nueva página del texto (beta)
Inglés (pdf)
Artículo en XML
Referencias del artículo
Traducción automática
Enviar artículo por email
Citado por SciELO 
Similares en
SciELO 
Permalink
1. Introduction
Since its formation, the Earth has undergone many changes, among which one of the most remarkable has been the emergence of life. This must have occurred before 3500 Ma, the age of the rocks where the oldest known fossils have been found (Schopf, 2006). The event was preceded by a period called "chemical evolution", which involved chemical reactions among components of the ocean, the lithosphere and the early atmosphere. Life possibly emerged as a result of chemical processing in these media, with a continuous increase in molecular complexity (Morowitz, 2002). Multiple examples of water-based systems in which life could have arisen include seas and oceans, lakes, pools and ponds (plus other ephemeral water bodies), and intertidal zones.
The quest to explain how life originated on Earth is an old and unsolved topic. Several scientific hypotheses try to explain how life emerged as a result of the physicochemical interactions between organic molecules and the physical environment. The experimental evidence of chemical synthesis under conditions that possibly existed on early Earth provides support for the hypothesis of chemical evolution. The precise mechanism that led to the transformation of organic compounds to simple biological entities is still an open problem nonetheless. Two theories attempt to explain how life appeared: the "gene first" theory and the "metabolism first" approach (e.g. , Bada and Lazcano, 2003; Delaye and Lazcano, 2005). According to the genetic approach, prebiotic soup produced organic compounds that gave rise to the first genetic systems; whereas the metabolic theory proposes the existence of a rudimentary primary metabolism (Orgel, 2000, Lazcano, 2010; Luisi, 2014). Both theories have been supported by several experimental as well as theoretical results (Orgel, 1998). Such is the case of the synthesis of many important organic compounds for present living beings - including some amino acids, nitrogenous bases, carboxylic acids and sugars. However, the formation of biological polymers, and how the relationship between proteins and nucleic acids evolved is still a far too complex and unresolved problem.
Either theory requires the synthesis of organic matter, which necessarily involves the participation of an energy source. Although there are many plausible energy sources for the generation of organic compounds, one of the most conspicuous sources on Earth is its internal heat, which is released in many environments. Such is the case of anomalously high thermal gradients around volcanoes and volcanic hot springs (Lathe, 2004, 2005), which can be subaerial and submarine - some with temperatures that may range from 90° to > 400 °C (Russell and Hall, 1997; Kelley et al., 2001). Processes driven by external sources for energy include many other interfaces such as those between rocks, water, air, and snow-air (Muller and Schulze-Makuch, 2006). From the point of view of prebiotic chemistry, high temperature gradients would have provided the necessary energy flux to promote chemical reactions. But at the same time, however, such gradients could have been harmful to organic compounds, thus promoting the degradation of the synthesized products (Muller and Schulze-Makuch, 2006).
For many years, a common feature in prebiotic experiments was their lack of any geological context. The discovery of hydrothermal vents in mid-ocean ridges in the 1970s led many scientists to propose that these systems would make plausible geological sites for the origin of life (Holm and Charlou, 2001; Holm and Andersson, 2005). Most of these vents build up mineralized chimneys by means of hot fluids associated with Earth's mantle magmas at temperatures above 300 °C. French (1968, 1970) suggested that hydrothermal systems might have allowed organic compounds to be synthesized by non-biological processes through Fischer-Tropsch-like reactions (FT). These reactions involve gases at high pressures and temperatures, as well as minerals like siderite and other carbonates, sulfates, iron oxides, and some silicates. Those processes could have taken place at some depth within the Earth's crust, where organic matter could be produced, and then transferred to the surface (Holm and Andersson, 2005). Furthermore, Corliss et al.(1981) suggested that hydrothermal vents along mid-ocean ridges were likely sites for the origin and evolution of life. Also, Baross and Hoffman (1985) suggested that submarine hydrothermal systems have retained some characteristics, making them environments where reactions and evolutionary processes can be examined.
