Early onshore basaltic alteration and its natural hydrogen potential in the Asal–Ghoubbet rift, Republic of Djibouti.

Abstract

<p>The East African Rift (EAR) is a large opening system that allows the observation of all stages of rift evolution from continental opening in the south to oceanization in the north (Ethiopia-Djibouti). Also, the Asal–Ghoubbet active rift, in the Republic of Djibouti, is composed of a magmatic crust and tends to evolve into an oceanic crust. It’s a site of interest for geothermal energy and natural hydrogen. Previous studies have indicated that dihydrogen (H<sub>2</sub>) emanates from this rift. However, the well-known serpentinization reaction is not the mechanism generating H<sub>2</sub> at this site. Rather, the H<sub>2</sub> is generated as follows: (1) by alteration of basaltic lava at depth <em>via </em>reaction with seawater flowing from Ghoubbet Bay towards Lake Asal; (2) by simple degassing of the volcanic chamber located a few kilometers below the Fiale Caldera in the rift axis; or (3) as a result of pyritization processes <em>via </em>the oxidation of H<sub>2</sub>S.</p><p>Drill cuttings from the Fiale 1 (F1) and Gale le Goma 1 (Glc1) geothermal wells (located on the inner and outer rift margins, respectively) were analyzed to determine where H<sub>2</sub> is generated. Total rock analyses indicated distinct zones at depths of 464 m and 280 m for F1 and Glc1, respectively, representing the boundary between the Asal and Stratoïd Basalts. <sup>57</sup>Fe Mössbauer analyses show a decrease in the percentage of Fe<sup>3+</sup> at depth, indicating that Fe<sup>2+</sup>-rich material, particularly in the Stratoïd Basalts, may be a source of H<sub>2</sub>.</p><p>Based on well data from the rift center and the outer rift margin, it is evident that H<sub>2</sub> is present at the surface in the rift axis and that this area offers good remnant potential because of the presence of Fe-rich chlorite. Conversely, few H<sub>2</sub> emissions were measured at the surface on the outer rift margins, although well data showed some H<sub>2</sub> (~0.25%) at depth. The presence of a cap rock in the rift axis has not yet been proven; however, the high loss on ignition and the mineralogy in well Glc1 may indicate that the rocks are sufficiently altered to offer potential as a seal. If so, the rift margins would offer greater exploration potential than the rift center.</p>