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Serpentinite partially made of chrysotile, from Slovakia
Maurienne valley
, Savoie, French Alps
Sample of serpentinite from the Golden Gate National Recreation Area, California, United States
upper mantle dunite peridotite
that has been multiply metamorphosed during the Devonian, Permian, and Mesozoic.
Tightly folded serpentinite from the Tux Alps, Austria. Closeup view about 30 cm × 20 cm (11.8 in × 7.9 in).

Serpentinite is a

sea floor at tectonic plate boundaries.[2] Serpentinite has been called serpentine or serpentine rock, particularly in older geological texts and in wider cultural settings.[3][4][5][6][7]

Formation and petrology

Serpentinization is a form of low-temperature

native iron;[9] methane and other hydrocarbon compounds; and hydrogen sulfide.[10]

During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure.

exothermic and rock temperatures can be raised by about 260 °C (500 °F),[12] providing an energy source for formation of non-volcanic hydrothermal vents.[14] The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.[15][12]

The final mineral composition of serpentinite is usually dominated by lizardite, chrysotile, and magnetite. Brucite and antigorite are less commonly present. Lizardite, chrysotile, and antigorite all have approximately the formula Mg3(Si2O5)(OH)4 or (Mg2+, Fe2+)3Si2O5(OH)4, but differ in minor components and in form "crystal habit".[16] Accessory minerals, present in small quantities, include awaruite, other native metal minerals, and sulfide minerals.[8]

Formation of serpentine minerals

Olivine is a solid solution of forsterite, the magnesium endmember of (Mg2+, Fe2+)2SiO4, and fayalite, the iron endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks.[17] Serpentinite can form from olivine via several reactions:

Forsterite3 Mg
2SiO
4 + silicon dioxideSiO
2 + 4 H
2O → serpentine2 Mg
3Si
2O
5(OH)
4

(Reaction 1a)
Forsterite2 Mg
2SiO
4 + water3 H
2O → serpentineMg
3Si
2O
5(OH)
4 + bruciteMg(OH)
2

(Reaction 1b)

Reaction 1a tightly binds silica, lowering its

X-ray diffraction, and it is easily altered under surface weathering conditions.[20]

A similar suite of reactions involves pyroxene-group minerals:

Enstatite3 MgSiO
3 + silicon dioxideSiO
2 + H
2O → talcMg
3Si
4O
10(OH)
2

(Reaction 2a)
Enstatite6 MgSiO
3 + 3 H
2O → serpentineMg
3Si
2O
5(OH)
4 + talcMg
3Si
4O
10(OH)
2

(Reaction 2b)

Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over.[21] When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:

Forsterite6 Mg
2SiO
4 + talcMg
3Si
4O
10(OH)
2 + water9 H
2O → serpentine5 Mg
3Si
2O
5(OH)
4

(Reaction 3)

This reaction requires higher temperatures than those at which brucite forms.[20]

The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed 600 °C (1,112 °F) during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.[22]

Breakdown of diopside and formation of rodingites

Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down according to the reaction

Diopside3 CaMgSi
2O
6 + 6 H+
 → serpentineMg
3Si
2O
5(OH)
4 + 3 Ca2+
 + H
2O + silicon dioxide4 SiO
2

(Reaction 4)

This raises both the pH, often to very high values, and the calcium content of the fluids involved in serpentinization. These fluids are highly reactive and may transport calcium and other elements into surrounding mafic rocks. Fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and depleted in silica, called rodingites.[23]

Formation of magnetite and hydrogen

In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the

reducing conditions.[18] Under these conditions, water is capable of oxidizing ferrous (Fe2+
) ions in fayalite. The process is of interest because it generates hydrogen gas:[24][25]

Fayalite3 Fe
2SiO
4 + water2 H
2O → magnetite2 Fe
3O
4 + silicon dioxide3 SiO
2 + hydrogen2 H
2

(Reaction 5)

However, studies of serpentinites suggest that iron minerals are first converted to ferroan brucite, that is, brucite containing Fe(OH)2,[26] which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization:[27][28]

6 Fe(OH)
2ferrous hydroxide → 2 Fe
3O
4magnetite + 4 H
2Owater + 2 H
2hydrogen

(Reaction 6)

Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between 200 and 315 °C (392 and 599 °F).[29] If the original ultramafic rock (the protolith) is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen.[30]

Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite).[31] During serpentinization, the rock is enriched in chlorine, boron, fluorine, and sulfur. Sulfur will be reduce to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as anhydrite.[13] The sulfides produced include nickel-rich sulfides, such as mackinawite.[32]

Methane and other hydrocarbons

Laboratory experiments have confirmed that at a temperature of 300 °C (572 °F) and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization.[10] One reaction pathway is:[27]

forsterite18 Mg
2SiO
4 + fayalite6 Fe
2SiO
4 + 26 H
2O + CO
2 → serpentine12 Mg
3Si
2O
5(OH)
4 + magnetite4 Fe
3O
4 + methaneCH
4

(Reaction 7)

Metamorphism at higher pressure and temperature

Lizardite and chrysotile are stable at low temperatures and pressures, while antigorite is stable at higher temperatures and pressure. Its presence in a serpentinite indicates either that serpentinization took place at unusually high pressure and temperature or that the rock experienced higher grade metamorphism after serpentinization was complete.[22]

Infiltration of CO2-bearing fluids into serpentinite causes distinctive talc-carbonate alteration. Brucite rapidly converts to magnesite and serpentine minerals (other than antigorite) are converted to talc. The presence of pseudomorphs of the original serpentinite minerals shows that this alteration takes place after serpentinization.[33]

Serpentinite may contain

phyllosilicate mineral), tremolite (Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2), and metamorphic olivine and diopside (calcium-rich pyroxene). This indicates that the serpentinite has been subject to more intense metamorphism, reaching the upper greenschist or amphibolite metamorphic facies.[34]

Above about 450 °C (842 °F), antigorite begins to break down. Thus serpentinite does not exist at higher metamorphic facies.[15]

Extraterrestrial production of methane by serpentinization

The presence of traces of

ALH 84001 meteorite, which came from Mars, shows that indeed the organic matter it contains was formed by serpentinization, not by life processes.[37][38]

Using data from the Cassini probe flybys obtained in 2010–12, scientists were able to confirm that Saturn's moon Enceladus likely has a liquid water ocean beneath its frozen surface. A model suggests that the ocean on Enceladus has an alkaline pH of 11–12.[39] The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, that leads to the generation of H
2, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.[39][40]

Occurrence

Ophiolite of the Gros Morne National Park, Newfoundland. Ophiolites characteristically have a serpentinite component.

Serpentinite can be formed wherever ultramafic rock is infiltrated by water poor in carbon dioxide.

subduction zones
.

Conditions are highly favorable for serpentinization at slow to ultraslow spreading mid-ocean ridges.

crustal extension is high compared with the volume of magmatism, bringing ultramafic mantle rock very close to the surface where fracturing allows seawater to infiltrate the rock.[14]

Serpentinized ultramafic rock is found in many

pillow basalts, and a layer of deep water sediments containing radiolarian ribbon chert.[42]

Serpentinization is nearly total in the forearc mantle of subduction zones. Here mantle rock is cooled by the subducting slab to temperatures at which serpentinite is stable, and fluids are released from the subducting slab in great quantities into the ultramafic mantle rock. Direct evidence that serpentinization is taking place in the Mariana Islands island arc is provided by the activity of serpentinite mud volcanoes. Xenoliths of harzburgite and (less commonly) dunite are occasionally erupted by the mud volcanoes, giving clues to the nature of the protolith.[43]

Because serpentinization increases the volume and lowers the density of the original rock, serpentinitization may lead to uplift that creates coastal ranges above mantle forearcs.[43] Further uplift can bring serpentinite to the surface when subduction ceases, as has taken place with the serpentinite exposed at the Presidio of San Francisco.[44]

Moho discontinuity, in which seismic velocity abruptly decreases across the crust-mantle boundary, which is the opposite of the usual behavior. The serpentinite is highly deformable, creating an aseismic zone in the forearc, and the presence of serpentinite may limit the maximum depth of megathrust earthquakes.[45] Serpentinization at slow spreading mid-ocean ridges can cause the seismic Moho discontinuity to be placed at the serpentinization front, rather than the base of the crust as defined by normal petrological criteria.[46][13] The Lanzo Massif of the Italian Alps shows a sharp serpentinization front that may be a relict seismic Moho.[47]

