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Antigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low‐strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring‐shear experiments on antigorite and antigorite‐quartz mixtures at temperatures between 20 and 500°C at 150 MPa effective normal stress. Pure antigorite is strain hardening, with frictional coefficient (μ) > 0.5, and developed cataclastic microstructures. In contrast, antigorite‐quartz mixtures (10% quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200°C to 0.22 at 500°C. Antigorite‐quartz mixtures developed foliation similar to natural serpentinite shear zones. Although antigorite‐quartz reactions may form mechanically weak talc, we only find small, localized amounts of talc in our deformed samples, and room temperature friction is higher than expected for talc. Instead, we propose that the observed weakening at temperatures ≥200°C primarily results from silica dissolution leading to a lowered pore‐fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution‐precipitation creep. We suggest that under our experimental conditions, efficient dissolution‐precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional‐viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution‐precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example, in faults juxtaposing serpentinite with quartz‐bearing rocks. Plain Language Summary: Some major, plate‐boundary fault zones are very weak, and in some places this is thought to be because they contain the mineral serpentine. Antigorite is a common form of serpentine at depth, but in many rock deformation experiments, antigorite is found to be relatively strong. In natural fault zones, however, antigorite can be highly deformed at seemingly low stress. The motivation for this study was to explore whether antigorite can be weak also in the laboratory. Because laboratory experiments occur relatively fast, the mechanism of deformation may be different from in nature. To address this, we mixed antigorite with quartz, which changes the pH and promotes dissolution of antigorite. We found that when deforming antigorite mixed with quartz, the experimental fault was very weak, because the dissolution of antigorite removed sticking points ‐ a process that is also documented in natural faults. The experiments also produced minor talc, from the reaction of antigorite with quartz, but too little to affect strength. We suggest that it is the activation of dissolution and precipitation of antigorite which allows deformation of major serpentine‐bearing faults at low stress at temperatures of several hundred degrees Celsius. Key Points: At 20°C and 500°C antigorite is strain‐hardening, develops cataclastic microstructures and has a frictional coefficient of ∼0.5Samples of antigorite mixed with quartz are strain‐weakening, and the friction coefficient decreases from 0.36 at 200°C to 0.22 at 500°CThe weakness of quartz‐antigorite is associated with activation of dissolution‐precipitation creep [ABSTRACT FROM AUTHOR]

Antigorite is the high‐temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in the subducting oceanic lithosphere. It has a wavy crystal structure along its a‐axis and several polysomes with different m‐values (m = 13–24) have been identified in nature. The m‐value is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit primarily depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of pressure and temperature. However, it is not well understood which m‐values of antigorite are stable at different pressure and temperature conditions. To investigate the pressure dependence of the stability of different m‐values in antigorite, we performed first‐principles calculations for several polysomes (m = 14–19) at high pressure from 0 to 14 GPa and compared their enthalpies at static 0 K. We found that although the energy differences between polysomes are small, polysomes with larger m‐values are more stable at ambient pressure, while polysomes with smaller m‐values are more stable at elevated pressures. This suggests that the structure of antigorite in the oceanic lithosphere subducting into the deep Earth may gradually evolve into a different polysome structure than the antigorite samples observed at ambient or near‐surface pressure conditions. These changes in the m‐values are accompanied by a minor dehydration reaction. By modulating the available amount of free water in the system, antigorite polysomatism may influence the distribution of intermediate‐depth seismicity, such as the observance of double seismic zones. Plain Language Summary: Antigorite is a hydrous mineral known for being an important carrier of water into the Earth's interior and a seismicity trigger at intermediate‐depths down to ∼200 km. However, its crystal structure is complex (e.g., Uehara, 1998), and thus it is not well understood which structural variant (polysome) of antigorite is stable or dominant at high pressures and temperatures during subduction. In this study, several crystal structures of antigorite were calculated by first‐principles calculations, and their enthalpies (i.e., Gibbs free energy of a system at static 0 K) were compared to determine the stable structure as a function of pressure. The results indicate that the structure and chemical composition of antigorite likely changes under high pressure, resulting in the gradual release of water during the subduction process. As antigorite is thought to be the main trigger of seismicity in the subducting lithospheric mantle, its metasomatism may contribute to explain the reported distributions of intermediate‐depth earthquakes. Key Points: Antigorite crystal structures with various values of m (14–19) were determined by first‐principles calculation under pressureThe relative enthalpy shows that antigorite with smaller m‐values are stabilized with increased pressureAntigorite in suducting slabs may gradually dehydrate under high pressure as a result of changes to stable m‐values [ABSTRACT FROM AUTHOR]

Weak serpentine minerals affect the mechanical behavior of serpentinized peridotites at depth, and may play a significant role in deformation localization within subduction zones, at local or regional scale. Mixtures of olivine with 5, 10, 20 and 50 vol. % fraction of antigorite, proxies for serpentinized peridotites, were deformed in axial shortening geometry under high pressures (ca. 2–5 GPa) and moderate temperatures (ca. 350°C), with in situ stress and strain measurements using synchrotron X‐rays. We evaluate the average partitioning of stresses at the grains scale within each phase (mineral) of the aggregate and compare with pure olivine aggregates in the same conditions. The in situ stress balance is different between low antigorite contents up to 10 vol. %, and higher contents above 20 vol. %. Microstructure and stress levels suggest the deformation mechanisms under these experimental conditions are akin to (semi)brittle and frictional processes. Unlike when close to dehydration temperatures, hardening of the aggregate is observed at low serpentine fractions, due to an increase in local stress concentrations. Below and above the 10–20 vol. % threshold, the stress state in the aggregate corresponds to friction laws already measured for pure olivine aggregates and pure antigorite aggregates respectively. As expected, the behavior of the two‐phase aggregate does not evolve as calculated from simple iso‐stress or iso‐strain bounds, and calls for more advanced physical models of two‐phase mixtures. Plain Language Summary: In subduction zones, a tectonic plate plunges beneath another one, and result in large mechanical stresses. These conditions can lead to earthquakes or ground displacements observable at human timescales. Measuring how viscous the rocks within subduction zones are, may help understand these events. Serpentinite are rocks from subduction zones which contain a variable amount of a weak minerals, including serpentine, and other stronger minerals. Using deformation experiments this study seeks to measure serpentinites viscosity, as a function of the amount of serpentine. At low serpentine content (at least up to 10%, but lower than 20 vol.%) and under temperature relevant for cold subduction zones, we found that the rock remains as hard as the strongest mineral which bears the whole load, and can even become harder because of grain‐scale stress concentrations. At 50% volume fraction of serpentine, the rock has the same viscosity as serpentine itself. These measurements may for instance help larger scale numerical models of interseismic processes that happen between earthquakes, in subduction zones. Key Points: In situ stresses within antigorite + olivine aggregates deformed under high pressures and temperaturesStress partitionning changes with a threshold between 10% and 20 % antigorite volume fractionOlivine controls stress before this threshold and the aggregate hardens relative to pure olivine [ABSTRACT FROM AUTHOR]

The crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs. Plain Language Summary: This study focuses on understanding why rocks in the forearc regions near subduction zones, where one tectonic plate slides beneath another, have a specific type of olivine arrangement known as type‐B olivine fabric. This arrangement is responsible for explaining why seismic velocities have a certain pattern in those areas. However, there has been disagreement about how this arrangement of olivine comes about. To investigate this, we conducted experiments where we subjected rocks rich in antigorite to high temperatures, causing them to transform into olivine. During these experiments, we observed changes in the size and shape of olivine grains, and their arrangement shifted from type‐C to type‐B as we adjusted the temperature and duration of the experiments. We found a relationship between the shape of olivine grains and how they were arranged. This change in arrangement can be explained by factors like the grains growing in different directions, rotating, and coming together in a specific way. Our discovery suggests that this particular olivine arrangement can occur during the transformation from antigorite to olivine at relatively low temperatures. This arrangement plays a more significant role for the trench‐parallel seismic anisotropy in the subduction forearc regions than we previously understood. Key Points: Dynamic dehydration of antigorite triggers the evolution of olivine crystallographic preferred orientation (CPO) from type‐C to type‐BTransitional type‐C olivine CPO is induced by topotactic and anisotropic growth of olivine during antigorite dehydrationType‐B olivine CPO is the final form of antigorite dehydration generated by anisotropic growth, grain rotation, and oriented coalescence [ABSTRACT FROM AUTHOR]

Research on polymer matrix composites with excellent tribological properties has received increasing attention in recent years. In this study, antigorite and wollastonite mineral dual-phase-reinforced polytetrafluoroethylene (PTFE) matrix composites were prepared by filling PTFE with mineral powders using ball-milling, cold-pressing, and pressureless sintering methods. The phase structure, microstructure, chemical composition, Shore hardness, and tribological behavior of the composites rubbed against steel balls under dry friction conditions were investigated. The results show that the composites have a dense structure and uniform distribution of mineral phases, with a Shore hardness of 62–68.8, an increase of 18.7–23.7% compared to pure PTFE. Compared with the addition of mono antigorite or wollastonite, the composites prepared by simultaneously filling the two minerals exhibited excellent tribological properties. The average friction coefficient and wear volume of the (10Atg + 20Wl)/PTFE composites were reduced by 44.2% and 71.4%, respectively, compared to those of pure PTFE. A dense and continuous tribofilm composed mainly of SiO2, MgSiO3, Mg2SiO4, MgO, CaO, CaMg(SiO3)2, and CaF2 was formed on the worn surfaces of both the dual-phase-reinforced PTFE matrix composites and counterpart steel balls during the friction process. The reduction in friction and wear is attributed to the reinforcement effect of the minerals on the PTFE matrix as well as the complex physical and chemical reactions at the friction interface stimulated by the synergistic effects between the two minerals. The addition of wollastonite reduced the phase transition temperature of antigorite, promoting more tribochemical reaction products with good abrasion resistance and friction-reducing properties, which contributed to the excellent tribological behavior of the composites. [ABSTRACT FROM AUTHOR]

