H2 generation versus H2 consumption in volcanic gas systems: A case study in the Afar hot spot in Djibouti

The Djibouti area is located in the Afar depression which corresponds to the core of a major hot spot at the junction of three rifts: The Red Sea oceanic rift, the Gulf of Aden oceanic rift and the East African continental rift. In the extension of the Gulf of Aden, the opening of the Gulf of Tadjourah in the last 30 million years was followed by the establishment of parallel NW-SE oriented depressions (Gobaad, Hanle, Gaggade, Assal-Ghoubbet). This region is characterised by high heat flow, complex volcanic and tectonic activity, and a dense network of faults favorable to the development of hydrothermal activities. A field survey was conducted to study the gas composition and origin in thermal springs and fumaroles along a transect running from the Lake Abbhé to Djibouti. Hydrogen traces were detected at fumaroles and hot springs associated with hydrothermal systems at the edges of the Gobaad, Hanlé, Gaggade, Assal-Ghoubbet and Arta area. The processes governing to the origin of H₂ and its consumption at different levels of the magmatic-hydrothermal systems are discussed. H₂ is interpreted as related to several processes including, chemical equilibrium in magmatic gases, alteration of Fe^II-rich rocks, and oxidation of volcanic H₂S. Methane associated with the hydrogen has mostly an abiotic origin and could result from the reaction of hydrogen with carbon oxides. Also, helium emanations have been observed notably on the sides of the Lake Abbhé in boiling hot springs. Gas samples have shown R/RA isotopic ratios of helium up to 10, which is are characteristic values for helium of mantle origin characterized an ³He enrichment with respect to air values.