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Serpentinites: Essential Roles
in Geodynamics, Arc Volcanism,
Sustainable Development,
and the Origin of Life
Stéphane Guillot1 and Keiko Hattori2
1811-5209/13/0009-095$2.50
S
Chrysotile veins in
lizardite serpentinite
from Munro Township,
Ontario, Canada.
PHOTO COURTESY OF
B ENOIT SAUMUR
DOI: 10.2113/gselements.9.2.95
In subduction zones, serpentinites
occur near the base of the mantle
wedge at depths greater than 10
km. In some forearc environments, buoyant serpentinites have
been exhumed to the seafloor
along normal faults and through
tectonic processes, such as subduction erosion and the return flow
of subducted material in a subduction channel (the low-viscosity
interface between the two plates).
Good examples of the latter are the
serpentinite diapirs in the Mariana
forearc, where serpentinites from
KEYWORDS : serpentinization, geochemical cycle, arc magma, seismicity,
the base of the mantle wedge
nickel ore, CO2 sequestration, origin of life
have ascended to the seafloor.
The upward momentum of these
serpentinites was such that they
ORIGIN OF SERPENTINITE
produced “seamounts” and “mud
volcanoes” with up to 2000 m of relief. The discovery of
Serpentinites—water-rich rocks composed mostly of
these modern serpentinites has provided useful inforserpentine-group minerals (chrysotile, lizardite and antigmation for identifying the protoliths of serpentinites in
orite)—are present in almost all continents and island arcs.
They form large massifs and belts (e.g. the Great Serpentinite ancient terranes and the geological setting for serpentinization in the geological past (e.g. Hattori and Guillot 2007)
Belt in Cuba and Hispaniola; Saumur et al. 2010), as well
as tabular bodies along faults and shear zones. The associ- Ancient serpentinites on continents are well exposed in
ation of serpentinite with mafic volcanic rocks and chert
suture zones associated with the closure of paleo-oceans;
has long been known to Alpine geologists, who referred to
examples are serpentinites in northern India, Turkey, and
them collectively as ophiolites or the Steinmann Trinity.
the Appalachians (Coleman 1977). Kelemen and Matter
The recognition of this association has led to extended (2008) estimated that the Semail ophiolite in the Sultanate
discussions on the origin of serpentinite. One theory held of Oman and the United Arab Emirates (the world’s largest
that serpentinite crystallizes directly from a serpentinite
ophiolite at ~10,000 km 2 ) contains ~5 × 1016 kg of partly
magma, but this idea was dispelled by Tuttle and Bowen’s
serpentinized peridotite to a depth of 3 km. The Massif
(1958) classic laboratory study, which established the du Sud in New Caledonia, the world’s longest continuous
low-temperature stability range of serpentine. Nowadays,
ophiolite, covers an area of ~6000 km 2 . This ophiolite
the mineralogy of serpentinite is known to result from the
consists mostly of partially to totally hydrated mantle
relatively low-temperature hydration of peridotite, during
peridotite. Smaller ophiolites occur on all continents
which Mg-rich olivine and orthopyroxene are altered
(FIG. 1), including Antarctica. Our fi rst-order approximato serpentine minerals with the approximate formula tion suggests that >3% of the Earth’s surface is made up
Mg3Si2O5 (OH) 4 (Evans et al. 2013 this issue).
of serpentinite and serpentinized peridotite.
erpentinites are rocks consisting mostly of the serpentine-group minerals
chrysotile, lizardite and antigorite. They are formed by the hydration
of olivine-rich ultramafic rocks and they contain up to ~13 wt% H2O.
They have long been used by many cultures as building and carving stones.
Serpentinites play essential roles in numerous geological settings. They act as
a lubricant along plate boundaries during aseismic creep and contribute to the
geochemical cycle of subduction zones. In the mantle, they are a reservoir of
water and fluid-mobile elements. Serpentinites can produce nickel ore where
weathered, and they can sequester CO2 where carbonated. They may have
provided an environment for the abiotic generation of amino acids on the
early Earth and other planets, potentially leading to the development of life.
The ocean floor is commonly composed of volcanic rocks
formed at ridges, but the production of volcanic rocks is
low at slow-spreading ridges, where mantle peridotites
are abundantly exposed. The hydration of these mantle
peridotites forms serpentinites on the seafloor, and
serpentinites may constitute up to 25% of the top part
of the oceanic lithosphere (Cannat et al. 2010) (FIG. 1).
