Search

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]

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]

• 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]

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]

Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6–1.9 GPa and 680–710 °C). The mean δ56Fe of all the Cerro del Almirez meta-serpentinites (+0.05 ± 0.01‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03‰). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al 2 O 3 /SiO 2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34–67 ppm) and isotope signatures (δ66Zn = +0.18 – +0.55‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ66Zn = +0.16 ± 0.06‰). The Atg-serpentinites ([Zn] = 34–46 ppm; δ66Zn = +0.23 ± 0.06‰) display similar [Zn] and δ66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ66Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ66Zn = +0.30 ± 0.06‰) and Chl-harzburgites with granofels ([Zn] = 38–59 ppm; δ66Zn = +0.33 ± 0.04‰) or spinifex ([Zn] = 48–66 ppm; δ66Zn = +0.43 ± 0.09‰) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29–45 ppm) and a positive correlation exists between [Zn] and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO 3 (H 2 O)5+] or [ZnCO 3 (H 2 O) 3 ]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism. [ABSTRACT FROM AUTHOR]

• Tonian La Tuna Ophiolite, Uruguayan shield. • U-Pb dating of zircon from metasomatites. • Main age = 743 ± 2 Ma. • Zircon ɛHf (T) = +4.9 - +11.2, U/Yb<0.1; tourmaline δ11B = +12.1‰ to + 18.1‰. • Adamastor Ocean crust + mantle in the Dom Feliciano Belt. Accretionary to collisional orogenies such as the Brasiliano Orogeny of eastern South America present the opportunity to distinguish between oceanic-continental (accretionary) and continental (collisional) processes. However, the multitude of geotectonic processes involved and the reduced degree of preservation of the accretionary stage restrain the interpretation of evidence. This study focuses on a metasomatite associated with the La Tuna Ophiolite in the southern Dom Feliciano Belt of the Uruguayan Shield, as these can provide crucial insights into the accretionary stage of the Brasiliano Orogeny. The metasomatite occurs as a meter-sized massive tourmalinite body surrounded by a meter-wide chloritite lens, both enclosed by the La Tuna serpentinite. Our SEM-EDS and EPMA data revealed that the serpentines from the La Tuna ophiolite are antigorite and lizardite, while chlorite was identified as clinochlore. Abundant inclusions of sheridanite chlorite in dravite tourmaline suggests that tourmaline overgrew chlorite. Dravite crystals revealed the presence of three diffuse zones with minor compositional variations, mainly enrichments in Ca and Fe towards the rims. The U-Pb LA-ICP-MS dating of metasomatic zircon yielded ages of 743.2 ± 3.4 Ma (n = 22) for the chloritite and 743.1 ± 2.4 Ma (n = 31) for the tourmalinite. High δ11B values (+12.1‰ to + 18.1‰) of tourmaline (LA-ICP-MS) suggest that the metasomatic event was caused by seawater-derived fluids in a fluid-dominated environment. The single metasomatic U-Pb age combined with the high δ11B values suggest that the metasomatites underwent more intense metasomatic events than those associated with the Bossoroca (−8.5‰ to +1.8‰) and the Ibaré ophiolites (+3.2‰ to +5.2‰) further north in the São Gabriel Terrane of the Dom Feliciano Belt. Nonetheless, a few spots in tourmaline located within fracture zones and in contact with chlorite displayed negative (c. −5‰) and low positive δ11B values (c. 2‰). These are interpreted as overprinting by crustal fluids, either during the tectonic emplacement of the La Tuna Ophiolite or during exhumation. The calculated T DM Hf model ages for zircon in tourmalinite and chloritite range from 0.74 to 1.09 Ga and their ɛHf(T) values range from +4.9 to +11.2, indicating derivation from depleted mantle sources. The data corroborate earlier results indicating that the associated La Tuna amphibolites have composition similar to N-MORB. Furthermore, the trace element composition of zircon in the metasomatites is consistent with an oceanic environment (U/Yb < 0.1). Geotectonically, the La Tuna metasomatites formed at 743 ± 2 Ma in an oceanic setting, most likely in a juvenile Adamastor Ocean basin. During the collisional stage of the Brasiliano Orogeny, the La Tuna Ophiolite was tectonically emplaced within the <666 Ma La Micaela Schist of the Paso del Drágon Complex. As such, this study on the La Tuna Ophiolite contributes direct evidence on the oceanic evolution leading up to the accretionary stage of the Brasiliano Orogeny and provides new constraints on the transition towards the collisional stage. [ABSTRACT FROM AUTHOR]

This work provides the first comprehensive petrological and multi-scale structural investigation of a suite of ultramafic bodies and enclosing metasediments from the Late Paleozoic accretionary prism of South-Central Chile. The targeted outcrops are located in the La Cabaña area and are characterized by the presence of three main types of ultramafic rocks: (1) partially serpentinized massive peridotites , (2) antigoritic schistose serpentinites , and (3) antigoritic mylonitic serpentinites. Partially serpentinized massive peridotites and schistose serpentinites recorded a common first hydration event with the formation of Fe2+ porous rim in chromites (at ~500 °C), followed by static partial transformation of olivine to lizardite (below ~300 °C) and formation of rims of Fe3+-rich chromite and magnetite around chromites. A second event of serpentinization linked to fluid passage through zones with focused deformation resulted in the partial transformation of partially lizarditized massive peridotites into antigoritic schistose serpentinites (at ~320–400 °C). This second event took place within the shallower portion of the serpentinitic subduction channel, near the base of the accretionary prism and was characterized by the formation of tubular folds made up of lenses of olivine-lizardite massive rocks embedded in a matrix of schistose serpentinites. Antigoritic mylonitic serpentinites encountered in the matrix of schistose serpentinites may locally contain metamorphic olivine (i.e., olivine-bearing mylonitic serpentinites), registering higher temperature conditions at medium pressures (~600 °C, 11 kbar) that prevailed during the initial immature stage of development of the subduction system. Moreover, some mylonitic serpentinites containing Ti-clinohumite, Ti-chondrodite and metamorphic olivine still record higher pressure conditions (>15 kbar, i.e., >50 km depth for ~500 °C), which are related with a deformational event taking place when the subduction system had already cooled. These medium- and high-P mylonitic serpentinites preserved kinematic indicators of burial, whereas their exhumation along the subduction channel towards the shallower schistose serpentinites near the base of the accretionary prism could take place through discrete shear zones possibly aided by large sheath folds. Once incorporated in the accretionary prism (c. 285 Ma ago), the serpentinites shared metamorphism and deformation with the enclosing metasediments. Subvertical shortening related to uplift processes within the accretionary system took place in the stability field of antigorite (>300 °C) and, later, a subhorizontal E-W shortening prevailed in more superficial conditions. • Shallow portions of the subduction channel can be captured in the accretionary prism. • Hanging mantle lenses are dragged and tectonically mixed in the subduction channel. • Medium and high-T serpentinites can exhume to shallow portions of the channel. • Within-channel exhumation occurred in sheared zones or within the sheath folds head. • Localized deformation and fluid pathways characterize shallow subduction channel. [ABSTRACT FROM AUTHOR]

