties.
Results:
Estimates are made for this transboundary region of the population potentially exposed to hazardous salinity levels. The findings indicate that about 11.6 million people (∼7% of the total population), including 400,000 infants and half a million pregnant women, rely on groundwater for drinking and live in areas of high groundwater salinity (EC > 1500 µS/cm). Somalia is the most affected and has the largest number of people potentially exposed. Around 50% of the Somali population (5 million people) may be exposed to unsafe salinity levels in their drinking water. In only five of Somalia's 18 regions are less than 50% of infants potentially exposed to unsafe salinity levels. The main drivers of high salinity include precipitation, groundwater recharge, evaporation, ocean proximity, and fractured rocks. The combined overall accuracy and area under the curve of multiple runs is ∼ 82%.
Conclusions:
The modelled groundwater salinity maps for three different salinity thresholds in the Horn of Africa highlight the uneven spatial distribution of salinity in the studied countries and the large area affected, which is mainly arid flat lowlands. The results of this study provide the first detailed mapping of groundwater salinity in the region, providing essential information for water and health scientists along with decision-makers to identify and prioritize areas and populations in need of assistance.
... Eritrea is estimated to have high a renewable energy potential such as wind, solar, and geothermal. For instance, it has a high wind energy potential along the Red Sea coastal regions [20,21], and the geothermal energy potential is also expected in hot springs and fumaroles of the Red Sea coast [18,22,23]. The mitigating problems related to the fossil fuel energy, the government of Eritrea has taken some efforts to encourage the importance of alternative energies, particularly the solar energy [20]. ...
The remote sensing technology is suitable to analyze the potential of renewable energies such as solar energy, and play a great role to minimize global warming worldwide. However, in Eritrea, exploration of the potential of solar energy using such an advanced technique is so limited. In this work, a digital elevation model (DEM) is applied to estimate the potential of solar energy in Eritrea at a regional level for the photovoltaic system. The ArcGIS and ENVI softwares are used to compute the solar radiation from the DEM data. In addition, the global horizontal irradiation (GHI) is adopted to evaluate the suitability of the solar energy potential for the photovoltaics development. The results obtained reveal that the annual average maximum and minimum GHI of the three regions (Maekel, South, and Anseba) in the highlands and the three regions (Ghash Barka, Norther Red Sea, and Southern Red Sea) in the lowlands range between 2775 kWh/m2 and 870 kWh/m2, and between 2238 kWh/m2 and 830 kWh/m2, respectively. Thus the results show that Eritrea is suitable to develop the photovoltaics (PVs) electricity where the 69% and 31% part of the country is categorized as excellent suitable and highly suitable to generate the PVs power. Therefore, this research work might be significant for the decision-makers and to serve as a reference data for further studies in this area.
... For example, in the greater rift valley region, in Ethiopia, Kenya and Tanzania heat from geothermal activity and carbon dioxide influx from deeper earth accelerate the dissolution of the otherwise less soluble volcanic rocks and result in high salinity groundwaters (Darling et al., 1996). Geothermal induced salinization of groundwater is commonly observed, for example, in Ethiopia (Darling et al., 1996), Kenya (Darling et al., 1996), Tanzania, Djibouti (Awaleh et al., 2017) and Eritrea (Lowenstern et al., 1999). Groundwater residence time controls the degree to which groundwater salinity increases due to rock dissolution. ...
In Africa groundwater is the principal source of drinking water (https://washdata.org/) and has significant potential to improve food production as a source of irrigation water. Although volumes of stored groundwater are high when compared to surface water, not all the available groundwater is suitable for irrigation. One of the principal factors that limit the development of groundwater is salinity, but quantitative evidence regarding the scale of salinization in Africa has been lacking. This paper presents an initial analysis of the extent of groundwater salinization in Africa, bringing together groundwater salinity data (n = 12,255) from across the continent. In our dataset c.80% of the samples have electrical conductivity values less than 2000 μS/cm. Samples with high conductivity values of more than 2000 μS/cm are restricted to limited geological and climatic environments. The data reveals salinity does not affect the water security status of most countries in Africa. However, salinity compounds water insecurity issues in arid low groundwater recharge environments. The paper discusses the role of anthropogenic processes such as river valley water resources development, extensive groundwater pumping, inter-basin groundwater transfer, and irrigation in altering the salinity of groundwater bodies. The paper further elucidates the origin of groundwater salinity by critically reviewing the natural and human-induced factors that control the salinization of the various groundwater bodies across Africa. Existing case studies reveal several causes of salinization, including i) human-induced salinization, ii) climate and hydrological change-induced paleo groundwater salinity, iii) rock dissolution, and iv), saltwater encroachment.
... Anhydrous alums are less common in volcanic fumaroles: godovikovite has been described in the Phlegrean fields and La Fossa cra-ter on the island of Vulcano (Mindat.org database, 2019), while its hydrated form (tschermigite) has been found in the thermal fields of southern Kamchatka peninsula (Zhitova et al., 2019), east African rift (Lowenstern et al., 1999) and possibly Merapi volcano, Indonesia (Kavalieris, 1994). Steklite, KAl(SO 4 ) 2 , the K-analogue of godovikovite, has been identified in the fumarole deposits of Tolbachik volcano, Kamchatka (Murashko et al., 2013). ...
Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the composition and flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (MI MIII (XVIO4)2 with MI = NH4⁺, Na⁺, K⁺; MIII = Al³⁺, Fe³⁺ and XVI = S⁶⁺, Mo⁶⁺) in the 330 °C fumaroles of the Lascar volcano (Chile). The alum occurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rock cations within the crust. The average growth rate is estimated at ca. 0.3 μm/day, based on the 19-year-long activity of the degassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreases towards crust rim. The crust selectively concentrates Tl, V and Te (thousands of μg/g) and to a lesser extent Mo (hundreds of μg/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the MI, MIII and XVI sites of alum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates. Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Such surface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggest that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that the partition coefficients of Tl (3.3 × 10⁷), V (1.1 × 10⁷) and Te (0.6 × 10⁷) between crust surface and gases are two to four orders of magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alums lose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano (5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrous alums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes, reducing the atmospheric emissions of these three elements.
... However, this activity is punctuated, as no significant deformation was recorded along the range during long periods (e.g., 1993(e.g., -1997(e.g., : Amelung et al., 2000. From south to north, these edifices are comprised of: Tat 'Ali, Hayli Gubi, Ale Bagu, Erta 'Ale, Borale 'Ale, Dalafilla, Alu, Gada 'Ale, and Alid (Duffield et al., 1997;Hagos et al., 2016;Lowenstern et al., 1999). Alid lies across a salt plain from the rest of the range, and might be considered its own sector, and indeed is unusual in that the core of Alid 'volcano' is actually a domed intrusive core draped with volcanic products (Duffield et al., 1997). ...
The Afar Depression is a broad region of subdued topography wherein the Red Sea, Gulf of Aden, and East African Rift System converge at a triple junction. This mature sector of the East African Rift System is characterized by attenuated continental lithosphere, with the implication that lavas erupted here can provide insights into the composition of sub-lithospheric magma reservoirs. Magmas in Afar and Yemen largely have geochemical characteristics consistent with a mixture of melts derived from the Afar plume, and from a hybrid endmember comprised of the depleted upper mantle and Pan-African lithosphere. Lavas erupted in the Erta ‘Ale range do not comport with this model and instead appear to have been derived from melting of a lithospheric mantle metasome that is likely related to prior Afar plume-lithosphere interaction. The extant data show no significant change in the reservoirs contributing to magmatism during the development of the rift in this region, however the surface distribution of magmatism reveals a relationship between rift evolution and magmatism. In Yemen, magmatic activity is limited to the regional Mid Miocene Resurgence event at ca. 10 Ma, and small-volume alkaline volcanism along the evolving Gulf of Aden margin from 6 Ma to present. In contrast, a much more extensive magmatic record is preserved in Afar. Coincident with the development of the rift margin, initial magmatism in Afar during this period (20–10 Ma) was dominantly in the form of large silicic eruptions, with lesser volumes of crustally contaminated Mablas basalts. The development of rift-marginal basins at ca. 7 Ma was coincident with the widespread eruption of fissural basalt of the Dalha Series. The Dalhoid Series represents a pulse of magmatism from ca. 5.6 to 3.9 Ma, which correlates with the initiation of sedimentation within the Sagantole Formation. The initiation of the lower member of the Afar Stratoid Series at ca. 3.9 Ma is broadly contemporaneous with sedimentation within the Hadar Formation, while the upper member (2.6 Ma) correlates with the Busidima Formation. The Gulf Basalt Series (2.8–0.3 Ma) temporally overlaps the upper member of the Afar Stratoid Series, but represents the first manifestations of the coincidence of strain localization and magmatism within rift axial grabens. The subsequent Axial Series (ca. 0.7 Ma to present), manifests as basaltic cones and central silicic volcanoes within three zones aligned with the Main Ethiopian Rift, Red Sea Rift, and Gulf of Aden. The evolution in Afar from initial large-scale silicic volcanic events to basin-wide fissural basaltic flows, to modern zones of focused magmatism, broadly parallels the localization of strain during the same period. This intimate relationship between the mechanism of strain accommodation and surface manifestation of magmatism in Afar has the implication that the modern zones of focused magmatism and strain are the pre-cursors of oceanic spreading centers. While Afar is clearly at an advanced stage in the rifting process, whether these modern zones are the direct precursors to oceanic spreading centers remains unresolved.
... The depression originated during the Lower Miocene, 23-25 Mya (Barberi et al., 1972), and deposits of evaporites up to ~3 km thick (Bonatti et al., 1971) attest to an intermittent marine connection to the Red Sea. These connections persisted until the Late Pleistocene (~32 000 BP), when the rise of the Alid volcanic center blocked the northern waterway (Lowenstern et al., 1999) and triggered the rapid (~4000 years) desiccation of the basin (Bonatti et al., 1971). After the Last Glacial Maximum event of ~20 000 BP, with a rise in aridity, the Early Holocene saw an increased amount of groundwater as a result of deglaciation (~9800 BP: Gasse, 1974), which is likely to have caused the formation of an extensive palaeo-lake flooding the area below the −50 m contour until ~7300 BP (Chiozzi et al., 2017). ...