Abstract—The Maksyutov eclogite–blueschist Complex is characterized by a complicated fold-and-thrust structure that has been formed during the Late Devonian collision between the subducting Baltica margin (East European Plate) and the Magnitogorsk island arc. Eclogites are the most studied rocks of the Complex; their formation and exhumation are usually associated with the collisional stage of the orogen development. At the same time, the origin of meta-ultramafic rocks, which together with eclogites form sheets and boudins within metasedimentary rocks (schists and quartzites), still remains unknown. This paper presents the results of the first detailed petrological study of meta-ultramafic rocks represented by antigorite−chlorite and magnesite−antigorite meta-harzburgites, and chlorite−antigorite metaorthopyroxenite. Mineral compositions and textural relationships between minerals in the meta-harzburgites indicate at least two stages of rock transformations. Minerals of the early mineral paragenesis (first stage)—olivine, accessory chromite, and low-fluorine Ti-clinohumite – have a metamorphic genesis; ultrahigh-pressure (UHP) conditions of their formation are discussed. Partial replacement of olivine by orthopyroxene-bearing parageneses with Cr−Al antigorite and/or high-chromium chlorite is established for the second stage. The phase equilibria modeling using the Perple_X software package demonstrates that formation of antigorite−orthopyroxene paragenesis was associated with Si−Al metasomatism at: T ~ 630°С, P ~ 2 GPa, log ~ −0.6, log ~ −2.5. It is important to note that the mineral paragenesis is highly sensitive to : a slight decrease in log relative to the above value would lead to the growth of olivine with antigorite, and an increase would lead to the growth of orthopyroxene. The latter may explain the formation of meta-orthopyroxenites, which are widely distributed among the meta-ultramafic rocks of the Maksyutov Complex. Similar calculations performed for the range of = 0.01–0.05 in H2O–CO2 fluid showed the replacement of silicate minerals by magnesite under the established thermodynamic conditions. Carbonation and Si−Al metasomatism are specific features of high-pressure transformations of meta-ultramafic rocks, which have not been established in the associated eclogites, quartzites, and shales. Such selective fluid influence on different rock types is interpreted as a result of their different tectono-metamorphic evolution: meta-ultramafic rocks are fragments of the suprasubduction mantle, which were tectonically juxtoposed with the rocks of the subducting plate (eclogites and metasedimentary rocks). [ABSTRACT FROM AUTHOR]

Serpentines are geologically important minerals, and antigorite (Atg), lizardite (Lz) and chrysotile (Ctl) are the three key varieties. Their quick and accurate identification with micro-Raman spectroscopy requires to consider the effects of different crystallographic orientations and different chemical compositions. By collecting from existing literatures all Raman spectroscopic data and compositional data acquired from the same or identical Atg, Lz and Ctl samples, we critically examined the compositional effects for the first time, and found that some compositional parameters like the Al2O3 and Cr2O3 contents have significant impacts on the Raman features. Taking into account the effects of both compositional difference and crystallographic orientation difference, we propose two identification schemes for Atg, Lz and Ctl: the first one uses those weak but characteristic Raman peaks at 1200–1000 cm−1, and the second one uses those intense and unanimously-observed Raman peaks at ∼688, 378 and 229 cm−1. As for the first identification scheme, no peak at 1200–1000 cm−1 suggests the presence of Lz; a single peak at ∼1045, at ∼1070, or at ∼1106 cm−1 indicates the presence of Atg, Lz, or Ctl, respectively; two Raman peaks at ∼1040 and 1070 cm−1 implies the presence of Lz; the occasionally observed one single peak at ∼1040 cm−1 may imply the presence of either Atg or Lz, which can be sought out by resorting to the peak position ratio R∼1045/688 (Atg having R∼1045/688 > ∼1.521 whereas Lz attaining R∼1045/688

The continental mid‐lithosphere discontinuity (MLD) is widely detected within cratons, with the dominant depth range of 70–100 km and a significant reduction of shear‐wave velocity of 2%–12%. However, the formation mechanism and corresponding strength of the MLD are widely debated, which may strongly affect the roles of MLD in craton evolution. The comparisons among variable mechanisms indicate that the strength of the MLD varies from the relatively high viscosity of wet olivine to the rather low viscosity of antigorite. Thus, systematic numerical modeling has been conducted with the MLD of contrasting strengths, that is, the wet olivine‐induced MLD or antigorite‐induced MLD, to investigate the roles of MLD in the craton instability under variable tectonic regimes (stable, extension, compression, mantle flow traction, or mantle plume). The models show that the cratonic lithosphere with wet olivine‐induced MLD maintains its stability under all the tectonic regimes. In contrast, the antigorite‐induced MLD with lowest viscosity could significantly promote the decoupling of lithosphere, and facilitate the lithospheric deformation. However, lithospheric delamination only occurs with the rather weak MLD interacting with the sub‐plate asthenosphere upwelling during craton extension or mantle plume activity. The sufficient amount of melts is essential for this process, which requires a large amount of extension or a mantle plume with rather high temperature anomaly and large size. Therefore, craton destruction is still difficult, and requires additional strict conditions. This may explain the general stability of most cratons with widespread MLDs. Plain Language Summary: Geophysical observations have revealed a seismic discontinuity within cratons, which is defined as continental mid‐lithosphere discontinuity (MLD). The MLD is often considered to be rather weak, which could facilitate the lithospheric delamination, and even the craton destruction. However, most cratons preserve the stability during long‐term evolution. Thus, the viscosity of the MLD and its roles in craton evolution require further investigations. In this study, we have conducted systematic numerical modeling to investigate the effects of the MLD on craton instability under variable tectonic regimes. The wet olivine‐induced MLD and antigorite‐induced MLD are two end‐members of MLD with contrasting strengths, which are compared in the present models. The model results indicate that only the connection and interaction between the weak antigorite‐induced MLD and the sub‐plate asthenosphere could cause craton destruction during tectonic extension or upwelling of mantle plume. The sufficient amount of melts is essential for this process to destroy the lower part of lithosphere. Thus, craton destruction requires additional strict conditions, which may explain the stability of most cratons on the Earth, although with the presence of MLD. Key Points: The viscosity of mid‐lithosphere discontinuity varies from relatively high viscosity of wet olivine to the rather low value of antigoriteThe wet olivine‐induced mid‐lithosphere discontinuity could not lead to craton destruction in any tectonic regimesCraton destruction occurs with the presence of antigorite‐induced mid‐lithosphere discontinuity combing additional strict conditions [ABSTRACT FROM AUTHOR]

Serpentinites play an important role in the delivery of water into subduction zones. In addition, serpentinites also contain ferric Fe and can transport significant redox potential. We present high-pressure and high-temperature experiments and Mössbauer spectroscopy measurements on natural lizardite and antigorite samples equilibrated at various oxygen fugacities in order to quantify the relationship between the oxygen fugacity f (O2) and the Fe3+/Fetot ratio in these two phases. In antigorite, Fe3+ partitions into the octahedral site and is charge balanced by tetrahedral Al. In lizardite, tetrahedral Fe3+ is observed only at low temperature as well as under high f (O2), whereas Fe3+ prefers the octahedral site at temperatures exceeding 500 °C and at 3 to 5 GPa. Although metastable, lizardite remains in redox equilibrium in our experiments at conditions above the lizardite to antigorite phase transformation at 300 °C and demonstrates a similar stability to antigorite. The Al concentration of lizardite is found to be temperature dependent, and it was possible to reequilibrate the Fe3+/Fetot ratio of lizardite from 0.1 to 0.9 by using redox buffers such as Fe metal, graphite, graphite–calcite, Re–ReO2 and Ru–RuO2. Our experiments on antigorite demonstrate that antigorite does not adjust its Al concentration on experimental time scales. Since Fe3+ is charge balanced by Al, it was also not possible to manipulate the Fe3+/Fetot ratio of antigorite. The coexisting phases, however, show chemical equilibration with this antigorite composition. We have retrieved the standard Gibbs energy for Fe3+- and Al-endmembers of antigorite and lizardite and calculated the metamorphic evolution of subducting serpentinites. The lizardite to antigorite transformation does not cause a decrease in the bulk Fe3+/Fetot ratio under f (O2) buffered conditions, in contrast to observations from some natural settings, but does result in the formation of additional magnetite due to antigorite having a lower Fe3+/Fetot ratio than lizardite at equilibrium. If the f (O2) of antigorite serpentinite is buffered during subduction, such as due to the presence of graphite and carbonate, the bulk Fe3+/Fetot ratio decreases progressively. On the other hand, in a closed system where the bulk serpentinite Fe3+/Fetot ratio remains constant, the f (O2) increases during subduction. In this scenario, the f (O2) of an antigorite serpentinite with a typical Fe3+/Fetot ratio of 0.4 increases from the fayalite–magnetite–quartz to the hematite–magnetite f (O2) buffer during dehydration. These f (O2) results confirm earlier inferences that fluids produced by antigorite dehydration may not contain sufficient oxidised sulphur species to oxidise the mantle wedge. Sufficiently high levels of f (O2) to mobilise oxidised sulphur species may be reached upon antigorite dehydration, however, if closed system behaviour maintains a high bulk redox potential across the lizardite to antigorite phase transformation. Alternatively, oxidation of the mantle wedge might be achieved by oxidising agents from sources in subducted oceanic crust and sediments. [ABSTRACT FROM AUTHOR]

