Water, Air, Earth and Cosmic Radiation

Abstract

In the context of the origin of life, rocks are considered mainly for catalysis and adsorption-desorption processes. Here it is shown how some rocks evolve in energy and might induce synthesis of molecules of biological interest. Radioactive rocks are a source of thermal energy and water radiolysis producing molecular hydrogen, H₂. Mafic and ultramafic rocks evolve in water and dissolved carbon dioxide releasing thermal energy and H₂. Peridotites and basalts contain ferromagnesian minerals which transform through exothermic reactions with the generation of heat. These reactions might be triggered by any heating process such as radioactive decay, hydrothermal and subduction zones or post-shock of meteorite impacts. H₂ might then be generated from endothermic hydrolyses of the ferromagnesian minerals olivine and pyroxene. In both cases of mafic and radioactive rocks, production of CO might occur through high temperature hydrogenation of CO₂. CO, instead of CO₂, was proven to be necessary in experiments synthesizing biological-type macromolecules with a gaseous mixture of CO, N₂ and H₂O. In the geological context, N₂ is present in the environment, and the activation source might arise from cosmic radiation and/or radionuclides. Ferromagnesian and radioactive rocks might consequently be a starting point of an hydrothermal chemical evolution towards the abiotic formation of biological molecules. The two usually separate worlds of rocks and life are shown to be connected through molecular and thermodynamic chemical evolution. This concept has been proposed earlier by the author (Bassez J Phys: Condens Matter 15:L353–L361, 2003, 2008a, 2008b; Bassez Orig Life Evol Biosph 39(3–4):223–225, 2009; Bassez et al. 2011; Bassez et al. Orig Life Evol Biosph 42(4):307–316, 2012, Bassez 2013) without thermodynamic details. This concept leads to signatures of prebiotic chemistry such as radionuclides and also iron and magnesium carbonates associated with serpentine and/or talc, which were discussed at the 2014 European Astrobiology Network Association conference on Signatures of Life.

Introduction: Chemical Setting

A great number of laboratory experiments have produced prebiotic molecules thought to be necessary for the construction of living organisms. Since 1989, Kobayashi and coworkers have conducted experiments in the gas phase (see for example Kobayashi et al. 2008). When a mixture of CO and N₂ or NH₃ above liquid water is irradiated with energy typical of cosmic radiation such as protons, He ions, electrons, γ rays, soft X-rays and UV radiation, deposits of macromolecular structures are formed. These deposits occur with UV light acting on NH₃ but not on N₂. They produce amino acids and nucleic acids bases and they transform into aggregates upon heating (Kurihara et al. 2012). Production occurs with CO but not with CO₂. The primitive Earth’s atmosphere may have contained essentially carbon dioxide. CO might have been present through CO₂ photolysis as proved by observations of the modern atmosphere (Clerbaux et al. 2005). However diffusion processes in the gas phase might have distributed molecules over a wide area. Questions are: which medium is able to concentrate molecules for reactions to occur and how can CO be produced in this medium? Cavities inside rocks might be an answer to the first question. Supercritical water (374 °C, 22.1 MPa) which has greatly reduced polarity can also concentrate apolar molecules such as CO₂, H₂ and N₂. As discussed previously, water of low polarity and cavities inside rocks might offer special locations for molecular concentration (Bassez 2003). Consequently surface and sub-surface rocks are media where molecules can concentrate and reactions might occur. Sub-surface water might easily reach temperatures as high as 500-800 °C, and pressures above critical point, for instance in zones of subduction, hydrothermal vents or geysers. Surface rocks are likely also to be exposed to supercritical water. For instance, meteorite impacts might generate post-shock pressures with temperatures as high as 500 °C and above (French 1998). Heating due to impact during Late Heavy Bombardment contributed to increase surface temperature. Ejecta temperatures have been calculated as a function of crater diameters on the Earth and the Moon: ca. 800 °C is proposed for a ca. 120 km crater (Abramov and Mojzsis 2012).

