Evidence of temperature increase of CO2-bearing fluids from Aluto-Langano geothermal field (Ethiopia): a fluid inclusions study of deep wells LA-3 and LA-6
Valori, Aldo; Teklemariam, Meseret; Gianelli, Giovanni
European Journal of Mineralogy Volume 4 Number 5 (1992), p. 907 - 920
published: Oct 14, 1992
manuscript accepted: Jan 15, 1992
manuscript received: Jun 27, 1991
ArtNo. ESP147050405003, Price: 29.00 €
Abstract The Aluto-Langano geothermal field is located within one of the most active volcanic areas of the Main Ethiopian Rift system. At Aluto, exploratory drilling has revealed the existence of high temperature (ca. 335 °C) two-phase fluids. This study is focused on wells LA-3 and LA-6, which are located on the main upflow zone of the geothermal system. Evidence for the occurrence of high intensity water-rock interaction is given by the presence of minerals such as calcite, chlorite, hematite, illite, quartz, sphene, epidote, garnet, prehnite, biotite and actinolite. Seven samples have been selected from drillcores recovered at different depths for petrographic observation, microthermometry and crushing experiments. Primary and essentially secondary fluid inclusions are recognized in authigenic calcite and quartz crystals as well as in igneous quartz crystals of the reservoir rock. Most of the inclusions are two-phase liquid-rich (liquid fraction: 0.8-0.9), but single-phase vapour inclusions are also present, especially in the upper part of the system. A total of 333 temperatures on cooling and 392 temperatures on heating were measured for liquid-rich (P and S) inclusions. The recognition of first melting indicates a fluid dominated by Na(K)Cl. Melting ice temperatures (Tmi) range from 0.0 to -1.9 °C. Clathrate was never observed. Homogenization temperatures (Th) range from 240 to 350 °C and are generally lower than the measured inhole temperatures at each depth. Both Tmi and Th values show bimodal frequency distributions suggesting that several generations of fluids have circulated early in the system. Crushing experiments and cooling demonstrate that important chemical changes have occurred from early hydrothermal fluid circulation to the present day, especially as regards PCO2∙ The Tmi-Th distribution defines a dilution trend between low salinity heated ground waters and a saline (about 3% NaCl eq.) high temperature fluid (350 °C) rising from a deep source. The mineral association calcite, epidote, garnet and/or prehnite is consistent with the initial very low CO2 content. The fluid inclusions study demonstrates heating up of the system from about 250 °C to the present-day temperatures of 320-335 °C. This heating could be explained by an intake of high enthalpy fluid in the upflow of the system.