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Stéphane Guillot - One of the best experts on this subject based on the ideXlab platform.
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Evidence for a serpentinized plate interface favouring continental subduction
'Springer Science and Business Media LLC', 2020Co-Authors: Zhao Liang, Stéphane Guillot, Malusà Marco, Yuan Huaiyu, Paul Anne, Lu Yang, Stehly Laurent, Solarino Stefano, Eva Elena, Lu GangAbstract:International audienceThe dynamics of continental subduction is largely controlled by the rheological properties of rocks involved along the subduction channel. Serpentinites have low viscosity at geological strain rates. However, compelling geophysical evidence of a Serpentinite channel during continental subduction is still lacking. Here we show that anomalously low shear-wave seismic velocities are found beneath the Western Alps, along the plate interface between the European slab and the overlying Adriatic mantle. We propose that these seismic velocities indicate the stacked remnants of a weak fossilised Serpentinite channel, which includes both slivers of abyssal Serpentinite formed at the ocean floor and mantle-wedge Serpentinite formed by fluid release from the subducting slab. Our results suggest that this serpentinized plate interface may have favoured the subduction of continental crust into the upper mantle and the formation/exhumation of ultra-high pressure metamorphic rocks, providing new constraints to develop the conceptual and quantitative understanding of continental-subduction dynamics
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high pressure Serpentinites a trap and release system controlled by metamorphic conditions example from the piedmont zone of the western alps
Chemical Geology, 2013Co-Authors: Romain Lafay, Stéphane Guillot, Marguerite Godard, Stephane Schwartz, Baptiste Debret, Fabien Deschamps, Christian NicolletAbstract:We provide new insights into the geochemistry of Serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These Serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustres complex and (3) Serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, Serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the Serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and Serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine Serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that Serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
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Rheology and tectonic significance of Serpentinite
Elements, 2013Co-Authors: Greg Hirth, Stéphane GuillotAbstract:Serpentinites occur in many active geologic settings and control the rheology of the lithosphere where aqueous fluids interact with ultramafic rocks. The crystal structure of serpentine-group minerals results in diagnostic physical properties that are important for interpreting a wide range of geophysical data and impart unique rheological behaviors. Serpentinites play an important role during continental rifting and oceanic spreading, in strain localization along lithospheric strike-slip faults, and in subduction zone processes. The rheology of serpentine is key for understanding the nucleation and propagation of earthquakes, and the relative weakness of Serpentinite can significantly affect geodynamic processes at tectonic plate boundaries.
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High-pressure Serpentinites, a trap-and-release system controlled by metamorphic conditions: Example from the Piedmont zone of the western Alps
Chemical Geology, 2013Co-Authors: Romain Lafay, Stéphane Guillot, Marguerite Godard, Stephane Schwartz, Baptiste Debret, Fabien Deschamps, Christian NicolletAbstract:We provide new insights into the geochemistry of Serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These Serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustrés complex and (3) Serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, Serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the Serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and Serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine Serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that Serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
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Contrasting origins of Serpentinites in a subduction complex, northern Dominican Republic
Geological Society of America Bulletin, 2010Co-Authors: B.m Saumur, Keiko Hattori, Stéphane GuillotAbstract:Serpentinites in a Tertiary subduction complex in the northern Dominican Republic contain low concentrations of incompatible elements in bulk-rock compositions and high Mg in relict silicate minerals. The forsterite component in olivine ranges from 89.0% to 90.8%, and the enstatite component in orthopyroxene ranges from 89.4% to 91.1%, suggesting that they are mantle peridotites. Two different protoliths are identified for the Serpentinites based on the bulk-rock compositions and spinel chemistry: abyssal peridotites and forearc mantle peridotites. Hydrated abyssal peridotites are voluminous and occur in ophiolite complexes in the northern terranes (Puerto Plata Complex and the northern part of the Rio San Juan Complex) and in Serpentinite mélanges in the central part of the Rio San Juan Complex. The Serpentinite mélanges contain fragments of high-pressure–low-temperature rocks and are interpreted to be tectonic mélanges, representing part of a Serpentinite subduction channel. The Serpentinites show moderate Al/Si weight ratios (0.026–0.081) in bulk rocks and moderate Cr# (atomic ratio of Cr/[Cr + Al] = 0.20–0.55) in spinel. Hydrated forearc mantle peridotites occur along major strike-slip faults, the Camú fault zone, and the Septentrional fault zone. They show low bulk-rock Al/Si weight ratios (up to 0.021), high concentration in Ir-group platinum group elements (13.1–24.6 ppb total), and high Cr# (0.48–0.67) in spinel. Raman spectroscopy and X-ray powder diffraction indicate that lizardite is the predominant serpentine species. The absence of antigorite suggests that these Serpentinites were derived from a shallow depth (
Keiko Hattori - One of the best experts on this subject based on the ideXlab platform.