The Danakil Depression of northeastern Africa is among the harshest environments on Earth. Yet, despite extreme aridity, this desert region hosts the endemic cichlid genus Danakilia. As currently recognized, the genus includes at least two populations of Danakilia franchettii from groundwater springs feeding Lake Afrera (Ethiopia), one population of Danakilia dinicolai from volcanic Lake Abaeded, and three recently discovered riverine populations (Eritrea) of uncertain taxonomic status. Here, we analyse the variation in shape of the body and lower pharyngeal jaw of all known populations, using a geometric morphometrics approach. We investigate whether body and lower pharyngeal jaw shape, two evolutionarily important traits in cichlids, vary within and between Danakilia populations; whether patterns of variation in these traits are consistent with local adaptation (genetic or plastic) or a neutral model of variation in geographical space; and whether these traits show consistent patterns of covariation that might result from pleiotropy or linkage. We found that shape variation between the Afrera and the Abaeded samples confirms current taxonomy. We observed a sharp separation of the southern Afrera populations from all northern populations for both traits, with a less pronounced separation among the northern populations, and a deeper body in some populations compared with the more elongated shape of other riverine populations. Significant variation between all populations is not easily explained by phenotypic plasticity, suggesting that populations might be on independent, possibly neutral or quasineutral, evolutionary trajectories and constitute separate, highly vulnerable units deserving of conservation efforts. © 2018 The Linnean Society of London, Biological Journal of the Linnean Society.
... Thick evaporitic formations reaching a depth of at least 3 km (Bonatti et al. 1971), attest to formerly marine conditions and profoundly affect the present-day geochemistry and ecology of the region. The Danakil basin separated from the Red Sea $32,000 BP with the uplift of the Alid volcanic center (Lowenstern et al. 1999), an event that triggered the final desiccation of the marine basin, which lasted only $4000 years (Bonatti et al. 1971). ...
The Danakil Depression in northeastern Africa represents one of the harshest arid environments on Earth, yet two genera of fishes, Danakilia (Cichlidae) and Aphanius (Cyprinodontidae), share its sparse aquatic habitats. The evolutionary history of these fishes is investigated here in the context of genetic, geological and paleoenvironmental information. We collected samples from seven sites and assessed phylogeographic relationships using concatenated COI and cytb mtDNA genes. Danakilia morphospecies show low differentiation at mitochondrial markers, but variation is partitioned between a northern cluster containing D. dinicolai plus three undescribed riverine populations, and a southern cluster including two creek populations of D. franchettii separated by the hypersaline waters of Lake Afrera. Aphanius displayed four genetically distinct clades (A. stiassnyae in Lake Afrera; one distributed across the entire area; one in Lake Abaeded; and one in the Shukoray River), but without clear large-scale geographic structure. However, Danakil Aphanius are clearly differentiated from A. dispar sensu stricto from the Sinai Peninsula. Geological evidence suggests that after the Late Pleistocene closure of the Danakil-Red Sea connection, increased post-glacial groundwater availability caused the formation of a brackish paleo-lake flooding the entire region below the –50 m contour. Fish populations previously isolated in coastal oases during glaciation were able to mix in the paleo-lake. Subsequently, in a more arid phase starting ∼7300 BP, paleo-lake regression isolated fishes in separate drainages, triggering their still ongoing diversification.
... Agriculture drainage, sewage and seawater evaporites fields are drawn according to Vengosh (2014). Afar Hydrothermal waters describes waters of meteoric origin, whereas mean ± standared deviation crosses depict geothermal or regional waters from Afar with variable seawater contributions (Awaleh et al., 2016;Lowenstern et al., 1999;SanJuan et al., 1990;UNDP 1973). Afar evaporites (Binega, 2006;Martini, 1977), Basalts (Awaleh et al., 2016) and Main Ethiopian Rift (M.E.R.; Gianelli and Teklemariam, 1993;Pittalis et al., 2016) fields are also shown for comparison. ...
... Agriculture drainage, sewage and seawater evaporites fields are drawn according to Vengosh (2014). Afar Hydrothermal waters describes waters of meteoric origin, whereas mean ± standared deviation crosses depict geothermal or regional waters from Afar with variable seawater contributions (Awaleh et al., 2016;Lowenstern et al., 1999;SanJuan et al., 1990;UNDP 1973). Afar evaporites (Binega, 2006;Martini, 1977), Basalts (Awaleh et al., 2016) and Main Ethiopian Rift (M.E.R.; Gianelli and Teklemariam, 1993;Pittalis et al., 2016) fields are also shown for comparison. ...
... The Red Sea Rift is characterized, to the north, by the Erta Ale Range, which separates the Ethiopian Plateau from the Danakil Block. The Erta Ale Range consists of~NW-SE aligned volcanoes, largely with basaltic composition, and hydrothermal spots associated with fracturing and diking above oceanic crust ( Fig. 18; Barberi and Varet, 1972a;Barrat et al., 1998;Oppenheimer and Francis, 1998;Lowenstern et al., 1999;Acocella, 2006;Carniel et al., 2010). InSAR data recorded during the last decade have captured repeated deformation (magma withdrawal, dike emplacement and sill inflation) along this portion of the range (Amelung et al., 2000;Nobile et al., 2012;Pagli et al., 2012), often accompanied by seismic activity (e.g. ...
Plate boundaries are the most active, unstable and hazardous areas on Earth. The aim of this study is twofold.
1) Provide an overview of the main structural features along divergent and convergent plate boundaries and their relationships to volcanism. The considered divergent plate boundaries include the continental East African Rift System (EARS), the transitional Afar Rifts and slow (Iceland) and fast (East Pacific Rise) oceanic ridges. The analysis of the convergent plate boundaries refers to the extensional (Taupo Volcanic Zone, New Zealand), strike-slip (Sumatra), contractional (NE Japan) and more complex (Central Andes) volcanic arcs.