Such ideas began to gain importance when Wächtershäuser (1988 a, b) hypothesized that the first organisms on Earth could have been thermophilic and chemoautotrophic beings. These organisms could have had a primitive metabolism that took place on the surface of solid particles, especially sulfides (most likely, iron sulfides) that formed from hydrothermal venting (Wächtershäuser 1988 a, b). However, these ideas remain controversial and there is no compelling evidence to support that the first organisms were thermophilic. Regardless of how life originated, this process must be thermodynamically favored or it would have been unlikely (Martin and Russell, 2007). The chemical and thermal dynamics in hydrothermal vents makes such environments highly suitable thermodynamically for chemical evolution processes to take place. Therefore, thermal energy flux is a permanent agent and contributed to the evolution of the planet, including prebiotic chemistry.
2. Hydrothermal systems and prebiotic synthesis
A hydrothermal system is an environment where hot fluids circulate below the Earth's surface and may (or not) reach the surface as hot springs or vents. The two main components of a hydrothermal system are a heat source, and a fluid phase. In addition, fluid circulation requires faults, fractures and permeable lithologies (Pirajno, 2009).
Such systems can be classified according to their tectonic setting, the characteristics of their emplacement, and the sources for fluids, among other geological variables. As for the matter of prebiotic synthesis, a variety of such environments have been invoked as likely sites to harbor the plausible chemical reactions thus involved. These are submarine and subaerial hydrothermal systems, and similar systems associated with impact cratering.
2.1. Subaqueous hydrothermal systems
These hydrothermal systems (submarine or sublacustrine) can be divided into those linked to magmatism as both source for heat and chemical components, and those associated with venting of basinal brines. Such environments correspond, respectively, to volcanogenic massive sulfide (VMS) and sedimentary-exhalative (SEDEX) deposits and their present-day analogues. Paleo-hydrothermal systems associated with metalliferous deposits in black shales may be likely candidates as well. All theoretical and experimental approaches to prebiotic reactions have been carried out considering VMS-like hydrothermal systems, while neglecting the others. SEDEX systems and those associated with metalliferous black shales provide all geological and physicochemical characteristics that would have favored prebiotic reactions as effectively as VMS systems, like the necessary temperature gradients, euxinic environments, and a wide range of depths of formation (see Table 1). The problem in the involvement of SEDEX systems with prebiotic reactions resides in the age of the oldest examples of such systems, as no known deposits are older than late Paleoproterozoic (ca . 1.8 Ga; Lydon, 1996). Metalliferous black shales can be significantly older (middle Paleoproterozoic, ca. 2.1 Ga or older; Mossman et al., 2005) than SEDEX deposits. However, neither type has yet been found to be old enough as to be coeval with prebiotic processes or, least of all, be involved with them. In contrast, Archean VMS deposits are numerous (ca. 3.5 Ga; Barrie and Hannington, 1999). The striking lack of Archean SEDEX deposits can be associated with the limiting effect of high reduced iron contents on the activity of reduced sulfur in anoxic oceans (sic , Goodfellow, 1992) in which metals in hydrothermal fluids [...] were dispersed because a lack of reduced sulfur to precipitate them (sic , Misra, 1999). Therefore, it is likely that SEDEX-type hydrothermal systems did effectively exist during the Archean, despite being unable to generate sulfide deposits because reduced sulfur in the oceans would have been previously "sequestered" by iron to precipitate iron sulfides directly from seawater. After the oxygenation of Earth's oceans SEDEX deposits formed, during worldwide anoxic events of the Paleozoic, as might also be the case for Proterozoic deposits (Misra, 1999).
Table 1 Comparison between shallow hydrothermal environments (both fossil ore deposits and present examples). Based loosely on Misra (1999), Jébrak and Marcoux (2008) and Pirajno (2009).
Submarine vents in association with magmas at or near mid-ocean ridges are usually described as the likeliest hydrothermal systems to be associated with the emergence of life on Earth (i.e. Corliss et al., 1981, Nisbet and Sleep, 2001). Besides their possible role in the emergence and development of life on Earth, hydrothermal vents could sustain living organisms on Europa or Mars (Pope et al., 2006; Chyba and Phillips, 2007). The most common case for the formation of submarine hydrothermal vents occurs when seawater migrates through fractures into the crust and reaches the vicinity of a magmatic intrusion. While approaching the magmas, water is heated up by the anomalously high thermal gradient induced by their emplacement. Additionally, magmas release aqueous fluids upon their cooling down. Therefore, hydrothermal fluids in magma-related systems may come from either magmatic or marine sources. Similarly, sulfur, iron, copper, zinc and other metals may come from magmas, ocean water or host rock leachates. The dissolved minerals nourish chemosynthetic bacteria that constitute the base of the food chain for a variety of invertebrates, including large tubeworms (Levin, 2009).