In the subducting oceanic lithosphere, a significant part of the seismicity is triggered in the mantle, especially along the upper and lower Wadati-Benioff planes. Several studies have investigated the potential involvement of dehydration reactions in the triggering mechanism of mantle earthquakes. Recent experimental results reveal that, under subduction conditions, mechanical instabilities nucleate in strong stable mineral aggregates during the destabilization of minor amounts of antigorite, i.e. the high-temperature serpentine, through a stress transfer, without any fluid overpressure. Here I confront these laboratory results to seismological observations. On one hand, most of the natural hydrous magnesium silicates seem to be known, with experimentally deduced stability limits up to 7 GPa, at least, available as relatively accurate estimates. On the other hand, recent achievements in thermal structure of subduction zones combined with precise hypocentre relocation give access to pressure and temperature conditions at earthquakes hypocentres. A series of P-T diagrams summarizes the stability limit of minerals that may be part of natural peridotites or serpentinites with variable compositions at pressures from 0.5 to 6 GPa and temperatures from 200 to 950 °C, and compares it with seismicity. Both hydrous and anhydrous phases are considered. A myriad of minor metamorphic reactions could participate in a transformation-driven stress transfer, even if the stability limits of serpentine minerals seem to correlate with most of the observed seismicity. • Minor mineral phases play a major role in the triggering mechanism of mantle earthquakes. • A myriad of metamorphic reactions may participate in transformation-driven stress transfers within the subducting mantle. • Mantle seismicity within slabs is far from fitting the ideal dehydration curve of antigorite in a P-T diagram. • Considering variable paragenesis, chemistry and H2O saturation, serpentine stability limits correlate with most seismicity. • The lower Wadati-Benioff plane is due to both the dehydration reactions and the hydration limit of the subducting plate. • The upper plane of cold subducting mantle could be due to H2O transfer from low-grade serpentine minerals to antigorite. • The sharp seismicity increase at 90 km, 3 GPa, beneath Japan and Chile, would indicate H2O transfer to HP hydrous phases. [ABSTRACT FROM AUTHOR]

Abstract Much attention has been paid to deal with cesium from the serious concern over the environmental threat as a well-known radionuclide. Radioactive cesium is usually precipitated and concentrated from aqueous waste before the final processing such as verification at high temperature for permanent storage underground somewhere. Few choices are available due to the high solubility of almost all the cesium compounds. A new idea is developed to use insoluble magnesium source to increase the efficiency of precipitating cesium as struvite (CsMgPO 4.6H 2 O). Activated antigorite (Mg 3 Si 2 O 5 (OH) 4) by ball-milling was used together with phosphoric acid to combine cesium chloride and form struvite at nearly stoichiometric ratio, even by simple agitation at room temperature. It is more interesting to notice that other alkali metals of potassium and sodium do not form stable struvite phase and remain in solution, giving a very high selectivity of cesium removal over potassium and sodium. This method may be applied both in cesium removal from the radioactively contaminated source and cesium concentration for metallurgical processing. Graphical abstract Unlabelled Image Highlights • Cesium was precipitated by using mechanochemically activated antigorite. • High efficient precipitation at room temperature. • High selectivity of cesium removal over other alkali metals. • Developed an environment-friendly method for antigorite application. [ABSTRACT FROM AUTHOR]

The environmental impact of natural occurrences of asbestos (NOA) and asbestos-like minerals is a growing concern for environmental protection agencies. The lack of shared sampling and analytical procedures hinders effectively addressing this issue. To investigate the hazard posed by NOA, a multidisciplinary approach that encompasses geology, mineralogy, chemistry, and toxicology is proposed and demonstrated here, on a natural occurrence of antigorite from a site in Varenna Valley, Italy. Antigorite is, together with chrysotile asbestos, one of the serpentine polymorphs and its toxicological profile is still under debate. We described field and petrographic analyses required to sample a vein and to evaluate the NOA-hazard. A combination of standardized mechanical stress and automated morphometrical analyses on milled samples allowed to quantify the asbestos-like morphology. The low congruent solubility in acidic simulated body fluid, together with the toxicity-relevant surface reactivity due to iron speciation, signalled a bio-activity similar or even greater to that of chrysotile. Structural information on the genetic mechanism of antigorite asbestos-like fibres in nature were provided. Overall, the NOA site was reported to contain veins of asbestos-like antigorite and should be regarded as source of potentially toxic fibres during hazard assessment procedure. [Display omitted] • Site-specific impact of a natural occurrence of asbestos (NOA) should be assessed. • An approach to evaluate the NOA-hazard in a natural site is presented. • A procedure to assess if a mineral exhibits asbestos-like properties is proposed. • A vein of asbestos-like antigorite is proved to pose a potential environmental hazard. [ABSTRACT FROM AUTHOR]

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H 2 O, whereas Ol hydration was characterized by gains of Si and H 2 O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18 – 1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3 – 1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism. • The Chandman meta-peridotite body interacted with slab-derived fluids. • Calcium metasomatism of the mantle predated the main serpentinization event. • Fluid flow pathways into the mantle changed from grain boundaries to fracture networks. • Large fluid flux from slab involves mantle metasomatism and hydration. [ABSTRACT FROM AUTHOR]

Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpentinites. Bulk rock and mineral major element chemistry linked to detailed petrography reveals that silicate mineral Mg# (100 ⁎ [Mg/(Mg+Fe)] molar ) are higher than bulk rocks due to the presence of magnetite, and olivine Mg# are lower in Chl-harzburgite than in Atg-serpentinite. The amount of magnetite is lower in Chl-harzburgite (1.4 vol%) than in Atg-serpentinite (2.8 vol%), resulting in reactive bulk rock compositions with Mg# of 92.7 and 96.0, respectively. Pseudosection modelling employing these reactive bulk compositions yields a small temperature field at 670 °C, 1.6 GPa where Atg-serpentinite and Chl-harzburgite coexist at the same metamorphic conditions. Thus, the antigorite dehydration front represents a compositional boundary rather than a thermal front (isograd). We suggest that this compositional boundary represents an oxidation front established upon serpentinisation at the ocean floor. Previous studies have shown that with increasing extent of serpentinisation increasing amounts of magnetite are formed concomitant with an increase in the Mg# of coexisting silicates. During subduction of such heterogeneous ultramafic rocks antigorite dehydration will be a continuous reaction that can occur over up to ∼40 °C, controlled by variations in Mg# of the reactant silicates. The difference in the amount of magnetite between Atg-serpentinite and Chl-harzburgite is thus not related to a change in redox budget of subducting serpentinites at Almirez imposed by the antigorite dehydration reaction. Rather, mass balance considerations suggest that ferric iron from antigorite may actually be the most prominent contribution to the redox budget of the antigorite dehydration reaction. Our findings also imply that direct comparison between Atg-serpentinites and Chl-harzburgites to infer geochemical changes associated with prograde dehydration reactions may lead to erroneous conclusions, including estimates on element loss mediated by aqueous fluid escape and associated changes in redox budget based on Fe 3+ /Fe tot . [ABSTRACT FROM AUTHOR]

Antigorite-bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones. Here we present a detailed study of the microstructures and seismic properties in a sample of antigorite-olivine schist previously studied by Kern et al. (1997, 2015). We have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections. Microstructures indicate that deformation is localized in the bands with high antigorite fractions, resulting in strong crystallographic preferred orientations (CPOs) with point maxima of poles to (100) parallel to lineation and poles to (001) to the foliation normal. Olivine CPO suggests deformation under high temperature and low stress, with a [100] fiber texture. The CPO strength varies with grain size, but is strong even in fine-grained antigorite, and larger grains tend to display higher internal misorientation. Orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis, (100) ol ||(001) atg being more frequent than [001] ol ||[010] atg . Two new orientation relationships between olivine and antigorite have been documented. Seismic velocities decrease while anisotropy increases with increasing antigorite modal content. Antigorite grain shape has a weak effect on seismic velocities, but it is important on anisotropy. Comparison between CPO-derived seismic velocities using Voigt, Reuss, Hill averages and geometric mean only showed good agreement in 1/3 of experimental velocities. If the crack porosity of 1.63% measured experimentally at 600 MPa was used in the self-consistent model with two crack orientations with planes normal to Z and Y, good match with all experimental velocities was achieved. The self-consistent model implies important crack porosity in the foliation plane at 600 MPa that reduces Vp normal to the foliation by 0.3 km/s. [ABSTRACT FROM AUTHOR]

