A Polyextreme Hydrothermal System Controlled by Iron: The Case of Dallol at the Afar Triangle

Abstract

One of the latest volcanic features of the Erta Ale range at the Afar Triangle (NE Ethiopia) has created a polyextreme hydrothermal system located at the Danakil depression on top of a protovolcano known as the dome of Dallol. The interaction of the underlying basaltic magma with the evaporitic salts of the Danakil depression has generated a unique, high-temperature (108 °C), hypersaline (NaCl supersaturated), hyperacidic (pH values from 0.1 to −1.7), oxygen-free hydrothermal site containing up to 150 g/L of iron. We find that the colorful brine pools and mineral patterns of Dallol derive from the slow oxygen diffusion and progressive oxidation of the dissolved ferrous iron, the iron-chlorine/-sulfate complexation, and the evaporation. These inorganic processes induce the precipitation of nanoscale jarosite-group minerals and iron(III)-oxyhydroxides over a vast deposition of halite displaying complex architectures. Our results suggest that life, if present under such conditions, does not play a dominant role in the geochemical cycling and mineral precipitation at Dallol as opposed to other hydrothermal sites. Dallol, a hydrothermal system controlled by iron, is a present-day laboratory for studying the precipitation and progressive oxidation of iron minerals, relevant for geochemical processes occurring at early Earth and Martian environments.

Hyperdiverse archaea near life limits at the polyextreme geothermal Dallol area

“…2a; Fig. S3a): 1) In agreement with recent observations, Dallol ponds were characterized by NaCl supersaturated brines highly enriched in Fe with different oxidation states, largely explaining color variation (14). Potassium and sulfur were also abundant (Table S2).…”

“…S1). Dallol chimneys and hyperacidic ponds can appear and desiccate in a matter of days or weeks, generating a variety of evaporitic crystalline structures observable in situ (14). Likewise, very active, occasionally explosive (salt 'bombs'), hydrothermal activity characterized by hot (110°C), slightly acidic (pH~4.4), black hypersaline fluids was detected in the Black Mountain area in 2016 ('Little Dallol'; sample BL6-01; Figs.…”

Origin and Evolution of the Halo-Volcanic Complex of Dallol: Proto-Volcanism in Northern Afar (Ethiopia)

“…30 km south to Dallol, on the northern side of the Erta Ale volcanic chain. In addition to geological observations, field and laboratory analyses were performed to characterize the local physicochemistry of the different local polyextreme environments, determine whether life can thrive under those conditions (Belilla et al, 2019) and study brine chemical evolution and the dynamics of mineral precipitation (Kotopoulou et al, 2019). During the third field trip (12-26 January 2019), we revisited the previously sampled areas and we extended our exploration to the Round Mountain and to different lakes around the Erta Ale volcanic range: Lake Assale (Karum), Lake Bakili and several unnamed small lakes ( Figures 1A,B).…”

Discovery of Afifi, the shallowest and southernmost brine pool reported in the Red Sea

“…A hydrothermal system would justify the high values of oxygen, but not those of hydrogen 50 . In fact, subaerial hydrothermal water in the same general basin of the Red Sea-crossing basalt and evaporitic sediment, has high δ 18 O values of around +10‰ (V-SMOW) but little change in δD 51,52 . The Afifi brine pool and column waters showed a linear relationship between δ 18 O and δD values that is compatible with evaporation processes (δD = 4.82 δ 18 O + 3.67; R 2 = 0.95, Fig.…”

“…The geological setting of the Afifi pool is between Plio-Pleistocene reef platforms underlain by Miocene layered evaporites, including salt 9 . Salt tectonic processes formed diapirs that flowed on the seafloor 52 , and the dissolution of salt may have formed the brines that accumulated in the Afifi bathymetric depression. We suggest that where the interface between overlaying Red Sea waters and the brine pool waters is located.…”