The importance of submarine hydrothermal vents for studies related to the origin of life lies in: (1) providing hot water from shallow to surficial environments, (2) upwelling fluids interact with seawater, provide nutrients, are agents for chemical imbalance, thus potentially allowing the synthesis of organic compounds, (3) they produce a rapid crystallization of carbonates and silicates at low temperatures, which increases the local potential to preserve microbial organisms as fossils and their chemical signatures, despite later diagenetic or low-grade metamorphic processes (Pope et al., 2006), and (4) according to some authors, the oldest forms of terrestrial life might have been autotrophic-thermophiles (Pope et al. , 2006).
The so-called "black smokers" are the most conspicuous submarine hydrothermal manifestations (Figure 1). These are hydrothermal fumaroles with abundant sulfides in suspension that upon precipitation form mounds along favorable faults or within submarine calderas. In association with divergent margins, these are usually located close to mid-ocean ridges. In these fumaroles, due to their proximity to magmas, water may attain temperatures above 400 °C and low pH (see Table 1). The latter facilitates the leaching of iron and other metals as the water seeps through the country rocks. Such fluids come in contact with cold seawater, thus generating a rapid nucleation of sulfides and other minerals and resulting in a turbid suspension resembling a cloud of black smoke.
Figure 1 Structural section that combines evidence from active submarine magmatic-hydrothermal vents and from fossil volcanogenic massive sulfide (VMS) deposits, especially in Kuroko-type settings, including all typical styles of mineralization and hydrothermal assemblages. Based on Lydon (1988) and Hannington et al. (1995). Key: T = temperature.
Another type of hydrothermal vents is dubbed "white smokers", which are generally distant from their heat source (Figure 1); therefore, their temperatures are lower than those in black smokers. Black and white smokers may coexist in the same hydrothermal field, but they generally represent proximal and distal vents to the main upflow zone, respectively (Figure 1). However, white smokers correspond mostly to waning stages of such hydrothermal fields, as magmatic heat sources become progressively more distant from the source (due to magma crystallization) and hydrothermal fluids become dominated by seawater instead of magmatic water (see references in Table 1 for VMS systems). The temperature in white smokers can be as low as 40° to 75 °C and are alkaline (pH between 9 and 9.8; Kelley et al., 2001). Mineralizing fluids from this type of vents are rich in calcium and they form dominantly sulfate-rich (i.e ., barite and anhydrite) and carbonate deposits. These may form giant chimneys, the largest of which stand almost 60 m above the bottom of the ocean (Figure 2) at the Lost City hydrothermal field (Kelley et al. , 2001). Hydrothermal fluids in this location contain methane, ethane and propane, and organic acids such as formate and acetate form in association with this hydrothermal system (Proskurowski et al., 2008).
Figure 2 Size of hydrothermal chimneys in the Lost City Hydrothermal Field, compared to those of a blue whale, an orca, a 17-story building, and a diver.
The global frequency distribution of depths for the occurrence of actualistic examples of VMS deposits (magmatic-hydrothermal seafloor vents, either black or white smokers) shows a dominant range between 2000 and 3000 m (Figure 3; also, Figures 2 and 3 in Hannington et al., 2005), which is consistent with calculated depths of fossil examples (see Table 1 and references therein). In spite of this, much shallower examples are known both in present-day manifestations (Figure 3) as well as in fossil VMS deposits (e.g. , Camprubí et al., 2017). Their global frequency distributions in latitude and longitude, however, are not expected to be clear because such spatial variables do not govern any geological characteristics for the location of venting. In fact, the present-day convergent or divergent margins that are capable of sustaining hydrothermal manifestations analogous to VMS deposits (see Figure 1 in Hannington et al. , 2005) can be found in a wide variety of latitudes and longitudes, including divergent margins within the Arctic Circle and the convergent margin adjacent to the Antarctic Peninsula. However, the latitudinal distribution of those vents shows a bimodal pattern between 40°N and 40°S (Figure 2), which includes different types of both convergent and divergent margins (see Figure 1 in Hannington et al. , 2005). The distribution is deemed to be an artifact due to the difficulties of carrying out oceanographic campaigns in extremely low latitudes, either north or south, in comparison with peri-equatorial regions.