Virtually unlimited volumes of serpentine resources stimulate search for new procedures for its processing, including those involving thermal activation. Along with the calcination parameters (temperature, time), the crystal-chemical features of the initial serpentines are an important factor. They predetermine the sequence of the formation of new phases in the course of thermolysis and the content of these phases in the heat treatment product. The influence of the calcination temperature on the thermal decomposition of a serpentine mineral, antigorite, was studied by differential scanning calorimetry, X-ray diffraction analysis, and Mössbauer spectroscopy. Amorphous antigorite is not formed in the course of thermolysis, and the intermediate amorphous magnesia–silicate phase is a mixture of two dehydroxylates differing in the ability to react with acid solutions. The activity of the calcination products was determined by two different methods. The optimum calcination temperature is 750°C; it ensures the maximal content of the amorphous active magnesia–silicate phase. [ABSTRACT FROM AUTHOR]

Fluids released through the dehydration of serpentinite can be rich in Cl−, which enables the significant mobility of Cr in subduction zones. However, the Cr isotope behavior accompanying the mobility of Cr during serpentinite dehydration is still poorly constrained. Here, we report high‐precision Cr isotope data for a unique suite of serpentinites that represent metamorphic products at different depths in oceanic subduction zones. Low‐grade serpentinites affected by significant Cr loss during serpentinization exhibit remarkably higher δ53Cr, while samples with Cr contents >∼1,800 ppm typically preserve mantle‐like δ53Cr. Antigorite serpentinites have an average δ53Cr value of −0.17‰ ± 0.19‰ (n = 12, 2SD), which is statistically lower than those of low‐grade serpentinite (−0.05‰ ± 0.30‰, n = 80, 2SD) and higher‐grade chlorite harzburgite (−0.10‰ ± 0.27‰, n = 22, 2SD). This suggests that resolvable Cr isotope fractionation occurs during serpentinite dehydration, which is explained by the variability of Cr isotope behavior in the presence of Cl‐bearing fluids at different dehydration stages. No obvious Cr isotope fractionation was found during chlorite harzburgite dehydration, probably related to the limited Cr mobility in a Cl‐poor fluid. Other processes, such as melt extraction, external fluid influx and retrograde metamorphism, have negligible effects on the Cr isotope systematics of meta‐serpentinites. Fluids released by serpentinite dehydration may have a great effect on the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas. Plain Language Summary: Serpentinite plays an important role in crust–mantle interaction and arc magmatism in oceanic subduction zones. Serpentinite‐derived fluids potentially mobilize Cr in the form of Cl complexes. However, there is limited information available about the Cr isotope compositions of subducted serpentinites, hindering the understanding of Cr isotope behavior during the dehydration of serpentinite. Here, we first report high‐precision Cr isotope data for a unique suite of (meta‐)serpentinite that encompasses low‐pressure oceanic serpentinite to metamorphic products of complete dehydration at great depth. Our results fill the gap in the Cr isotope composition of meta‐serpentinites metamorphosed under diverse P–T conditions in oceanic subduction zones. We found that there is variable and significant Cr isotope fractionation during prograde dehydration of serpentinites. Such serpentinite‐derived fluids may contribute to the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas. Key Points: First high‐precision Cr isotope compositions on a unique suite of meta‐serpentinitesOceanic serpentinites with significant Cr loss tend to exhibit remarkably higher δ53CrSignificant Cr isotope fractionation occurs during prograde dehydration of serpentinites in subduction zones [ABSTRACT FROM AUTHOR]

Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction. However, the fate of magnetite and other oxides at higher pressure and temperature conditions has remained underexplored. We present a comprehensive petrological and geochemical study of oxide-sulfide-silicate mineral assemblages in metaperidotites beyond antigorite- and chlorite-out reactions (T = 650–850 °C and P = 1–3 GPa). Several ultramafic lenses, covering different bulk rock compositions and extents of oxidation upon oceanic serpentinization, were investigated from the Central Alps, Switzerland. Results point to two endmember scenarios: (i) Most frequently, metaperidotites have olivine with a Mg# of 89–91 (defined as molar Mg/(Mg + Fetot) × 100) and contain low oxide modes (0.06–1.41 vol.%), hematite is absent, and redox conditions are weakly oxidized and buffered by orthopyroxene-olivine-magnetite. (ii) Rare occurrence, high olivine Mg# > 94.5 metaperidotites display coexisting hematite and magnetite, high oxide modes (up to 4 vol.%), and redox conditions are hematite-magnetite (HM) buffered (Δlog10fO2,QFM of + 3 to + 4). Spinel displays evolving compositions from magnetite over chromite to Al-Cr-spinel, roughly correlating with increasing temperature. Most of the samples buffered by the olivine-orthopyroxene-magnetite assemblage contain coexisting pentlandite ± pyrrhotite, thus identifying stable sulfides beyond antigorite dehydration for these weakly oxidized samples (Δlog10fO2,QFM < 2.5). No sulfides were recognized in the highly oxidized sample. The transition of magnetite to chromite at around 700 °C goes along with a shift in fO2 to lower values. At the prevailing oxygen fugacity in the weakly oxidized metaperidotites sulfur in a coexisting fluid is always present in its reduced form. However, oxidized sulfur can be stable in the dehydration fluids released from highly oxidized serpentinites. [ABSTRACT FROM AUTHOR]

Research on polymer matrix composites with excellent tribological properties has received increasing attention in recent years. In this study, antigorite and wollastonite mineral dual-phase-reinforced polytetrafluoroethylene (PTFE) matrix composites were prepared by filling PTFE with mineral powders using ball-milling, cold-pressing, and pressureless sintering methods. The phase structure, microstructure, chemical composition, Shore hardness, and tribological behavior of the composites rubbed against steel balls under dry friction conditions were investigated. The results show that the composites have a dense structure and uniform distribution of mineral phases, with a Shore hardness of 62–68.8, an increase of 18.7–23.7% compared to pure PTFE. Compared with the addition of mono antigorite or wollastonite, the composites prepared by simultaneously filling the two minerals exhibited excellent tribological properties. The average friction coefficient and wear volume of the (10Atg + 20Wl)/PTFE composites were reduced by 44.2% and 71.4%, respectively, compared to those of pure PTFE. A dense and continuous tribofilm composed mainly of SiO2, MgSiO3, Mg2SiO4, MgO, CaO, CaMg(SiO3)2, and CaF2 was formed on the worn surfaces of both the dual-phase-reinforced PTFE matrix composites and counterpart steel balls during the friction process. The reduction in friction and wear is attributed to the reinforcement effect of the minerals on the PTFE matrix as well as the complex physical and chemical reactions at the friction interface stimulated by the synergistic effects between the two minerals. The addition of wollastonite reduced the phase transition temperature of antigorite, promoting more tribochemical reaction products with good abrasion resistance and friction-reducing properties, which contributed to the excellent tribological behavior of the composites.

Phosphorus (P) and nitrogen (N) can be simultaneously recovered by struvite (MgNH4PO4·6H2O) crystallization, and this has attracted more and more attention. However, the cost of producing struvite from wastewater is high due to the chemical inputs, which makes the struvite process unattractive in terms of economic benefits. Thus, it is important to develop a new struvite technology with low cost and simple operation. In this work, the antigorite was activated by a ball-milling process. The crystal form of the antigorite was destroyed by the mechanical forces, leading an increase in the dissolution of Mg2+ and OH− in the structure. The concentration of Mg2+ dissolved into water increases with the milling time, as well as the pH. Additionally, the dissolution of the activated antigorite depends on the concentration of phosphate-ammonium. The responsive dissolution makes the activated antigorite adaptively adjust the dissolution of Mg2+ and OH− and maintain a stable recovery of nitrogen and phosphorus from wastewater. The performance of P and N recovery was investigated by moderating variables, including the molar ratio of N/P, activation time, reaction time, and dosage. The results showed that the activated antigorite in the wastewater can be automatically converted into struvite. Actual swine wastewater was treated by the activated antigorite process, and a recovery rate of 98% was achieved by adding 1.2 g/L activated antigorite. The economic evaluation shows that the cost of the activated antigorite process is only 36% of an ordinary chemical process (MgSO4 and NaOH). [ABSTRACT FROM AUTHOR]

A low-probability but serious self-extinction of smouldering incense sticks is an undesirable characteristic. This paper reports the investigations on its cause and remedy, conducted for an incense stick manufacturer. Measurement of smouldering rate and surface temperature of glowing incense stick tip of several known compositions were made by recording the time required for the propagation of smoldering front and using thermal camera respectively. Several possible explanations like presence of (a) phosphorous, potassium, and sodium related compounds, (b) inadvertent inclusion of inorganic compounds, and others ruled out the role of phosphorous and related compounds, and a simple role of inorganic compounds despite the fact that defective incense sticks invariably contained large fraction of silica (SiO2) as revealed by scanning electron microscopy/ energy dispersive X-ray diffraction (SEM/EDX). Ultimately the issue was traced to the presence of antigorite that must have got infused into the incense sticks through termite mud into the raw materials. X-ray diffraction (XRD) of incense stick samples confirmed silica is in the form of α-quartz associated with antigorite in all defective samples and also in termite mud infested sample. Thermal studies of the incense sticks using differential thermal analysis (DTA) show endothermic decomposition of defective samples at 550 to 580°C and is also confirmed through differential scanning calorimetry (DSC) which shows an endothermic peak around 576°C corresponding to the endothermic phase transformation temperature of antigorite. It is therefore inferred that the presence of materials like antigorite in combination with α-quartz in incense sticks produce significant endothermal decomposition leading to self-extinction. The primary practical cause has been traced to termite mud infusion into the raw materials used for making the incense sticks. [ABSTRACT FROM AUTHOR]