The B-type olivine fabric (i.e., the [010] axes subnormal to foliation and the [001] axes subparallel to the lineation) has been regarded as an important olivine fabric for interpreting global trench-parallel S-wave polarization in fore-arc regions. However, strong serpentinization and cold temperature environment in the mantle wedge should inhibit development of the B-type olivine fabric that requires high temperature to activate solid-state plastic deformation. Here we report fabrics of olivine and antigorite generated at low temperatures (300–370 °C) during serpentinization in a fossil mantle wedge of the Val Malenco area, Central Alps. Olivine in the serpentine matrix develops a pronounced B-type fabric, while antigorite in the same matrix displays a strong crystallographic preferred orientation (CPO) with the (001) planes and the [010] axes subparallel to foliation and lineation, respectively. The following evidence leads to the conclusion that the B-type olivine fabric results from dissolution creep assisted by grain boundary sliding (GBS) and grain rotation, rather than solid-state plastic deformation: (1) serpentinization took place at low temperatures and a fluid-enriched environment, ideal for dissolution-precipitation creep; (2) the voids and zigzag boundaries along the interface between antigorite and olivine suggest a fluid dissolution reaction; (3) the primary coarse olivine develops a nearly random fabric, indicating the B-type fabrics in the fine-grained olivine may not be inherited fabrics. These results document for the first time the B-type olivine CPO formed by dissolution creep at low temperatures during serpentinization and provide a mechanism to reconcile petrofabric observations with geophysical observations of trench parallel fast S-wave seismic anisotropy in fore-arc mantle wedge regions. [ABSTRACT FROM AUTHOR]

Dehydration of antigorite in subduction zones releases a large amount of aqueous fluid and volatile elements, which can potentially oxidize the mantle wedge. The redox capacity of three synthetic serpentinites with variable Fe total , Fe 3+ and S − contents is investigated using XANES spectroscopy at both, Fe and S K -edges. Experiments are performed between 450 and 900 °C, at 2 GPa and f O 2 ∼ QFM-2 ; conditions similar to those encountered in subduction zones. Redox reactions in the synthetic serpentinites, which involve Fe and S can be summarized as follows: 1) the reduction of (S − )-pyrite into (S 2− )-pyrrhotite (∼450 °C), with ∼4.4 mg/g of the sulfur degassed most likely as H 2 S, 2) the consumption of magnetite that reacts with antigorite to form Fe-rich olivine (<500 °C), 3) the reduction of (Fe 3+ )-antigorite into (Fe 2+ )-antigorite (∼580 °C), occurring about 100 °C below the temperature of antigorite breakdown, 4) the main (Fe 2+ )-antigorite breakdown that forms olivine and enstatite (∼675 °C), and 5) the decomposition of minor amounts of (Fe 2+/3+ )-clinochlore (∼800 °C). The bulk Fe 3+ /Fe total ratio is found to decrease with run temperature from ∼0.82–0.97 depending on the hydrous starting material, down to 0.1–0.2 in the high-temperature anhydrous assemblages. The evolution of mineral modes and Fe 3+ /Fe total with temperature in our synthetic samples shows similar trends to what has been reported in serpentinite rocks collected, for example, along a metamorphic transect in the western Alps. We show that a large amount of O 2 -equivalent – up to 10 mol/kg of rock – can be generated at temperature around 450 °C due to the presence of oxides and sulfides such as magnetite and pyrite. Owing to the poor capacity of aqueous fluid to transfer redox conditions, we surmise that this O 2 -equivalent is “consumed” at the scale of the lithospheric-mantle top which is partially serpentinized and therefore bear strong redox gradients. [ABSTRACT FROM AUTHOR]

Intra-oceanic subduction initiation may occur where a transform fault or a fracture zone is located between the younger and older lithospheres. However, the mechanisms and conditions controlling the initiation of subduction remain debated. Here, we present two-dimensional visco–elasto–plastic numerical models of vertically-forced (or spontaneous) subduction initiation along such a pre-existing fault zone where slab-derived fluids enter the overlying mantle wedge. The models show that just after sinking of the older lithosphere, hydrothermal alteration to form serpentine (antigorite) or talc occurs within a juvenile, thin mantle wedge, resulting in the development of a low-viscosity shear zone (1019–1020 Pa s) at its base. The low-viscosity shear zone acts to decouple the mantle wedge from the slab, thereby facilitating slab subduction. Continued slab subduction generates shallow (<50 km depth) flux melting in the inner part of the mantle wedge at 3–5 Myr after the start of the model run, consistent with the timing of boninitic magmatism after the onset of subduction initiation in the Izu–Bonin–Mariana system. Our models that incorporate water release from the slab require average fracture zone (or transform fault) shear strengths of ≤7 MPa to attain a self-sustaining subduction zone. The fault strengths might be achieved in nature where seawater infiltrates deep into the fault zone. • Effects of slab-derived fluids on intra-oceanic subduction initiation are evaluated. • Hydrothermal alteration in an infant mantle wedge facilitates the initiation. • A self-sustained subduction is achieved with a weak (≤7 MPa strength) fracture zone. [ABSTRACT FROM AUTHOR]

The electrical conductivities of antigorite with magnetite fractions of 0, 5, 10, 20, 25 and 30 vol% were measured before and after dehydration at 2.0 GPa and temperatures of 573–1073 K, and the oxygen fugacity of the aggregates after dehydration was estimated. Our results indicate that before dehydration, the electrical conductivity was significantly enhanced when the connectivity threshold for magnetite exceeded 21 vol%. The interaction between the magnetite and the fluid liberated the magnetite from the antigorite, resulting in a decrease in the magnetite fraction. The iron in the magnetite was dissolved in the liquid, increasing the oxygen fugacity. The oxygen fugacity of the aggregates increased with increasing magnetite content, and the oxygen fugacity of the oxidizing fluid released by the decomposition of the serpentine was between QFM + 1.5 and QFM + 2.8 (QFM = Quartz-Fayalite-Magnetite oxygen buffer), which caused an increase in the electrical conductivity of the aggregates of up to 0.1 S/m. Based on geophysical observations, we conclude that ∼2 vol% of oxidizing fluid with QFM + 2.8 can generate a high conductivity anomaly of ∼10–0.5 S/m in a warm subduction zone, such as the Southwest Japan and Cascadia subduction zones. • The electrical conductivities of antigorite containing different proportions of magnetite were measured after dehydration. • The oxygen fugacity of the oxidizing fluid released by the serpentine-magnetite was estimated. • The electrical conductivity anomaly in the mantle wedge was interpreted to be oxidizing fluid. [ABSTRACT FROM AUTHOR]