Experiments show that methane is obtained from hydrogenation of CO₂ dissolved in hydrothermal water at 390 °C and 400 bars (Foustoukos and Seyfried 2004). In this reaction, CO might be an intermediate, since hydrocarbons can be obtained from CO gas through Fisher-Tropsch Type reactions. CO has also been formed at atmospheric pressure through catalytic hydrogenation of CO₂ and a flow of H₂ at 500 °C (Chen et al. 2000). H₂ production is consequently a necessary step for the formation of CO. The question how to obtain CO from CO₂ consequently becomes the question: how can H₂ be obtained in a geological context of rocks and heated water? In this article, it is shown that radioactive rocks are a source of thermal energy and water radiolysis producing molecular hydrogen, H₂ and that mafic and ultramafic rocks evolving in water and dissolved carbon dioxide are a source of thermal energy and H₂. Consequently, radioactive and mafic rocks might induce synthesis of molecules of biological interest. The excitation sources might arise from cosmic radiation and/or radioactive rocks. In the context of the origin of life, minerals have been considered in catalysis and adsorption-desorption processes (Cleaves et al. 2012; Pandey et al. 2013). Here it is the energy produced by their chemical evolution which is mainly analyzed. A scenario is proposed for syntheses of biological-type macromolecules following meteorite impacts during Late Heavy Bombardment. It is concluded that geological sites where iron and magnesium carbonates are observed in association with serpentine and/or talc, might have been in the past, local sites of high local heat production at the origin of H₂ and CO and molecules of biological interest. It is also concluded that in the absence of ferromagnesian rocks, occurrence of long-lived and now-extinct radionuclides are also to be considered for production of H₂, CO and biological-type molecules.

Thermodynamic Results and Discussions

Serpentinization reactions of rocks are usually written globally following Eq. (1). Olivine and pyroxene react with water and CO₂ to produce other minerals, magnetite, carbonates and H₂.

$$ \begin{array}{c}\kern1em \mathrm{olivine}+\mathrm{pyroxene}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\kern0.28em \Longleftrightarrow \kern0.28em \mathrm{serpentine}\kern1em \\ {}\kern1em +{\left(\mathrm{FeMg}\right)}_2{\mathrm{Si}\mathrm{O}}_4+\left(\mathrm{FeMg}\right){\mathrm{Si}\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\kern0.28em \Longleftrightarrow \kern0.28em {\left({\mathrm{Fe}}^{2+}{\mathrm{Fe}}^{3+}\mathrm{Mg}\right)}_3{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{O}\mathrm{H}\right)}_4\kern1em \\ {}\kern1em \mathrm{talc}+\mathrm{brucite}\kern0.36em +\mathrm{magnetite}+\mathrm{quartz}+\mathrm{carbonates}+\mathrm{hydrogen}\kern1em \\ {}\kern1em {\left(\mathrm{FeMg}\right)}_3{\mathrm{Si}}_4{\mathrm{O}}_{10}{\left(\mathrm{O}\mathrm{H}\right)}_2+\left(\mathrm{FeMg}\right){\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{Fe}}_3{\mathrm{O}}_4\kern0.36em +{\mathrm{Si}\mathrm{O}}_2+{\mathrm{Fe}\mathrm{CO}}_3+{\mathrm{MgCO}}_3+{\mathrm{H}}_2\kern1em \end{array} $$

(1)

The author calculated values of thermodynamic functions for some elementary reactions of the iron and magnesium endmembers of olivine and pyroxene (2) to (9). Enthalpies Δ_r H°, entropies Δ_r S° and free enthalpies Δ_r G°, are calculated for hydrolysis and carbonation reactions of the olivines fayalite (Fe₂SiO₄) and forsterite (Mg₂SiO₄) and for the pyroxenes ferrosilite (FeSiO₃) and enstatite (MgSiO₃). Individual values for products and reactants are taken from tabulated data at 298.15 K and 10⁵ Pa (Robie and Hemingway 1995). Values for SiO₂ are those of quartz. Units of Δ_r H°, Δ_r S° and Δ_r G°, are given per mole of advancement of the reaction.

Hydrolysis of Olivine

$$ \begin{array}{c}\kern1em \ast \mathbf{3}/\mathbf{2}\ \mathbf{F}{\mathbf{e}}_{\mathbf{2}}\mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{4}}\kern1em +\kern1em {\mathbf{H}}_{\mathbf{2}}\mathbf{O}\ \to \kern0.5em \mathbf{F}{\mathbf{e}}_{\mathbf{3}}{\mathbf{O}}_{\mathbf{4}}+\mathbf{3}/\mathbf{2}\ \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}+{\mathbf{H}}_{\mathbf{2}}\kern1em \\ {}\kern1em {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ}=+21.35\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern4.5em {\Delta}_{\mathrm{r}}\mathrm{S}{}^{\circ}=+42.85\ \mathrm{J}.{\mathrm{K}}^{-1}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern1em \\ {}\kern1em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ}={\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}}\kern0.5em =8.6\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern1.5em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ}={\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ}-\mathrm{T}.{\Delta}_{\mathrm{r}}\mathrm{S}{}^{\circ}=8.57\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern1em \end{array} $$