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Abyssal Serpentinites: Transporting Halogens from Earth’s Surface to the Deep Mantle
MDPI AG, 2019Co-Authors: Lilianne Pagé, Keiko HattoriAbstract:Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor Serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from Serpentinites during the lizardite⁻antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from Serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for Serpentinite thickness of the metamorphosed slab (500 m), antigorite Serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle
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Tracing halogen and B cycling in subduction zones based on obducted, subducted and forearc Serpentinites of the Dominican Republic.
Scientific reports, 2017Co-Authors: Lilianne Pagé, Keiko HattoriAbstract:Serpentinites are important reservoirs of fluid-mobile elements in subduction zones, contributing to volatiles in arc magmas and their transport into the Earth's mantle. This paper reports halogen (F, Cl, Br, I) and B abundances of Serpentinites from the Dominican Republic, including obducted and subducted abyssal Serpentinites and forearc mantle Serpentinites. Abyssal Serpentinite compositions indicate the incorporation of these elements from seawater and sediments during serpentinization on the seafloor and at slab bending. During their subduction and subsequent lizardite-antigorite transition, F and B are retained in Serpentinites, whilst Cl, Br and I are expelled. Forearc mantle Serpentinite compositions suggest their hydration by fluids released from subducting altered oceanic crust and abyssal Serpentinites, with only minor sediment contribution. This finding is consistent with the minimal subduction of sediments in the Dominican Republic. Forearc mantle Serpentinites have F/Cl and B/Cl ratios similar to arc magmas, suggesting the importance of Serpentinite dehydration in the generation of arc magmatism in the mantle wedge.
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Serpentinite: What, Why, Where?
Elements, 2013Co-Authors: Bernard W. Evans, Keiko Hattori, Alain BaronnetAbstract:Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe 2+ to Fe 3+ . Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H 2 . The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in Serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge Serpentinites and arc magmas suggest a linkage between the dehydration of Serpentinite and arc magmatism.
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Contrasting origins of Serpentinites in a subduction complex, northern Dominican Republic
Geological Society of America Bulletin, 2010Co-Authors: B.m Saumur, Keiko Hattori, Stéphane GuillotAbstract:Serpentinites in a Tertiary subduction complex in the northern Dominican Republic contain low concentrations of incompatible elements in bulk-rock compositions and high Mg in relict silicate minerals. The forsterite component in olivine ranges from 89.0% to 90.8%, and the enstatite component in orthopyroxene ranges from 89.4% to 91.1%, suggesting that they are mantle peridotites. Two different protoliths are identified for the Serpentinites based on the bulk-rock compositions and spinel chemistry: abyssal peridotites and forearc mantle peridotites. Hydrated abyssal peridotites are voluminous and occur in ophiolite complexes in the northern terranes (Puerto Plata Complex and the northern part of the Rio San Juan Complex) and in Serpentinite mélanges in the central part of the Rio San Juan Complex. The Serpentinite mélanges contain fragments of high-pressure–low-temperature rocks and are interpreted to be tectonic mélanges, representing part of a Serpentinite subduction channel. The Serpentinites show moderate Al/Si weight ratios (0.026–0.081) in bulk rocks and moderate Cr# (atomic ratio of Cr/[Cr + Al] = 0.20–0.55) in spinel. Hydrated forearc mantle peridotites occur along major strike-slip faults, the Camú fault zone, and the Septentrional fault zone. They show low bulk-rock Al/Si weight ratios (up to 0.021), high concentration in Ir-group platinum group elements (13.1–24.6 ppb total), and high Cr# (0.48–0.67) in spinel. Raman spectroscopy and X-ray powder diffraction indicate that lizardite is the predominant serpentine species. The absence of antigorite suggests that these Serpentinites were derived from a shallow depth (