Interactions between aqueous fluids and ultrabasic rocks are essential processes in a broad range of contexts including hydrothermal alteration on the parent body of carbonaceous chondrites, at mid-oceanic ridge, and in subduction zones. Tracking these processes and understanding reaction kinetics require knowledge of the diffusion of water in rocks, and of isotope fractionation in major minerals forming under hydrous conditions, such as serpentines. We present a study of D/H inter-diffusion in antigorite, a common variety of serpentine. Experiments were performed in a belt apparatus at 315 °C, 450 °C and 540 °C and at 3.0 GPa on natural antigorite powders saturated with interstitial D 2 O. An experiment was performed in a diamond anvil cell at 350 °C and 2.5 GPa on an antigorite single-crystal loaded with pure D 2 O. D/(D + H) ratios were mapped using Raman spectroscopy for the experiments at 315 °C, 450 °C and 540 °C and by NanoSIMS for the experiment at 350 °C. As antigorite is a phyllosilicate, diffusion coefficients were obtained for crystallographic directions parallel and perpendicular to the silicate layers (perpendicular and parallel to the c ∗ -axis, respectively). Arrhenius relations for D/H inter-diffusion coefficients were determined to be D D/H (m 2 /s) = 4.71 × 10 −2 × exp(−207(−33/+58) (kJ/mol)/ RT ) and D D/H (m 2 /s) = 1.61 × 10 −4 × exp(−192(−34/+93) (kJ/mol)/ RT ) perpendicular and parallel to the c ∗ -axis, respectively, and D D/H (m 2 /s) = 7.09 × 10 −3 × exp(−202(−33/+70) (kJ/mol)/ RT ) for the bulk diffusivity. Assuming D/H inter-diffusion coefficients for antigorite are the same for all serpentine species, closure temperature and diffusion durations are applied to hydrothermal alteration in the oceanic lithosphere, and in CI, CM and CR chondrites. Closure temperatures lie below 300 °C for terrestrial hydrothermal alteration and depend on serpentine variety because they have different typical grain sizes. Closure temperatures lie below 160 °C for carbonaceous chondrites, indicating that D/H isotopic exchange may have persisted down to very low temperatures in their parent bodies. Local D/H isotopic compositions may be associated with grain size heterogeneities in carbonaceous chondrites due to protracted alteration of fine-grained material with the lowest closure temperatures ( ca 80 °C). [ABSTRACT FROM AUTHOR]

Metaophiolites that consist mainly of serpentinites or metabasites outcrop in the East Thessaly region, Central Greece. These formations, along with some ophiolite outcrops, have been variably emplaced onto the Pelagonian tectonostratigraphic zone as dispersed and deformed thrust sheets. Based upon their estimated metamorphic degree, serpentinites from the metaophiolites and ophiolitic units of East Thessaly have been divided into three groups: Group-1 serpentinites from East Othris, include lizardite and antigorite in balanced amounts, defining greenschist facies metamorphic conditions (~ 320–340 °C, P ≈ 6–7 kbar). Group-2 serpentinites are marked by further prevalence of antigorite over lizardite, suggesting upper-greenschist to lower-blueschist facies metamorphism (~ 340–370 °C, P ≈ 8–10 kbar). Group-3 serpentinites are mainly characterized by the predominance of antigorite corresponding to blueschist facies metamorphism (~ 360–400 °C, P ≈ 11–12 kbar). The chemical composition and mineral chemistry of the East Thessaly serpentinites suggest that their protoliths were highly depleted harzburgites. Group-1 serpentinites exhibit higher Mg/Si ratio values and LOI compared to serpentinite Groups-2 and -3, due to increasing metamorphic conditions of the latter groups. The prominent Cs, U, Pb, As and Sb enrichments point to subduction-related serpentinites that were subjected to fluid/rock interactions. The East Thessaly serpentinites also seem to have undergone deserpentinization retrograde metamorphism (estimated at P < 8 kbar and T < 350 °C). Retrograde metamorphism also had a significant impact on the rodingite intrusions hosted within the serpentinites, forming metarodingites through late-stage derodingitization processes. [ABSTRACT FROM AUTHOR]

Episodic Tremor and Slip (ETS) events display a rich behaviour of slow and accelerated slip with simple oscillatory to complicated chaotic time series. It is commonly believed that the fast events appearing as non volcanic tremors are signatures of deep fluid injection. The fluid source is suggested to be related to the breakdown of hydrous phyllosilicates, mainly the serpentinite group minerals such as antigorite or lizardite that are widespread in the top of the slab in subduction environments. Similar ETS sequences are recorded in different lithologies in exhumed crustal carbonate-rich thrusts where the fluid source is suggested to be the more vigorous carbonate decomposition reaction. If indeed both types of events can be understood and modelled by the same generic fluid release reaction AB ( solid ) ⇌ A ( solid ) + B ( fluid ) , the data from ETS sequences in subduction zones reveal a geophysically tractable temporal evolution with no access to the fault zone. This work reviews recent advances in modelling ETS events considering the multiphysics instabilities triggered by the fluid release reaction and develops a thermal-hydraulic-mechanical-chemical oscillator (THMC spring) model for such mineral reactions (like dehydration and decomposition) in Megathrusts. We describe advanced computational methods for THMC instabilities and discuss spectral element and finite element solutions. We apply the presented numerical methods to field examples of this important mechanism and reproduce the temporal signature of the Cascadia and Hikurangi trench with a serpentinite oscillator. [ABSTRACT FROM AUTHOR]

Antigorite is common in hydrated mantle domains and commonly shows strong alignment either due to deformation or growth in a preferred orientation. The alignment of antigorite imparts a strong anisotropy to the host rock affecting physical properties. A quantitative analysis of how antigorite affects these properties requires a reliable measurement of the crystallographic preferred orientation of antigorite, and EBSD analysis is the most widely used technique. Potential problems include (i) mis-indexing, which can add significant uncertainties to the results; and (ii) sample preparation, which may affect the measured CPO in particular for automated mapping. Combining results derived from X-ray goniometry, EBSD and U-stage techniques with FIB-TEM analysis leads to the following conclusions concerning these two issues. (i) There is a significant issue with mis-indexing a - and b- axes due to rotational similarities about the c -axis. Similar problems may also affect the c -axes measurements but this is less significant than the a - and b- axes when data are filtered using lower MAD values. Filtering using MAD values of <0.7° can significantly change the resulting CPO. (ii) Sample preparation can also affect the measured CPO: sections prepared parallel to the foliation show only minor differences with MAD values of <2.0°. Mis-indexing problems can be minimized by using an MAD value of <0.7° and analysing thin sections cut parallel to the foliation. [ABSTRACT FROM AUTHOR]

A series of dehydration experiments in the piston-cylinder apparatus was carried out at 2 GPa and 550–850 °C on a natural antigorite sample mixed with 5 wt.% of magnetite. Chemical analyses of experimental products show a progressive decrease of the Mg# in antigorite and clinopyroxene between 550 and 675 °C, whereas the Mg# of olivine increases. The observed behavior of Mg# signifies Fe–Mg exchange between coexisting minerals. At higher temperatures, between 700 and 850 °C, compositions remain stable for all minerals in experimental assemblages. Thermodynamic parameters of the ferrous antigorite end-member were refined with the use of Holland and Powell (1998) data set and added to the antigorite solid solution. Good agreement between theoretical calculations performed for the studied bulk composition and experimental results confirms extrapolated thermodynamic data for Fe-antigorite. Constrained parameters allowed to calculate phase relationships for various serpentinite compositions. First, we assessed the effect of bulk iron content, from 0 to 10 wt.% FeO, on the stability field of antigorite. The results show significant decrease of the antigorite thermal stability with increasing bulk Fe content. Second, we demonstrated the influence of bulk iron content on dehydration reactions in subduction zones along typical thermal gradients. Dehydration observed in pure MSH (MgO–SiO 2 –H 2 O) systems comprised of antigorite appears as a univariant reaction, which happens at 710 °C/3.7 GPa and 640 °C/6 GPa in “hot” and “cold” subduction, respectively. In contrast, more complex in composition Fe-bearing serpentinites show spread dehydration profiles through divariant reactions from ~ 300 °C/0.8 GPa to 700 °C/3.6 GPa and from 450 °C/4 GPa to 650 °C/7.4 GPa for “hot” and “cold” thermal gradients respectively. A comparison between depths of “water-release events” and “earthquake occurrence” in the South Chile slab (“hot” subduction) highlights a clear correlation between each other, suggesting a possible contribution of the dehydration of Fe-bearing serpentinites to the occurrence of seismic events. In contrast, and based on the same comparison applied to the Kermadec slab, our results demonstrate that there is no correlation in depth between serpentinite dehydration and seismicity in “cold” subduction zones. [ABSTRACT FROM AUTHOR]