(2)

$$ \begin{array}{c}\hfill *\ \mathbf{M}{\mathbf{g}}_{\mathbf{2}}\mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{4}}\kern0.5em + \mathbf{3}/\mathbf{2}\ {\mathbf{H}}_{\mathbf{2}}\mathbf{O}\kern0.5em \to\ \mathbf{1}/\mathbf{2}\ \mathbf{M}{\mathbf{g}}_{\mathbf{3}}\mathbf{S}{\mathbf{i}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{5}}{\left(\mathbf{O}\mathbf{H}\right)}_{\mathbf{4}}\kern0.75em +\kern0.75em \mathbf{1}/\mathbf{2}\ \mathbf{M}\mathbf{g}{\left(\mathbf{O}\mathbf{H}\right)}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 40.55\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern1.75em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}}\kern0.5em = - 23.7\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\hfill \end{array} $$

(3)

Carbonation of Olivine

$$ \begin{array}{c}\hfill *\ \mathbf{F}{\mathbf{e}}_{\mathbf{2}}\mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{4}}\kern0.75em + \mathbf{2}\ \mathbf{C}{\mathbf{O}}_{\mathbf{2}}\kern1.25em \to \kern1em \mathbf{2}\ \mathbf{F}\mathbf{e}\mathbf{C}{\mathbf{O}}_{\mathbf{3}}\kern0.5em + \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 157.3\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern2em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}}\kern0.5em = - 55.8\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\hfill \end{array} $$

(4)

$$ \begin{array}{c}\hfill *\ \mathbf{M}{\mathbf{g}}_{\mathbf{2}}\mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{4}} + \mathbf{2}\ \mathbf{C}{\mathbf{O}}_{\mathbf{2}}\kern1.5em \to \kern1em \mathbf{2}\ \mathbf{M}\mathbf{g}\mathbf{C}{\mathbf{O}}_{\mathbf{3}}\kern0.75em +\kern0.75em \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 177.3\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern1.5em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}} = - 72.9\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\hfill \end{array} $$

(5)

Hydrolysis of Pyroxene

$$ \begin{array}{c}\kern1em \ast \mathbf{3}\ \mathbf{F}\mathbf{eSi}{\mathbf{O}}_{\mathbf{3}}\kern0.5em +{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\kern0.5em \to \kern0.5em \mathbf{F}{\mathbf{e}}_{\mathbf{3}}{\mathbf{O}}_{\mathbf{4}}+\mathbf{3}\ \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}+{\mathbf{H}}_{\mathbf{2}}\kern1em \\ {}\kern1em {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ}=+23.6\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern5em {\Delta}_{\mathrm{r}}\mathrm{S}{}^{\circ}=+47.8\ \mathrm{J}.{\mathrm{K}}^{-1}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern1em \\ {}\kern1em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ}={\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}}\kern0.5em =+9.5\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern1em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ}={\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ}-\mathrm{T}.{\Delta}_{\mathrm{r}}\mathrm{S}{}^{\circ}=9.35\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern1em \end{array} $$

(6)

$$ \begin{array}{c}\hfill *\ \mathbf{3}/\mathbf{2}\ \mathbf{M}\mathbf{gSi}{\mathbf{O}}_{\mathbf{3}}\kern1em + {\mathbf{H}}_{\mathbf{2}}\mathbf{O}\kern0.5em \to\ \mathbf{1}/\mathbf{2}\ \mathbf{M}{\mathbf{g}}_{\mathbf{3}}\mathbf{S}{\mathbf{i}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{5}}{\left(\mathbf{O}\mathbf{H}\right)}_{\mathbf{4}}\kern0.5em + \mathbf{1}/\mathbf{2}\ \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 31.15\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\kern2.75em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}} = - 19.8\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\hfill \end{array} $$

(7)

Carbonation of Pyroxene

$$ \begin{array}{c}\hfill *\ \mathbf{Fe}\mathbf{Si}\mathbf{O}\mathbf{3} + \mathbf{C}{\mathbf{O}}_{\mathbf{2}}\kern0.5em \to\ \mathbf{F}\mathbf{e}\mathbf{C}{\mathbf{O}}_{\mathbf{3}} + \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 77.9\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern2.25em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}} = - 26.7\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\hfill \end{array} $$

