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Bernard W. Evans - One of the best experts on this subject based on the ideXlab platform.
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Lizardite versus antigorite serpentinite magnetite hydrogen and life
Geology, 2010Co-Authors: Bernard W. EvansAbstract:The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50–300 °C), where Lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich Lizardite, magnetite, and dihydrogen gas (±brucite) are products of the reaction. At higher temperatures (400–600 °C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.
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Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?)
Geology, 2010Co-Authors: Bernard W. EvansAbstract:The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50–300 °C), where Lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich Lizardite, magnetite, and dihydrogen gas (±brucite) are products of the reaction. At higher temperatures (400–600 °C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.
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Magnetite-free, yellow Lizardite serpentinization of olivine websterite, Canyon Mountain complex, N.E. Oregon
American Mineralogist, 2009Co-Authors: Bernard W. Evans, Scott M. Kuehner, Anastasia ChopelasAbstract:We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored Lizardite (X Fe = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite’s identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the Lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe 3+ exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T , f H 2 O , f H 2 , and a SiO 2 attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-Lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization.
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Control of the Products of Serpentinization by the Fe2+Mg –1 Exchange Potential of Olivine and Orthopyroxene
Journal of Petrology, 2008Co-Authors: Bernard W. EvansAbstract:It is argued that the high-Mg content (mg-number1⁄4 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental FeMg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite.This causes hydrogen to be evolved in an oxygen-conserved reaction.The low-variance mineral assemblage Olþ SrpþBrcþMag sets the chemical potentials of H2O, SiO2 and O2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe3þ content of the serpentine minerals, particularly Lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe/SFe ratios40 4, with an average value (0 69) similar to that of magnetite (0 67). This feature may be attributed to the presence of high-Fe Lizardite, as has been found in Mo« ssbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs SFe a.p.f.u.) for olivinepseudomorph Lizardite and, with some exceptions, for bastite Lizardite, that show a substitution of the cronstedtite component (Fe charge-balanced onTand M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite Lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe for Fe. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in Lizardite seems to be limited to a few cases of Lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than Lizardite, facts that may be attributed to the greater Fe content of Lizardite. Chrysotile veins provide the best record available to us of the environmental FeMg 1 exchange potential in the pore fluid attending serpentinization.This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe can lead to inappropriate petrological conclusions in the case of serpentinites.
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the serpentinite multisystem revisited chrysotile is metastable
International Geology Review, 2004Co-Authors: Bernard W. EvansAbstract:The two rock-forming polymorphs of serpentine Mg3Si2O5(OH)4, Lizardite and chrysotile, occur in nature in virtually identical ranges of temperature and pressure, from surficial or near-surficial environments to temperatures perhaps as high as 400°C. Laboratory evidence indicates that Lizardite is the more stable at low temperatures, but the difference in their Gibbs free energies is not more than about 2 kJ in the 300-400°C range. Above about 300°C, antigorite + brucite is more stable than both; in other words, chrysotile is nowhere the most stable. The crystal structures of Lizardite and chrysotile give rise to contrasting crystallization behaviors and hence modes of occurrence. The hydration of peridotite at low temperature results in the growth of Lizardite from olivine, and (commonly topotactically) from chain and sheet silicates, although the MgO-SiO2-H2O (MSH) phase diagram predicts antigorite + talc in bastite. The activity of H2O during serpentinization may be buffered to low values by the solids,...
Baptiste Debret - One of the best experts on this subject based on the ideXlab platform.
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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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Evolution of Fe redox state in serpentine during subduction
Earth and Planetary Science Letters, 2014Co-Authors: Baptiste Debret, Stéphane Schwartz, Christian Nicollet, Manuel Muñoz, Muriel Andreani, Nathalie Bolfan-casanova, Julie Carlut, Nicolas TrceraAbstract:Serpentinites are an important component of the oceanic lithosphere that formed at slow to ultra-slow spreading centers. Serpentine could thus be one of the most abundant hydrous minerals recycled into the mantle in subduction zones. Prograde metamorphism in subducted serpentinites is characterized by the destabilization of Lizardite into antigorite, and then into secondary olivine. The nature of the fluid released during these phase transitions is controlled by redox reactions and can be inferred from oxidation state of Fe in serpentine minerals. We used bulk rock analyses, magnetic measurements, SEM observations and μXANES spectroscopy to establish the evolution of View the MathML sourceFe2O3Tot(BR) and magnetite content in serpentinite and Fe oxidation state in serpentine minerals from ridge to subduction settings. At mid-ocean ridges, during the alteration of peridotite into serpentinite, iron is mainly redistributed between magnetite and oceanic serpentine (usually Lizardite). The Fe3+/FeTotal ratio in Lizardite and the modal percentage of magnetite progressively increase with the degree of local serpentinization to maxima of about 0.8 and 7 wt%, respectively, in fully serpentinized peridotites. During subduction, the View the MathML sourceFe2O3Tot(BR) of serpentinite remains constant (∼7–10 wt%, depending on the initial Fe content of the peridotite) while the modal percentage of magnetite decreases to less than 2% in eclogite facies rocks. The Fe3+/FeTotal ratio in serpentine also decreases down to 0.2–0.4 in antigorite at eclogite facies. Our results show that, in the first 70 km of subduction, the transition from Lizardite to antigorite is accompanied by a reduction of Fe in bulk rock samples and in serpentine minerals. This redox reaction might be coupled with the oxidation of reduced oceanic phases such as sulfides, and the formation of oxidized fluids (e.g. SOX, H2O, COX). At greater depths, the beginning of antigorite dehydration leads to an increase of Fe3+/FeTotal in relict antigorite, in agreement with the preferential partitioning of ferric iron into serpentine rather than into olivine.