Mineral dehydration in the subduction zone enormously affects Earth's geodynamics and the global geochemical cycles of elements. This work uses Raman spectroscopy and X-ray diffraction to investigate the dehydration process of antigorite under compression and shear loading conditions in a rotational diamond anvil cell (RDAC) at room temperature. In order to compare the shear effects, T301 stainless steel and Kapton plastic are applied as the gasket materials. In the experiment using a high-strength T301 stainless steel gasket, two new broad OH-stretching peaks of H2O and H3O2− appear at 3303 and 3558 cm−1, respectively, at 1.7 GPa. The original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 remain, while the central pressure is increased to 8.0 GPa, and the largest pressure gradient is about 2.5 GPa in the sample chamber. In another experiment with a low-strength gasket of Kapton plastic, two new OH-stretching broad peaks of H2O and H3O2− also start to appear at 3303 and 3558 cm−1, respectively, at a lower pressure of 0.3 GPa, but the original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 almost completely vanish as the central pressure reaches 3.0 GPa, with the largest pressure gradient at around 4.8 GPa. The comparison between the two experiments shows that antigorite is easier to dehydrate in the chamber of a Kapton plastic gasket with a larger gradient of shear stress. However, its axial compression stress is lower. The high-pressure Raman spectra of MgO2(OH)4 octahedron and SiO4 tetrahedron in the low wavenumber zones (100–1200 cm−1) combined with the micro-beam X-ray diffraction spectrum of the recovered product strongly support the structural breakdown of antigorite. This investigation reveals that the water-bearing silicate minerals have strong shear dehydration in the cold subduction zone of the plate, which has important applications in predicting the physical and chemical properties of subduction zones and deducing the rate of plate subduction. [ABSTRACT FROM AUTHOR]

Antigorite dehydration is a process able to release, in comparison with other minerals, the highest amount of H2O from a subducting slab. The released fluid delivers critical elements (e.g., S, Cu, and REE) to the overlying subarc mantle, modifying the mantle source of arc magmas and related ore deposits. Whether antigorite breakdown produces oxidising or reducing fluids is debated. Whereas previous studies have investigated antigorite dehydration in serpentinites (i.e., in a (C)AMFS-H2O system), this contribution is devoted to the CMFS-COHS carbonate system, which is representative of the metacarbonate sediments (or carbonate-dominated ophicarbonate rocks) that sit atop the slab. Thermodynamic modelling is used to investigate the redox effect of the carbonate-buffered antigorite dehydration reactions (i.e., brucite breakdown and antigorite breakdown) on electrolytic fluid geochemistry as a function of P-T-fO2. The influence of P-T-fO2 conditions on the solubility of C and S, solute-bound H2 and O2, fluid pH, the average valence states of dissolved C and S, and the fluid redox budget indicates that, in metacarbonate sediments, the CaCO3+antigorite reaction tends to produce reducing fluids. However, the redox state of such fluids is buffered not only by the redox state of the system but also, most importantly, by concomitantly dissolving redox-sensitive minerals (i.e., carbonates, graphite, pyrite, and anhydrite). A qualitative correlation between the redox state of the system and the possible depth of fluid release into the mantle wedge is also derived. [ABSTRACT FROM AUTHOR]

Major, minor, and trace element geochemistry as well as iron oxidation state and isotopes were investigated in serpentinites and olivine‐talc fels present along a metamorphic gradient in the Bergell contact aureole (Central European Alps) to evaluate element mobility during serpentine. This aureole is an ideal target to study dehydration of mantle rocks due to the increase in temperature from greenschist facies conditions (350°C) to amphibolite facies conditions (750°C) at low pressures of 0.4 GPa. Petrography and geochemistry document several events of fluid–rock interaction and metamorphism. Serpentinization of the mantle rocks started on the ocean floor. Subsequent Alpine regional metamorphism led to the formation of antigorite‐serpentinites containing olivine and diopside. These antigorite‐serpentinites were transformed into olivine‐talc fels in a large part of the contact‐aureole. Bulk‐rock major and trace element compositions maintain the geochemical signature of the precursor antigorite‐serpentinites. No apparent changes are indeed observed despite the fact that major dehydration reactions occurred. In addition, changes neither in Fe3+/Fetot ratio nor in δ56Fe values were observed. Local composition variations of antigorite‐serpentinites and olivine‐talc fels reflect chemical heterogeneities related to protolith composition and serpentinization processes on the ocean floor prior to contact metamorphism. Hence, prograde dehydration reactions occurring during contact metamorphism did not induce substantial element mobility, change in redox state, or isotopic fractionation in these contact metamorphic rocks. [ABSTRACT FROM AUTHOR]

To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO3. For a bulk CaCO3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1–2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H2-rich fluids. [ABSTRACT FROM AUTHOR]

The active Sagaing fault in Myanmar defines the boundary between the Indian Plate and the Eurasian Plate and causes seismic damage in the major cities of Myanmar. Small bodies of serpentinite occur along the fault. We for the first time investigated the highly sheared serpentinite bodies in the Sheinmagar area and Yega Inn area along the Sagaing fault. Extensively sheared/brecciated serpentinites and related rocks, such as talc and/or chlorite-bearing rocks contains small rock fragments of serpentinites. Serpentine texture and mineral chemistry indicate that the protolith of these serpentinites were mainly harzburgite with minor amounts of dunite, some of which are cut by gabbroic veins. No shape-preferred orientation of the antigorite is present, indicating that the serpentinization was occurred under relatively static conditions. Protolith and serpentine minerals are similar to those of the jadeitite-bearing serpentinites in the north of the Sagaing fault (the Jade Mine belt). Chemical variations of spinels in the studied area are within the compositional range of forearc peridotites and those in the mantle section of nearby ophiolites. After the formation of antigorite serpentinite under static conditions, these serpentinites were subsequently, but locally deformed, probably due to the activity of the Sagaing fault, resulting in the formation of serpentinite schist/brecciated rock. The presence of the less-deformed antigorite serpentinite in the sheared/brecciated zone indicates the strain localization mainly along the surrounding serpentine-talc (±chlorite) schistose rocks, which is probably formed by the reaction between serpentinite and country rocks. Further studies are needed to better understand whether the distribution of serpentinized peridotites cause variations in the activity of the Sagaing fault. [ABSTRACT FROM AUTHOR]

On nowadays multiphase and the facies heterogeneity of the formations are distinguished at the study of kimberlite pipes. Most researchers associate the formation of diamonds only with the mantle source. To date, satellite minerals with specific compositions associated with kimberlite diamonds have been identified as deep mantle diamond association. They are extracted from the concentrate of the kimberlites heavy fraction and may reflect the diamond grade of the pipe. For some minerals in the diamond association, however, they can not be reliable. Some researchers also revealed shallow diamond associations, related to the formation of serpentine, calcite, apatite, and phlogopite. There is recent data on the formation of diamonds in rocks of the oceanic crust. In the last years microdiamonds were identified in chromites of the oceanic crust in association with antigorite formed at 350-650 °C and 0.1-1.6 GPa. As a result, the authors established a postmagmatic kimberlitic stage of diamond formation associated with secondary mineral associations based on the experimental and mineralogical data for the conditions of the shallow upper mantle and crust. Mineralogical and petrographic studies of Angolan kimberlite pipe show that antigorite is the indicator mineral of this stage.

The distribution of earthquakes at intermediate depths corresponding to pressures <2 GPa in several hot subduction zones (such as Cascadia and southwestern Japan) coincides with the breakdown of antigorite to forsterite and talc; thus, this reaction may have triggered these earthquakes. However, previous studies have overlooked the potential significance of this reaction. Here, we performed a series of time-dependent dehydration experiments on antigorite at a pressure of 200 MPa and a temperature range of 500-650 °C. The results show that dehydration is controlled by a heterogeneous nucleation and growth mechanism and has an activation energy of 354 ± 24 kJ/mol. The formation of fine-grained forsterite and large talc crystals is consistent with kinetic results indicating Avrami exponents n = -- 1.4-1.1 and ~2.7, respectively. Fluid production rates at 600 and 650 °C are 2.54 x 10-6 and 4.69 x 10-5 mfluidm-3cks-1, respectively, which are much faster than those of mantle deformation, causing high fluid pressure in hot subducting mantle but not necessarily embrittlement. We emphasize the role of kinetic mechanisms in controlling the grain sizes of reaction products, which likely determine the mechanical behavior of serpentinized fault zones. Superplasticity or velocity weakening of fine-grained forsterite and velocity weakening of antigorite by water and/or talc may be responsible for earthquake nucleation and propagation in a heterogeneous system, which can be either dehydration products within a serpentinized fault zone or the mixture of antigorite fault and surrounding peridotite in hot subduction zones (<2 GPa). [ABSTRACT FROM AUTHOR]

