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

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

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

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

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

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

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

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

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

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]

The determination of the mechanical properties of serpentinites is essential toward the understanding of the mechanics of faulting and subduction. Here we present the first in situ tensile tests on antigorite in a transmission electron microscope. A push‐to‐pull deformation device is used to perform quantitative tensile tests, during which force and displacement are measured, while the evolving microstructure is imaged with the microscope. The experiments have been performed at room temperature on 2 × 1 × 0.2 μm3 beams prepared by focused ion beam. The specimens are not single crystals despite their small sizes. Orientation mapping indicated that several grains were well oriented for plastic slip. However, no dislocation activity has been observed even though the engineering tensile stress went up to 700 MPa. We show also that antigorite does not exhibit a purely elastic‐brittle behavior since, despite the presence of defects, the specimens accumulate permanent deformation and did not fail within the elastic regime. Instead, we observe that strain localizes at grain boundaries. All observations concur to show that under these experimental conditions, grain boundary sliding is the dominant deformation mechanism. This study sheds a new light on the mechanical properties of antigorite and calls for further studies on the structure and properties of grain boundaries in antigorite and more generally in phyllosilicates. Key Points: No dislocation activity has been observed although tensile stress reached 700 MPaThe behavior of antigorite is not purely elastic‐brittle despite the presence of defects in the specimens (pores and damage)The main deformation mechanism is by grain boundary sliding [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]

The transition between antigorite‐serpentinite and chlorite‐harzburgite at Cerro del Almirez (Betic Cordillera, Southern Spain) exceptionally marks in the field the front of antigorite breakdown at high pressure (~16–19 kbar) and temperature (~650°C) in a paleosubducted serpentinite. These ultramafic lithologies enclose three types of metarodingite boudins of variable size surrounded by metasomatic reaction rims. Type 1 Grandite‐metarodingite (garnet+chlorite+diopside+titanite±magnetite±ilmenite) mainly crops out in the antigorite‐serpentinite domain and has three generations of garnet. Grossular‐rich Grt‐1 formed during rodingitization at the seafloor (<2 kbar, ~150–325°C, ~FMQ buffer). During subduction, the alternating growth of Grt‐2b (richer in andradite and pyralspite components than Grt‐1) and Grt‐3 (very rich in andradite component) reflects the change from internally buffered metamorphic conditions (>10 kbar, ~350–650°C, ~FMQ buffer) to influx events of oxidizing fluids (fO2 ~HM buffer) released by brucite breakdown in the host antigorite‐serpentinite. Type 2 Epidote‐metarodingite (epidote+diopside+titanite±garnet) derives from Type 1 and is the most abundant metarodingite type enclosed in dehydrated chlorite‐harzburgite. Type 2 formed by increasing μSiO2 (from −884 to −860 kJ/mol) and decreasing μCaO (from −708 to −725 kJ/mol) triggered by the flux of high amounts of oxidizing fluids during the high‐P antigorite breakdown in serpentinite. The growth of Grt‐4, with low‐grandite and high‐pyralspite components, in Type 2 metarodingite accounts for progressive reequilibration of garnet with changing intensive variables. Type 3 Pyralspite‐metarodingite (garnet+epidote+amphibole+chlorite±diopside+rutile) crops out in the chlorite‐harzburgite domain and formed at peak metamorphic conditions (16–19 kbar, 660–684°C) from Type 2 metarodingite. This transformation caused the growth of a last generation of pyralspite‐rich garnet (Grt‐5) and the recrystallization of diopside into tremolitic amphibole at decreasing fO2 and μCaO (from −726 to −735 kJ/mol) and increasing μMgO (from −630 to −626 kJ/mol) due to chemical mixing between the metarodingite and the reaction rims. The different bulk Fe3+/FeTotal ratios of antigorite‐serpentinite and chlorite‐harzburgite, and of the three metarodingite types, reflect the highly heterogeneous oxidation state of the subducting slab and likely point to the transfer of localized oxidized reservoirs, such as metarodingites, into the deep mantle. [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]

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]

