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.

Figure 1

The hydrothermal system of Dallol within the Erta Ale range and maps of the active hydrothermal features of January 2017. (a) Dallol dome located in Danakil depression within the Erta Ale volcanic range of the Afar Triangle. (b) Geological map of the wider area of the Dallol dome, including Black and Yellow Lakes. (c) Aerial image of Dallol dome, showing the active areas in January 2017. (d) Map of January 2017 of the hydrothermal activity showing the referred sampling sites within the text, spring (S1), gray spring (S3) and a system of four successive pools (P1–P4).

Figure 2

Representative image of the hydrothermal system of Dallol. Fumaroles and active hydrothermal chimneys forming terraces and pools are shown at the background of the image. Moving downward from the terraces, the oxidation of the Fe mineral precipitates is manifested by the change in the color from white to green, yellow, and finally to red. Pillars formed by exhausted springs of past hydrothermal activity are shown at the upper left and right of the image. The height of the right pillar is 1.5 m.

Figure 3

Comparative bar graph of sulfate, chloride, fluoride, and iron concentrations of Dallol (purple-shaded area) and other hyperacidic volcanic sites and the Red Sea brines. The values for Copahue, Poas,^, Ruapehu, Nevado del Ruiz, Ebeko, Santa Ana, Kawah Ijen, Red Sea brines, and Dallol (this study) acidic fluids are shown in ppm.

Figure 4

Fe(II)/Fe(III) ratio, pH, O₂ concentration, and temperature of four successive pools of a typical spring-terrace system. Pool 1 (P1), the upper pool (oxygen-free spring is at the left of the pool, out of the field of view), is dominated by Fe(II) species of light green color. Moving downstream from the spring to pool 3 (P3), temperature is decreasing and the atmospheric O₂ diffusion results to the formation of Fe(III) species and to pH drop. Ultimately, the lowest pool 4 (P4) is characterized by Fe(III), the precipitation of which darkens the color of the pool, lowers further the pH, increases the temperature, and consumes the oxygen.

Figure 5

Raman and UV–vis spectra of the spring water (S1) and the four gradual pools (P1–P4) of Figure 1 and Figure 4. (a) Region of the Raman spectra showing the main vibrations of the Fe aqueous complexes. The concentration of the Fe(III) is increasing gradually from pool 2 to pool 4, as shown by the Fe(III)-Cl stretching vibration band at 312 cm^–1. This correlates with the decrease of the intensity of the Fe(II)–OH₂ stretching vibration at 370 cm^–1. (b) UV-absorbance spectra showing the increase of the band centered at 330 nm from the spring and upper pool to the lowest pools, due to Fe(II) oxidation and Cl-complexation.

Figure 6

Pictures, microscopic images, and mineral composition of different Dallol patterns. (a) Water-lily structures created by subaqueous hydrothermal activity. (b) Halite flowers with variable degree of oxidation. (c) Halite with egg-like shapes forming around gas vents. (d) Twisted hollow tubes of halite. (e) Halite pearls. (f) Efflorescences. (g) Chocolate bar cracks formed during crystallization of an Fe-rich brine or melt. (h) FESEM and Fe, K, S, Cl EDX map of a jarosite-rich efflorescence. (i) FESEM image of jarosite spherules. (j) FESEM image of part of a jarosite spherule decorated by akaganeite particles. (k) TEM/HAADF image of the akaganeite nanoparticles. (l) hematite spindles. (m) Micro-Raman spectrum of the hematite spindles. Mineral abbr.: Hl=Halite, Jrs=Jarosite, Na-Jrs=Natrojarosite, Ak=Akaganeite, Gt=Goethite, Fe-OH=Fe-oxyhydroxides, Fe-O=Fe-oxides, Hem=Hematite, Syl=Sylvite, Gp=Gypsum, Anh=Anhydrite, Car=Carnallite, S=Sulfur, Fe-/Mg-/Ca-Sil=Fe-/Mg-/Ca-silicates.

Figure 7

Stable isotope data for the Dallol hydrothermal brines and Black and Yellow Lake waters (2016 and 2017 campaigns). Dallol spring water has δD‰ values similar to the local meteoric water. The anomalous high δ¹⁸O values of the hot springs indicate the interaction of the meteoric water with the underlying basaltic flows and evaporites. The δ¹⁸O‰ and δD values of the Dallol pools are higher due to evaporation processes (δ¹⁸O > +10 ‰, δD > −10 ‰). Yellow and Black Lake waters exhibit relatively negative values for both δD (−25‰ to −48‰) and δ¹⁸O (−1‰ to −7‰) showing distinct hydrological processes with respect to the Dallol dome. MWL, Meteoric Water Line; AAMWL, Addis Ababa Meteoric Water Line. Magmatic and primary magmatic waters are plotted after refs (−.)