The influence of structural features of three serpentine-group minerals (antigorite, chrysotile, and lizardite) on the hydration of heat-treated materials and the formation of magnesium silicate binder has been studied. Initial serpentine samples have been fired in the interval 550–800 °C with a step of 50 °C; acid neutralization capacity (ANC) values have been determined for all samples. Antigorite samples (SAP) have exhibited a maximum reactivity at a temperature of 700 °C (ANC 7.7 meq/g). We have established that the acid-neutralizing capacity of chrysotile and lizardite samples in the temperature range of 650–700 °C differ slightly; the capacity varied in the interval of 19.6–19.7 meq/g and 19.6–19.7 meq/g, respectively. The samples obtained at optimal temperatures (antigorite—700 °C, lizardite, and chrysotile—650 °C) have been studied. Heat-treated serpentines have interacted with water vapor for a year; serpentine hydration has been investigated. The strength characteristics of the resulting binder agents were studied after 7, 28, 180, and 360 days. Upon hardening within 7 days, the strengths of the SAP and SCH samples have been almost the same (2.2 MPa), whereas this indicator for the SLH and SLK samples has been significantly lower (0.5 MPa). After hardening for over a year, the chrysotile sample SCH had the highest strength (about 8 MPa), whereas the strength of antigorite SAP was 3 MPa. The samples of initial, heat-treated, and hydrated heat-treated serpentines have been studied using XRD, differential scanning calorimetry, and surface texture analysis. The serpentine structure is crucial in destroying the mineral crystal lattice during heat treatment. In contrast to heat-treated chrysotile and lizardite, antigorite did not adsorb water. Structural features of chrysotile provided the highest compressive strength of the binding agent compared with antigorite and lizardite. The acid-neutralizing ability of lizardite was noticeably higher than antigorite, whereas its compressive strength was lower due to the layered mineral structure and impurities. We have established that the minerals' structural features are crucial for the hydration of heat-treated serpentines; the structure determines material utilization in various environmental technologies. [ABSTRACT FROM AUTHOR]

The most abundant serpentine mineral in subduction settings, antigorite has one of the highest water storage capacities and is involved in seismicity. Seismic wave velocities of antigorite are important for detecting and quantifying serpentinization within the mantle wedge and the subducting oceanic plate. At present, the elastic properties of antigorite at high pressures and temperatures are unclear. In this study, we have investigated pressure-volume-temperature (P-V-T) data and thermodynamic properties of antigorite using first-principles molecular dynamics (FPMD) simulations. Using these simulations results, we computed the relevant thermoelastic parameters and estimated compressional and shear wave velocities (v P and v S) of antigorite in subduction conditions. A simplified velocity model of antigorite with its coexisting mantle anhydrous phases was introduced to help us understand the potential effect of serpentinization on the seismic velocity of mantle rocks. Combined with seismic observations, we re-evaluated some velocity anomalies within forearc mantle wedges and established reliable serpentinization budgets. These results can provide preliminary evaluations and reliable constraints on serpentinization and water content in mantle rocks, which has important implications for understanding global plate dynamics and the deep water cycle. • The thermodynamic properties of antigorite were determined using first principles calculations. • The seismic wave velocities of antigorite at high temperatures and pressures were esitimated. • New models were introduced to provide constraints for detection of serpentinized mantle. [ABSTRACT FROM AUTHOR]

We review the main petrologic and geochemical features of magmatic rocks from continental arcs compared to their intra-oceanic equivalents. Both types share a number of characteristics, including their typical HFSE-depleted incompatible element patterns and isotopic Sr, Nd, Pb and Hf features, intermediate between those of N-MORB and continental crust. They are thought to derive from the partial melting of the mantle wedge metasomatized by fluids emanating from the downgoing plate. They are dominantly hydrous fluids provided by the breakdown of amphibolites and antigorite serpentinites, but a significant role is attributed to felsic hydrous melts derived from oceanic metabasalts and associated sediments. The more enriched character of magmas from continental arcs with respect to those from intra-oceanic arcs is ascribed to their genesis from a less depleted mantle wedge in a milder thermal regime, to the enrichment of their source via subduction erosion of the continental margin, and finally to their contamination during ascent and storage within a thicker crust. • The main petrologic and geochemical features of magmatic rocks from continental and intra-oceanic arcs. • They share similar characteristics, including their typical HFSE-depleted incompatible element patterns and isotopic Sr, Nd, Pb and Hf features. • They have intermediate geochemical characteristics between N-MORB and continental crust. • They derive from the partial melting of the metasomatized mantle wedge. • Compared with intra-oceanic arc the continental ones are more enriched. [ABSTRACT FROM AUTHOR]

The Puebla‐Morelos (Mexico) 2017 earthquake nucleated ∼250 km inland from the trench within the Cocos oceanic plate mantle. Here, we argue that the Puebla‐Morelos (Mexico) 2017 earthquake resulted from changes in effective stress due to the reaction brucite + antigorite = olivine + H2O leading to the reactivation of pre‐existing seafloor faults. Fluid release (∼185 kg of H2O per m3 of subducted hydrated harzburgite) and volume increase (ΔVrsolid+fluid = ∼0.8%) likely occur along subducted seafloor‐inherited faults. The amount of H2O released, and magnitude of volume change depends on the degree of faults hydration; only highly hydrated (>40% of hydration) faults will stabilize brucite and experience this reaction at depth. The brucite + antigorite dehydration reaction may be key for intermediate seismicity worldwide. Plain Language Summary: The Puebla‐Morelos (central Mexico) Mw7.1 earthquake occurred in an uncommon locality in Mexico compared to most of the catastrophic earthquakes that have occurred in this part of the world—it nucleated ∼250 km inland, almost below Mexico City. In this paper, we explore the role of mineralogical changes occurring in the Cocos tectonic plate, which is currently descending below the North America plate. Our model tracked the changes in the minerals within the rocks and show that the earthquake might have been triggered by the physical changes associated with the mineral reaction: brucite + antigorite = olivine + H2O. We suggest that this mineral reaction primarily occurred along faults in the subducting oceanic floor along which the mantle lithosphere was hydrated prior to subduction. The brucite + antigorite dehydration reaction may be key for intermediate seismicity worldwide. Key Points: The dehydration of antigorite + brucite has a positive total volume change and leads to fluid‐induced fault reactivationWe hypothesize that fluid‐induced fault reactivation triggered the Puebla‐Morelos 2017 earthquakeThe antigorite + brucite reaction only happens in highly hydrated regions of the upper lithospheric mantle [ABSTRACT FROM AUTHOR]

The metamorphic evolution of serpentinite has a fundamental bearing on geochemical cycles, mechanical behaviour and seismic properties of the Earth. Phase equilibria in the system MgO–Al2O3–SiO2–H2O (MASH) provide a 'backbone' for phase equilibria in larger systems that are applicable to serpentinites and also many whiteschists. Phase equilibria for MASH, and departures from MASH, are calculated using the Holland and Powell data set, tc‐ds62, and new activity–composition relations for antigorite and talc. Phase relations are presented using PT projections and compatibility diagrams to highlight the role of P, T and composition on the equilibrium mineral assemblages stable in these chemical systems at high pressure. These phase relations provide the context for pseudosection and other modelling focused on individual rock compositions. [ABSTRACT FROM AUTHOR]

Hydration/dehydration of mantle peridotite significantly influences the mechanical properties at subduction zone interfaces. In particular, brucite is mechanically weak, antigorite + brucite occurs in shallow mantle wedges, and dehydration of this assemblage to form olivine is often observed. However, the bulk-rock strength of brucite-bearing serpentinite is poorly constrained due to the difficulty of crystallographic preferred orientation (CPO) measurements of brucite, and the dehydration of brucite + antigorite has not been investigated experimentally. Within the hydrated mantle in subduction zones, brucite disappears at lower temperatures than antigorite. Therefore, previous studies using natural brucite-bearing serpentinites have been limited despite the potential importance of brucite in subduction zones. Here, we succeeded in conducting electron backscatter diffraction (EBSD) analyses of brucite within mantle wedge-derived serpentinite samples (Shiraga body) from the subduction-type Sanbagawa metamorphic belt and, for the first time, conducted experiments on the dehydration of antigorite + brucite using natural samples of brucite-bearing serpentinite subjected to experimental conditions of 500 °C and 0.9 GPa using a Griggs-type apparatus. By directly comparing the texture using scanning electron microscopy (SEM)-EBSD before and after the dehydration reaction, we reveal that grains of metamorphic olivine were nucleated within existing brucite grains and show the importance of olivine topotactic growth from brucite. As a similar texture is observed in naturally formed metamorphic olivine in the Shiraga body, the occurrence of pre-existing brucite and its topotaxy with olivine when it reacts with antigorite to form olivine may be an important feature of the development of olivine CPO within the mantle wedge corner. [ABSTRACT FROM AUTHOR]

Antigorite serpentinite is expected to occur in parts of subduction plate boundaries, and may suppress earthquake slip, but the dominant deformation mechanisms and resultant rheology of antigorite are unclear. An exhumed plate boundary shear zone exposed near Nagasaki, Japan, contains antigorite deformed at 474°C ± 30°C. Observations indicate that a foliation defined by (001) crystal facets developed during plate‐boundary shear. Microstructures indicating grain‐scale dissolution at high‐stress interfaces and precipitation in low‐stress regions suggest that dissolution‐precipitation creep contributed to foliation development. Analysis of crystal orientations indicate a small contribution from dislocation activity. We suggest a frictional‐viscous rheology for antigorite, where dissolution‐precipitation produces a foliation defined by (001) crystal facets and acts to resolve strain incompatibilities, allowing for efficient face‐to‐face sliding between facets. This rheology can not only explain aseismic behavior at ambient plate boundary conditions, but also some of the contrasting behaviors shown by previous field and laboratory studies. Plain Language Summary: The mineral antigorite is important along certain types of tectonic plate boundary, but how antigorite responds to tectonic forces is uncertain. We tried to improve our understanding of this, which might help to better understand how movement occurs between tectonic plates. Rocks that occur along a beach near Nagasaki, Japan, represent an ancient tectonic plate boundary and some of these rocks contain abundant antigorite. We walked along this beach, collecting samples of the antigorite and measuring geological structures. By examining micro‐scale structures within our samples, and comparing with our observations from the beach, we were able to identify the physical mechanisms by which antigorite accommodated movement between the ancient tectonic plates. This mechanism allows movement at a rate that depends on the amount of slip (strain) the rock has accommodated, and the level of driving force, which is related to how fast the slip is. The mechanism we identified provides an explanation for the lack of earthquakes along high‐strain, low driving‐force regions of plate boundaries where antigorite occurs. Key Points: Outcrops of deformed antigorite serpentinite provide a chance to constrain deformation mechanisms at natural shear zone conditionsMicrostructural observations indicate that the dominant deformation mechanisms are dissolution‐precipitation creep and frictional slidingDevelopment of a fine‐grained and foliated fabric with progressive strain leads to reduced effective viscosity [ABSTRACT FROM AUTHOR]

