Water-rock interaction processes in the Aluto-Langano geothermal field (Ethiopia)

Gianelli, Giovanni; Teklemariam, Meseret

doi:10.1016/0377-0273(93)90007-e

Cited by 60 publications

(65 citation statements)

References 18 publications

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“…Aluto has an internally drained central depression (herein referred to as a caldera floor) that has been partially in-filled by alluvium and reworked material from the surrounding surface volcanic deposits. While there are no detailed accounts of the eruptive history of the complex, descriptions of the geology of Aluto are in regional-scale mapping reports (e.g., Dakin and Gibson, 1971;Di Paola, 1972;Kebede et al, 1985); studies of the Ziway-Shala lake basin system (Gasse and Street, 1978;Street, 1979;Le Turdu et al, 1999;Benvenuti et al, 2002;Gibert et al, 2002), and publications related to geothermal development on Aluto (Gebregzabher, 1986;Valori et al, 1992;Gizaw, 1993;Gianelli and Teklemariam, 1993;Teklemariam et al, 1996;Saibi et al, 2012). This work and our own field investigations suggest that the Aluto volcanic complex has undergone several cycles of explosive and effusive volcanism throughout its history and that these eruptive phases have been dominantly silicic in composition.…”

Section: Aluto Volcano-geological Overview and Deep Well Observationsmentioning

confidence: 98%

Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

et al. 2015

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In volcanically and seismically active rift systems, preexisting faults may control the rise and eruption of magma, and direct the flow of hydrothermal fluids and gas in the subsurface. Using high-resolution airborne imagery, field observations, and CO 2 degassing data on Aluto, a typical young silicic volcano in the Main Ethiopian Rift, we explore how preexisting tectonic and volcanic structures control fluid pathways and spatial patterns of volcanism, hydrothermal alteration and degassing. A new light detection and ranging (lidar) digital elevation model and evidence from deep geothermal wells show that the Aluto volcanic complex is dissected by rift-related extensional faults with throws of 50-100 m. Mapping of volcanic vent distributions reveals a structural control by either rift-aligned faults or an elliptical caldera ring fracture. Soil-gas CO 2 degassing surveys show elevated fluxes (>>100 g m -2 d -1 ) along major faults and volcanic structures, but significant variations in CO 2 flux along the fault zones reflect differences in near-surface permeability caused by changes in topography and surface lithology. The CO 2 emission from an active geothermal area adjacent to the major fault scarp of Aluto amounted to ~60 t d -1 ; we estimate the total CO 2 emission from Aluto to be 250-500 t d -1 . Preexisting vol canic and tectonic structures have played a key role in the development of the Aluto volcanic complex and continue to facilitate the expulsion of gases and geothermal fluids. This case study emphasizes the importance of structural mapping on active rift volcanoes to understand the geothermal field as well as potential volcanic hazards.

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Section: Aluto Volcano-geological Overview and Deep Well Observationsmentioning

confidence: 98%

Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

et al. 2015

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“…Fournier, 1985;Gianelli and Teklemarian, 1993). This hypothesis is supported by several lines of evidence.…”

Section: Constraints From δ 18 θ Signaturesmentioning

confidence: 65%

Origin of Authigenic Carbonates in Eocene to Quaternary Sediments from the Arctic Ocean and Norwegian-Greenland Sea