Serpentine minerals serve as a Mg donor in carbon capture and storage by mineralisation (CCSM). The acid-treatment of nine comprehensively-examined serpentine polymorphs and polytypes, and the subsequent microanalysis of their post-test residues highlighted several aspects of great importance to the choice of the optimal feed material for CCSM. Compelling evidence for the non-uniformity of serpentine mineral performance was revealed, and the following order of increasing Mg extraction efficiency after three hours of acid-leaching was established: Al-bearing polygonal serpentine (< 5%) ≤ Al-bearing lizardite 1T (≈ 5%) < antigorite (24–29%) < well-ordered lizardite 2H 1 (≈ 65%) ≤ Al-poor lizardite 1T (≈ 68%) < chrysotile (≈ 70%) < poorly-ordered lizardite 2H 1 (≈ 80%) < nanotubular chrysotile (≈ 85%). It was recognised that the Mg extraction efficiency of the minerals depended greatly on the intrinsic properties of crystal structure, chemistry and rock microtexture. On this basis, antigorite and Al-bearing well-ordered lizardite were rejected as potential feedstock material whereas any chrysotile, non-aluminous, widely spaced lizardite and/or disordered serpentine were recommended. The formation of peripheral siliceous layers, tens of microns thick, was not universal and depended greatly upon the intrinsic microtexture of the leached particles. This study provides the first comprehensive investigation of nine, carefully-selected serpentine minerals, covering most varieties and polytypes, under the same experimental conditions. We focused on material characterization and the identification of the intrinsic properties of the minerals that affect particle's reactivity. It can therefore serve as a generic basis for any acid-based CCSM pre-treatment. [ABSTRACT FROM AUTHOR]

Reduction of friction and wear by lamellar solid powders in lubricating system is a valid approach that can allow to formulate energy saving lubricants. An experimental study has been made of the friction properties of antigorite powders having flaked-bladed morphology and 0.86 µm of the mean particle size. The results showed that antigorite transformed to forsterite (at 793 °C) and enstatite+forsterite (at 831 °C). The friction coefficient, the wear volume and power consumption of the driving motor in system with antigorite powders were respectively reduced by 19.3%, 33.33% and 4.5–5.0% as compared with base oils. Results are explained using particle spacing-polishing and friction-induced formation of like-ceramic film mechanism. [ABSTRACT FROM AUTHOR]

Serpentinite rocks are considered to be a suitable raw material for Carbon Capture and Mineralization (CCM), which has been recognized as a potential method for the management of the industrial CO 2 -emissions. Different serpentine minerals, that form serpentinite rocks, feature divergent characteristics that influence the eventual suitability of a serpentinite rock for CCM. The present study focuses on these characteristics, sub-divides them and explains how each characteristic influences the suitability. The carbonation method employed in this study is developed at Åbo Akademi University. The good, yet sometimes inconsistent results gained with this method, gave grounds to a closer mineralogical study. Samples in this study consist of an antigorite dominating serpentinite, a lizardite dominating serpentinite, an antigorite seam, a lizardite rock and a serpentinite rock with high water content (15.15%). A serpentinite tailing is used as a base sample. Mineralogical studies were executed with an optical microscope, Scanning Electron Microscope with Energy Dispersive X-Ray Spectrometer (SEM-EDX) and X-ray Diffraction (XRD). The results state that the characteristics that influence the suitability of a serpentinite rock for mineral carbonation are the mineralogical structure, parent rock and later transformation of the rock through metamorphosis or other type of alteration. The features that positively influence the suitability are more common in lizardite than in antigorite. As serpentinite rocks are widely available, additional knowledge on their behavior based on the mineralogical characteristics, and understanding these variables, enables improvement of the method and widens its range of application. The results can also be used as guidelines to other rocks that are found to be possible raw materials for the carbonation method. The results do not apply to carbonation processes only, but also other purposes, for example among industrial exploitations of quarried materials and their further utilization as profitable goods. [ABSTRACT FROM AUTHOR]

Large parts of the shallow mantle wedge are thought to be hydrated due to release of fluids from the subducting slab and serpentinization of the overlying mantle rocks. If serpentinization proceeds under low SiO 2 activity, brucite can be a major phase in the low-temperature (< 450 °C) part of the serpentinized mantle wedge, but only very few natural examples have been documented. A combined petrological, geochemical, and geological study shows that brucite is widely distributed in the wedge mantle-derived Shiraga metaserpentinite body in the Sanbagawa metamorphic belt of SW Japan. Thermodynamic modeling combined with bulk rock composition and point counting indicates that the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~ 10–15 vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m-thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H 2 O budget and mechanical properties of serpentinite; these should be taken into consideration when examining the behavior of the shallow mantle wedge. [ABSTRACT FROM AUTHOR]

Understanding the surface and structural properties of kaolinite/halloysite nanostructures is of utmost importance from the points of view of nanohybrid preparation. The formation of nano-sized particles depends on the quality and quantity of bonding sites as well as on the required degree of dispersity. The basic goal of this work is the study of the effect of the morphology (as secondary structure) on the surface properties as well as the demonstration of the applicability of inverse gas chromatography (IGC) to characterize exfoliated surfaces. The surface properties of nanostructures made of kaolinites of varying structural order were investigated with N 2 adsorption and inverse gas chromatography (IGC). The spectral changes of the OH groups sensitive to the changes in the outer chemical environment were followed by FTIR spectroscopy. The nanostructures made of a highly ordered kaolinite (HI = 1.4) were investigated as a function of the way of energy-input, as well. Exfoliation resulted in a mixture of nanoparticles with pseudo-hexagonal and tubular morphology. The pseudo-hexagonal morphology favored the formation of pores with cylindrical symmetry. Structural order can primarily influence the pore size distribution, the dispersion component of surface energy and the acidity parameter. The deformation band of the inner OH groups can be considered as an indicator of the changes in the secondary structure. The relative band intensity of the inner OH groups increased with the increase of the structural order. [ABSTRACT FROM AUTHOR]

Serpentinite rock dissolution experiments at 25 °C and 2 < pH < 9 were used to retrieve antigorite dissolution rates, as its high abundance and comparatively fast dissolution dominate element release from the rock. Retrieved antigorite dissolution rates at pH 2.02 and 3.06 are similar to previous literature data for serpentine minerals at corresponding pH. In contrast, retrieved rates at higher pH are considerably lower than corresponding literature data. Nonetheless, the retrieved rates at pH 5.76 and 7.28 are consistent with the linear log rate – pH relationship of Orlando et al. (2011), suggesting that rates follow a single ‘acidic’ mechanism to a pH of at least 7.3. The relatively low dissolution rates of serpentine minerals found in this study suggests that significantly more time is required for serpentine weathering than that estimated using the dissolution rates of these solids reported by Marini (2006) based on a review of literature data. As such, using enhanced serpentine weathering as a carbon capture/storage strategy may be less efficient than previously assumed. [ABSTRACT FROM AUTHOR]