2) Propose an original and innovative frame to understand tectono-magmatic processes along plate boundaries, based on two major points. a) Magmatism may effectively control the development of plate boundaries. At immature continental divergent plate boundaries (as the non-magmatic portions of the EARS), regional extension plays a major role in extending the upper crust; however, along mature continental and oceanic rifts magmatism is most effective in spreading plates apart through dikes. At convergent plate boundaries, the possibility to develop extensional, strike-slip, contractional and oblique systems along the volcanic arc provides highly variable structural conditions controlling magmatism. At least three magmatic processes (diking, sill emplacement, heat-induced strain localization) may control the development of volcanic arcs. In addition, megathrust earthquakes may induce transient extension, enhancing volcanism. Therefore, the evolution of both divergent and convergent plate boundaries is significantly magma-controlled and magmatic processes seem largely self-sustained, requiring limited direct tectonic contribution. b) Magmatism along divergent and convergent plate boundariesold usually focuses in magmatic systems, associated with a dominant volcano. Magmatic systems provide a convenient framework to relate the geometric and compositional features of a magmatic plate boundary to its tectonic setting. In particular, the larger the opening rate, the lower is the aspect ratio A (A = width W/length L) of the magmatic system. Therefore, the linear magmatic accretion of divergent plate boundaries and extensional arcs is replaced by a punctiform accretion in contractional arcs.
Several aspects (magmatic rates, lateral vs. vertical dike propagation along divergent plate boundaries, structural control on volcanism along strike-slip arcs) are still poorly defined and should be the target of future research.
The BrineMine Project is a German-Chilean multidisciplinary research project realized by research and industry partners. The focus is developing strategies for raw material and water extraction from geothermal springs (Brine Mining) in Chile. The topics can be separated into a geological/geochemical part and a mechanical engineering part, which are processed in close cooperation by the project consortium. In the first part, the economic potential of the dissolved raw materials in thermal spring waters in Chile is assessed by analyzing existing geochemical data of different sites. This is complemented by hydrogeochemical and geophysical exploration campaigns. The second part focuses on the development, construction and implementation of a prototype for pre-treatment and concentration of geothermal brines. With the comprehensive expertise of the team, a treatment strategy was developed and tested in a geothermal power plant, enabling controlled silica precipitation in order to overcome this limiting factor for geothermal energy production and associated raw material extraction.
In this study, successful milestones of the BrineMine project are presented. The economic potential of elements in Chilean thermal waters is demonstrated. Additionally, the global potential of Brine Mining is outlined. The development of the silica treatment strategy is further described, as well as a possible integration of a prototype into an operating geothermal power plant. Finally, the construction and implementation of a large-scale first-generation prototype are presented with promising field results.
The BrineMine project is a 3-year BMBF-funded research project that scientifically investigates the material use of geothermal water. The project is a bi-national research project between German and Chilean research and industry partners. The project is divided into two main topics: 1) Determination of the potential of thermal waters with regard to the extraction of economically relevant raw materials and 2) Pre-treatment of thermal waters prior to the extraction of raw materials. The aim of the project is the development, installation and commissioning of a prototype at selected geothermal sites in Chile and Germany.
It is well known that the highly mineralized thermal waters extracted from geothermal energy sources sometimes contain significant concentrations of economically strategic substances such as lithium, rubidium, antimony, tungsten, etc. The BrineMine project aims to describe the occurrence of these substances in geothermal waters qualitatively and quantitatively against the background of raw material extraction. For this purpose, a database will be created based on literature values and own hydrochemical sampling campaigns to statistically describe the composition of different geothermal waters.
The northern Afar extremity, also called Danakil, extending into Ethiopia and Eritrea, is characterised by a wide lowland area, up to 250 km long and increasing in width from north to south up to 70 km, occupied along its median axis by a salt plain 120 km long and 30 km wide on average, with minimum altitudes of 120 m below sea level, corresponding to a former gulf of the Red Sea that dried out around 25,000 years ago.
Three new improved equations of the Na/K geothermometer were developed through computational (artificial neural networks) and statistical tools (based on ordinary linear regression) from the analysis of a more representative world geochemical database (n=212) than hitherto used. The new Na/K geothermometers are given by the following equations: t° C=883/log(Na/K)+0.908-273.15 t° C=1273.2[tanh(-104144-log(Na/ K)-0.5642)]+1156.9 t° C=883(±15)/log(Na/K)+0.894 (±0.032)-273.15 These new geothermometric equations were successfully evaluated and compared with measured deep temperature wellbore logs using a different geochemical database (n=112) to avoid the training and regression bias. The obtained results clearly show that the new geothermometers systematically provide better and reliable estimations of the deep equilibrium temperatures (for temperatures over 160 °C) than the equations previously reported in the geothermal literature. Details of the theoretical basis of the Na/K geothermometer, the computational and geochemometric methodologies used, as well as the validation and comparison results are outlined in this work.
