Abstract
An alternative hypothesis for the origin of the banded iron formations and the synthesis of prebiotic molecules is presented here. I show the importance of considering water near its supercritical point and at alkaline pH. It is based on the chemical equation for the anoxic oxidation of ferrous iron into ferric iron at high-subcritical conditions of water and high pH, that I extract from E-pH diagrams drawn for corrosion purposes (Geophysical Research s Vol 15, EGU2013–22 Bassez 2013, Orig Life Evol Biosph 45(1):5-13, Bassez 2015, Procedia Earth Planet Sci 17, 492-495, Bassez 2017a, Orig Life Evol Biosph 47:453-480, Bassez 2017b). The sudden change in solubility of silica, SiO2, at the critical point of water is also considered. It is shown that under these temperatures and pressures, ferric oxides and ferric silicates can form in anoxic terrains. No FeII oxidation by UV light, neither by oxygen is needed to explain the minerals of the Banded Iron Formations. The intervention of any kind of microorganisms, either sulfate-reducing, or FeII-oxidizing or O2-producing, is not required. The chemical equation for the anoxic oxidation of ferrous iron is applied to the hydrolyses of fayalite, Fe2SiO4 and ferrosilite, FeSiO3. It is shown that the BIF minerals of the Hamersley Group, Western Australia, and the Transvaal Supergroup, South Africa, are those of fayalite and ferrosilite hydrolyses and carbonations. The dissolution of crustal fayalite and ferrosilite during water-rock interaction needs to occur at T&P just below the critical point of water and in a rising water which is undersaturated in SiO2. Minerals of BIFs which can then be ejected at the surface from venting arcs are ferric oxide hydroxides, hematite, FeIII-greenalite, siderite. The greenalite dehydrated product minnesotaite forms when rising water becomes supersaturated in SiO2, as also riebeckite and stilpnomelane. Long lengths of siderite without ferric oxides neither ferric silicates can occur since the exothermic siderite formation is not so much dependent in T&P. It is also shown that the H2 which is released during hydrolysis/oxidation of fayalite/ferrosilite can lead to components of life, such as macromolecules of amino acids which are synthesized from mixtures of (CO, N2, H2O) in Sabatier-Senderens/Fischer-Tropsch & Haber-Bosch reactions or microwave or gamma-ray excitation reactions. I propose that such geobiotropic synthesis may occur inside fluid inclusions of BIFs, in the silica chert, hematite, FeIII-greenalite or siderite. Therefore, the combination of high-subcritical conditions of water, high solubility of SiO2 at these T&P values, formation of CO also at these T&P, high pH and anoxic water, leads to the formation of ferric minerals and prebiotic molecules in the process of geobiotropy.
Highlights
I present an alternative to the usually accepted theory that ferric oxides form in the presence of UV light, oxygen, or microorganisms, introducing my 2013 proposition that they form through anoxic oxidation of FeII in high-subcritical water and high pH, and that this process can lead to the formation of prebiotic molecules in the process of geobiotropy
Conclusion on the Mineral Syntheses In the scenario for the formation of BIFs, upon the experiments described in the above paragraphs on Solubilities of Amorphous Silica and Quartz as a function of T&P, amorphous silica and quartz dissolve in hsc water and not in low supercritical water
Considering the studies reported in the paragraphs on the Solubility of Amorphous Silica versus Quartz, the silica which is dissolved in hsc water, interacts with the ocean cool water in a quenching process, and is distributed in equal amounts of amorphous and crystalline quartz
Summary
I present an alternative to the usually accepted theory that ferric oxides form in the presence of UV light, oxygen, or microorganisms, introducing my 2013 proposition that they form through anoxic oxidation of FeII in high-subcritical water and high pH, and that this process can lead to the formation of prebiotic molecules in the process of geobiotropy. Since 2013, I show in conferences and articles that ferric compounds can form under anoxic conditions, at high T ~300°C–350 °C, high P ~10MPa–25 MPa and high pH ~9.5–14 This proposition is based on a new redox equation that I extract from E-pH diagrams drawn for the system Fe-H2O (Cook and Olive 2012). The diagrams are posted at the URL address in Bassez (Bassez 1998–2018: La Géobiotropie) This equation which represents the oxidation of FeII into FeIII in the absence of oxygen and with the release of H2, led to the thermodynamic studies of hydrolyses and carbonations of silicates (Bassez 2013–Bassez 2017a, b) and to the new concept of geobiotropy (Bassez 2016a, b, 2017a, b). In this article I show one part of this new domain: how the alkaline anoxic abiotic oxidation of ferrous iron can bring new insights into the understanding of the Banded Iron Formations and the origin of life
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