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.
Highlights
The dome of Dallol lies in the Danakil depression at the extension of the Main Ethiopian Rift (MER) of the Afar Triangle (Figure 1a)
We report on a terrestrial hydrothermal system that discharges hyperacidic, hypersaline and oxygen-free brines that contain up to 150 g/L of iron
We demonstrate that the hyperacidic pH, the brine evolution, the color palette, and the mineral paragenesis are controlled by inorganic processes, related to iron oxidation and iron complexation with chlorides and sulfates
Summary
The dome of Dallol lies in the Danakil depression at the extension of the Main Ethiopian Rift (MER) of the Afar Triangle (Figure 1a). For the analysis of δD and δ18O in water, an aliquot of water (0.7 μL) was injected onto a ceramic column containing a glassy carbon tube at 1450 °C to produce H2 and CO gases.[15] A hightemperature reactor (TC/EA) coupled online via a ConFlo III interface to a Delta XP isotope ratio mass spectrometer (Thermo-Finnigan, Bremen). These gases were separated by chromatography using a helium carrier gas stream. High-resolution transmission electron microscopy (HRTEM), along with fast fourier transform images (FFT), and selected area electron diffraction (SAED) were performed to investigate the crystal structure of the studied nanophases
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