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pressure temperature estimates of the Lizardite antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure–temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous lab-oratory experiments, the stabilityfields of these species remain poorly constrained. This paper presents pet-rological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from theAlpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressureand temperature conditions from sub-greenschist (Pb4 kbar, T~200–300 °C) to eclogite facies conditions(P>20 kbar, T>460 °C) along a subduction geothermal gradient. We use the observed mineral assemblagein natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matterin associated metasediments to constrain the temperature of the Lizardite to antigorite transition at highpressures. We show that below 300 °C, Lizardite and locally chrysotile are the dominant species in themesh texture. Between 320 and 390 °C, Lizardite is progressively replaced by antigorite at the grain bound-aries through dissolution–precipitation processes in the presence of SiO2enrichedfluids and in the cores ofthe Lizardite mesh. Above 390 °C, under high-grade blueschist to eclogite facies conditions, antigorite is thesole stable serpentine mineral until the onset of secondary olivine crystallization at 460 °C
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
Anne Line Auzende - One of the best experts on this subject based on the ideXlab platform.
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pressure temperature estimates of the Lizardite antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure–temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous lab-oratory experiments, the stabilityfields of these species remain poorly constrained. This paper presents pet-rological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from theAlpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressureand temperature conditions from sub-greenschist (Pb4 kbar, T~200–300 °C) to eclogite facies conditions(P>20 kbar, T>460 °C) along a subduction geothermal gradient. We use the observed mineral assemblagein natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matterin associated metasediments to constrain the temperature of the Lizardite to antigorite transition at highpressures. We show that below 300 °C, Lizardite and locally chrysotile are the dominant species in themesh texture. Between 320 and 390 °C, Lizardite is progressively replaced by antigorite at the grain bound-aries through dissolution–precipitation processes in the presence of SiO2enrichedfluids and in the cores ofthe Lizardite mesh. Above 390 °C, under high-grade blueschist to eclogite facies conditions, antigorite is thesole stable serpentine mineral until the onset of secondary olivine crystallization at 460 °C
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites.
Lithos, 2012Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P < 4 kbar, T ~ 200-300°C) to eclogite facies conditions (P > 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
Stéphane Schwartz - One of the best experts on this subject based on the ideXlab platform.
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Evolution of Fe redox state in serpentine during subduction
Earth and Planetary Science Letters, 2014Co-Authors: Baptiste Debret, Stéphane Schwartz, Christian Nicollet, Manuel Muñoz, Muriel Andreani, Nathalie Bolfan-casanova, Julie Carlut, Nicolas TrceraAbstract:Serpentinites are an important component of the oceanic lithosphere that formed at slow to ultra-slow spreading centers. Serpentine could thus be one of the most abundant hydrous minerals recycled into the mantle in subduction zones. Prograde metamorphism in subducted serpentinites is characterized by the destabilization of Lizardite into antigorite, and then into secondary olivine. The nature of the fluid released during these phase transitions is controlled by redox reactions and can be inferred from oxidation state of Fe in serpentine minerals. We used bulk rock analyses, magnetic measurements, SEM observations and μXANES spectroscopy to establish the evolution of View the MathML sourceFe2O3Tot(BR) and magnetite content in serpentinite and Fe oxidation state in serpentine minerals from ridge to subduction settings. At mid-ocean ridges, during the alteration of peridotite into serpentinite, iron is mainly redistributed between magnetite and oceanic serpentine (usually Lizardite). The Fe3+/FeTotal ratio in Lizardite and the modal percentage of magnetite progressively increase with the degree of local serpentinization to maxima of about 0.8 and 7 wt%, respectively, in fully serpentinized peridotites. During subduction, the View the MathML sourceFe2O3Tot(BR) of serpentinite remains constant (∼7–10 wt%, depending on the initial Fe content of the peridotite) while the modal percentage of magnetite decreases to less than 2% in eclogite facies rocks. The Fe3+/FeTotal ratio in serpentine also decreases down to 0.2–0.4 in antigorite at eclogite facies. Our results show that, in the first 70 km of subduction, the transition from Lizardite to antigorite is accompanied by a reduction of Fe in bulk rock samples and in serpentine minerals. This redox reaction might be coupled with the oxidation of reduced oceanic phases such as sulfides, and the formation of oxidized fluids (e.g. SOX, H2O, COX). At greater depths, the beginning of antigorite dehydration leads to an increase of Fe3+/FeTotal in relict antigorite, in agreement with the preferential partitioning of ferric iron into serpentine rather than into olivine.