Harnessing the geothermal potential in Eritrea has a significant impact on the economic development of Eritrea as the country relies its electric power generation from imported refined petroleum products, which is wholly based on oil burning plants. The impact can be viewed both the unreliable cost fluctuation mainly increment of fuel cost coupled with the unfriendly environment. Therefore geothermal energy will have an important input in alleviating expenditure on foreign currency while safeguarding the environment. The tectonic setting and geological makeup of the south-eastern coastal zone of Eritrea is a favourable site for having geothermal resources that are potential for the development of geothermal resources mainly for electrical generation and geothermal utilization. Alid and Nabro-Dubbi fields are the notable places with ample geothermal manifestations. There are considerable low temperature-thermal springs potential for recreation spas, health and mineral water bottling, and etc, occur at around the Asmara-Massawa highway and close to Gulf of Zula, which do not show any immediate association with recent magmatism. The completion of some of the surface studies on Alid prompts here to concentrate on the recent work performed. A more than 250°C reservoir temperature was estimated using gas geothermometers from Alid geothermal prospect. As recommended from the earlier studies, a geophysical assessment was to define the recently an assessment of fracture analysis was conducted since geothermal reservoir is stored in some favourable geological structures. Thorough assessments of Rose diagram and fault and fracture (FFD) analyses have been performed to know areas of up-flow zone. A 70°N trending high fracture zone is well marked on the FFD contour plot. The low resistivity anomaly of the preliminary resistivity survey conducted on the Ghinda-Darere is in line with the contour plot. The hydrogeological assessment performed regionally indicates that the recharge area is mainly from three catchments mainly the input is from highland area. The resistivity survey that was conducted recently has availed an interesting anomaly at the rift floor and opened a wider perspective in exploration.
and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N2 show a distinct hot-spot signature ( ). The δ13C–CO2 is approximately −3.6‰ and δ34ST is approximately +3.3‰. The δD/δ18O of fumarole H2O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from 3H analyses is ≤40 years. Fumarolic activity at Alcedo is controlled by a caldera-margin fault containing at least seven hydrothermal
explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993–1994 produce approximately
15 m3/s of steam, yet discharge temperatures are ≤97°C. Water content of the total gas is 95–97 mol.%, noncondensible gas is 92–98 mol.%
CO2, and sulfur gas is dominated by H2S. Relative amounts of He, Ar, and N2 show extensive mixing between hot spot and air or air-saturated meteoric water components but the average . The δ13C–CO2 is approximately −3.5‰ and δ34ST is approximately −0.8‰. The δD/δ18O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3–5‰ in 18O compared with local meteoric water. 3H indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry
indicates a reservoir temperature of 260–320°C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing
up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development
of this resource.
Geothermal energy, which is an abundant resource in the Earth's crust, has emerged as an important and clean alternative source of energy for electrical power generation and direct use (Muffler, 1981). Hot waters from thermal springs and shallow wells have been used extensively since prehistoric times for bathing, washing and cooking. For the last 50 years many countries have also used low enthalpy geothermal water (20°-150°C) for agriculture, aquaculture, space heating, fruit drying, paper manufacturing and other applications (Fridleifsson, 1998). Electrical power generation using geothermal steam started at Larderello, Italy in 1912 (Ellis and Mahon, 1977), and currently a total of over 8000 MW of electricity are generated from geothermal systems in 22 countries, with the USA, Philippines, Italy, Mexico, Indonesia, Japan and New Zealand being the main producers (Duffield et al., 1994; Wright, 1998). The σ2H, σ18O and tritium values of water, especially when integrated with the concentrations and isotopes of solutes and gases are powerful geochemical tools for determining the origin, nature, distribution and interactions of fluids in geothermal systems. The stable isotopes of water are especially useful because the relations governing their distribution in present-day surface and shallow groundwaters of an area (the local meteoric water) as well as their modifications in aquifers and geothermal systems are reasonably well known. The isotopes of meteoric water may be modified by evaporation and mixing in shallow aquifers and by mixing, boiling, and isotopic exchange with minerals in geothermal systems. For review articles and many references, see Truesdell and Hulston, 1980; Fournier, 1989; D'Amore, 1992; Kharaka and Thordsen, 1992, and Giggenbach, 1983. In this summary review, we will discuss the major historical developments in the application of water isotopes and other geochemical tools to the study of geothermal systems. We will cover briefly the major advances in the last 25 years, including determination of isotopic and chemical compositions of solutes and noble gases, application of vapour-phase tracers, and improved geochemical modelling of chemical reactions in reservoir processes. These advances, as well as exploration in deep portions of known geothermal systems have led to a better understanding of vapour-dominated systems, associated ore deposits, the influence of magmatic heat sources, the origins of highly corrosive fluids, scaling and corrosion. These advances have helped to optimize injection strategies for improved production and helped to decrease waste disposal costs. Finally, we will discuss an integrated approach, using water isotopes and other geochemical and hydrologic parameters to investigate the recharge to the hydrothermal system at Yellowstone National Park, USA, the largest geothermal system in the world.
The Geysers geothermal area has been an active producer of steam and hot water since the late Tertiary or early Quaternary time. The source of the energy supplying this geothermal area is thought to be a hot rock mass of some unknown depth below the underlying Franciscan rocks. The primary source of water in this natural steam generator is considered to be a combination of meteoric water and condensed underground steam in equilibrium with the loss of energy to the atmosphere. This escaping steam and hot water, channeled upward along faults and fractures in the underlying rocks, has provided a natural energy source and a chemical laboratory capable of forming a variety of interesting minerals. These minerals, most of which are sulfates containing ammonia, have been quite abundant until the 1970s. Since then they have decreased in quantity due to the increased diversion of the steam for electrical generation. -from Authors
Most data on the stable isotope composition of deep-seated carbon and sulphur come from igneous rocks and low-temperature gases. Very little information comes from high-temperature gases which escape from magmas (T = 1,000±200 °C), even though these are the best specimens of primary igneous fluids. Moreover, the only available 13C/12C data on carbon in high-temperature basaltic gases2–4 are largely divergent: low ratios from −14 to −26‰ versus PDB were measured in gases from Hawaii2,3,5, which is consistent with the values for reduced carbon in basalts6, whereas higher ratios around −6.5‰,typical of carbon in most of carbonatites7,8, diamonds9 and kimberlites10, have been found in gases from northern Afar4 (P.A. and M. Javoy, in preparation). The high-temperature gas samples collected from the November 1978 Asal rift eruption, southern Afar1, have provided new isotope data on basaltic igneous volatiles, released from upper mantle magmatism11.