Notable occurrences of serpentinite are found at

Newfoundland ophiolites, and the Main Ophiolite Belt of New Guinea.[49]

Hydrothermal vents and mud volcanoes

A white carbonate spire in the Lost City vent field

The formation of serpentinite is highly exothermic, releasing up to 40 kilojoules (9.6 kcal) per mole of water reacting with the rock. This corresponds to a release of about 660 MJ/m3 and can raise the rock temperature by about 260 °C (500 °F),[15] providing an energy source for formation of non-volcanic hydrothermal vents.[14] The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.[15][12]

Deep sea hydrothermal vents located on serpentinite close to the axis of mid-ocean ridges generally resemble

black smokers located on basalt, but emit complex hydrocarbon molecules. The Rainbow field of the Mid-Atlantic Ridge is an example of such hydrothermal vents. Serpentinization alone cannot provide the heat supply for these vents, which must be driven mostly by magmatism. However, the Lost City Hydrothermal Field, located off the axis of the Mid-Atlantic Ridge, may be driven solely by heat of serpentinization. Its vents are unlike black smokers, emitting relatively cool fluids (40 to 75 °C (104 to 167 °F)) that are highly alkaline, high in magnesium, and low in hydrogen sulfide. The vents build up very large chimneys, up to 60 meters (200 ft) in height, composed of carbonate minerals and brucite. Lush microbial communities are associated with the vents. Though the vents themselves are not composed of serpentinite, they are hosted in serpentinite estimated to have formed at a temperature of about 200 °C (392 °F). Sepiolite deposits on mid-ocean ridges may have formed through serpentinite-driven hydrothermal activity.[13] However, geologists continue to debate whether serpentinization alone can account for the heat flux from the Lost City field.[50]

The forearc of the Marianas subduction zone hosts large serpentinite mud volcanoes, which erupt serpentinite mud that rises through faults from the underlying serpentinized forearc mantle. Study of these mud volcanoes gives insights into subduction processes, and the high pH fluids emitted at the volcanoes support a microbial community.[51]

Serpentinite thermal vents are a candidate for the environment in which life on Earth originated.[51] Most of the chemical reactions necessary to synthesize acetyl-CoA, essential to basic biochemical pathways of life, take place during serpentinization.[52] The sulfide-metal clusters that activate many enzymes resemble sulfide minerals formed during serpentinization.[53]

Experimental drilling into the gabbro layer of oceanic crust near mid-ocean ridges has demonstrated the presence of a sparse population of hydrocarbon-degrading bacteria. These may feed on hydrocarbons produced by serpentinization of the underlying ultramafic rock.[54][55]

Ecology

Serpentinite ecosystem in the south of New Caledonia
Main article: Serpentine soil

Soil cover over serpentinite bedrock tends to be thin or absent. Soil with serpentine is poor in calcium and other major plant nutrients, but rich in elements toxic to plants such as chromium and nickel.[56] Some species of plants, such as Clarkia franciscana and certain species of manzanita, are adapted to living on serpentinite outcrops. However, because serpentinite outcrops are few and isolated, their plant communities are ecological islands and these distinctive species are often highly endangered.[44] On the other hand, plant communities adapted to living on the serpentine outcrops of New Caledonia resist displacement by introduced species that are poorly adapted to this environment.[57]

Serpentine soils are widely distributed on Earth, in part mirroring the distribution of ophiolites. There are outcroppings of serpentine soils in the Balkan Peninsula, Turkey, the island of Cyprus, the Alps, Cuba, and New Caledonia. In North America, serpentine soils also are present in small but widely distributed areas on the eastern slope of the Appalachian Mountains in the eastern United States, and in the Pacific Ranges of Oregon and California.[citation needed]

Uses

Decorative stone in architecture and art

Drinking cups, evidences of serpentinite turning in Zöblitz

Serpentine group minerals have a

Larissa, Greece.[59]
Serpentinites are used in many ways in the arts and crafts. For example, the rock has been turned in Zöblitz in Saxony for several hundred years.[60]

Carving stone tools, oil lamp-known as the Qulliq and Inuit sculpture

The Inuit and other indigenous people of the Arctic areas and less so of southern areas used the carved bowl shaped serpentinite qulliq or kudlik lamp with wick, to burn oil or fat to heat, make light and cook with. The Inuit made tools and more recently carvings of animals for commerce.[61]

  • Magnetic serpentine walrus
    Magnetic serpentine walrus
  • Inuit Elder tending the Qulliq, a ceremonial oil lamp made of serpentinite.
    Inuit Elder tending the Qulliq, a ceremonial oil lamp made of serpentinite.