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Contrasting origins of Serpentinites in a subduction complex, northern Dominican Republic
Geological Society of America Bulletin, 2009Co-Authors: B.m Saumur, Keiko Hattori, Stéphane GuillotAbstract:Serpentinites in a Tertiary subduction complex in the northern Dominican Republic contain low concentrations of incompatible elements in bulk-rock compositions and high Mg in relict silicate minerals. The forsterite component in olivine ranges from 89.0% to 90.8%, and the enstatite component in orthopyroxene ranges from 89.4% to 91.1%, suggesting that they are mantle peridotites. Two different protoliths are identifi ed for the Serpentinites based on the bulk-rock compositions and spinel chemistry: abyssal peridotites and forearc mantle peridotites. Hydrated abyssal peridotites are voluminous and occur in ophiolite complexes in the northern terranes (Puerto Plata Complex and the northern part of the Rio San Juan Complex) and in Serpentinite melanges in the central part of the Rio San Juan Complex. The Serpentinite melanges contain fragments of high-pressure‐low-temperature rocks and are interpreted to be tectonic melanges, representing part of a Serpentinite subduction channel. The Serpentinites show moderate Al/Si weight ratios (0.026‐0.081) in bulk rocks and moderate Cr# (atomic ratio of Cr/[Cr + Al] = 0.20‐0.55) in spinel. Hydrated forearc mantle peridotites occur along major strike-slip faults: the Camu fault zone, and the Septentrional fault zone. They show low bulk-rock Al/Si weight ratios (up to 0.021), high concentration in Ir-group platinum group elements (13.1‐24.6 ppb total), and high Cr# (0.48‐0.67) in spinel. Raman spectroscopy and X-ray powder diffraction indicate that lizardite is the predominant serpentine species. The absence of antigorite suggests that these Serpentinites were derived from a shallow depth (
Baptiste Debret - One of the best experts on this subject based on the ideXlab platform.
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iron and zinc stable isotope evidence for open system high pressure dehydration of antigorite Serpentinite in subduction zones
Geochimica et Cosmochimica Acta, 2021Co-Authors: Baptiste Debret, Vicente Lopez Sanchezvizcaino, Carlos J Garrido, Marielaure Pons, Pierre Bouilhol, Edward C Inglis, Helen M. WilliamsAbstract:Abstract Subducted Serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in Serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-Serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-Serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing Serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6–1.9 GPa and 680–710 °C). The mean δ56Fe of all the Cerro del Almirez meta-Serpentinites (+0.05 ± 0.01‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03‰). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34–67 ppm) and isotope signatures (δ66Zn = +0.18 – +0.55‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ66Zn = +0.16 ± 0.06‰). The Atg-Serpentinites ([Zn] = 34–46 ppm; δ66Zn = +0.23 ± 0.06‰) display similar [Zn] and δ66Zn values to those of slab Serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ66Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ66Zn = +0.30 ± 0.06‰) and Chl-harzburgites with granofels ([Zn] = 38–59 ppm; δ66Zn = +0.33 ± 0.04‰) or spinifex ([Zn] = 48–66 ppm; δ66Zn = +0.43 ± 0.09‰) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and Serpentinites (29–45 ppm) and a positive correlation exists between [Zn] and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ66Zn from Atg-Serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus Serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.