(8)

$$ \begin{array}{c}\hfill *\ \mathbf{Mg}\mathbf{Si}{\mathbf{O}}_{\mathbf{3}} + \kern0.5em \mathbf{C}{\mathbf{O}}_{\mathbf{2}}\kern1em \to\ \mathbf{M}\mathbf{g}\mathbf{C}{\mathbf{O}}_{\mathbf{3}} + \kern0.5em \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 84.9\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern2.25em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}} = - 33.1\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\hfill \end{array} $$

(9)

Serpentine produced through exothermic hydrolysis of olivine and pyroxene can spontaneously lead to talc through reaction (10) that is slightly exothermic.

$$ \begin{array}{c}\hfill *\ \mathbf{M}{\mathbf{g}}_{\mathbf{3}}\mathbf{S}{\mathbf{i}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{5}}{\left(\mathbf{O}\mathbf{H}\right)}_{\mathbf{4}}\ \left(\mathbf{serp}\right) + \mathbf{2}\ \mathbf{S}\mathbf{i}{\mathbf{O}}_{\mathbf{2}}\kern0.5em \to\ \mathbf{M}{\mathbf{g}}_{\mathbf{3}}\ \mathbf{S}{\mathbf{i}}_{\mathbf{4}}{\mathbf{O}}_{\mathbf{10}}{\left(\mathbf{O}\mathbf{H}\right)}_{\mathbf{2}}\kern0.5em \left(\mathbf{talc}\right) + {\mathbf{H}}_{\mathbf{2}}\mathbf{O}\hfill \\ {}\hfill {\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} = - 4.4\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\kern2.25em {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ} = {\sum}_{\mathrm{i}}{\upnu}_{\mathrm{i}}.{\Delta}_{\mathrm{f}}\ \mathrm{G}{{}^{\circ}}_{\mathrm{i}} = - 12.3\ \mathrm{kJ}.\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1\ }\hfill \end{array} $$

(10)

Recent experiments confirm Eq. (5). When forsterite was exposed to water and CO₂ at 120 °C and 80 bars over 7 days, crystals of magnesite and spherical particles of SiO₂ were observed (Aaberg et al. 2013).

For both olivine and pyroxene production of H₂ occurs through hydrolysis of the iron silicate. Both reactions at 298 K are endothermic and not spontaneous. Calculations with Robie and Hemingway’s data show that they do not proceed spontaneously even at the Curie temperature for the magnetic transition of magnetite, 838 K, where cation reordering occurs (Levy et al. 2012). However hydrolyses of the magnesium silicates are both exothermic and spontaneous. Carbonations of the iron and magnesium endmembers of olivine are highly exothermic and proceed spontaneously. Hydrolyses of the Mg-olivine and Mg-pyroxene lead to production of serpentine and consecutively of talc. Consequently, the six exothermic reactions including hydrolyses of forsterite and enstatite and carbonations of fayalite and forsterite and of ferrosilite and enstatite should induce the endothermic hydrolysis of the iron silicates fayalite and ferrosilite, with the resulting production of H₂ which is experimentally observed. Through these six exothermic reactions, heat is released. An approximate calculation of temperature elevation can be attempted using the amount of heat released during spontaneous carbonation of forsterite and considering that this heat is only absorbed by water (eq. 11). Using the value of the enthalpy which remains after the energy corresponding to entropy is removed, i.e., using the free enthalpy value Δ_r G° = − 72.9 kJ.mol⁻¹ (eq.5), temperature raises up to ca.700 °C. More precise values may be obtained using more elaborate calculations considering that heat is also used for heating minerals and considering that inside some specific rock cavities, it is neither H nor G, but energy U and free energy F, and also heat capacity C_v which should be considered. Calculations using the YMP database for CO₂ sequestration shows that an injected system of 100 bar pCO₂ at 90 °C would enhance olivine carbonation by a factor of 75 compared to the system of 100 bar pCO₂ at 30 °C (Paukert et al. 2012). Consequently, carbonation might be triggered by any heating process, such as focused solar radiation, radioactive decay, subduction zones as for Isua Supercrustal Belt, hydrothermal activity encountered in vents or geysers or veins as for the mafic and ultramafic geological environment of the australian 3.446 Ga Kitty’s Gap chert, Warrawoona, Pilbara (Bassez et al. 2011) and also as a consequence of meteorite impacts on the Earth and extraterrestrial objects. High amount of heat can be locally released and help production of CO. Consequently, geological sites where iron and magnesium carbonates are observed in association with serpentine and/or talc, might have been in the past, local sites of high local heat production at the origin of H₂ and consecutively CO and biological-type molecules.