ROLE OF SERPENTINITE IN GEODYNAMICS
Large earthquakes occur near the interfaces between
convergent plates. Examples include the earthquake
offshore from Sumatra in December 2004 (magnitude ~9),
the Tohoku earthquake near Japan in March 2011 (magnitude ~9), and the earthquake near Parkfield, California, in
1857 (magnitude ~8). Friction between two plates causes
brief (<10 minutes) to long-term (>1 year) seismic activity,
along with phenomena of intermediate duration, such as
tremors and slow-slip events (Shelly et al. 2007). Movement
along the upper surface of a subducting plate is facilitated
by a mechanically weak mineral, such as serpentine, talc,
or another phyllosilicate, thus producing aseismic slip
1 CNRS, ISTerre, Université de Grenoble
1381 rue de la Piscine, Grenoble cédex 9, France
E-mail: [email protected]
2 Department of Earth Sciences, University of Ottawa
140 Louis-Pasteur, Ottawa, ON K1N 6N5, Canada
E-mail: [email protected]
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ROLE OF SERPENTINITE
IN GEOCHEMICAL CYCLES
Map of serpentinite occurrences on the seafloor and
continents. The oceanic lithosphere is color coded for
spreading rate (Cannat et al. 2010), and black lines show the ridge
axes. The major occurrences of serpentinite are shown by yellow
circles for seafloor sites (courtesy of Javier Escartín) and by green
lines on continents (based on Coleman 1977). Serpentinites are also
present on the ocean floor in the forearc regions of intraoceanic,
western Pacific arcs (Fryer et al. 1999).
FIGURE 1
It has been known for some time that serpentinites are
present in subduction zones; however, only recently has
their relationship to arc magmatism (Hattori and Guillot
2003) and global geochemical cycles (Hattori and Guillot
2007) been recognized. Serpentinite formed near the
seafloor incorporates not only water but also fluid-mobile
elements (such as B, Li, As, Sb, Pb, U, Cs, Sr, and Ba) into the
serpentine structure. Thus, serpentinization in the oceanic
lithosphere transfers elements from the hydrosphere to the
lithosphere (FIG. 2). Consequently, the subduction of serpentinized mantle peridotite transports large amounts of water
and fluid-mobile elements from the ocean to the mantle.
The pore fluids expelled from sediments in subduction zones
at relatively shallow depth (10–30 km) are released upward
into the overlying forearc mantle, resulting in the formation
of serpentinite bodies near the base of the mantle wedge
(FIG. 2). This process also transfers fluid-mobile elements
together with water into the overlying mantle (Schmidt and
Poli 2003). Therefore, all serpentinites, independent of their
place of origin (seafloor or subduction zone) and ultimate
geological setting, are moderately to strongly enriched
in fluid-mobile elements (e.g. Hattori and Guillot 2003;
Deschamps et al. 2013).
(Hirth and Guillot 2013 this issue). Furthermore, serpentinite is buoyant compared to anhydrous peridotite and
weakens the physical strength of a subduction channel,
thus leading to ductile behavior. Because of these properties, the presence of serpentinite in and near a subducting
slab enables subducted oceanic and continental rocks to
be exhumed to the surface (Gorczyk et al. 2007; Guillot
et al. 2009).
Arc magmas contain high concentrations of fluid-mobile
elements, such as As, Pb, and Sr, that were ultimately derived
from the subducted sediments and slab (e.g. Leeman 1996);
however, the mechanism of transfer of these elements from
the surface of the subducting slab to magma in the interior
of the mantle wedge has been debated. The enrichment
patterns of many fluid-mobile elements in mantle-wedge
serpentinites are strikingly similar to the element patterns
in magmas at volcanic fronts (Hattori and Guillot 2003;
Evans et al. 2013). This evidence suggests a link between the
dehydration of serpentinite and arc magmas. Serpentinite
contains over 13 wt% H2O, which is released during dehydration at high pressures (30–100 km depth) and/or temperatures. The ascent of these fluids (plus fluid-mobile elements)
Schematic cross section of a subduction zone illustrating the geochemical characteristics of different
serpentinites. The transfer of fluid-mobile elements is also noted
(after Deschamps et al. 2013).
FIGURE 2
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Sheared serpentinite in the Rivière-des-Plantes ultramafic
complex of the Southern Québec Ophiolite Belt,
Canadian Appalachians. PHOTO COURTESY OF ALAIN TREMBLAY
FIGURE 3
Cross section of the serpentinite sole of the Koniambo
massif, New Caledonia; the serpentinite is mined by
Koniambo Nickel SAS. PHOTO COURTESY OF PIERRE GAUTIER
FIGURE 4
into the interior of the hot mantle wedge contributes to
partial melting and subsequent arc magmatism (Hattori and
Guillot 2003; Evans et al. 2013).
many practical ways, such as for railway ballast and building
stone. The asbestiform types (e.g. chrysotile) of the serpentine-group minerals have been used extensively for their
thermal and electrical insulating properties. Unfortunately,
fi ne fibrous asbestos, especially the amphibole type, has
been linked to malignant mesothelioma of the lungs (e.g.