Figure 3 Frequency histograms for the geographical distribution of submarine hydrothermal vents worldwide, regarding (A) latitude, (B) longitude, and (C) depth, constructed by using the InterRidge Vents Database 3.3 (http://vents-data.interridge.org) dated April 30th 2014.
2.2. Subaerial hydrothermal systems
Hydrothermal manifestations are abundant in subaerial settings, particularly in association with convergent plate boundaries (that is, continental and island volcanic arcs), but also in transform boundaries. For instance, hydrothermal activity is known to occur in the Salton Sea, in association with the San Andreas fault system but, unlike volcanic arcs, this case is normally placed among modern equivalents to SEDEX deposits and subaqueous brine pools (see Table 1). The most relevant and numerous recent/active hydrothermal fields are found in geothermal and magmatic-hydrothermal contexts, which are normally considered as the modern analogues of low-sulfidation and high-sulfidation epithermal deposits, respectively (which, in both cases, may include intermediate-sulfidation deposits; Simmons et al., 2005; Camprubí and Albinson, 2006, 2007; Sillitoe, 2015). The uppermost part of such systems has a tendency to display wide variations in temperature, salinity, volatile content, pH and redox potential, hence the broad range in reactivity between the associated hydrothermal fluids and host rocks. Such variables are largely controlled by the vertical or lateral nearness of hydrothermal discharge zones to their parental intrusions (see Figure 1in Sillitoe, 2015) and the geological and hydrological characteristics in each area (White and Hedenquist, 1990). Besides the broad temperature and salinity gradients that may occur in the actual variety of such environments, the occurrence of deep hypogene low- to intermediate-sulfidation fluids (generally near-neutral and reduced; geothermal context) or high- to intermediate-sulfidation fluids (acidic and oxidized; magmatic-hydrothermal context) determines (1) the possible zonation of alteration assemblages around the fluid conduits, and (2) the mineralogy of the mineral precipitates (if any) that may occur on the surface. In addition to the "original" physicochemical characteristics of hydrothermal fluids, their chemical characteristics may vary depending on the occurrence of (relatively) near-surface boiling, which may generate H2SO4-rich fluids locally in steam-heated grounds (shallow hypogene acidic fluids), independently from the composition of pre-boiling fluids (e.g. , Sillitoe, 2015). This means that hydrothermal fluids of any kind that undergo boiling may generate acidic fluids upon condensation of boiled-off steam, and the associated alteration assemblages and surficial hydrothermal features.
Acidic fluids from either deep or shallow hypogene sources generate alteration assemblages that result from extremely reactive to relatively mild reactions between fluids and host rocks, from proximal to distal areas to hydrothermal upflow, respectively. No surface sinter deposits, either carbonate- or silica-rich, can be expected from highly reactive high-sulfidation type fluids. In this environment, silica is the only residue after extreme acid leaching of every other mineral, or as a late overprint. Common hydrothermal manifestations of the high-sulfidation type are high-temperature solfataras and fumaroles centered on recent volcanic edifices, and hyper-acidic crater lakes. Near-neutral low-sulfidation type fluids, on the contrary, may develop sinter deposits in hot spring environments unless the hydrothermal discharge occurs in high-relief terrains. Common hydrothermal manifestations of the low-sulfidation type are hot springs and geysers. Common manifestations associated with steam-heated grounds are fumaroles, steaming grounds, and mud pots (or mud "volcanoes"). The position of the groundwater table is sensitive to seasonal variations in rainwater availability, climatic and tectonic changes, or several other phenomena (e.g. , Sillitoe, 2015).
2.3. Impact cratering and hydrothermal systems
Impact cratering is a major geological process that occurs on the Earth and all other solid bodies of the Solar System. Impact cratering alludes to a process in which an asteroid or comet at high speed collides with a planetary surface, producing a cavity known as an impact crater (Melosh et al., 1990; Melosh, 2011). Temperatures and pressures produced by the collision are typically > 10000 K and a few hundred GPa, respectively. For comparison, in the Earth's core, the pressure is estimated at ~350 GPa and temperature at ~5000 K. Under these conditions, both the target and projectile material melt or vaporize near the point of contact (Collins et al., 2012).