Chow¹,

Morad²,

Al‐Aasm³

1996

Proceedings of the Ocean Drilling Program

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Authigenic carbonates occur in thin layers, nodules, and burrows in fine-grained siliciclastic sediments of middle Eocene to Quaternary age that were recovered from the Arctic Ocean and Norwegian-Greenland Sea during Leg 151. At Site 913 (East Greenland Margin), concentrically zoned microspherules and rhombohedrons of ferroan rhodochrosite and manganoan siderite, 20-600 µm in diameter, are the main authigenic carbonates in the sedimentary succession. They grew displacively within the host clays and silts, and have locally coalesced to form aggregates. At Sites 909 (Fram Strait) and 911 (Yermak Plateau), very fine to fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate in the sedimentary succession. It occurs as interparticle cement, disseminated crystals, and rounded intraclasts in host clays, silts, and muds. Minor micritic or radialfibrous calcite and dolomite were also found at these three sites. Most of these carbonates are interpreted to have precipitated at shallow-burial depths during early diagenesis.All authigenic carbonates (rhodochrosite, siderite, and calcite) from Site 913 are strongly enriched in 12 C (δ 13 C = -22.9 to -13.8%c) and show decreasing δ 13 C values with depth below the seafloor. This evidence, along with authigenic pyrite found in microspherule cores, suggests that dissolved carbon was derived from the oxidation of marine organic matter in the bacterial sulfate-reduction zone and during the early stages of methanogenesis. However, low organic matter content and headspace methane concentrations suggest additional derivation of dissolved carbon from thermogenic methane. The very low δ 18 θ values (-11.8%c) for rhodochrosite and the close association with barite in voids and fractures suggest rhodochrosite precipitation from Mn-charged, hydrothermal fluids, possibly related to volcanic activity associated with seafloor spreading. The δ 13 C values of most siderites from Site 909 are close to -6%c, which suggests an origin in the Fe-reduction, suboxic zone, possibly only centimeters or decimeters below the seafloor. However, minor occurrences of siderite with very low δ 13 C values (-21.8%c), along with their close association with authigenic pyrite, indicate additional derivation of dissolved carbon from bacterial sulfate-reduction and the early stages of methanogenesis. The importance of anaerobic microbial processes is further corroborated by the moderate to high organic matter content and headspace methane concentration at Site 909. The range of δ 18 θ values for siderite from Sites 909 and 911 (-10.3 to +5.0%o) appears to be related to both increasing burial temperatures and interaction between labile volcaniclastic sediment and marine pore waters.

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“…The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage (Teklemariam et al 1996). Such alterations are the result of high geothermal discharge with the reservoir temperature registered at 360°C by Gizaw (1993) and 335°C by Gianelli and Teklemariam (1993). It is evident that the geothermal system of the Sodere-Gergedi area is much lower than the Aluto-Langano system and the rate of alteration is consequently low.…”

Section: Geological and Hydrogeological Settingmentioning

confidence: 91%

“…This potential has been estimated to be in the range of 1,000 Mega Watts (MW) (UNDP 1973;Chernet and GebreEgziabher 1983;Gianelli and Teklemariam 1993;Gizaw 1993;Teklemariam et al 1996; Ayele et al 2002). Other localized/small-area, lowbudget and less-extensive studies have been conducted in areas such as Boku, Gergedi and Gemeto (close to Awassa) thermal springs with a view to small-scale utilization of the resource for household, therapeutic and recreational purposes (Alemayehu and Vernier 1997;Haile et al 2002).…”

Section: Introductionmentioning

confidence: 98%

The interference of a deep thermal system with a shallow aquifer: the case of Sodere and Gergedi thermal springs, Main Ethiopian Rift, Ethiopia

Haile

Abiye

2012

Hydrogeol J

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An integrated survey program involving geological, hydrogeological and geophysical techniques has been employed to characterize the aquifer geometry, recharge and circulation dynamics of thermal springs within a shallow aquifer system in Ethiopia. The selected springs for the case study are Sodere and Gergedi, which are situated within the tectonically active Main Ethiopian Rift (MER). Geologically, the studied springs are located on PlioQuaternary volcanic rocks. The geophysical results indicate the presence of subsurface weak zones represented by extensional tectonics and weathering zones which are responsible for thermal water circulation and facilitate recharge from the adjacent surface-water bodies. The structures inferred by the resistivity survey, both sounding and electrical tomography, present contrasts in rock resistivity response. The anomalous zones in the magnetic data are in good agreement with the zones that are revealed by geological mapping and surface manifestation of the thermal water discharge zones. The shallow aquifer of the central MER is under the influence of thermal water, which increases the groundwater temperature and mineral content.

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Water-rock interaction processes in the Aluto-Langano geothermal field (Ethiopia)

Cited by 60 publications

References 18 publications

Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

Origin of Authigenic Carbonates in Eocene to Quaternary Sediments from the Arctic Ocean and Norwegian-Greenland Sea

The interference of a deep thermal system with a shallow aquifer: the case of Sodere and Gergedi thermal springs, Main Ethiopian Rift, Ethiopia

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