Structures analogous to oceanic spreading ridges, transform faults, and "leaky" fracture zones have been identified in Afar. They were formed during the past 3 to 4 m.y. and permit identification of the present plate boundaries within Afar and determination of the amount of spreading in this interval. At least two microplates are required in the zone of the junction of the African, Arabian, and Somalian plates. Variations of the spreading rate along single ridges or from one ridge to another, ridge jumping, migration of spreading with time, and counterclockwise rotation of the microplates are inferred by comparing volcanological, chronological, and geochemical data with aeromagnetic data. Deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates, whose interiors are also systematically affected by regional faulting and locally by volcanism and transverse faulting. This indicates that although plate tectonics can explain most of the Afar features it fails when applied at such a small scale.
Several domains of contrasted extensional deformation have been identified in the southern Afar triangle (East Africa) from fault patterns analyzed with panchromatic stereoscopic SPOT (Système Probatoire d'Observation de la Terre) images. Stretching directions and statistical orientation and offset variations of faults fit with the Danakii "crank-arm" model of Sichler: A 10° sinistral rotation of the Danakil block explains the fault geometry and dextral block rotation in the southern part of the Afar triangle, as well as the oblique extension in the Tadjoura Gulf. Analogue modeling supports this interpretation.
The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within approximately 4 months by a large increase in He-3/He-4 in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 R(A) in July 1990 and subsequently declined to values near 5 R(A). Potential sources of the He-3-rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a He-3-rich ps chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 R(A)), the persistence of relatively high values (>5 R(A)) over a period of 3 years, the increase in the flux of total He relative to gases in air-saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased He-3/He-4 ratios in the fumarolic ps, and that the intrusive process may have persisted for approximately 1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome-south moat region during the 1989-1991 period resulted in at most relatively small changes in He-3/He-4 in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of He-3-rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.
The rates of chemical reactions between aqueous sulfates and sulfides are essentially identical to sulfur isotopic exchange rates between them, because both the chemical and isotopic reactions involve simultaneous oxidation of sulfide-sulfur atoms and reduction of sulfate-sulfur. The rate of reaction can be expressed as a second order rate law: R = k·[∑SO42−]·[∑S2−], where R is the overall rate, k is the rate constant and [∑SO42−] and [∑S2−] are molal concentrations. We have computed the rate constants from the available experimental data on the partial exchange of sulfur isotopes between aqueous sulfates and sulfides using the rate law established by us: , where t is time and α0, α, and αe are, respectively, the fractionation factors at t = 0 (the initial condition), at the end of experiment, and at equilibrium. The equilibrium fractionation factor can be expressed as: (T in Kelvin).
A steady-state model of the brine waters in Atlantis II Deep is presented. The deuterium and oxygen-18 concentrations in the water, and the dissolved argon content suggest a relative warm near surface Red Sea water as the source of the brine. In evaluating the overall environmental situation in the Red Sea in terms of temperature and salinity, the probable source lies about 800km to the south near the Strait of Bab el Mandeb. By integrating isotope data, and the trace and major element spectra of the brine, the origin, age and history of the brine waters become apparent.
Gathered here are those chemical properties which are not covered under the general headings of phase equilibria and crystal structure. The chemical properties of a feldspar can be considered from many viewpoints: thus a crystal chemist emphasizes the chemical bonding, whereas a geochemist emphasizes the geo-chemical and petrological affinities. Ideally each feldspar would be characterized by its content of all 92 naturally-occurring elements; the distribution of these elements among the sites of the crystal structure; the valence state; and the isotopic distribution. In chemically-zoned specimens, the spatial distribution is also important. Of course, the amount of information obtained on a particular specimen depends on the special interests of the investigator and on the equipment available. Most of the available data were collected in a casual manner, and there are only a few comprehensive studies. The quality of the data is highly variable as outlined in the last chapter. Few early investigators provided the information necessary for judging the quality of their data. Let it suffice that readers should be cautious. A pessimist might claim that all the early data should be abandoned. An optimist might point to the great advances made by the early workers, and ask for new careful studies of matters of particular interest.
Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende-biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self-sealing of near-surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated northwest-trending Sierra Nevada frontal faults that tap hot water at depth.
Using data from selected geothermal fields in Iceland, several gas geothermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The effects of gases from other sources, e.g. magmatic fluid entering reservoirs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give results reasonably close to the inflow temperatures, and are not constrained by the need to know thermodynamic parameters, are used to estimate subsurface temperatures from fumarole steam composition. It is suggested that geothermometers based on H2 and H2S concentrations are relatively effective. The CO2, H2S and H2 geothermometers are also evaluated by thermodynamic simulation using plausible mineral buffers. The CO2 geothermometer generally gives slightly low temperatures for values in the range 100–200°C. The calibration of the existing H2S and H2 geothermometers needs improvement when reservoir temperatures are higher than 220°C. The mineral buffer controlling H2S in reservoirs seems to be pyrite + magnetite + epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing temperature.