The Río Guanajibo serpentinite body (RGSB) near Mayagüez, Puerto Rico, is part of an ophiolite mélange thrust in an oceanic convergent zone. The aim of this study was to characterize the extent and chronology of serpentinization within this peridotite mass. Mineralogy, microstructures, and veining episodes within the RGSB were characterized using optical microscopy, x-ray diffraction (XRD), scanning electron microscopy (SEM), and structural analyses. This study identified, for the first time, all three serpentine polymorphs (i.e., antigorite, chrysotile, lizardite) in serpentinite samples collected from Puerto Rico. Lizardite, the initial serpentine mineral formed from widespread hydration of olivine, was found throughout serpentinite samples. Chrysotile was the most abundant polymorph observed in sheared serpentinite samples, consistent with conditions favoring low fluid to rock ratios, supersaturation and abundant porosity. Antigorite was observed as a replacement texture in serpentinites that were not exposed to greenschist facies metamorphic conditions, and were frequently found in veins with a shear component. The results indicate that metamorphic conditions do not exclusively dictate polymorph formation. The mineralogy and textures observed within the different vein generations reflect the formation conditions, and deformational mechanisms, that occurred during the serpentinization process; six veining episodes (V1 – V6) were identified and grouped into four stages of serpentinization. Stage one (V1 and V2 type veins) represents the earliest stages of serpentinization and was characterized by microscopic fracture networks that formed as a result of cracking during the initial hydration of olivine under low water/rock ratios. During stage two (V3 and V4 type veins), fibrous crack - seal veins formed to accommodate continued volume expansion, via incremental fracture openings, caused by continued hydration of olivine. The ascension of serpentinite into the upper lithosphere was inferred to occur during Stage three; V5 type veins are associated with this stage. Textures and vein morphologies, representing supersaturated conditions and a decrease in temperature, were observed. Stage 4 (V6 type veins) was characterized by shear deformation features, which formed as a result of thrusting associated with the emplacement of the RGSB or Late Eocene transpression and fault reactivation along the Caribbean plate boundary. [ABSTRACT FROM AUTHOR]

Light noble gas (He–Ne–Ar) solubility has been experimentally determined in a range of materials with six-member, tetrahedral ring structures: beryl, cordierite, tourmaline, antigorite, muscovite, F-phlogopite, actinolite, and pargasite. Helium solubility in these materials is relatively high, 4 × 10 −10 to 3 × 10 −7 mol g −1 bar −1 , which is ∼100 to 100,000× greater than He solubility in olivine, pyroxene, or spinel. Helium solubility broadly correlates with the topology of ring structures within different minerals. Distinctive He–Ne–Ar solubility patterns are associated with the different ring structure topologies. Combined, these observations suggest ring structures have a strong influence on noble gas solubility in materials and could facilitate the recycling of noble gases, along with other volatiles (i.e., water, chlorine, and fluorine), into the mantle. Measurements of Ne and Ar solubility in antigorite, however, are highly variable and correlated with each other, suggesting multiple factors contribute the solubility of noble gases in serpentine-rich materials. [ABSTRACT FROM AUTHOR]

The Cerro del Almirez massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain) is composed of antigorite (Atg-) serpentinite and chlorite (Chl-) harzburgite separated by a thin reaction front formed in a palaeo-subduction setting. These ultramafic rocks preserve unique prograde structures related to the pre- and peak high-pressure event (1.6–1.9 GPa and 680–710 °C). The oldest subduction-related structures are preserved in Atg-serpentinites: a penetrative S 1 foliation and associated L 1 stretching lineation that formed in a non-coaxial regime with a top-to-the-W sense of shear. This planar–linear fabric is crosscut by olivine ± Ti-clinohumite veins, formed during the prograde breakdown of brucite and pre-metamorphic clinopyroxene, which form a system of veins decimetres long. They record embrittlement processes due to the release of 6 vol.% of H 2 O associated with brucite dehydration. The growth of prograde olivine and/or tremolite porphyroblasts is syn- to post-kinematic in relation to the S 1 foliation. Further reactions at higher temperature related to the complete breakdown of the Atg (i.e. Atg-out) are post-kinematic to S 1 . Prograde Chl-harzburgite is crosscut by sets of conjugate zones associated with grain-size reduction of olivine grains. These grain-size reduction zones are interpreted as brittle structures generated by hydrofracturing due to overpressure fluids in a compressional setting at low-differential stresses. Structures related to the exhumation process are mainly preserved in the metasedimentary host rock, where an S 2 /L 2 planar–linear fabric developed within a shear zone dominated by a non-coaxial regime with a top-to-the-W sense of movement in a transpressional regime. Peak metamorphic conditions deduced for the schists are similar in pressure (1.3–1.9 GPa) but lower in temperature (560–590 °C) compared to the ultramafic rocks in contact with them, suggesting a major shear zone at the base of the ultramafic massif during D 2 deformation in a right-handed transpressional regime. [ABSTRACT FROM AUTHOR]

We performed deformation-DIA experiments on antigorite serpentinite at pressures of 1–3.5 GPa and temperatures of between 400 and 650 °C, bracketing the stability of antigorite under subduction zone conditions. For each set of pressure–temperature ( P – T ) conditions, we conducted two runs at strain rates of 5 × 10 − 5 and 1 × 10 − 4 s − 1 . We complemented our study with a sample deformed in a Griggs-type apparatus at 1 GPa and 400 °C ( Chernak and Hirth, 2010 ), and with natural samples from Cuba and the Alps deformed under blueschist/eclogitic conditions. Optical and transmission electron microscopies were used for microstructural characterization and determination of deformation mechanisms. Our observations on experimentally deformed antigorite prior to breakdown show that deformation is dominated by cataclastic flow with observable but minor contribution of plastic deformation (microkinking and (001) gliding mainly expressed by stacking disorder mainly). In contrast, in naturally deformed samples, plastic deformation structures are dominant (stacking disorder, kinking, pressure solution), with minor but also perceptible contribution of brittle deformation. When dehydration occurs in experiments, plasticity increases and is coupled to local embrittlement that we attribute to antigorite dehydration. In dehydrating samples collected in the Alps, embrittlement is also observed suggesting that dehydration may contribute to intermediate-depth seismicity. Our results thus show that semibrittle deformation operates within and above the stability field of antigorite. However, the plastic deformation recorded by naturally deformed samples was likely acquired at low strain rates. We also document that the corrugated structure of antigorite controls the strain accommodation mechanisms under subduction conditions, with preferred inter- and intra-grain cracking along (001) and gliding along both a and b . We also show that antigorite rheology in subduction zones is partly controlled by the presence of fluids, which can percolate within the exhumation channel via deformation-induced interconnected porosity. [ABSTRACT FROM AUTHOR]