The subduction of serpentinized slabs is the dominant process to transport "water" into Earth's mantle, and plays a pivotal role for subduction dynamics. Antigorite, the most abundant serpentine mineral in subduction settings, may imprint a seismic signature on serpentinized slabs, making them seismically distinguishable from the dry, non‐serpentinized ones. However, the complete single‐crystal elasticity of antigorite has not been experimentally constrained at high pressures, hindering the use of seismological approaches to detect serpentinization in subducting slabs. Here, we report the full elastic stiffness tensor of antigorite by single‐crystal Brillouin spectroscopy and X‐ray diffraction up to 7.71(5) GPa. We use our results to model seismic properties of antigorite‐bearing rocks and show that their seismological detectability depends on the geometrical relation between seismic wave paths and foliation of serpentinized rocks. In particular, we demonstrate that seismic shear anisotropy shows low sensitivity to serpentinization for a range of relevant geometries. Plain Language Summary: The subduction of serpentinized slabs plays a key role in the deep recycling of water into the Earth's interior. Antigorite is the main serpentine mineral in subducting slabs, and the most important carrier of water. Antigorite‐bearing rocks are predicted to have a distinct seismic signature, potentially allowing them to be detected with seismological approaches. However, our current knowledge on seismic properties of antigorite‐bearing rocks is limited, mostly hampered by a lack of experimental constraints on single‐crystal elasticity of antigorite at relevant pressures. In this study, state‐of‐the‐art techniques were employed to produce the first experimental description of the complete high‐pressure elasticity of antigorite single crystals. Our experimental data set was implemented in the modeling of seismic properties of antigorite‐bearing rocks at pressures relevant for subduction. Our results were used to discuss the relation between seismic wave path and shear wave anisotropy in serpentinized slabs, and challenge the use of shear wave splitting as a proxy for serpentinization in slabs. Key Points: Single‐crystal elasticity of antigorite at high pressures is determined by Brillouin spectroscopy and X‐ray diffraction experimentsSeismic signature of serpentinized slabs is constrained in a relevant composition‐pressure spaceSerpentinization in slabs may be undetectable through shear wave anisotropy [ABSTRACT FROM AUTHOR]

Mineral dehydration in the subduction zone enormously affects Earth’s geodynamics and the global geochemical cycles of elements. This work uses Raman spectroscopy and X-ray diffraction to investigate the dehydration process of antigorite under compression and shear loading conditions in a rotational diamond anvil cell (RDAC) at room temperature. In order to compare the shear effects, T301 stainless steel and Kapton plastic are applied as the gasket materials. In the experiment using a high-strength T301 stainless steel gasket, two new broad OH-stretching peaks of H2O and H3O2− appear at 3303 and 3558 cm−1, respectively, at 1.7 GPa. The original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 remain, while the central pressure is increased to 8.0 GPa, and the largest pressure gradient is about 2.5 GPa in the sample chamber. In another experiment with a low-strength gasket of Kapton plastic, two new OH-stretching broad peaks of H2O and H3O2− also start to appear at 3303 and 3558 cm−1, respectively, at a lower pressure of 0.3 GPa, but the original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 almost completely vanish as the central pressure reaches 3.0 GPa, with the largest pressure gradient at around 4.8 GPa. The comparison between the two experiments shows that antigorite is easier to dehydrate in the chamber of a Kapton plastic gasket with a larger gradient of shear stress. However, its axial compression stress is lower. The high-pressure Raman spectra of MgO2(OH)4 octahedron and SiO4 tetrahedron in the low wavenumber zones (100–1200 cm−1) combined with the micro-beam X-ray diffraction spectrum of the recovered product strongly support the structural breakdown of antigorite. This investigation reveals that the water-bearing silicate minerals have strong shear dehydration in the cold subduction zone of the plate, which has important applications in predicting the physical and chemical properties of subduction zones and deducing the rate of plate subduction.

Novel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10−9 − 10−4 1/s), and high pressure (1 GPa) in a Griggs‐type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, by applying 8–12 stress‐strain steps per temperature. The microstructures of deformed samples share features documented in previous work (e.g., shear microcracks), and highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models. Plain Language Summary: The rheology of Antigorite, a hydrous mineral that is present on top of subducting slabs and in the stagnant mantle wedge, could control subduction structure. Instead of deforming antigorite with our motor set to a constant speed, we redesigned the deformation assembly and machine to accurately servo‐control the stress on deforming samples. Measuring strain rate at several stresses and temperatures allows us to construct a flow law to extrapolate behavior to subduction zone stresses/strain rates. The microstructure of samples cut open after deformation suggests antigorite rheology is controlled by the resistance of defects to movement along the crystallographic sheet structure. Key Points: Isotropic antigorite was deformed at constant stress to low strainSlip along basal planes and kinks allow plastic deformationA low temperature plasticity (LTP) creep law describes deformation rheology well [ABSTRACT FROM AUTHOR]

We present new experimental data on the effect of F on the stability of antigorite and its breakdown products at high pressures (2–6 GPa) and high temperatures (570–850 °C). The experiments show that F does not affect the stability of antigorite, but addition of F to the system affects which minerals are formed when antigorite breaks down. In a F-free system and in a system with intermediate F contents (2 wt% F), antigorite breaks down to olivine and orthopyroxene, but in a F-rich system (5 wt% F), antigorite breaks down to other hydrous and F-bearing mineral assemblages which include chlorite, clinohumite and humite-group minerals (HGM). Since the latter mineral phases are stable at higher pressures and temperatures, and contain more F than antigorite, significant amounts of F and potentially other halogens can be retained in the subducting slab and transported deep into the mantle and possibly even into the Earth’s transition zone. [ABSTRACT FROM AUTHOR]

Picrolite artefacts comprise some of the most distinctive material remains in the prehistory of the island of Cyprus, in the Eastern Mediterranean. Picrolite exploitation dates from at least 12,000 years ago for the manufacture of personal ornaments and items with a symbolic function. It is commonly assumed that picrolite nodules were collected in secondary deposits on an ad hoc basis. This narrative, however, ignores the fact that picrolite carriers can only be found in very specific locations on the island, discrete from each other. Here we report initial outcomes of the application of handheld portable X-ray fluorescence (HHpXRF) and synchrotron-based X-ray fluorescence spectroscopy (SR-μXRF) to the analysis of picrolite raw materials performed at the newly opened PUMA beamline of the SOLEIL Synchrotron Radiation Facility. Our work refines the basic characteristics of the elemental constituents of the picrolite raw material and highlights key micro-structural differences between two distinct source regions on the Troodos Massif in western Cyprus. Picrolite source characterisation is expected to contribute significant new knowledge to the study of rare raw material consumption, prehistoric social organisation, networking and possible long-distance exchange of this idiosyncratic raw material within and beyond the island's geographic boundaries. [ABSTRACT FROM AUTHOR]

At 1.5-3 GPa, the HT ht is in the range of 540-570 °C in the presence of the NaCl-H SB 2 sb O fluid, whereas the minimal temperature of the Atg degradation is 700 °C in the NaCl-free system (run 2d, Fig. Regarding antigorite, the calculations based on an empirical equation of state for NaCl-H SB 2 sb O fluids predict the suppression of the dehydration temperature by about 70 °C at 2-3 GPa even in a relatively diluted NaCl-H SB 2 sb O solution ([18]). Keywords: Antigorite; decomposition; subduction zone; NaCl aqueous fluid; H2O activity; high pressure; high temperature; in situ X-ray diffraction EN Antigorite decomposition subduction zone NaCl aqueous fluid H2O activity high pressure high temperature in situ X-ray diffraction 1074 1079 6 06/01/22 20220601 NES 220601 Introduction The serpentine group minerals, phyllosilicates with the ideal formula Mg SB 3 sb Si SB 2 sb O SB 5 sb (OH) SB 4 sb , are major constituents of hydrated ultramafic rocks of the oceanic lithosphere. Antigorite decomposition in the presence of halite The HP-HT diffraction patterns collected on antigorite mixed with the NaCl-H SB 2 sb O fluid (runs 1s-3s, Table 1) are presented in Figures 3 to 5. [Extracted from the article]

Antigorite is the main carrier of water in Earth's subduction zones. The dehydration processes of antigorite were investigated by carrying out in situ phase transition experiments using a dynamic diamond anvil cell, with a time-resolved Raman scattering system, at 0.3–10 GPa and 396–1100 K. Three typical phase transformation reactions occurred within the P–T range of this study, corresponding to three reaction products. At low pressures (<0.7 GPa), antigorite transfers to talc and forsterite; as the temperature increases, the talc disappears and a combination of forsterite and clinoenstatite occurs. At moderate pressures (1.8–7.5 GPa), antigorite dehydrates into forsterite and clinoenstatite as temperatures increase; with the continuous increase in pressure, the dehydration products become clinoenstatite and phase A. At high pressures (>8.6 GPa), the products of the dehydration phase transition of antigorite are consistently clinoenstatite and phase A. Compared with the previous studies carried out by large-volume presses (such as a multi anvil press and a piston-cylinder press), the reaction to produce phase A occurs at higher P–T conditions, and the stable temperature region for talc as a dehydration product is narrower. Moreover, large quantities of pores with 5–10 μm in diameter formed in dehydration products, supporting the hypothesis that intermediate-depth earthquakes may result from dehydration embrittlement. The precise phase boundary determined by this in situ study provides a better understanding of the dehydration phase transition behavior and geological phenomena exhibited by antigorite under different pressure and temperature conditions. [ABSTRACT FROM AUTHOR]

