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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] E LEMENTS , V OL . 9, PP. 95–98 95 A PR IL 2013 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 E LEMENTS 96 A PR IL 2013 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). E LEMENTS 97 A PR IL 2013 (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) REFERENCES Butt CRM, Cluzel D (2013) Nickel laterite ore deposits: Weathered serpentinites. Elements 9: 123-128 Cannat M, Fontaine F, Escartín J (2010) Serpentinization and associated hydrogen and methane fluxes at slow spreading ridges. In: Rona PA, Devey CW, Dyment J, Murton BJ (eds) Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. American Geophysical Union Geophysical Monograph 188, pp 241-264 Coleman RG (1977) Ophiolites: Ancient Oceanic Lithosphere? Springer-Verlag, Berlin, 229 pp Deschamps F, Godard G, Guillot S, Hattori K (2013) Geochemistry of subduction zone serpentinites: A review. Lithos in press Ehlmann BL, Mustard JF, Murchie SL (2010) Geologic setting of serpentine deposits on Mars. Geophysical Research Letters 37: doi: 10.1029/2010GL042596 Evans BW, Hattori K, Baronnet A (2013) Serpentinite: What, why, where? Elements 9: 99-106 Fryer P, Wheat CG, Mottl MJ (1999) Mariana blueschist mud volcanism: Implications for conditions within the subduction zone. Geology 27: 103-106 Fubini B, Fenoglio I (2007) Toxic potential of mineral dusts. Elements 3: 407-414 Goff F, Lackner KS (1998) Carbon dioxide sequestering using ultramafic rocks. Environmental Geosciences 5: 89-101 Gorczyk W, Guillot S, Gerya TV, Hattori K (2007) Asthenospheric upwelling, oceanic slab retreat, and exhumation of E LEMENTS 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. UHP mantle rocks: Insights from Greater Antilles. Geophysical Research Letters 34: doi: 10.1029/2007GL031059 Guillot S, Hattori K, Agard P, Schwartz S, Vidal O (2009) Exhumation processes in oceanic and continental subduction contexts: a review. In: Lallemand S, Funiciello F (eds) Subduction Zone Dynamics. Springer-Verlag, Berlin, pp 175-204 Hattori K, Guillot S (2003) Volcanic fronts form as a consequence of serpentinite dehydration in the forearc mantle wedge. Geology 31: 525-528 Hattori K, Guillot S (2007) Geochemical character of serpentinites associated with high- to ultrahigh-pressure rocks in the Alps, Cuba, and the Himalayas: Recycling of elements in subduction zones. Geochemistry, Geophysics, Geosystems 8: doi: 101029/2007GC001594 Hirth G, Guillot S (2013) Rheology and tectonic significance of serpentinite. Elements 9: 107-113 Kelemen PB, Matter J (2008) In situ carbonation of peridotite for CO2 storage. 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Elements 9: 115-121 Saumur B-M, Hattori K, Guillot S (2010) Contrasting origins of serpentinites in a subduction complex, northern Dominican Republic. Geological Society of America Bulletin 122: 292-304 Schmidt MW, Poli S (2003) Generation of mobile components during subduction of oceanic crust. Treatise on Geochemistry 3: 567-593 Schulte M, Blake D, Hoehler T, McCollom T (2006) Serpentinization and its implications for life on the early Earth and Mars. Astrobiology 6: 364-376 Shelly DR, Beroza GC, Ide S (2007) Non-volcanic tremor and low-frequency earthquake swarms. Nature 446: 305-307 Sleep NH, Meibom A, Fridriksson TH, Coleman RG, Bird DK (2004) H2 -rich fluids from serpentinization: Geochemical and biotic implications. Proceedings of the National Academy of Sciences 101: 12818-12823 Tuttle OF, Bowen NL (1958) Origin of Granite in the Light of Experimental Studies in the System NaAlSi3O8 KAlSi3O8 -SiO2 -H2O. Geological Society of America Memoir 74, 151 pp A PR IL 2013