Since Alvarez et al. (1980) proposed a possible extraterrestrial impact to explain the mass extinction at the K-T (Cretaceous-Cenozoic) boundary; extensive work has been carried out to try to understand the consequences of an impact due to objects larger than one km in diameter. An object 10 km across, similar to the one that formed the Chicxulub crater in the Yucatán Peninsula, would throw a huge amount of dust into the air, thus blocking sunlight and inhibiting photosynthesis and cutting off heterotrophs' primary food source. Also, the ejecta that failed to escape Earth's gravitational pull would reenter the atmosphere and raise the atmospheric temperature enough to cause global forest fires (Melosh et al., 1990; Kring and Durda, 2002). Although, at first glance, such impacts may prevent life from developing instead of being favorable for it, some positive effects can be expected. For example, in recent years scientists have retaken the idea that prebiotic compounds, water, and other gaseous components that are now present on Earth were brought and deposited by asteroids and comets that collided with the planet (Bernstein et al., 1999; Horneck, 2006).
The first authors who propose that impact craters may produce hydrothermal systems were AlDahan (1990), Kring (1995), and McCarville and Crossey (1996). This was sustained because pressures and temperatures during the contact and compression stage are capable of melting the country rocks (Melosh et al., 1990). Such melts act as heat sources for hydrothermal systems in impact craters as the associated high geothermal gradient is established (Newsom, 2012). This proposal has been supported by hydrothermal deposits that have been recognized in the Chicxulub, Sudbury and Haughton impact craters (Kring, 1995; Osinski et al., 2005; Ames et al., 2006; Parnell et al., 2006). In fact, there is evidence of impact-induced hydrothermal systems in 60 out of the 181 confirmed terrestrial impact craters (Kirsimäe and Osinski, 2012).
The life span calculated for these systems is in the order of 104 - 105 years if the crater is 100 km in diameter, and up to 106 years for a 180 km crater like Chicxulub (Daubar and Kring, 2001). Magma-related hydrothermal activity may normally last in the order of 105 - 106 years, even several million years, for epithermal and porphyry-related deposits, including similar subaerial active geothermal systems (Cathles et al., 1997; Arehart et al., 2002; Camprubí and Albinson, 2007; Baumgartner et al., 2009; Redmond and Einaudi, 2010; Chiaradia et al., 2013). Single fossil VMS deposits also exhibit similar age spans (e.g. , Ross et al., 2014; Belford et al., 2015), and thus the temporal likelihood for all these types of hydrothermal systems to have roles in prebiotic chemistry is essentially the same.
Besides Earth, the other place in the inner Solar System that presents evidence of impact-induced hydrothermal systems is Mars (Rathbun and Squyres, 2002; Pirajno, 2009). As less energy is necessary to produce an impact crater of any size on Mars than on Earth, so would be the volume of molten rock that could be produced in Mars. Therefore, it is expected that for similar craters on Mars and Earth, the hydrothermal activity on Mars would be shorter in duration than on Earth by about an order of magnitude (Pope et al., 2006). Another interesting aspect about craters on Mars is that they could have been occupied by paleolakes. These could have existed over thousands of years as crater lakes, much like as on Earth, covered by an ice layer and heated from below by impact melts (Kirsimäe and Osinski, 2012). Phyllosilicates have been detected in impact-induced hydrothermal systems on Mars (Kirsimäe and Osinski, 2012); these minerals have been proposed as natural catalysts for prebiotic reactions.
It has been considered that the main sources for fluids in an impact-induced hydrothermal system would be seawater and meteoric water. The chemical composition of these fluids depends on the composition of the ground that received the impact (or target material). In this kind of systems, Ca-Mg or K-rich minerals will dominate depending on the target material, the size of the impact crater, the sources for fluids and the evolution of the hydrothermal system. Usually, Na-K minerals dominate at the beginning of the lifetime of the hydrothermal system, and evolve into Ca-Mg minerals at the last phase of the impact cooling. For example, in the early stages of such hydrothermal activity, magmatic feldspars (plagioclase, microcline) are replaced by hydrothermal alkali feldspars (albite, "pericline"- and "adularia"-type potassium feldspar). During waning stages of hydrothermal systems, calcite and dolomite can precipitate depending on the enrichment of Ca and Mg in the fluids (Kirsimäe and Osinski, 2012).