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Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366
Lithos, 2019Co-Authors: Baptiste Debret, Elmar Albers, Bastien Walter, Roy E. Price, Jaime D. Barnes, Hugues Beunon, Sébastien P. Facq, David P. Gillikin, Nadine Mattielli, Helen M. WilliamsAbstract:Abstract The Mariana forearc is a unique setting on Earth where Serpentinite mud volcanoes exhume clasts originating from depths of 15 km and more from the forearc mantle. These peridotite clasts are variably serpentinized by interaction with slab derived fluid, and provide a record of forearc mantle dynamics and changes in geochemistry with depth. During International Oceanic Discovery Program (IODP) Expedition 366, we recovered serpentinized ultramafic clasts contained within Serpentinite muds of three different mud volcanoes located at increasing distance from the Mariana trench and at increasing depth to the slab/mantle interface: Yinazao (distance to the trench: 55 km / depth to the slab/mantle interface: 13 km), Fantangisna (62 km / 14 km) and Asut Tesoru (72 km / 18 km). Four different types of ultramafic clasts were recovered: blue Serpentinites, lizardite-Serpentinites, antigorite/lizardite- and antigorite-Serpentinites. Lizardite-Serpentinites are primarily composed of orange serpentine, forming mesh and bastite textures. Raman and microprobe analyses revealed that these textures contain a mixture of Fe-rich brucite (XMg ~ 0.84) and lizardite/chrysotile. Antigorite/lizardite- and antigorite-Serpentinites record the progressive recrystallization of mesh and bastite textures to antigorite, magnetite and pure Fe-poor brucite (XMg ~ 0.92). Oxygen isotope compositions of clasts and pore fluids showed that the transition from lizardite to antigorite is due to the increase in temperature from 200 °C to about 400 °C within the forearc area above the slab/mantle interface. Lizardite-, antigorite/lizardite- and antigorite-Serpentinites displayed U-shaped chondrite normalized Rare Earth Element (REE) patterns and are characterized by high fluid mobile element concentrations (Cs, Li, Sr, As, Sb, B, Li) relative to abyssal peridotites and/or primitive mantle. The recrystallization of lizardite to antigorite is accompanied by a decrease in Cs, Li and Sr, and an increase in As and Sb concentrations in the bulk clasts, whereas B concentrations are relatively constant. Some clasts are overprinted by blue serpentine, often in association with sulfides. Most of these blue Serpentinites were recovered at Yinazao and the uppermost units of Fantangisna and Asut Tesoru suggesting alteration in the shallower portions of the forearc, possibly during exhumation of the clasts. This episode of alteration resulted in a flattening of REE spectra and an increase of Zn concentrations in Serpentinites. Otherwise, no systematic changes of ultramafic clasts chemistry or mineralogy were observed with increasing depth to the slab. The samples document previously undescribed prograde metamorphic events in the shallow portions of the Mariana subduction zone, consistent with a continuous burial of the serpentinized forearc mantle during subduction. Similar processes, induced by the interaction with fluids released from the downgoing slab, likely occur in subduction zones worldwide. At greater depth, breakdown of brucite and antigorite will result in the massive transfer of fluids and fluid mobile elements, such as As, Sb and B, to the source of arc magmas.
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Assessing sulfur redox state and distribution in abyssal Serpentinites using XANES spectroscopy
Earth and Planetary Science Letters, 2017Co-Authors: Baptiste Debret, Muriel Andreani, Adelie Delacour, Stephane Roumejon, Nicolas Trcera, Helen Williams, Eimf Edinburgh Ion MicroprobeAbstract:Sulfur is one of the main redox sensitive and volatile elements involved in chemical transfers between earth surface and the deep mantle. At mid-oceanic ridges, sulfur cycle is highly influenced by Serpentinite formation which acts as a sink of sulfur under various oxidation states (S2-, S-, S-0 and S6+). Sulfur sequestration in Serpentinites is usually attributed to the crystallization of secondary minerals, such as sulfides (e.g. pyrite, pyrrhotite) or sulfates (e.g. anhydrite). However, the role of serpentine minerals as potential sulfur carriers is not constrained. We investigate the distribution and redox state of sulfur at micro-scale combining in situ spectroscopic (X-ray absorption near-edge structure: XANES) and geochemical (SIMS) measurements in abyssal Serpentinites from the SWIR (South West Indian Ridge), the Rainbow and the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) areas. These Serpentinites are formed in different tectono-metamorphic settings and provide a meaningful database to understand the fate of sulfur during seafloor serpentinization. XANES spectra of Serpentinite powders show that the sulfur budget of the studied samples is dominated by oxidized sulfur (S6+ /Sigma S = 0.6-1) although sulfate micro phases, such as barite and anhydrite, are absent. Indeed, mu-XANES analyses of mesh, bastite and antigorite veins in thin sections and of serpentine grains rather suggest the presence of S6+ ions incorporated into serpentine minerals. The structural incorporation of S in serpentine minerals is also supported by Xray fluorescence mapping revealing large areas (1600 mu m(2)) of Serpentinite where S is homogeneously distributed. Our observations show that serpentine minerals can incorporate high S concentrations, from 140 to 1350 ppm, and that this can account for 60 to 100% of the sulfur budget of abyssal Serpentinites. Serpentine minerals thus play an important role in S exchanges between the hydrosphere and the mantle at mid-oceanic ridges and may participate to S recycling in subduction zones. (C) 2017 The Authors. Published by Elsevier B.V.
