User:Maryland72/Draft-Serpentinization

Serpentinization is an alteration process where a low-silica, ultramafic rock (typically peridotite) is altered with the addition of water into the mineral crystal structure of the rock. The resulting product is serpentinite. Ultramafic rocks are rich in minerals olivine and pyroxene, typically found in the lower oceanic crust and upper mantle. The interaction with water results in oxidation of the ferrous iron within olivine and pyroxene, producing precipitations of ferric iron in magnetite and other minerals and releases H₂. Serpentinization occurs in many different geologic settings on Earth, including subduction zones, mid-ocean ridges, and ophiloites. It also has important implications in origin and evolution of early life on Earth.

Process

Serpentinization process has the general reaction: olivine + pyroxene + H₂O → serpentine ± brucite ± magnetite + H₂, where serpentine refers to one or more members of a mineral group including lizardite, antigorite, and chrysotile. Olivine is stable in the presence of water at higher temperatures, so regions of serpentinization are restricted to temperatures below ~330-400°C (McCollom and Bach 2009). The aqueous alteration of olivine and pyroxene (common minerals in ultra-mafic rocks) produces H₂. Additional reactions with the H₂ can produce methane, CH₄, through the reduction of CO₂, or other aqueous or gaseous carbon bearing molecule, present in the system. (http://onlinelibrary.wiley.com/doi/10.1111/gfl.12159/full) One reaction that can produce methane is the represented by the following: CO₂ + 4H₂ → CH₄ + 2 H₂O This reaction is slightly inhibited below temperatures ~350°C, and requires a long residence time to convert CO₂ to CH₄. The reaction is catalyzed by a various minerals, such as awaruite (a Ni-Fe alloy), or through the intervention of biological organisms.

When serpentinization occurs at temperatures lower than ~200°C, the reaction creates strong alkaline conditions (high pH values, commonly above 10). At higher temperatures, the pH decreases and only moderately alkaline conditions are maintained. These reactions result in rocks composed primarily of serpentine, brucite, and magnetite, as well as fluid with high pH, and enriched in hydrogen and methane.

Geological Settings

Serpentinization can occur in locations where fluids circulate through ultramafic rocks. These locations are typically regions where tectonics have led to uplift and exposure of mantle rocks, or deep in Earth's surface where fluids circulate through the subsurface. Slight variations of basic serpentinization reactions described above are found in the following geological settings.

Subduction Zones

During subduction, fluids from the down-going slab are released and can interact with the upper mantle rocks above causing serpentinization (Kelley et al 2005).

Mid-Ocean ridges

Serpentinization occurs in areas along mid-ocean ridges where the ultra-mafic rocks are exposed to circulating hydrothermal fluids through seafloor spreading. Some areas off-ridge can also host serpentinization reactions when mantle rock is displaced near seafloor surface due to tectonic movements. This mantle rock is then free to interact with seawater.

Ophiolites

Main article ophiolities Ophiolites are sections of oceanic crust and upper mantle that have been thrust onto continental landmasses by tectonic processes. The presence of ultra-mafic rock in these sections can undergo serpentinization when exposed to groundwater circulation. Alkaline springwater, with elevated H₂ and CH₄ levels, was found to be the result of serpentinization in the Coast Range and Josephine ophiolites of western United States.

Biological Consequences

A variety of energy sources for microbes that may thrive in deep subsurface regions are produced at sites where serpentinization occurs. Abiotic methane (CH₄) and hydrogen (H₂). This abiotic source of energy can also have important implications for potential life on other planets. (DOI: 10.1111/gfl.12159)

Hydrogen and Methane

While H₂ is a direct product of serpentinization, methane (CH₄) can form via Fischer–Tropsch Type reactions (a process that produces a variety of hydrocarbons through a series of chemical reactions).(Cite - http://onlinelibrary.wiley.com/doi/10.1111/gfl.12159/full) Some chemosynthetic microorganisms can metabolize energy from H₂ and CH₄ through redox reactions with electron acceptors such as O₂ and SO₄^2-. Organisms capable of metabolizing H₂ and CH₄ include hydrogen oxidizers, methans-oxidizers, metahnogens, and sulfate reducers. The microbial composition at ultra-mafic hydrothermal vents differs from that of deep-sea hydrothermal vents where serpentinization is no occurring. At these non-mafic sites, organisms dominantly metabolize hydrogen sulfide (H₂S) as the electron donor in this habitat.

The pH of the system caused by serpentinization also likely impacts the microbial community. Lower pH fluids at high-temperature hydrothermal vent systems have a more diverse microbial community. At lower-temperature vents like Lost City (<90°C) there is very low microbial diversity present, where the pH is more alkaline and inorganic carbon levels are very low due to carbonate precipitation. (Schrenk 2013) Presently, studies on microbial communities have been mostly associated with deep-sea hydrothermal vent systems. Although it is likely that active microbial populations are present in many serpentinite subsurface environments and alkaline springs on land.<

Early Earth and other planets

Chemosynthetic microbial communities, like those supported by serpentinization, have been suggested to be modern analogs of communities that existed on early Earth. The composition of early Earth lava was more ultra-mafic than they are today, presently they are more basaltic in composition with a higher silica content. With ultra-mafic rocks more widespread, it is likely that serpentinization and therefore the production of H₂ and CH₄ was more prevalent. Serpentinization has been proposed to be a source of energy for ancient life on Earth. It may be one of the most abundant sources of energy in earth Earth, potentially until the onset of photosynthesis.

Ultra-mafic rocks are likely to be present on other planetary bodies as well. If exposed to aqueous fluids, serpentinization and the production of H₂ and CH₄ is a potential source of energy for extraterrestrial life. Serpentinization is thought to occur on Mars and is a possible source for the CH₄ in the martian atmosphere. (Onstott)