In this work, new and superb magnetic adsorbent has been tailored via one-step dry-coating process. The developed magnetic adsorbent was fabricated from Antigorite, Bentonite, and Iron oxide for the first time, showing excellent capability for adsorption of positively and negatively charged contaminants. The structural and surface characteristics of the adsorbent were analyzed by X-ray fluorescence, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared, scanning electron microscopy-energy dispersive X-ray spectrometer, and Brunauer–Emmett–Teller analysis techniques. Zeta potential and particle size were sufficiently studied. Adsorption parameters for removal of both cations (Cd2+, Pb2+) and anions (AsO 4 3− and CN−) were investigated. The maximum adsorption capacities were about 219.2 mg/g for Cd2+, 201.2 mg/g for Pb2+, 182.6 mg/g for AsO 4 3−, and 157.7 mg/g for CN−. Freundlich isotherm was the best model to describe the chemisorption behavior of the studied heavy metals. The composite exhibited high regeneration efficiencies of about 92.8% and 91.3% for removal of Cd2+ and AsO 4 3−, respectively after six reuse cycles. The composite could be efficiently implemented, easily separated and successfully recycled exhibiting 90% removal efficiency after treatment of real electroplating wastewater. [Display omitted] • New cationic-anionic magnetic adsorbent prepared from natural minerals via cost-effective dry process. • Functional and morphological characterizations confirmed the formation of the composite. • The developed composite revealed amphoteric characteristics over a wide range of pH (2–8). • Maximum adsorption capacities for Cd2+, Pb2+, AsO43- and CN- were 219.2, 201.2, 182.6 and 157.7 mg/g, respectively. • This composite exhibits excellent reusability and superior performance for treatment of electroplating wastewater. [ABSTRACT FROM AUTHOR]

Optimisation of heat treatment of serpentine is critical for the development of large-scale CO 2 sequestration by mineral carbonation. [Display omitted] • Full amorphisation of antigorite cannot be achieved before the onset of forsterite formation. • Aluminium-rich antigorite shows significantly higher thermal stability than lizardite. • The activation energy for dehydroxylation of antigorite varies between 290 and 515 kJ mol−1. • Forsterite begins to form at 700 °C when only 49% antigorite converted to amorphous phase. • High Al 2 O 3 antigorite is more suitable for mineral carbonation at high pressure conditions. Heat-treatment of serpentine minerals generates structural amorphicity and increases reactivity during subsequent mineral carbonation, a strategy for large-scale sequestration of CO 2. This study employs thermal analyses (TGA-DSC) in conjunction with in-situ synchrotron powder X-ray diffraction (PXRD) to record concurrent mass loss, heat flow, and mineralogical changes during thermal treatment of antigorite. Isoconversional kinetic modelling demonstrates that thermal decomposition of antigorite is a complex multi-step reaction, with activation energies (E α) varying between 290 and 515 kJ mol−1. We identify three intermediate phases forming during antigorite dehydroxylation, a semi-crystalline chlorite-like phase (γ -metaserpentine) showing an additional reaction pathway for the decomposition of Al 2 O 3 -rich antigorite into pyrope, and two distinct amorphous components (α and β -metaserpentine) which convert into forsterite and enstatite at higher temperature, respectively. The combination of isoconversional kinetics with in-situ synchrotron PXRD illustrates, for the first time, that local crystal structure changes, related to intermediate phase and forsterite formation, are responsible for the steep increase in activation energy above 650 °C and only 49% dehydroxylation can be achieved prior to this increase. This suggests that the high thermal stability of Al 2 O 3 -rich antigorite would severely limit Mg extraction during application of mineral carbonation under flue gas conditions. [ABSTRACT FROM AUTHOR]

It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite + brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite + brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries. [ABSTRACT FROM AUTHOR]

Abstract: Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7μgg−1, with a δ11B of +22‰) until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid, capable of providing the distinctive δ11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite–chlorite–orthopyroxene–olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ11B=−3 to +6‰), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P–T side of the isograd. 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-system conditions. Serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a mélange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of 11B-rich fluids. [Copyright &y& Elsevier]

Abstract: Seismic velocity is one of the most important sources of information about the Earth's interior. For its proper interpretation, we must have a thorough understanding of the dependence of seismic velocity on microstructural elements, including the modal composition, the crystal preferred orientation (CPO), the grain shape, the spatial distribution of mineral phases, etc. The conventional Voigt, Reuss and Hill averaging schemes take into account only the modal composition and the CPO. The information about the Earth's interior is thus poorly constrained. For a better interpretation, it is critical to have a calculation method which accounts for the grain shape and the spatial distribution of mineral phases, etc. We propose a calculation method which accounts for the grain shape of strongly dimensionally anisotropic minerals like micas and serpentines. Our method can be applied to a distributed geometrical orientation of mineral grains. Comparison was made between calculated and measured velocities in three antigorite–serpentinite mylonites. Judging from the root mean square relative error, our method provides velocities closer to measured values than the Voigt, Reuss and Hill averaging schemes. The input of the grain shape considerably improves the prediction of seismic properties. However, large discrepancies (>0.1 km/s) between measured and calculated velocities can be seen in some directions. The discrepancies might come from microstructural elements which were not considered in the calculation (layer structures and cracks). [Copyright &y& Elsevier]

This paper discusses two possible pathways of loss of the two main gases from Titan׳s post-accretional atmosphere, methane (CH4) and ammonia (NH3), by the mechanisms of thermal escape and emission from the interior coupled with thermal escape. The results give the decline of initial atmospheric gas masses to their present-day levels of 0.1bar CH4 and 1.4bar N2 (or equivalent 1.7bar NH3, as a precursor of N2). From the published data on planetary and Titan׳s accretion rates, the accretion temperature was estimated as T ac =355 to 300K. In the first 0.5–0.6Myr after accretion, Titan׳s surface cools to 150K and it takes about 5Myr to cool to near its present temperature of 94K. The present-day internal composition corresponds to the accreted Titan made of two solids, antigorite and brucite, that account for 59.5wt%, and an outer shell of an aqueous solution of NH3+(NH4)2SO4 accounting for 40.0wt%, and methane for a much smaller fraction of 0.6wt%. In thermal escape of CH4 and NH3, based on the Maxwell–Boltzmann distribution of gas-molecule velocities, the initial gas mass N 0 in the atmosphere is lost by a first-order flux, N t =N 0 exp(−kt), where t is time (yr) and k (yr−1) is a rate parameter that depends on temperature, gas molecular mass, atmosphere thickness, and Titan׳s escape velocity. The computed initial T ac =355K is too high and the two gases would be lost from the primordial atmosphere in several hundred years. However, emissions of CH4 and NH3 from the interior, at reasonable rates that do not deplete the Titan gas inventory and function for periods of different length of time in combination with thermal escape, may result in stable CH4 and NH3 atmospheric masses, as they are at the present. The periods of emissions of different magnitudes of CH4 range from 6×104 to 6×105 yr, and those of NH3 are 55,000–75,000yr. At the lower T ac =300K, thermal escape of gases alone allows their atmospheric masses to decrease from the primordial to the present-day levels in 50,000–70,000 years, when Titan׳s temperature has decreased to 245–255K. Below this temperature, the NH3 atmospheric mass is comparable to the present-day N2 mass. Thermal escape does not contradict the existence of the photolytic sink of CH4 in the cooled Titan atmosphere. The thermal escape mechanism does not require arbitrary assumptions about the timing of the start and duration of the gas emissions from the interior. [ABSTRACT FROM AUTHOR]

Abstract: 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%. [Copyright &y& Elsevier]

Abstract: This review examines studies on the dehydroxylation of serpentine minerals published in the open literature from 1945 to 2013, with brief description of earlier work. Presently, the energy cost and technological complications, required to amorphise serpentine minerals by dehydroxylation, prevent their large-scale application for sequestering of CO2. The focus of the review is on thermal dehydroxylation, although mechanical dehydroxylation by grinding and shock, as well as thermomechanical dehydroxylation are also covered. We discuss the chemical and physical transformations involving the proposed mechanisms, thermal stability, reaction kinetics, the formation of intermediates and products, associated heat requirements, factors that influence the reaction, as well as associated enhancements in both dissolution and carbonation. The primary factor controlling the availability of Mg for either extraction or carbonation is structural disorder. The review demonstrates that, activation processes must avoid recrystallisation of disordered phases to fosterite and enstatite, and minimise the partial pressure of water vapour that engenders reverse reaction. [Copyright &y& Elsevier]