The Erta' Ale volcano has provided a series of high quality volcanic gas samples. Thermodynamic calculations show that homogeneous equilibrium is established within the gaseous phase, the oxidation state of which is consistent with the oxygen fugacity in equilibrium with the basalt (10−9.33atmat 1130°C). Moreover, carbon and sulphur stable isotope analyses permit to define gases of true magmatic origin and conversely, to get information on the isotopic state of mantle volatile elements.
The Guanacaste Geothermal Province encompasses three major geothermal systems, each centered on its respective volcanic structure: Rincón de la Vieja to the NW, Miravalles in the center and Tenorióto the SE. Each shows corresponding sets of surface manifestations: vapor discharges from fumaroles and steam-heated pools at altitudes > 500 m; lower temperature SO4Cl springs on the lower slopes of the respective volcano; and cooler neutral Cl springs to the S of the volcanic chain, at altitudes < 500 m. The production of HCO3-rich waters is limited to a narrow belt stretching to the S of Miravalles volcano. Chemical and isotopic evidence suggests that the neutral Cl waters, also discharged from deep wells, are derived from a more primitive ClSO4 water formed by transfer of readily mobilised, originally magmatic constituents to deeply circulating groundwater. Isotopic evidence suggests that this groundwater is derived largely from areas to the north of the volcanic chain. The presence of immature waters within drillable depth is indicated by the incursion of an acid SO4 water into a well at Miravalles after deepening to 2000 m. As the Cl waters rise, they start to boil. The separated vapors feed the fumaroles and steam-heated features at the surface, the residual water travels underground to be discharged some 5–25 km to the S. The uniformity in chemical and isotopic compositions of the neutral Cl waters is compatible with the assumption that the individual thermal systems are linked at depth to form large, contiguous geothermal reservoirs beneath the Guanacaste Geothermal Province.
A general description of methods, techniques, and apparatus used for the sampling, chemical analysis, and data reporting of geothermal gases and fluids is given. Step-by-step descriptions of the procedures are included in the appendixes.
This report describes methods used by the Stable Isotope and Gas Geochemistry
Laboratory of the U.S. Geological Survey for routine preparation, collection and analysis of
geothermal gas samples. Analyses of gases produced in geothermal systems provide information
on mineral-fluid and fluid-fluid interactions that occur below the earth's surface. Because certain
gases are ubiquitous in geothermal systems, routine analyses of these gases can be used to help
characterize geothermal systems and to determine temporal and spatial changes in these systems.
Geothermal gases that are routinely analyzed include: water vapor (H2O), carbon dioxide
(CO2), hydrogen sulfide (H2S), ammonia (NH3), hydrogen (H2), helium (He), argon (Ar),
oxygen (O2), nitrogen (N2), and methane (CH4). H2O, CO2 and H2S generally comprise
ninety-eight percent or more of these gases. Geothermal systems may also yield carbon
monoxide (CO), hydrocarbon gases other than CH4, and inert gases other than He and Ar. C2-
C7 hydrocarbons (hydrocarbon molecules containing from two to seven carbon atoms) may be
present, but are not routinely determined. CO and inert gases, other than He and Ar, are
measured elsewhere on selected samples; they will not be discussed here.
Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ34S, and 60 of these also for δ18O in the sulfate ion. Detailed δ34S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ34S. A summary curve for δ18O also shows definite variations, some at different times than δ34S, and always smaller. The measured δ34S and δ18O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ18O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ34S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ18O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ34S and δ18O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ34S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).
This study aims at developing methods that will lead to the generation of a reliable mean annual precipitation data base for Ethiopia. Multiple regression models have been formulated that explain the mean annual rainfall as a function of elevation and geographical location. The estimations, based on yearly values from a data set of 63 Ethiopian rainfall stations with records between 1969 and 1985, were developed for the whole country as well as for the already existing Food and Agricultural Organization (FAO) rainfall pattern regions and a new zonation derived by principal component and common factor analyses (PCA/CFA). In the PCA/CFA study, monthly rainfall values between 1968 and 1985 for 43 stations were used. The optimal zonation was derived by testing 36 different combinations resulting in different rainfall pattern regions. The alternatives tested were: correlation and covariance dispersion matrices, PCA and CFA eigentechni-ques, unrotated and rotated (Varimax and Direct Oblimin) components/factors and number of possible significant components/factors (3, 7, and 11). Principal component analysis of covariance matrix, Varimax rotation and seven extracted components gave by far the best relationship between mean annual rainfall, elevation, and geographical location. Models explaining at least 72 per cent of the variation in rainfall were constructed for regions covering about 98 per cent of the country, which is better than models based on the FAO rainfall pattern regions (69 per cent explained variation for 81 per cent of the country) and a model for the whole country (66·5 per cent explained variation).
Gas compositions of Cerro Prieto wells in 1977 reflected strong boiling in the reservoir around wells M-20 and M-25. This boiling zone appeared to be collapsing in 1982 when a number of wells in this area of the field were shut-in. In 1977 and 1982, gas compositions also showed boiling zones corresponding to faults H and L postulated by Halfman et al. (1982). Four gas geothermometers were applied, based on reservoir equilibria and calculated fugacities. The Fisher — Tropsch reaction predicted high temperatures and appeared to re-equilibrate slowly, whereas the H2S reaction predicted low temperatures and appeared to re-equilibrate rapidly. Hydrogen and NH3 reactions were intermediate. Like gas compositions, the geothermometers reflected reservoir processes, such as boiling. Surface gas compositions are related to well compositions, but contain large concentrations of N2 originating from air dissolved in groundwater. The groundwater appears to originate in the east and flow over the production field before mixing with reservoir gases near the surface.