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pressure temperature estimates of the Lizardite antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure–temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous lab-oratory experiments, the stabilityfields of these species remain poorly constrained. This paper presents pet-rological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from theAlpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressureand temperature conditions from sub-greenschist (Pb4 kbar, T~200–300 °C) to eclogite facies conditions(P>20 kbar, T>460 °C) along a subduction geothermal gradient. We use the observed mineral assemblagein natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matterin associated metasediments to constrain the temperature of the Lizardite to antigorite transition at highpressures. We show that below 300 °C, Lizardite and locally chrysotile are the dominant species in themesh texture. Between 320 and 390 °C, Lizardite is progressively replaced by antigorite at the grain bound-aries through dissolution–precipitation processes in the presence of SiO2enrichedfluids and in the cores ofthe Lizardite mesh. Above 390 °C, under high-grade blueschist to eclogite facies conditions, antigorite is thesole stable serpentine mineral until the onset of secondary olivine crystallization at 460 °C
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites.
Lithos, 2012Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P < 4 kbar, T ~ 200-300°C) to eclogite facies conditions (P > 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
Christian Nicollet - One of the best experts on this subject based on the ideXlab platform.
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Evolution of Fe redox state in serpentine during subduction
Earth and Planetary Science Letters, 2014Co-Authors: Baptiste Debret, Stéphane Schwartz, Christian Nicollet, Manuel Muñoz, Muriel Andreani, Nathalie Bolfan-casanova, Julie Carlut, Nicolas TrceraAbstract:Serpentinites are an important component of the oceanic lithosphere that formed at slow to ultra-slow spreading centers. Serpentine could thus be one of the most abundant hydrous minerals recycled into the mantle in subduction zones. Prograde metamorphism in subducted serpentinites is characterized by the destabilization of Lizardite into antigorite, and then into secondary olivine. The nature of the fluid released during these phase transitions is controlled by redox reactions and can be inferred from oxidation state of Fe in serpentine minerals. We used bulk rock analyses, magnetic measurements, SEM observations and μXANES spectroscopy to establish the evolution of View the MathML sourceFe2O3Tot(BR) and magnetite content in serpentinite and Fe oxidation state in serpentine minerals from ridge to subduction settings. At mid-ocean ridges, during the alteration of peridotite into serpentinite, iron is mainly redistributed between magnetite and oceanic serpentine (usually Lizardite). The Fe3+/FeTotal ratio in Lizardite and the modal percentage of magnetite progressively increase with the degree of local serpentinization to maxima of about 0.8 and 7 wt%, respectively, in fully serpentinized peridotites. During subduction, the View the MathML sourceFe2O3Tot(BR) of serpentinite remains constant (∼7–10 wt%, depending on the initial Fe content of the peridotite) while the modal percentage of magnetite decreases to less than 2% in eclogite facies rocks. The Fe3+/FeTotal ratio in serpentine also decreases down to 0.2–0.4 in antigorite at eclogite facies. Our results show that, in the first 70 km of subduction, the transition from Lizardite to antigorite is accompanied by a reduction of Fe in bulk rock samples and in serpentine minerals. This redox reaction might be coupled with the oxidation of reduced oceanic phases such as sulfides, and the formation of oxidized fluids (e.g. SOX, H2O, COX). At greater depths, the beginning of antigorite dehydration leads to an increase of Fe3+/FeTotal in relict antigorite, in agreement with the preferential partitioning of ferric iron into serpentine rather than into olivine.
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pressure temperature estimates of the Lizardite antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure–temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous lab-oratory experiments, the stabilityfields of these species remain poorly constrained. This paper presents pet-rological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from theAlpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressureand temperature conditions from sub-greenschist (Pb4 kbar, T~200–300 °C) to eclogite facies conditions(P>20 kbar, T>460 °C) along a subduction geothermal gradient. We use the observed mineral assemblagein natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matterin associated metasediments to constrain the temperature of the Lizardite to antigorite transition at highpressures. We show that below 300 °C, Lizardite and locally chrysotile are the dominant species in themesh texture. Between 320 and 390 °C, Lizardite is progressively replaced by antigorite at the grain bound-aries through dissolution–precipitation processes in the presence of SiO2enrichedfluids and in the cores ofthe Lizardite mesh. Above 390 °C, under high-grade blueschist to eclogite facies conditions, antigorite is thesole stable serpentine mineral until the onset of secondary olivine crystallization at 460 °C
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites
Lithos, 2013Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.
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Pressure-temperature estimates of the Lizardite/antigorite transition in high pressure serpentinites.
Lithos, 2012Co-Authors: Stéphane Schwartz, Bruno Reynard, Stéphane Guillot, Romain Lafay, Baptiste Debret, Christian Nicollet, Pierre Lanari, Anne Line AuzendeAbstract:Serpentine minerals in natural samples are dominated by Lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P < 4 kbar, T ~ 200-300°C) to eclogite facies conditions (P > 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the Lizardite to antigorite transition at high pressures. We show that below 300°C, Lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, Lizardite is progressively replaced by antigorite at the grain boundaries through dissolution-precipitation processes in the presence of SiO2 enriched fluids and through a solid-state transition in the cores of the Lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.