3. Minerals in hydrothermal vent systems
Mineral parageneses are a key for the definition of physicochemical parameters that characterize the conditions for mineral precipitation, including timing. Mineralization in submarine magmatic-hydrothermal systems is a product of the chemical and thermal exchange between the ocean, the lithosphere, and the magmas emplaced within it. Different mineral associations precipitate during the typical stages of mineralization that characterize the life span of such systems. For comprehensive reviews of this subject see Franklin et al. (1981, 2005), Lydon (1988), Ohmoto (1996), Barrie and Hannington (1999), Hannington et al. (2005). Minerals present in a hydrothermal system or a fossil VMS deposit are deposited passively or reactively. Mineral associations may vary (1) in different mineralized structures, either syngenetic (namely, passive precipitation in chimneys, mounds and stratiform deposits) or epigenetic (structures that correspond to feeder channels, and replacements of host rocks or pre-existing massive sulfide bodies), or structural zonation , (2) from proximal to distal associations with respect to venting areas within the same stratigraphic horizon , or horizontal zonation, (3) from deep to shallow associations (i.e. , stockworks to mounds), or vertical zonation , (4) from early and climactic to late stages of mineralization (dominated by sulfides, and sulfates or oxides, respectively), or temporal zonation , and (5) in various volcano sedimentary contexts, depending essentially on the composition of volcanic rocks and, ultimately, on the tectonomagmatic context. The most common minerals in ore-bearing associations of VMS deposits (non-metamorphosed or oxidized) and their modern analogues are pyrite, pyrrhotite, chalcopyrite, covellite, sphalerite, galena, tetrahedrite-tennantite, marcasite, realgar, orpiment, proustite-pyrargyrite, wurtzite, stannite (sulfides), Mn oxides, cassiterite, magnetite, hematite (oxides), barite, anhydrite (sulfates), calcite, siderite (carbonates) quartz and native gold, and are differently distributed in the various associations schematized above. The most common hydrothermal alteration assemblages are chloritic (including Mg-rich ones) and phyllic (dominated by "sericite", mostly illite), and also silicification, deep and shallow talcose alteration, and ferruginous (including Fe oxides, carbonates and sulfides) alteration.
Paleosurface features for the various types of subaerial/sublacustrine magmatic - hydrothermal systems (that is, epithermal-like systems) and their mineralogy were summarized in detail by Sillitoe (2015) as (1) steam-heated grounds, with opal/chalcedony, alunite, kaolinite and smectite, (2) groundwater table silicification, with opal/chalcedony, (3) lacustrine amorphous silica sediments, with opal and cristobalite, (4) hydrothermal eruption craters and breccias, with illite and smectite, (5) hot spring sinter, with opal/chalcedony, (6) hot spring travertine, with calcite and aragonite, (7) hydrothermal chert, with opal/chalcedony, and (8) silicified lacustrine sediments, with opal/chalcedony. See their occurrence and nature schematized in Figure 11 by Sillitoe (2015), as replacements, open-space or on-surface (subaerial or subaqueous) precipitation. Cases 6 to 8 (particularly case 6) occur distally to their hydrothermal upflow zone, which implies that their temperatures are lower than in proximal features, and their chemical characteristics attenuated by interaction with meteoric water. Such features and their particular mineral assemblages may be found topping various hydrothermal alteration assemblages in association with either acidic or near-neutral to alkaline fluids (high-sulfidation, and intermediate- to low-sulfidation fluids, respectively), but not necessarily. Alteration assemblages in the uppermost part of these systems are characteristically zoned as follows, from the central portion of hydrothermal upflow outwards into non-altered host rocks (see Stoffregen, 1987; Corbett and Leach, 1998; Camprubí and Albinson, 2006, 2007):
high-sulfidation systems: residual quartz (with opal, cristobalite and tridymite), advanced argillic (from silica + alunite, to alunite + kaolinite outwards), argillic (from kaolinite + silica, to kaolinite + silica + smectite outwards), illite- or smectite-rich phyllic, montmorillonite-to chlorite-rich propylitic alteration, including zeolites and carbonates (calcite and dolomite) in association with the most alkaline fluids;
intermediate- to low-sulfidation systems: phyllic to propylitic alteration, with the same mineral assemblages as those described for high-sulfidation systems.
