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.
Iron‐silica self‐organized membranes, so‐called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron‐silica membranes, grown from synthetic and natural, alkaline, serpentinization‐derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface‐area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron‐silica membranes might have assisted the generation and organization of the first biologically relevant organics.
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