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high pressure Serpentinites a trap and release system controlled by metamorphic conditions example from the piedmont zone of the western alps
Chemical Geology, 2013Co-Authors: Romain Lafay, Stéphane Guillot, Marguerite Godard, Stephane Schwartz, Baptiste Debret, Fabien Deschamps, Christian NicolletAbstract:We provide new insights into the geochemistry of Serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These Serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustres complex and (3) Serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, Serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the Serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and Serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine Serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that Serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
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three steps of serpentinization in an eclogitized oceanic serpentinization front lanzo massif western alps
Journal of Metamorphic Geology, 2013Co-Authors: Christian Nicollet, Stephane Schwartz, Muriel Andreani, Baptiste Debret, Marguerite GodardAbstract:The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean-continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated Serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and Serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive Serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta-ophicarbonates bordering the foliated Serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P-T (estimated in eclogitized metagabbros at 2-2.5 GPa and 550-620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated Serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into Serpentinite during alpine subduction and collision. Thus, the Serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo-Moho.
Marguerite Godard - One of the best experts on this subject based on the ideXlab platform.
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Fingerprinting and relocating tectonic slices along the plate interface: Evidence from the Lago Superiore unit at Monviso (Western Alps)
Lithos, 2020Co-Authors: Mattia Gilio, Marguerite Godard, Marco Scambelluri, Samuele Agostini, Thomas Pettke, Philippe Agard, Michele Locatelli, Samuel AngiboustAbstract:The Lago Superiore Unit (LSU, Monviso Massif, Italian Western Alps) is a section of fossil oceanic lithosphere equilibrated to eclogite facies conditions (550 °C – 2.8 GPa) during Alpine subduction (45–40 Ma). It is cut by two major shear zones, namely the Intermediate (ISZ) and Lower Shear Zone (LSZ), mostly consisting of Serpentinite. The lowermost, serpentine-rich, section of the Lago Superiore Unit, the Basal Serpentinite, separates the HP ophiolite domain from the underlying continental Dora-Maira Unit. Here we show that the LSZ and the Basal Serpentinite were active at different stages of the subduction and exhumation history of the complex. Most of retrograde deformation and mineral re-equilibration were localized in the LSZ. Channelized fluids percolating during this phase chemically homogenized the LSZ Serpentinites, that preserved their HP mineralogy only locally; the best-preserved relicts of the eclogite-facies high pressure stage within the LSZ Serpentinite are nodules of magnesite (representing former veins) and eclogite blocks. Differently, the underlying Basal Serpentinite largely escaped the exhumation-related processes and still records the prograde chemical and petrological history of the LSU Serpentinite, from ocean-floor hydration to HP metamorphic conditions. The Lago Superiore Unit thus represents a snapshot of major Alpine metamorphic and shearing events, from prograde subduction to exhumation. Its km-scale thickness, and the oriented antigorite fabric in the Lower Shear Zone and Basal Serpentinite makes it a good seismic reflector. This HP ophiolite complex can thus be used as proxy of a deep (70–80 km) Alpine-type subduction zone, and to better constrain and interpret seismic images of present-day convergent margins.
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Stability of antigorite Serpentinite and geochemical exchange with oceanic crustal rocks during ultrahigh- pressure subduction-zone metamorphism (Lago di Cignana Unit, Italian Western Alps)
Journal of Petrology, 2019Co-Authors: Mattia Gilio, Marguerite Godard, Marco Scambelluri, Samuele Agostini, Daniel Peters, Thomas PettkeAbstract:In the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing Serpentinites (the Cignana Serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this Serpentinite reveals a refertilized oceanic mantle peridotite protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana Serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this Serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, Serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The Serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the Serpentinites closer to the crustal LCU. The ZSZ and Cignana Serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana Serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana Serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction.