Additionally, all the features in the uppermost portions of epithermal deposits and their modern analogues may have anomalously high concentrations of Mn, As, Sb, Hg, Tl, Se, Au, Ag, Ga and W (Hedenquist et al., 2000; Sillitoe, 2015). These anomalies occur in association with minerals like pyrite, cinnabar, stibnite, orpiment, realgar, native sulfur, livingstonite, corderoite, several amorphous phases and, exceptionally, borates (Sillitoe, 2015, and references therein).
4. Hydrothermal vents and chemical evolution
In the field of chemical evolution, the search for a suitable location for chemical reactions to occur constitutes a major issue. Hydrothermal vents are environments that harbor very special physical, chemical and geological conditions, which could have been important for chemical evolution. On the one hand, the exergonic reactions in such systems could have been a source of free energy that promoted chemical reactions. This energy could eventually lead to the synthesis of prebiotic organic molecules, primarily from carbon dioxide reduction into hydrocarbons (Berndt et al., 1996), but also via methane oxidation and phosphate condensation (Russell et al., 2013).
On the other hand, probably one of the most important characteristics of hydrothermal vents is their high mineralogical diversity. A broad array of minerals could have acted as catalytic inorganic surfaces that favored the formation of organic molecules. Such mineralogical complexity implies the induction of important chemical gradients, thus favoring the interaction between electron donors (e.g. , methane, hydrogen, formate) and electron acceptors (e.g. , carbon dioxide, nitrate, nitrite, sulfite, native sulfur and ferric iron). As a consequence, these reactions would yield complex organic molecules aided by pH and thermal gradients, and perhaps also by mineral fracturing and fluid flow (Russell and Hall, 1997; Russell et al. , 2013).
There are numerous experiments proposed to test the role of hydrothermal vents in prebiotic synthesis. A review of the literature (Tables 2, 3 and 4) indicates several types of experiments that can be performed. The first group is related to decomposition experiments, sometimes referred as stability experiments (Table 2), and comprises the majority of available studies. In these experiments, the decomposition rate of biomolecules, mainly amino acids, is explored. The most surveyed one is the simplest amino acid: glycine. It is noticeable that the inclusion of mineral phases is considered fundamental in breakdown experiments. In this regard, the role of some iron-rich minerals has been explored (e.g. , magnetite, fayalite, pyrite, etc.); however, other minerals (e.g. , quartz, feldspars, muscovite, calcite) are also used. In this kind of experiments the formation of other amino acids is also pursued. An important finding is that there is an order in the decomposition rates of the selected molecules (Vallentyne, 1964; see Table 2), and that decomposition strongly depends on factors (such as pH, pressure, ionic strength, etc.) other than temperature. Another kind of information that can be obtained by means of these studies is the determination of kinetic parameters (Povoledo and Vallentyne, 1964) in order to predict the behavior of organic reactions.
Table 2 Experiments of decomposition and stability of organic molecules in physicochemical conditions that simulate those of hydrothermal vents.
Note: All amino acids are abbreviated according to the IUPAC indications.
Key: ABA = aminobutyric acid, Ala = alanine, Arg = arginine, Asp = asparagine, Cys = cysteine, Gaba = gamma-aminobutyric acid, Glu = glutamic acid, Gly = glycine, His = histidine, Hyp = hydroxyproline, Ile = isoleucine, Leu = leucine, Lys = lysine, Met = metionine, Nva = norvaline, Orn = ornithine, PCA = pyroglutamic acid, Phe = phenylalanine, Pro = proline, Pyr = pyruvate, Ser = serine, Thr = threonine, Tyr = tyrosine, Val = valine.