$$ {\Delta}_{\mathrm{r}}\mathrm{G}{}^{\circ}={\Delta}_{\mathrm{r}}\mathrm{H}{}^{\circ} - \mathrm{T}.{\Delta}_{\mathrm{r}}\mathrm{S}{}^{\circ}=\Delta \mathrm{H}{}^{\circ}\left(\mathrm{l}\mathrm{i}\mathrm{q},298\to 373\right) + \Delta \mathrm{H}{}^{\circ}\mathrm{v}\mathrm{a}\mathrm{p} + \Delta \mathrm{H}{}^{\circ}\left(\mathrm{g}\mathrm{a}\mathrm{z},373\to \mathrm{T}\right) $$

(11)

Analyses of the experimentally constructed thermodynamic equilibrium diagrams (E-versus pH) for iron and water, allow to understand the range of pH values where ferrous and ferric iron are present and consequently to understand the range of pH values where H₂ is released. E is the redox potential in volts, i.e., the electrical potential energy per unit charge. Experimental thermodynamic values were used to construct E-pH Pourbaix diagrams for the Fe-H₂O system around the critical point of water (Cook and Olive 2012). They considered the solid species Fe, Fe₂O₃ and Fe₃O₄ as for the system at 25 °C and 1 atm (Pourbaix 1963). E-pH diagrams are only very slighlty altered with pressure since the thermodynamic function ΔG, used for their construction, is relatively insensitive to pressure compared to temperature (Bassez 1998). Considering the diagram drawn at subcritical conditions, 350 °C and 250 bar (25 MPa) and for 10^-6 mol/kg, we observe that the line E-pH for the redox couple FeIII/FeII is below the redox line O₂/H₂O and within the range of stability of water with respect to the evolution of oxygen. Consequently Eq. (12) may be written for pH < 4. Considering the E-pH diagram for Fe-H₂O constructed at 25 °C and 1 atm (Pourbaix 1963), it is possible to write the same equation for pH < ~6.5. Another difference between the two diagrams is the occurrence at 350 °C of the electrochemical reaction which might be described by Eq. (13) for pH between 11.5 and 14. Indeed the redox line FeIII/FeII is located in the range below the redox line H⁺/H₂ and consequently below the range of stability of water with respect to the evolution of H₂.

$$ 4\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{O}}_2+4{\mathrm{H}}^{+}\kern0.5em \to\ 4\mathrm{F}\mathrm{eIII}+2{\mathrm{H}}_2\mathrm{O}\kern1.75em \mathrm{T}=350{}^{\circ}\mathrm{C}\kern1.5em \mathrm{p}\mathrm{H}<4 $$

(12)

$$ 2\ \mathrm{FeII}+2{\mathrm{H}}^{+}\to\ 2\mathrm{FeIII}+{\mathrm{H}}_{2\kern2em }\mathrm{T}=350{}^{\circ}\mathrm{C}\kern1.5em 11.5<\mathrm{p}\mathrm{H}<14 $$

(13)

Consequently H₂ seems to be released only at high pH and high temperature and only within a small range of alkaline pH values, 11.5 - 14. It is produced in connection with ferric iron. This pH area does not exist on the diagram constructed at low temperature, 25 °C and 1 atm. At low pH < 4, ferric iron seems to be formed only in the presence of O₂. This pH value decreases with increasing temperature, from 6.5 at 25 °C to 4 at 350 °C. This pH analysis shows the importance of considering water near its critical point, for release of H₂.

It appears that, when CO₂ is dissolved in water, these pH values are located approximately in same areas as those defined for binary diagrams of Fe-H₂O at 25 °C. Analysis of E-pH diagrams drawn for the ternary Fe-CO₂-H₂O system at 100 °C and 150 °C and at concentrations of dissolved iron of 10⁻⁵ mol.kg⁻¹ and dissolved carbonates of 10⁻³ mol.kg⁻¹ (Chivot 2004) leads to Eq. (12) which may be written for low pH, below 6, and to Eq. (13) which may be written for high pH.