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Linking Serpentinite geochemistry with tectonic evolution at the subduction plate-interface: The Voltri Massif case study (Ligurian Western Alps, Italy)
Geochimica et Cosmochimica Acta, 2016Co-Authors: E. Cannao, M. Scambelluri, S. Agostini, S. Tonarini, Marguerite GodardAbstract:Recent geochemical work shows that subduction-zone Serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of Serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B–O–H, Sr, Pb) of high-pressure Serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones. The Serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show undeformed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed Serpentinite has very high δ11BSRM951 (from +26‰ to +30‰), low Sr and Pb isotope ratios (87Sr/86Sr = 0.7053–0.7069; 206Pb/204Pb = 18.131–18.205) and low As and Sb contents (0.1 and 0.01 μg/g, respectively). Oxygen and hydrogen isotope compositions are +4.5‰ and −67‰, respectively. In contrast, mylonitic Serpentinite shows lower δ11B (from +22‰ to +17‰), significant enrichment in radiogenic Sr and Pb isotopes (87Sr/86Sr up to 0.7105; 206Pb/204Pb up to 18.725), and enrichment in As and Sb (1.3 and 0.39 μg/g, respectively). δ18O of the mylonitic Serpentinites reaches values of +5.9‰, whereas δD is comparable with that of undeformed rocks (approximately −70‰). In mylonitic Serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component. Our dataset shows that undeformed Serpentinite still preserves an oceanic geochemical fingerprint, whereas mylonitic Serpentinite is reset in its concentrations of FME and its B, Sr and Pb isotope compositions, due to interaction with sediment- and crust-derived fluids. The environment of this interaction is either compatible with (i) an outer-rise zone setting, with percolation of seawater-derived fluids enriched in sedimentary components into bending-related fault structures, or with (ii) subduction channel domains, where ascending sediment-derived slab fluids infiltrate slices of former oceanic Serpentinite accreted to the plate interface domain. Influx of sediment-derived subduction fluids along major deformation zones in Serpentinite modifies the element budget of the rocks, with important implications for element recycling and the tectonic history of Serpentinite. The B, Sr and Pb isotopic systematics, coupled with FME concentration in Serpentinites are particularly helpful geochemical tracers of interaction between different reservoirs in subduction-interface environments, and are more sensitive than the traditionally applied stable oxygen and hydrogen isotope compositions.
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high pressure Serpentinites a trap and release system controlled by metamorphic conditions example from the piedmont zone of the western alps
Chemical Geology, 2013Co-Authors: Romain Lafay, Stéphane Guillot, Marguerite Godard, Stephane Schwartz, Baptiste Debret, Fabien Deschamps, Christian NicolletAbstract:We provide new insights into the geochemistry of Serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These Serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustres complex and (3) Serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, Serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the Serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and Serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine Serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that Serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
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three steps of serpentinization in an eclogitized oceanic serpentinization front lanzo massif western alps
Journal of Metamorphic Geology, 2013Co-Authors: Christian Nicollet, Stephane Schwartz, Muriel Andreani, Baptiste Debret, Marguerite GodardAbstract:The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean-continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated Serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and Serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive Serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta-ophicarbonates bordering the foliated Serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P-T (estimated in eclogitized metagabbros at 2-2.5 GPa and 550-620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated Serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into Serpentinite during alpine subduction and collision. Thus, the Serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo-Moho.
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meta rodingite dikes as recorders of subduction zone metamorphism and Serpentinite dehydration voltri ophiolite italy
Chemical Geology, 2021Co-Authors: Anne A Haws, Marco Scambelluri, Paul G Starr, Besim Dragovic, Donato Belmonte, Mark J Caddick, Kirkland S Broadwell, Jay J Ague, Ethan F BaxterAbstract:Abstract The metamorphic devolatilization of Serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from Serpentinites, however, is challenging, as Serpentinite mineral assemblages are less conducive to extracting a detailed pressure-temperature-time (P-T-t) history. In this study, we use meta-rodingites (metasomatized gabbros) from the central portion of the Voltri Ophiolite in the Ligurian Alps (Italy), to investigate the P-T and geochronological record of subduction metamorphism and dehydration within the adjacent Serpentinites. These meta-rodingites underwent eclogite-facies metamorphic recrystallization to a garnet-clinopyroxene-chlorite-ilmenite assemblage, and are cross-cut by several generations of garnet-bearing veins. A comparison of the Voltri meta-rodingites with un-subducted