Swiss ovenstone

A variety of chlorite talc schist associated with Alpine serpentinite is found in Val d'Anniviers, Switzerland and was used for making "ovenstones" (German: Ofenstein), a carved stone base beneath a cast iron stove.[62]

Neutron shield in nuclear reactors

Serpentinite has a significant amount of

radiation shielding to protect operators from escaping neutrons.[63] Serpentine can also be added as aggregate to special concrete used in nuclear reactor shielding to increase the concrete density (2.6 g/cm3 (0.094 lb/cu in)) and its neutron capture cross section.[64][65]

CO2 sequestration

Because it readily absorbs carbon dioxide, serpentinite may be of use for sequestering atmospheric carbon dioxide.[66] To speed the reaction, the serpentinite may be reacted with carbon dioxide at elevated temperature in carbonation reactors. Carbon dioxide may also be reacted with alkaline mine waste from serpentine deposits, or carbon dioxide may be injected directly into underground serpentinite formations.[67] Serpentinite may also be used as a source of magnesium in conjunction with electrolytic cells for CO2 scrubbing.[68]

Cultural references

It is the state rock of California, USA and the California Legislature specified that serpentine was "the official State Rock and lithologic emblem."[4] In 2010, a bill was introduced which would have removed serpentine's special status as state rock due to it potentially containing chrysotile asbestos.[69] The bill met with resistance from some California geologists, who noted that the chrysotile present is not hazardous unless it is mobilized in the air as dust.[70] [needs update]

See also

References

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  2. ^ "Serpentine definition". Dictionary of Geology. Retrieved 23 October 2018.
  3. ^ "serpentine". Merriam-Webster.com Dictionary. Merriam-Webster. Retrieved 6 March 2022.
  4. ^ a b California Government Code § 425.2; see "CA Codes (Gov:420-429.8)". Archived from the original on 28 June 2009. Retrieved 24 December 2009.
  5. ^ Oakeshott, G.B. (1968). "Diapiric Structures in Diablo Range, California". AAPG Special Volume M8:Diapirism and Diapirs. 153: 228–243.
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  12. ^ a b c d Serpentinization: The heat engine at Lost City and sponge of the oceanic crust
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  23. ^ Frost & Beard 2007, pp. 1360–1362.
  24. ^ "Methane and hydrogen formation from rocks – Energy sources for life". Retrieved 6 November 2011.
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  34. ^ Moody 1976, p. 128.
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  36. ^ esa. "The methane mystery". European Space Agency. Retrieved 22 April 2019.
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  38. ^ Leah Crane (22 January 2022). "Mars: Organic compounds were made by water interacting with rocks". New Scientist.
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  40. ^ Wall, Mike (7 May 2015). "Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life". Space.com. Retrieved 8 May 2015.
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  61. ^ Kerr, A.; Squires, G.C. "Serpentinites and associated rock types near Hopedale, Nunatsiavut: Potential for artisanal carving-stone resources" (PDF). Geological Survey Report. 19 (1). Newfoundland and Labrador Department of Natural Resources: 39–57. Retrieved 3 September 2021.
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  69. ^ Fimrite, Peter (16 July 2010). "Geologists protest bill to remove state rock". San Francisco Chronicle. Retrieved 17 April 2018.
  70. ^ Frazell, Julie; Elkins, Rachel; O'Geen, Anthony; Reynolds, Robert; Meyers, James. "Facts about Serpentine Rock and Soil Containing Asbestos in California" (PDF). ANR Catalog. University of California Division of Agriculture and Natural Resources. Retrieved 17 April 2018.

External links

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