From E-pH diagrams constructed for ternary systems Fe-S-H₂O at 25 °C and 250 °C (Macdonald 1981, 1993) and at 75 °C, 100 °C and 150 °C (Chivot 2004) Eqs. (14) and (15) may be written. When sulfides are present, H₂ release seems to occur at low pH, 3.5 - 8, not because of transformation of ferrosilicates, but because of transformation of ferrosulfide. It is produced in connection with ferric iron at high temperature. The presence of O₂ is not required to transform ferrous iron into ferric iron.

$$ 3\mathrm{F}\mathrm{e}\mathrm{S} + 4{\mathrm{H}}_2\mathrm{O}\to \mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4 + 3\mathrm{S}{\mathrm{H}}_2+\kern0.5em {\mathrm{H}}_{2\kern0.75em }\kern1em 3.5 < \mathrm{p}\mathrm{H} < 8\kern1.5em \mathrm{T}=250{}^{\circ}\mathrm{C} $$

(14)

$$ \mathrm{F}\mathrm{e}\mathrm{S}+{\mathrm{H}}_2\mathrm{S}\to \mathrm{F}\mathrm{e}{\mathrm{S}}_2+{\mathrm{H}}_{2\kern5.5em }\ \mathrm{p}\mathrm{H}:\ 5.5-9.5\kern2.5em \mathrm{T}=25{}^{\circ}\mathrm{C} $$

(15)

Equation (14) is identical in redox to Eq. (13). The redox line FeIII/FeII is located in the range below the redox line H⁺/H₂ and consequently below the range of stability of water with respect to the evolution of H₂. Equation (15) can be written considering the redox couples SII/SI and H⁺/H₂. Equations (14) and (15) may be written for much lower pH values than in the absence of sulfides. Consequently, H₂ is shown to be released at lower pH when sulfides are present.

The above thermodynamic calculations show that a large amount of heat is locally produced through hydrolyses and carbonations of iron and magnesium olivine and pyroxene and especially through carbonation of magnesium olivine. This local heat might induce production of H₂ at high pH and high temperature near critical point of water, and consecutively production of CO. However another source of H₂ that has to be considered is water radiolysis. Gamma rays, X-rays and ionizing particles such as electrons, helium nuclei and neutrons induce water radiolysis and lead to the production of various species including protons H⁺, hydrogen radicals H^., hydroxyl radicals OH^., hydrated electrons e_hyd ⁻, O₂, hydrogen peroxide H₂O₂ and H₂. Production of H₂ is exponential in supercritical water. Water radiolysis is initiated when ionizing energy originates either from cosmic radiation or from radioactive decay. Now-extinct short-lived radionuclides present in the early solar system or during the Late Heavy Bombardment, LHB, and also long-lived radionuclides might participate both in water radiolysis for H₂ production and in gaseous mixture excitation for biological molecules production. Four long-lived radioisotopes were abundant when the Earth formed: ⁴⁰ K (half-life 1.277 × 10⁹a), ²³⁸U (half-life 4.51 × 10⁹a), ²³²Th (half-life 1.4 × 10⁹a) and ²³⁵U (half-life 7.1 × 10⁹a). These delivered enough energy to break OH bonds and create hydrogen radicals H^⋅ that may recombine into H₂. Such radionuclides are found in the ore zone of the 2.9 Ganni quartzites of the Witwatersrand basin, South Africa. The fracture water in this basin is a mixture of paleometeoric water which ranges in age from ~ 10 Ka to >1.5 Ma and is CO₂ rich, and of 2.0–2.3 Ga hydrothermal fluid which formed at ~ 250–300 °C. It contains abiogenic CH₄ and hydrocarbons, H₂, radiogenic gases and variable amounts of N₂ (Onstott et al. 2006). Samples from five gold mines and one coal mine, located at depths of 700 to 3300 m, were analyzed. They show that H₂ concentrations are not correlated with depth, salinity, pH, nor rock type, but that the highest concentrations are found in the deeper, nonmeteoric, older fracture water (Lin et al. 2005). This observation seems consistent with the reaction proposed above, of H₂ acting on CO₂ which might occur in meteoric water. Indeed the amount of H₂ should decrease following its consumption. Lin et al. proposed that “radiolytic H ₂ production more than suffices to explain the observed H ₂ concentration.” Consequently we may conclude that whenever radionuclides such as U, Th, or K, are present, there is enough H₂ avalaible through water radiolysis to produce CO from CO₂ in a locally warm environment. In turn CO can react with N₂ and H₂O to produce biological macromolecules after excitation by gamma rays arising from radionuclide decays or cosmic radiation if reactions occur on surfaces of the Earth and extraterrestrial objects.