Fubini and Fenoglio 2007).
In summary, serpentinites in oceanic lithosphere transport
fluid-mobile elements from seawater to the seafloor near
spreading ridges, from ridges to subduction zones, and from
subducting slabs to mantle wedges (FIG. 2). The eventual
dehydration of mantle-wedge serpentinites transfers their
contained water and fluid-mobile elements to arc magmas.
The increased interest in serpentinites is in part related to
their economic value (FIG. 4). Intensely weathered serpentinites are enriched in nickel (Ni); they account for nearly
60% of the world’s Ni production, and they will remain the
predominant source of Ni in the foreseeable future (Butt and
Cluzel 2013 this issue).
SERPENTINITE, CIVILIZATION,
AND SUSTAINABLE DEVELOPMENT
Serpentinites are fascinating and beautiful rocks. Since
antiquity, they have been used around the world—in the
Mediterranean, Asia–Oceania, and the Americas—to make
jewelry and ceremonial and ornamental carvings. Their
olive green color and smooth but scaly appearance (FIG. 3)
is the basis for their name, which comes from the Latin
serpentinus, meaning “serpent.” Serpentinites are also used in
FIGURE 5
In addition, partially serpentinized ultramafic rocks are
reactive and could be used to sequester anthropogenic CO2
and thus help to mediate its influence on global climate
Elevation map of Mars showing the locations of
serpentine-bearing materials (modified after Ehlmann
et al. 2010).
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(Power et al. 2013 this issue). The end products of the
reaction between CO2 and serpentinites are carbonates. Goff
and Lackner (1998) have suggested that CO2 -sequestration
schemes may be employed to extract strategically important metals, such as Ni, Co, and platinum-group elements,
as by-products of the process, since these metals are not
incorporated into carbonates.
The idea that serpentinization had a role in the origin of life
on Earth may also be applicable to other planets, as olivine is
a common phase in many meteorites and on other planets.
Serpentine is the dominant hydroxyl-bearing mineral in
carbonaceous chondrites, the most primitive meteorites
and those with the closest relationship to the solar nebula.
Nakhlite, a Martian meteorite, contains serpentine veins.
Serpentinite was recently identified on Mars using Mars
Reconnaissance Orbiter data (FIG. 5; Ehlmann et al. 2010):
serpentine is mixed with other alteration minerals and on
the flanks of the central topographic highs, in the ejecta of
impact craters, and in direct association with olivine-rich
rocks, all in Noachian (4.1–3.7 Ga) terranes (FIG. 5).
SERPENTINIZATION
AND THE ORIGIN OF LIFE
Serpentinization of peridotite is accompanied by the release
of hydrogen gas, a phenomenon recognized more than 50
years ago (Sleep et al. 2004; Evans et al. 2013). The hydrogen
and other reduced gases produced during serpentinization
provide a unique ecosystem on the deep seafloor (Ohara et
al. 2012). The presence of such conditions is now suggested
to be a factor in the origin of life on Earth and possibly other
planets (McCollom and Seewald 2013 this issue). Ultramafic
rocks and serpentinites are abundant in Archean greenstone
belts, including the Isua belt in Greenland, which contains
the oldest (3.81–3.70 Ga) supracrustal rocks on Earth. A
detailed geochemical study of serpentinites in the Isua belt
suggests that the fluids responsible for the hydration of the
ulramafic rocks were similar to those causing present-day
serpentinization in the Mariana forearc (Pons et al. 2011).
Considering the abundance of ultramafic rocks in the
Archean, serpentinization would have provided environments favorable for the abiotic production and stabilization
of amino acids, which are thought to be precursors to the
emergence of life on our planet (Schulte et al. 2006)
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CONCLUSIONS
In summary, serpentinites play versatile roles in many
scientific fields, including structural geology, rheology,
seismology, subduction zone geochemistry, marine
geochemistry, climate science, mineral deposits, astrobiology, and microbiology. Studies of serpentinites are
ongoing, yet fundamental questions remain, including the
details of the serpentinization process and the nature of the
serpentine mineral family.
ACKNOWLEDGMENTS
We thank the authors contributing to this issue of Elements
for their articles on this wide-ranging topic, the reviewers
for their input, and Principal Editor Georges Calas and
Managing Editor Pierrette Tremblay for their help and
patience in bringing this issue together.
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