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Gas emissions and sub-surface architecture of fault-controlled geothermal systems: a case study of the North Abaya geothermal area
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  • William Hutchison,
  • Euan Ogilvie,
  • Yafet G Birhane,
  • Peter H Barry,
  • Tobias P. Fischer,
  • Chris J Ballentine,
  • Darren J Hillegonds,
  • Juliet Biggs,
  • Fabien Albino,
  • Chelsea Cervantes,
  • Snorri Guðbrandsson
William Hutchison
University of St Andrews

Corresponding Author:wh39@st-andrews.ac.uk

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Euan Ogilvie
University of St Andrews
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Yafet G Birhane
CRPG, Université de Lorraine
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Peter H Barry
Woods Hole Oceanographic Institution
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Tobias P. Fischer
University of New Mexico
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Chris J Ballentine
University of Oxford
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Darren J Hillegonds
University of Oxford
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Juliet Biggs
University of Bristol, UK
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Fabien Albino
University of Grenoble Alpes, ISTerre
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Chelsea Cervantes
Reykjavik Geothermal
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Snorri Guðbrandsson
Reykjavik Geothermal
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Abstract

East Africa hosts significant reserves of untapped geothermal energy. Most exploration has focused on geologically young (<1 Ma) silicic caldera volcanoes, yet there are many sites of geothermal potential where there is no clear link to an active volcano. The origin and architecture of these systems is poorly understood. Here, we combine remote sensing and field observations to investigate a fault-controlled geothermal play located north of lake Abaya in the Main Ethiopian Rift. Soil gas CO2 and temperature surveys were used to examine permeable pathways and showed elevated values along a ~110 m high fault which marks the western edge of the Abaya graben. Ground temperatures are particularly elevated where multiple intersecting faults form a wedged horst structure. This illustrates that both deep penetrating graben bounding faults and near-surface fault intersections control the ascent of hydrothermal fluids and gases. Total CO2 emissions along the graben fault are ~300 t d–1; a value comparable to the total CO2 emission from silicic caldera volcanoes. Fumarole gases show δ13C of –6.4 to –3.8 ‰ and air-corrected 3He/4He values of 3.84–4.11 RA, indicating a magmatic source originating from an admixture of upper mantle and crustal helium. Although our model of the North Abaya geothermal system requires a deep intrusive heat source, we find no ground deformation evidence for volcanic unrest nor recent volcanism. This represents a key advantage over the active silicic calderas that typically host these resources and suggests that fault-controlled geothermal systems offer viable prospects for further exploration and development.