A step forward in prebiotic chemistry is the formation of more complex molecules: oligomers. The reason for this is that life is based in macromolecules. In consequence, the formation of oligomers in hydrothermal vents has also been explored (Table 3). Again, most experiments deal with glycine (e.g. , Huber and Wächtershäuser, 1998; Imai et al., 1999), but the oligomerization of alanine has also been addressed (Ogata et al., 2000; Kawamura et al., 2005). Oligomerization in hydrothermal vents is confirmed in some experiments (i.e. Islam et al., 2003; Kawamura et al. , 2005; see Table 3).
Table 3 Experiments of oligomerization and synthesis of organic molecules in physicochemical conditions that simulate those of hydrothermal vents.
Note: All amino acids are abbreviated according to the IUPAC indications.
Key: ABA = aminobutyric acid, Ala = alanine, AMP = adenosine monophosphate, Arg = arginine, Asp = asparagine, Cys = cysteine, Gaba = gamma-aminobutyric acid, Glu = glutamic acid, Gly = glycine, His = histidine, Hyp = hydroxyproline, Ile = isoleucine, Leu = leucine, Lys = lysine, Met = metionine, Nva = norvaline, Orn = ornithine, PCA = pyroglutamic acid, Phe = phenylalanine, Pro = proline, Pyr = pyruvate, Ser = serine, Thr = threonine, Tyr = tyrosine, Val = valine. PP i =Pyrophosphate.
Models for the synthesis of organic compounds include those associated with a variety of mineral-catalyzed reactions such as water-rock reactions (e.g. , serpentinization), Fischer-Tropsch reactions, and FeS-driven synthesis. Experiments listed on Table 3 show the synthesis of different types of amino acids, as well as lipids and hydrocarbons.
Experiments related to (1) the adsorption of organic molecules onto minerals, those related to explore the possible role of mineral channels in the concentration, and (2) the reactivity of some organics in hydrothermal vents are still scarce (Table 4).
Table 4 Compilation of various types of experiments in physicochemical conditions that simulate those of hydrothermal vents.
Note: All amino acids are abbreviated according to the IUPAC indications.
Key: ABA = aminobutyric acid, Ala = alanine, AMP = adenosine monophosphate, Arg = arginine, Asp = asparagine, Cys = cysteine, Gaba = gamma-aminobutyric acid, Glu = glutamic acid, Gly = glycine, His = histidine, Hyp = hydroxyproline, Ile = isoleucine, Leu = leucine, Lys = lysine, Met = metionine, Nva = norvaline, Orn = ornithine, PCA = pyroglutamic acid, Phe = phenylalanine, Pro = proline, Pyr = pyruvate, Ser = serine, Thr = threonine, Tyr = tyrosine, Val = valine.
Serpentinization has been proposed as a generator of drastic chemical, redox, pH, and thermal gradients (Russell et al., 2013). In this way, the weathering of mafic and ultramafic rocks constitutes a sink for CO2 (Berndt et al., 1996), because minerals present in basalts (olivine, plagioclase and augite) are highly reactive with CO2. It is likely that hydrothermal systems of the early Earth were largely hosted by olivine-rich ultramafic rocks (e.g. , komatiites) (Russell et al. , 2010). The formation of serpentine, brucite, and magnetite produces large amounts of hydrogen and methane (Kelley, 1996). The sustained production of hydrogen creates strongly reducing environments yielding a favorable environment for organic synthesis (Holm et al., 2015). This process could have contributed to the subsequent formation of organic compounds on early Earth.
5. Concluding remarks
The origin of life remains one of the most fundamental issues in science, branching out into a wide array of disciplines. Processes that might have contributed to the inventory of organic compounds, not only on early Earth but also on early Mars, include mineral-catalyzed reactions; such reactions may have occurred on submarine hydrothermal systems.
One of the principal reasons for the great deal of attention paid to hydrothermal systems is that their physical and chemical conditions could have favored the synthesis of organic compounds out of inorganic precursors. In these systems minerals are quite abundant, and minerals may have aided in the transition from geochemistry to biochemistry. Such environments are likely to have met the requirements for the occurrence of chemical evolution and the subsequent origin of life. The synthesis/destruction of organic compounds lingers as a controversial topic and many efforts are devoted to testing the role of these systems in prebiotic chemistry. A better understanding of reaction pathways that led to the synthesis of organic molecules in prebiotic hydrothermal vents requires additional laboratory experiments under physical and chemical conditions as close as possible to the actual ones