Antigorite–olivine aggregates were synthesized to serve as representative lithologies of subduction zones, which often suffered serpentinization due to the hydration of the mantle wedge. The electrical conductivities (EC) of antigorite–olivine aggregates with different antigorite proportions before and after dehydration at 2.0 GPa were measured. An increase in electrical conductivity prior to antigorite dehydration was due to Fe and Mg interdiffusion with an activation enthalpy of 130–200 kJ/mol. The electrical conductivity of the aggregates increased to 0.1 S/m due to antigorite dehydration. The electrical conductivity of 0.1 S/m derived from the laboratory experiments is consistent with geophysical observations. Combined cube model and modified Archie's law model with previous studies, the volume fractions of aqueous fluids in the mantle wedge of cold and hot subduction zones were estimated. • Antigorite–olivine aggregates were used as lithologies of subduction zones. • EC of antigorite-olivine aggregates were measured before and after dehydration. • The increase in EC before dehydration is due to the diffusion of Fe and Mg. • The volumes of fluid for the mantle wedges were estimated. [ABSTRACT FROM AUTHOR]

The occurrence of pervasive crystallization of antigorite associated to brittle tectonics is described for the first time in the External Ligurian peridotites of the Northern Apennine (Italy). Antigorite was found in a slide block of subcontinental peridotites included in the tectonized sedimentary mélange of the Leo Unit (western External Ligurian units). This mélange, as those from the External Ligurian units, originated in Santonian-early Campanian time span from a source area corresponding to the Ocean-Continent Transition Zone at the Adria plate margin. The antigorite was found within veins belonging to four systems, from V1 to V4, each characterized by different attitude and morphology. The morphology of the veins ranges from massive to fibrous, with the latter characterized by fibers both parallel and perpendicular to the vein walls. Rare occurrence of chrysotile and calcite were identified within the veins. The antigorite crystallization in the peridotites cannot be interpreted as related to an orogenic high-pressure metamorphism but most likely can be regarded as developed at shallow structural level in the late stage of the rifting phase when subcontinental mantle was progressively exposed at the surface, intruded by gabbro bodies, and cut by basaltic dykes. [ABSTRACT FROM AUTHOR]

Antigorite, a high-pressure polymorph of serpentine, is considered to be the most abundant hydrous mineral in subduction zones. Although antigorite dehydration is presumed as one of the origins of intermediate-depth earthquakes in the subduction zone, the amount of antigorite is uncertain because the amount of water infiltrated into the oceanic lithosphere is still debated. To investigate whether antigorite can be formed even with limited water availability, we conducted the axial deformation experiments of magnesium germanate at 1.2 GPa and T = 500-800 °C using a Griggs-type deformation apparatus. Magnesium germanate is an analog material of magnesium silicate, and the starting material was dried prior to experimentation. Nevertheless, the samples had initially high porosity, and hence a small amount of water (about 200 ppm wt H2O) was retained in the samples. In the samples deformed at 600 °C, stable slip occurred, and TEM analysis revealed that fine-grained platelets of germanate antigorite existed along the faults. A sharp absorption band assigned to the OH-stretching vibration of antigorite in Fourier transform infrared spectroscopic (FTIR) analysis also implies that antigorite was formed in the samples deformed at a temperature lower than 600 °C. Our results indicate that straininduced hydration of germanate olivine results in antigorite formation even with only a small amount of water present. Thus, partial serpentinization in the oceanic lithosphere can occur under slight water infiltration due to the high strain accumulated by subduction. [ABSTRACT FROM AUTHOR]

The influence of structural features of three serpentine-group minerals (antigorite, chrysotile, and lizardite) on the hydration of heat-treated materials and the formation of magnesium silicate binder has been studied. Initial serpentine samples have been fired in the interval 550–800 °C with a step of 50 °C; acid neutralization capacity (ANC) values have been determined for all samples. Antigorite samples (SAP) have exhibited a maximum reactivity at a temperature of 700 °C (ANC 7.7 meq/g). We have established that the acid-neutralizing capacity of chrysotile and lizardite samples in the temperature range of 650–700 °C differ slightly; the capacity varied in the interval of 19.6–19.7 meq/g and 19.6–19.7 meq/g, respectively. The samples obtained at optimal temperatures (antigorite—700 °C, lizardite, and chrysotile—650 °C) have been studied. Heat-treated serpentines have interacted with water vapor for a year; serpentine hydration has been investigated. The strength characteristics of the resulting binder agents were studied after 7, 28, 180, and 360 days. Upon hardening within 7 days, the strengths of the SAP and SCH samples have been almost the same (2.2 MPa), whereas this indicator for the SLH and SLK samples has been significantly lower (0.5 MPa). After hardening for over a year, the chrysotile sample SCH had the highest strength (about 8 MPa), whereas the strength of antigorite SAP was 3 MPa. The samples of initial, heat-treated, and hydrated heat-treated serpentines have been studied using XRD, differential scanning calorimetry, and surface texture analysis. The serpentine structure is crucial in destroying the mineral crystal lattice during heat treatment. In contrast to heat-treated chrysotile and lizardite, antigorite did not adsorb water. Structural features of chrysotile provided the highest compressive strength of the binding agent compared with antigorite and lizardite. The acid-neutralizing ability of lizardite was noticeably higher than antigorite, whereas its compressive strength was lower due to the layered mineral structure and impurities. We have established that the minerals’ structural features are crucial for the hydration of heat-treated serpentines; the structure determines material utilization in various environmental technologies.

Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 °C and 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite–hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr–V–Al–Fe3+. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89–91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90–91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz–fayalite–magnetite buffer. This is consistent with the observation that the Fe–Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized. [ABSTRACT FROM AUTHOR]

Antigorite dehydration is well known as a key process at convergent boundaries for the genesis of mantle wedge partial melting and intermediate‐depth earthquakes. However, the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and their effects on the seismic anisotropy of subducting slabs remain ambiguous and controversial. Here we report antigorite dehydration experiments on a foliated serpentinized peridotite at pressures of 0.3–6 GPa and temperatures of 700–900°C. Our results show that olivine CPO evolves from a fiber‐[100] type in fine grains to a type‐C like in coarse grains. We propose that the fiber‐[100] olivine CPO is developed by topotactic growth during incipient hydrostatic dehydration, while the orthorhombic type‐C like olivine CPO is developed by anisotropic growth resulting from an anisotropic fluid flow during later dehydration. The type‐C like olivine CPO in the antigorite‐rich layers after antigorite dehydration could contribute to the trench parallel fast shear wave seismic anisotropy at convergent plate boundaries. Plain Language Summary: We study the crystallographic preferred orientations and seismic anisotropies of dehydrated antigorite samples. Our results show that the CPOs of prograde mineral, olivine, evolve from the fiber‐[100] CPO in fine grains to the type‐C like CPO in coarse grains. The type‐C like olivine CPO after antigorite dehydration could cause trench‐parallel fast shear wave seismic anisotropy observed at convergent plate boundaries. Key Points: Olivine crystallographic preferred orientations (CPO) evolves from the fiber‐[100] CPO to the type‐C like CPO with increasing grain size during hydrostatic dehydration of antigoriteThe fiber‐[100] and type‐C like olivine CPOs are developed by topotactic growth and anisotropic growth, respectivelyThe prograde olivine with type‐C like CPO could contribute to trench parallel seismic anisotropy at convergent boundaries [ABSTRACT FROM AUTHOR]

Serpentinized peridotites in the Yangkou (YK), Suoluoshu (SLS) and Hujialin (HJL) areas in the Sulu ultrahigh-pressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton. Their protoliths, harzburgite and dunite, were variably hydrated by aqueous fluids released from subducting Yangtze continent. The rocks are enriched in fluid-mobile elements (FME) including Sb (42–333 times the depleted mantle value) and Pb (30–476 times). The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites, and greater than forearc mantle serpentinites from Marianas, suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones. Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area. The antigorite has highly enriched in Pb, U, Cs, and LREE, but not for the lizardite. The abundance of FME in two different species of serpentine reflects the different temperature of hydration. At temperature lower than 300 °C, formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures, such as Sb and As. When the temperature greater than 300 °C, formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab (or fragments), including Pb, U, Cs, LREE as well as Sb and As. The eventual breakdown of antigorite (600–700 °C) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.