Highlights: [•] EoS of antigorite at HPHT was determined by powder XRD in multi-anvil apparatus. [•] The dK/dT was reported for the first time based on a series of direct measurements. [•] The phase transition of antigorite was observed through volume softening behavior. [Copyright &y& Elsevier]

Abstract: Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep. [Copyright &y& Elsevier]

Abstract: Carbon Capture and Storage by mineralisation aims to reduce carbon dioxide emissions (CO2) by reacting CO2 with rocks rich in magnesium or calcium oxide and producing solid mineral carbonates, which can provide safe storage capacity. Recently, indirect mineral carbonation by pH swing mineralisation processes that use recyclable ammonium salts has shown promising results, but the process needs to be optimised. For example, the feasibility of this process in the presence of a mixture of NH3-salts has not been demonstrated. Accordingly, carbonation of rocks rich in magnesium and a mixture of NH4HCO3 and (NH4)2CO3 under different temperatures was investigated to reproduce a real scenario from an ammonia capture process. The highest ‘carbonation efficiency’ (expressed as the conversion of Mg ions to hydromagnesite) was 93.5% at 80°C and 1:4:3 as Mg:NH4 salts:NH3 molar ratio, while the ‘total CO2 captured’ was 62.6% under the same conditions, indicating that the process in presence of ammonium salts mixture is feasible. [Copyright &y& Elsevier]

Triaxial compression experiments on intact antigorite were performed under undrained conditions with a confining pressure of 100–400 MPa, temperature of 25–700 °C and strain rate of ∼1.5 × 10−6−10−5 s−1. We report a sequential transition in mechanical behavior from faulting to slow stick-slip behavior and then to stable sliding with increasing temperature. Syndeformational dehydration of antigorite at 100 MPa and 550–650 °C produces slow stick-slip, and the slip velocity and shear stress drop generally decrease with increasing temperature. At 100 MPa and 650 °C, the slip velocity and shear stress drop of slow stick-slip gradually decrease, implying a trend toward stable sliding, which might be related to a continuous increase in fluid pressure induced by the accumulation of fluid over time in the process of antigorite dehydration. Our results indicate that dehydration makes the sample weaker but more stable and does not cause embrittlement; therefore, an intermediate-depth earthquake in a subduction zone may be triggered by brittle deformation in strong peridotite surrounding dehydrating antigorite. Similar to the antigorite fault gouge previously studied, intact antigorite can also induce slow stick-slip under partial antigorite dehydration, indicating that slow earthquakes could be triggered in the mantle wedge of a hot subduction zone. • Intact antigorite samples were triaxially compressed at 100–400 MPa and 25–700 °C. • Warming causes these samples to be weaker but more stable, with no embrittlement. • Dehydration-induced earthquakes are likely to occur in surrounding strong peridotites. • Typical slow stick-slip events occur under partial dehydration conditions. • Such events may be responsible for slow earthquakes detected in the mantle wedge. [ABSTRACT FROM AUTHOR]

Abstract: Serpentine is considered the main hydrous mineral to cause significant weakening at the slab–mantle interface; however, despite the possible presence of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine species, respectively) at the base of the mantle wedge, the differences in rheological properties between these two serpentine types are poorly understood. To investigate the effect of serpentine speciation on slab–mantle decoupling, we performed a series of two-layer shear deformation experiments on liz/ctl, antigorite, and olivine at P =1GPa and T =250–300°C, using a Griggs-type solid-medium apparatus. With increasing shear strain (γ up to ~5), the liz/ctl and antigorite samples evolved to deform predominantly by slip or glide on the (001) basal plane, whereas the olivine sample deformed by a combination of brittle and crystal-plastic mechanisms. The contrasting amount of rotation of strain markers between layers demonstrates that the shear strain of liz/ctl is significantly lower than that of antigorite, possibly arising from differences in interlayer bond strength. The strain contrasts between serpentines and olivine were γ liz/ctl/γ ol =9–12 and γ atg/γ ol =1–2. Our results indicate that assuming a thin, serpentinized layer upon a subducting slab, significant decoupling can occur only where liz/ctl is stable in the layer. Thus, the distribution of serpentine species in the hydrated mantle wedge is a critical factor that controls the degree of decoupling along the slab–wedge interface, at depths below the seismogenic zone. [Copyright &y& Elsevier]

Abstract: Serpentinites are one of the major components of the oceanic lithosphere and are stable in the slab and the mantle wedge up to 100–150km depth in subduction zones. During oceanic mantle hydration and alteration, they trap trace and fluid mobile (FME: B, Li, As, Sb, Rb, Ba, Cs, Sr, U and Pb) elements that participate to elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context. The Lanzo massif is an eclogitized oceanic lithosphere that preserved its oceanic structure and recorded different steps of serpentinization/de-serpentinization from oceanic lizardite to prograde antigorite in subduction context, up to its dehydration and secondary olivine crystallization, and finally retrograde antigorite during massif exhumation. It constitutes a suitable place to study trace element behavior during serpentinization/de-serpentinization processes and associated chemical transfers between the different envelopes of the oceanic lithosphere and the mantle wedge. Geochemical analyses of serpentine and associated minerals show that the serpentinization/de-serpentinization of the Lanzo massif took place in a relatively closed system without significant trace element transfer between the different parts of the oceanic lithosphere. In the deeper part of the lithosphere, from the slightly serpentinized mantle peridotites (SSP, <20% serpentinization) to the paleo-Moho, composed of massive serpentinites (MS, 80% serpentinization), the trace elements mobility is reduced. The chemical composition of lizardite and antigorite is homogenized with the local degree of serpentinization: in SSP, serpentine veins composition is inherited from the host mineral while, in MS, their composition is homogenous between destabilized phases at the scale of the outcrop (~5m). In the shallowest part of the oceanic lithosphere, from the paleo-Moho to the oceanic paleo-seafloor, the serpentinites are foliated (FS, >90% serpentinization). In that zone, the alpine deformation enhances the mobility of trace elements and permits their redistribution and the homogenization of antigorite composition at massif scale. Locally, in the SSP and MS, the crystallization of metamorphic veins of ~1–2m corresponds to channelized fluid flows that allowed fluid transfers – and thereby trace elements – to longer distance. The successive crystallizations of antigorite and then olivine are accompanied by a diminution of some FME (B, Li, As, Sb, Ba, Rb) and Eu contents attesting that these elements are removed from slab to mantle wedge during subduction. [Copyright &y& Elsevier]

Highlights: [•] An in situ study of silicate mineral reactivity with dry and wet CO2 was performed. [•] The extents of reaction were strongly dependent on water concentration. [•] When water was present, thin water films formed on the surface of the minerals. [•] We show the first direct evidence of bicarbonate dissolved in the water films. [•] Antigorite's low reactivity was attributed to slow, incongruent mineral dissolution. [Copyright &y& Elsevier]

Serpentine minerals in natural samples are dominated by lizardite and antigorite. In spite of numerous laboratory experiments, the stability fields of these species remain poorly constrained. This paper presents petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P<4kbar, T~200–300°C) to eclogite facies conditions (P>20kbar, T>460°C) along a subduction geothermal gradient. We use the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter in associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures. We show that below 300°C, lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, lizardite is progressively replaced by antigorite at the grain boundaries through dissolution–precipitation processes in the presence of SiO2 enriched fluids and in the cores of the lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C. [ABSTRACT FROM AUTHOR]