seafloor rodingites from the Apennines reveals textural and geochemical evidence of complete metamorphic recrystallization of the original seafloor metasomatic assemblage during Alpine subduction. Phase equilibria modeling of the Voltri meta-rodingite sample suggests that the peak metamorphic assemblage recorded was stable in the range of ~450–600 °C, which is consistent with previous P-T estimates for gabbroic and ultramafic lithologies from the Voltri Ophiolite. A late prograde to peak metamorphic age of 40.6 ± 2.9 Ma was obtained from the meta-rodingite using Sm Nd garnet geochronology, which closely matches the ages of late prograde to peak metamorphism estimated by companion studies of associated eclogites from the central part of the Voltri Ophiolite. The overlap in P-T conditions and timing of peak metamorphism for samples across the area suggests subduction and exhumation of the central part of the Voltri Ophiolite as a coherent lithospheric slice. Dating of a garnetite vein cross-cutting the main meta-rodingite body suggests an influx of fluid at 37.7 ± 1.4 Ma, at or near peak eclogite-facies conditions. Enrichment in the vein selvage of both compatible transition metals (Cr, Ni, Cu) and fluid-mobile elements (Li, Rb, Cs, Ba) suggests the fluid carried both Serpentinite and sedimentary geochemical signatures and interacted with the main meta-rodingite body. We suggest that the most probable explanation for this fluid composition, and thus the source of the fluid, is the dehydration of the Voltri Serpentinites. These Serpentinites likely experienced an influx of sedimentary-derived, fluid-mobile-element-bearing fluids earlier in their history, either on the seafloor or most likely during the early stages of subduction. The presence of a sedimentary signature in this Serpentinite-derived fluid emphasizes the importance of subducted Serpentinites and their dehydration to the geochemical cycling of fluid-mobile elements in subduction zones. The widespread development of such vein systems within multiple meta-rodingites across the Voltri Ophiolite points to a large-scale fluid release event, likely resulting from the partial dehydration of brucite and antigorite within the surrounding Serpentinite hosts. By utilizing direct dating of vein mineralogy formed during Serpentinite dehydration, we provide one of the first geochronological records of Serpentinite dehydration and fluid release occurring during subduction.
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Fingerprinting and relocating tectonic slices along the plate interface: Evidence from the Lago Superiore unit at Monviso (Western Alps)
Lithos, 2020Co-Authors: Mattia Gilio, Marguerite Godard, Marco Scambelluri, Samuele Agostini, Thomas Pettke, Philippe Agard, Michele Locatelli, Samuel AngiboustAbstract:The Lago Superiore Unit (LSU, Monviso Massif, Italian Western Alps) is a section of fossil oceanic lithosphere equilibrated to eclogite facies conditions (550 °C – 2.8 GPa) during Alpine subduction (45–40 Ma). It is cut by two major shear zones, namely the Intermediate (ISZ) and Lower Shear Zone (LSZ), mostly consisting of Serpentinite. The lowermost, serpentine-rich, section of the Lago Superiore Unit, the Basal Serpentinite, separates the HP ophiolite domain from the underlying continental Dora-Maira Unit. Here we show that the LSZ and the Basal Serpentinite were active at different stages of the subduction and exhumation history of the complex. Most of retrograde deformation and mineral re-equilibration were localized in the LSZ. Channelized fluids percolating during this phase chemically homogenized the LSZ Serpentinites, that preserved their HP mineralogy only locally; the best-preserved relicts of the eclogite-facies high pressure stage within the LSZ Serpentinite are nodules of magnesite (representing former veins) and eclogite blocks. Differently, the underlying Basal Serpentinite largely escaped the exhumation-related processes and still records the prograde chemical and petrological history of the LSU Serpentinite, from ocean-floor hydration to HP metamorphic conditions. The Lago Superiore Unit thus represents a snapshot of major Alpine metamorphic and shearing events, from prograde subduction to exhumation. Its km-scale thickness, and the oriented antigorite fabric in the Lower Shear Zone and Basal Serpentinite makes it a good seismic reflector. This HP ophiolite complex can thus be used as proxy of a deep (70–80 km) Alpine-type subduction zone, and to better constrain and interpret seismic images of present-day convergent margins.
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Stability of antigorite Serpentinite and geochemical exchange with oceanic crustal rocks during ultrahigh- pressure subduction-zone metamorphism (Lago di Cignana Unit, Italian Western Alps)
Journal of Petrology, 2019Co-Authors: Mattia Gilio, Marguerite Godard, Marco Scambelluri, Samuele Agostini, Daniel Peters, Thomas PettkeAbstract:In the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing Serpentinites (the Cignana Serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this Serpentinite reveals a refertilized oceanic mantle peridotite protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana Serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this Serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, Serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The Serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the Serpentinites closer to the crustal LCU. The ZSZ and Cignana Serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana Serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana Serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction.