An example of scenario can be proposed. During LHB, impacts brought radionuclides which induced water radiolysis. Impacts are also at the origin of heat and pressure which turned water into the supercritical state. H₂ was released exponentially in supercritical water. The H₂O_sc medium concentrated CO₂, H₂, N₂ to produce biological-type molecules. The excitation source might have been radionuclides or cosmic radiation. In Archean times, 3.5 Ga ago, the Earth’s magnetic field was weaker than the present field value which is 30 μT. Several estimates were proposed for the field strength: 5μT which is 17 % that of present-day value (Hale 1987 and Yoshihara and Hamano 2004) and 50 to 70 % that of present-day value (Tarduno et al. 2010). In Archean times, the Sun’s luminosity was 18 to 25 % that of present day value (Le Hir et al. 2014) and little is known about coronal mass ejection and consequently about density and physical characteristics of particles emitted by the paleoarchean Sun. However, because of the low intensity of the geomagnetic field, cosmic radiation might have reached the surface of the Earth more easily 3.5 Ga ago. Cosmic radiation might have been the source of excitation of the gaseous mixtures located above the concentrated solutions. In Archean times, heat and excitation were consequently avalaible to produce molecules of biological interest and water radiolysis might have been induced either by radionuclides or by cosmic radiation.

CO₂ and N₂ were likely present in the early Earth’s atmosphere. Indeed, during Archean eon, paleoarchean era defined within the 2012 geological time scale standard as 4.03 to 3.49 Ga (Gradstein et al. 2012), and within the standard GTS2004 as 3.6 to 3.2 Ga, the partial pressure of CO₂ was likely several times higher than present, phanerozoic eon, holocene epoq. It has been recently proposed that 3.5 Ga ago, accounting for a 23 % weaker Sun, pCO₂ was ca. 11.5 mbar which is around 30 times higher than nowadays (Le Hir et al. 2014). Molecular N₂ was also present. 3.49 Ga ago, pN₂ is shown to be _“ lower than 1.1 bar, possibly as low as 0.5 bar” (Marty et al. 2013). Modern pN₂ is 0.8 bar.

Conclusion

Prebiotic molecules thought to be necessary for the construction of living organisms might be produced as a consequence of the chemical evolution of mafic, ultrmafic and radioactive rocks. H₂ might be released from hydrolysis of ferromagnesian rocks in the local heated medium of the exothermic reactions of carbonations of olivine and pyroxene and of hydrolysis of Mg-olivine and Mg-pyroxene and most probably from water radiolysis arising from cosmic radiation and/or gamma rays emitted by radioactive rocks. CO might consecutively be locally synthesized in hydrothermal processes where heat is produced. Surface rocks might encounter post-shock temperature and pressure which follow comets and meteorites impacts and which set water into the supercritical state, a medium for concentration of the apolar molecules, CO₂, N₂, and H₂, necessary for the synthesis of biological-type molecules. Subsurface rocks encounter heat in subduction zones or radioactive, hydrothermal and geysers areae. These heating processes might be also a triggering effect for enhancing rates of exothermic reactions, such as olivine carbonation. Molecules of biological interest might then be formed by excitation of mixtures of gaseous CO, N₂ and water located above solutions which are concentrated in cavities. They might dry or aggregate in the heated medium. The plausible excitation sources for surface rocks might be cosmic radiation and/or gamma rays emitted by now extinct short-lived radionuclides present on early Earth or during LHB, and also from long-lived radionuclides. For subsurface rocks, excitation might occur through radionuclides. Through the necessary ingredients, water, CO₂ and N₂ (air), ferromagnesian and radioactive rocks (earth) and cosmic radiation (fire), we recognize the four elements at the origin of everything proposed by Empedocles. The above described thermodynamic and molecular analyses allow the determination of signatures of prebiotic chemistry in geological sites throughout the universe. Such signatures might be iron and magnesium carbonates in association with serpentine and/or talc for H₂ production through ferromagnesian rocks and occurrence of long-lived and now-extinct radionuclides for H₂ production in the absence of ferromagnesian rocks.