Relationships between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932–83. Rainfall data consist of stations and regional indices calculated for three selected areas experiencing quite different rainfall patterns. The results indicate significant negative correlations between northern autumn rains in Somalia and the Southern Oscillation during the same season. These rains are also negatively correlated with pressure in the Western Indian Ocean, and positively correlated in the Eastern Indian Ocean. The reverse pattern is shown with sea-surface temperature. This agrees very well with the observations made in Kenya as far as the short rains of October-November are concerned.
Central Ethiopia summer rains, in contrast, indicate significant positive correlations with the Southern Oscillation, at zero and 3 months lag. El Niño years often correspond to drought years in this region. Evidence of an inverse relationship between the amount of rainfall in Ethiopia during summer (especially during September) and both the pressure and sea temperature over the Indian Ocean is also given. Correlations with the Arabian Sea are particularly strong.
However, teleconnections between summer rains in Northern Ethiopia (Eritrea) and the ENSO or the Indian Ocean fields are much weaker.
These results suggest that, although summer rains over Ethiopia are said to be related to the monsoon air flow from the Congo Basin and the Atlantic Ocean, there also exist quite strong connections with the surface conditions prevailing in the Indian and Pacific Oceans.
The chemical composition of gas mixtures emerging in thermal areas can be used to evaluate the deep thermal temperatures. Chemical analyses of the gas compositions for 34 thermal systems were considered and an empirical relationship developed between the relative concentrations of H2S, H2, CH4 and CO2 and the reservoir temperature. The evaluated temperatures can be expressed by: t°C = 24775 α + β + 36.05 -273 where α = 2 log CH4 CO2 -log H2 CO2-3 log H2S CO2 (concentrations in % by volume) and β = 7 log Pco2.
Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with ∼2.6 wt % dissolved H 2 O and a temperature near 870°C was intruded to within 2–4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.
Stable and radioactive isotope studies of ancient corals and mollusks from a fossil atoll in the Afar Rift indicate that final
separation of the Afar Depression from the Red Sea occurred not earlier than 32,000 years ago. Desiccation followed within
a few thousand years. The events recorded in the Afar Rift illustrate the processes occurring in the incipient stages leading
to the formation of an oceanic body by rifting of a continental block.
Deuterium and oxygen-18 measurements show that the Red Sea and Salton Sea brines are the results of a single process, the leaching of sediments by surface water circulating downward to a geothermal reservoir. The Salton Sea brine is derived from local precipitation but the Red Sea brine originates 1000 kilometers south of its basin, on the shallow sill which controls the circulation of the Red Sea. On this sill sea water penetrates a thick evaporite sequence to a depth of 2000 meters, and, driven by its increased density relative to sea water, flows northward to emerge in the brine-filled deeps.
The relationship between deuterium and oxygen-18 concentrations in natural meteoric waters from many parts of the world has
been determined with a mass spectrometer. The isotopic enrichments, relative to ocean water, display a linear correlation
over the entire range for waters which have not undergone excessive evaporation.
Geothermal potential of the Alid volcanic center
- M A Clynne
- W A Duffield
- R O Fournier
- L Giorgis
- C J Janik
- G Kahsai
- J Lowenstern
- K Mariam
- J G Smith
- T Tesfai
History and minerals of The Geysers
- G Dunning
- J F Cooper
Compilation of stable isotope fractionation factors of geochemicalinterest
- I Friedman
- J Oneil
Variations in δ18O values, water⧸rock ratios,and water flux in the Rico paleothermal anomaly
- P B Larson
- B S Zimmerman
A brief assessment of the Alid geothermal field: In: Report ES-10-94 of the IsraelMinistry of Energy and Infrastructure
- M Beyth
Preliminary assessment of the Alid geothermal field
- M Beyth
Geology of northern Afar (Ethiopia)
- Cnr-Cnrs
The Asal geothermal field (Republic ofDjibouti)
- H Correia
- C Fouillac
- A Gerard
- J Varet
Geology and geothermal potential of Alid volcaniccenter
- M A Clynne
- W A Duffield
- R O Fournier
- L Giorgis
- C J Janik
- G Kahsai
- J Lowenstern
- K Mariam
- J G Smith
- T Tesfai
Government of the State of (1995) Eritrea national map, 1:1,000
- Eritrea
Il vulcano Alid nella Colonia Eritrea
- A Marini
The weather over Eritrea, in Local Weather Prediction for the Red SeaCountries
- A H Michael
Scientific Party of the ManusFlux Cruise, 1997. Acidic and sulfate-richhydrothermal fluids from the Manus back-arc basin
- T Gamo
- K Okamura
- J-L Charlou
- T Urabe
- J-M Auzende
- J Ishibashi
- K Shitashima
- H Chiba
Geochemicalinvestigations of hydraulic connections between the Corwin Springs Known Geothermal ResourcesArea and adjacent parts of Yellowstone National Park
- Y K Kharaka
- R H Mariner
- T D Bullen
- B M Kennedy
- N C Sturchio
Volcanism and spreading in the northernmostsegment of the Afar rift (Gulf of Zula). In: Geodynamic evolution of the Afro Arabian rift system
- G Marinelli
- R Quaia
- R Santacroce