The serpentine minerals lizardite, polyhedral serpentine, chrysotile, antigorite and 15‐sector and 30‐sector polygonal serpentine have been studied by micro‐Raman spectroscopy, using selected samples, that had been previously characterized. The appropriate crystal orientations were determined by optical microscopy of petrographic sections. Oriented spectra, obtained using Nd‐YAG green laser radiation (532 nm), were deconvolved, extracting wavenumber and intensity values for the peaks, possibly overlapping and forming complex bands. Relevant Raman anisotropy is common and relevant in serpentine. Depending upon the orientation of the impinging laser, significant wavenumber shifts occur (up to 10 cm−1, mostly in polyhedral serpentine and lizardite). Furthermore, also, important intensity variations (up to 1 order of magnitude) occur in polyhedral serpentine, lizardite, chrysotile and antigorite as well. On the one hand, the possibly characteristic peaks have been identified and discussed. On the other hand, caution is suggested as far as the micro‐Raman characterization of polyphasic, variably oriented serpentine minerals is concerned. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of a natural antigorite sample from New Caledonia is compared to its theoretical counterpart computed for the pristine antigorite m = 17 polysome within the density functional perturbation theory framework. The theoretical model reproduces most of the bands related to Si-O stretching in the 800-1300 cm-1 range, OH libration, hindered OH translation and SiO4 bending in the 400-800 cm-1 range, and OH stretching in the 3500-3700 cm-1 range. Most of the observed bands have a composite nature involving several vibrational modes contributing to their intensity, except the apical and one of the basal Si-O stretching bands whose intensity is carried by a single mode. The peculiarity of the antigorite structure favors a localization of the Si-O and OH stretching modes in specific regions of the unit cell. Weaker Si-O stretching bands experimentally observed at 1205 and 1130 cm-1 are related to the occurrence of 6- and 8-reversals in the antigorite structure, respectively. The distribution of OH bond lengths leads to an asymmetric distribution of frequencies consistent with the width and the shape of the experimentally observed OH stretching band. It also leads to a strong distribution of OH libration frequencies ranging from 600 to 830 cm-1 explaining the asymmetry of the band observed at 648 cm-1 in the antigorite spectrum. [ABSTRACT FROM AUTHOR]

The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1,100–1,500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10−6–7.24 × 10−4 mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Plain Language Summary: As one of the most important hydrous minerals in subduction zone, antigorite has many geodynamic significances. However, low dehydration temperatures and complex crystal structure have long prevented good characterization of its peculiar rheological properties and its relationship to intermediate‐depth earthquakes. In this study, we deformed foliated iron‐poor antigorite. We found that all samples show stable fault slip dominated by a semibrittle regime and thus dehydration does not directly trigger earthquakes. Combined with the phase boundary information determined by phase equilibrium experiments under static compression, our results suggest that deformation geometry (such as compression, simple shear, and the direction of foliation plane with respect to the maximum compression axis) may change the phase stability of antigorite. As water or talc or a talc‐phase phase was evidenced previously to likely reduce the rate dependence, thus we proposed that in cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces earthquakes by producing such phases during shearing. In the semibrittle regime, small difference in strength between antigorite and dunite (σ(dunite)/σ(antigorite) < 1.6) suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Key Points: The strength of antigorite has weak strain rate and temperature dependences, as well as moderate pressure dependenceDehydration cannot directly trigger earthquakes and the deformation geometry may change the phase stability of antigoriteA thin antigorite layer is not sufficient to cause significant decoupling between the slab and the mantle wedge [ABSTRACT FROM AUTHOR]

• Massive CO 2 mobilisation from ophicarbonate around shallow intrusions. • Mixed H 2 O-CO 2 fluids dominate metamorphic carbon degassing at collisional plate boundaries. • Metamorphic ultramafic rocks with >50% tremolite or diopside likely were ophicarbonate. • Metamorphic CO 2 degassing may be highly relevant for the long-term global carbon cycle. The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO 2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650 °C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650 °C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid X CO2 values evolve from <0.09 to as high as ∼0.16, and the total H 2 O-CO 2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition. Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO 2 /MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr. The massive, contact metamorphic CO 2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid X CO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700 °C – 1.0 GPa and >600 °C – 0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800 °C – 1 GPa. Amagmatic CO 2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater table and diffuse outgassing. Despite the huge uncertainties associated with quantification of such metamorphic carbon fluxes, geological time scale global carbon cycling models should more rigorously explore the significance of these contributions. [ABSTRACT FROM AUTHOR]

Revalorization of mining residues is of central concerns to the mining industry and the environment. Specifically, environmental management of residual products from the exploitation of chrysotile in the Thetford Mines region is one of the government concerns in Quebec and Canada. This work uses mining wastes in a second resource generation for production of magnesium from cheap and health-friendly mineral sources; the goal being to produce chrysotile-depleted pre-concentrates for a use as precursors in the leach off extraction of magnesium. The concentration of lizardite/antigorite from chrysotile containing serpentine rock mine tailings originating from the Carey Mine site in East-Broughton (Québec) was carried out using a suite of hydrocyclone, settling/decantation and magnetic separations. Four size classes of the mining residue, namely (−3150,+1580), (−1580, +600), (−600, +300) and (−300, +150) μm, were tested with an aim to reduce the level of objectionable asbestos fibers to allow access to the safer Mg-bearing minerals contained in the mine waste sources. The asbestos fibers clean-up consisted of subjecting the sieved fractions to two hydrocyclone steps, six settling/decantation steps and two magnetic separation steps. The best results were achieved when the hydrocyclone separators led to Mg recovery of 85% (±4) for the coarsest size fraction size. Both hydrocyclone underflow streams underwent settling/decantation separations. The settling tests lasted 30 min and led to Mg recoveries of 82.5% (±1.8) of Mg in the ultimate concentrate. SEM characterizations revealed that it was possible to reduce substantially the amount of chrysotile fibers to render the coarse-sized fraction in the mining waste usable while significantly lowering the health risk of the fibers. A two-step magnetic separation was applied to the final settling/decantation underflow to remove magnetic minerals such as magnetite from the lizardite/antigorite concentrate. The final quasi-non-magnetic chrysotile-depleted lizardite/antigorite concentrate allowed sample recovery of 62.5% (±0.9) wt. of Mg. These preliminary results are intended as a first compulsory step in support of viable restoration and sustainable development scenarios for the Thetford Mines mining sites as second-breath sources for valuable magnesium. [ABSTRACT FROM AUTHOR]

Foliated antigorite serpentinites are contributing to seismic anisotropy in subduction zones. However, uncertainty remains regarding the development of antigorite crystal preferred orientations (CPO). This study analyzes the CPOs of antigorite and olivine in variably serpentinized samples of antigorite serpentinite from three localities in Southwest Japan, using U-stage, EBSD, and synchrotron X-rays. In all samples (010) poles show a maximum parallel to the lineation, and (001) poles concentrate normal to the foliation with a partial girdle about the lineation. (100) poles are less distinct and scatter widely. The three measurement methods give similar results, though orientation distributions from X-ray analysis which averages over larger sample volumes are more regular and weaker than U-stage and EBSD which focuses on local well-crystallized grains. In olivine-bearing serpentinite, the olivine CPO does not appear to control the antigorite CPO, though there are some topotactic relationships. The antigorite CPO formed contemporaneously with serpentinization and shear deformation. Based on our analyses, in a subduction zone mantle wedge, the (010) pole of antigorite is parallel to the direction of the subduction, and (001) lattice planes are parallel to the interface of the subducting oceanic plate. By averaging single crystal elastic properties over orientation distributions wave velocities have been calculated. Fast P velocities are parallel to the subduction direction and slow velocities are perpendicular to the subducting plate with an anisotropy of 10-15%. © 2013 Elsevier B.V.

Hydrous minerals in the hybridized zone at the slab-mantle interface play a critical role in retaining and transporting fluid mobile elements in subduction zones. The observed high B and As concentrations and high δ11B values in arc magmas were previously thought to have been resulted from the mechanical transport of forearc serpentinites down to subarc depths by mantle corner flows or through subduction channels because B, As, and Sb are mostly released during early stages of subduction. This study provides new insights into the recycling of B, As, and Sb in subduction zones via an in-situ study on a set of carbonate-bearing serpentinite samples from the Mianlue mélange complex in the Qinling Orogen. Petrological observations and bulk-rock Sr-Pb isotopes reveal that the Jianchaling serpentinites were formed by interactions between forearc peridotites and sediment-derived fluids at a shallow depth in the subduction channel. Some Jianchaling serpentinites experienced the lizardite to antigorite transition at 300–400 °C with infiltrations of CO 2 -rich fluids. In contrast, the Liangyazi antigorite serpentinites were produced at a greater depth. Antigorite grains in the Jianchaling serpentinites have lower Fe apfu than those in the Liangyazi serpentinites and most antigorite with geochemical data in the literature. Meanwhile, B, As, and Sb concentrations of Fe-poor antigorite from the Jianchaling serpentines (385–653 ppm B, 198–334 ppm As, and 6.24–8.01 ppm Sb) are not only higher than those of the Fe-rich antigorite from the Liangyazi serpentinites but also 2–3.5 times higher than those of the surrounding lizardite. The lower modal percentage of spinel and ferritchromite in antigorite veinlets, together with the lower bulk-rock FeO content of antigorite-lizardite serpentinites than that of lizardite serpentinites from the Jianchaling region, indicates that some FeO might be lost during the lizardite to antigorite transition. The presence of carbonate minerals in the antigorite veinlets and the remarkably high B, As, and Sb concentrations in the Jianchaling antigorite indicate that the penetrating fluids during the lizardite to antigorite transition are most likely derived from sediment and metabasite decarbonations at <350 °C. Infiltrations of CO 2 -rich fluids at the slab-mantle interface could buffer a relatively low pH to significantly enhance the mobility of Fe2+, which is produced by reduction of the cronstedtite component in lizardite or Fe3+ in magnetite during the lizardite to antigorite transition, and facilitate the formation of Fe-poor antigorite at the slab-mantle interface. The oxygen fugacity in such a CO 2 -rich water–rock system is also high enough for the occurrence of As5+ in the surrounding fluids. Therefore, once IVFe3+ is lost from the serpentine lattice structure during the lizardite to antigorite transition, B3+, As5+, and Sb5+ in the surrounding fluids can occupy the tetrahedral sites in the newly formed Fe-poor antigorite. The formation of Fe-poor antigorite at the slab-mantle interface is therefore made available to transport shallow slab-derived B, As, and Sb to a greater depth, where destabilization of antigorite at 600–700 °C liberates these elements into the arc magma source. [ABSTRACT FROM AUTHOR]