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Halogens and noble gases in Serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon
Geochimica et Cosmochimica Acta, 2018Co-Authors: Mark A. Kendrick, Marco Scambelluri, Jorg Hermann, Jose Alberto Padron NavartaAbstract:Ophiolitic Serpentinites and secondary peridotites formed by Serpentinite dehydration were investigated to improve constraints on the fates of noble gases and halogens during subduction zone metamorphism. The work extends previous studies to encompass F and four stages of serpentinization and Serpentinite dehydration including: (i) oceanic Serpentinites preserving the features of seafloor serpentinization; (ii) subducted high grade (olivine bearing) antigorite-Serpentinites; (iii) spinifex and granofels textured chlorite harzburgites; and (iv) a garnet peridotite. Serpentinites and secondary peridotites from different ophiolites are shown to have characteristic ranges of 40Ar/36Ar: chrysotile and antigorite Serpentinites from Erro Tobbio (Western Alps) have 40Ar/36Ar of ∼ 296–390; antigorite Serpentinites and chlorite harzburgites from Cerro del Almirez (Betic Cordillera) have 40Ar/36Ar of ∼ 340–600, and chlorite harzburgites and garnet peridotites from Cima di Gagnone (Swiss Alps) have 40Ar/36Ar of ∼ 600–1100. The variation of 40Ar/36Ar is unrelated to metamorphic grade at each locality but is broadly correlated with variation in other radiogenic isotopes (206Pb/204Pb and 87Sr/86Sr) between localities. This suggests excess 40Ar was derived from terrigenous sediments with characteristic ranges of 40Ar/36Ar and 87Sr/86Sr in different subduction zones. The secondary chlorite harzburgites have 20Ne/36Ar ratios of greater than seawater, contain parentless (or excess) 4He, and have higher F concentrations than any of the Serpentinites investigated. The 20Ne/36Ar is broadly correlated with 40Ar/36Ar in samples from Cerro del Almirez suggesting derivation of excess 40Ar and atmospheric 20Ne from a common source. The chlorite is shown to have higher concentrations of F, Ne and other noble gases than coexisting olivine and enstatite, which contain fluid-related inclusions. The high F content and high 20Ne/36Ar ratios of the chlorite harzburgites are ascribed to fluxing of dehydrating Serpentinites with F-, 40Ar-, 4He- and 20Ne-rich fluids derived from metasediments in the subducting slab, and an inferred high compatibility of F and Ne in chlorite. The garnet peridotite from Cima di Gagnone records the final and complete dehydration of Serpentinite. Based on the analysis of mineral separates minimally affected by retrogression (marked by garnet breakdown and the appearance of Cl-rich hornblende), nominally anhydrous garnet peridotite retains Cl, Br, I and non-radiogenic noble gas concentrations up to an order of magnitude higher than average depleted mantle. The data are consistent with serpentinised lithosphere and related secondary peridotites as major sources of deeply subducted seawater-derived volatiles in the Earth’s mantle. The data also demonstrate that the relative abundances of volatiles subducted into the mantle are controlled by multiple factors including: original seafloor alteration, the relative compatibilities of different noble gases and halogens in minerals forming during different stages of subduction and chemical exchange between different lithologies during subduction. The combination of these processes has produced elevated 20Ne/36Ar in chlorite harzburgites from two unrelated localities. This suggests that subduction of atmospheric Ne could be significantly more efficient than previously realised, which has implications for interpretation of the mantles primordial 20Ne/22Ne ratio and how the Earth accreted.
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Halogens and noble gases in Serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon
Geochimica et Cosmochimica Acta, 2018Co-Authors: Mark A. Kendrick, Marco Scambelluri, Jӧrg Hermann, Jose Alberto Padron NavartaAbstract:Ophiolitic Serpentinites and secondary peridotites formed by Serpentinite dehydration were investigated to improve constraints on the fates of noble gases and halogens during subduction zone metamorphism. The work extends previous studies to encompass F and four stages of serpentinization and Serpentinite dehydration including: (i) oceanic Serpentinites preserving the features of seafloor serpentinization; (ii) subducted high grade (olivine bearing) antigorite-Serpentinites; (iii) spinifex and granofels textured chlorite harzburgites; and (iv) a garnet peridotite.