Fluorine (50-650 ppm), bromine (0.03-0.3 ppm), iodine (0.03-0.4 ppm), boron (20-100 ppm) and nitrogen (5-45 ppm) concentrations are elevated in antigorite-serpentinites associated with the Tso Morari ultrahigh-pressure unit (Himalayas) exhumed from >100 km depth in the mantle wedge. These fluid-mobile elements are likely released with fluids from subducted marine sediments on the Indian continental margin to hydrate overlying forearc serpentinites at shallow depths. Of these, F and B appear to remain in serpentinites during the lizardite-antigorite transition. Our results demonstrate serpentinites as transient reservoirs of halogens, B, and N to at least 100 km depth in the mantle wedge, and likely deeper in colder slabs, providing a mechanism for their transport to the deeper mantle. [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]

We formulate a model for coupled deformation and dehydration of antigorite, based on a porosity-dependent yield criterion and including shear-enhanced compaction. A pore pressure and compaction instability can develop when the net volume change associated with the reaction is negative, i.e., at intermediate depth in subduction zones. The instability criterion is derived in terms of the dependence of the yield criterion on porosity: if that dependence is strong, instabilities are more likely to occur. We also find that the instability is associated with strain localization, over characteristic length scales determined by the hydraulic diffusivity, the elasto-plastic parameters of the rock, and the reaction rate. Typical lower bounds for the localization length are of the order of 10 to 100 for antigorite dehydration and deformation at 3 GPa. The fluid pressure and deformation instability is expected to induce stress buildup in the surrounding rocks forming the subducted slab, which provides a mechanism for the nucleation and propagation of intermediate-depth earthquakes. [ABSTRACT FROM AUTHOR]

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks. [ABSTRACT FROM PUBLISHER]

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]

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: 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]

Abstract: Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10–200km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up to faulting pattern needs to be established, especially for water cycling to the transition zone in the core of the slab. [Copyright &y& Elsevier]

Abstract: The Cerro del Almirez ultramafic massif is composed of antigorite serpentinite and chlorite harzburgite separated by a narrow transitional zone that marks the front of prograde serpentinite-dehydration at high pressure in a paleo-subduction setting. Concentrations of Sc and V indicate that the peridotite precursor of serpentinite experienced up to 20% partial melting in the spinel stability field at −2<Δlog fO2 FMQ <0. Peridotites underwent intense seafloor serpentinization in a fluid-dominated system. Olivine hydrolysis at ~200°C and pyroxene serpentinization at >350–400°C remobilized Ca and REE (especially LREE and Eu) and caused the progressive enrichment of Cs, Rb, Ba, U and Pb and locally the crystallization of talc by silica fluid addition. Transformation to antigorite serpentinite upon subduction led to Sr depletion, and Ti, Tm, Yb and Lu were remobilized at the sample scale during fluid-assisted crystallization of titanian clinohumite. The high-pressure prograde breakdown of antigorite to chlorite harzburgite preserved the REE fractionations and the characteristic negative Eu anomaly of precursor serpentinites. Relative enrichment of Th–U–Nb–Ta–Pb–Sr in chlorite harzburgite cannot be balanced by closed-system dehydration of serpentinite indicating that dehydration occurred in an open system involving external fluids equilibrated with crustal sources, such as rodingites and/or metasediments. Prograde chlorite harzburgite acts as a sink of these elements during open-system fluid flux along pathways of fluid discharge in the subducted slab. This process results in the recycling into the deep convective asthenospheric mantle of prograde harzburgite enriched in Th, U, HFSE and Pb relatively to oceanic depleted peridotite. This signature and the preservation in dehydrated subducted serpentinite of negative Eu anomaly and C–O–H–S crustal-like isotopic compositions make prograde serpentinite a potential reservoir for the balance of the Earth's HFSE budget and a potential source for diamonds with a low-pressure inherited geochemical signature. [Copyright &y& Elsevier]

Abstract: We report electron backscatter diffraction (EBSD) measurements of antigorite crystal preferred orientations (CPOs) from 7 samples of antigorite schist from serpentinite mélanges adjacent to the Motagua fault system in central Guatemala. The CPOs range from diffuse girdle to point distributions of (001)atg. Girdle distributions of (001)atg are not expected from deformation theory or experiments, suggesting that they are a result of growth rather than deformation, and may thus be topotactically related to the CPO of the olivine from which the antigorite grew. The calculated seismic anisotropy ranges from 6% to 28% in V P, and 5% to 33% in V S, and is highest for samples with a point maximum of (001)atg. For all samples the minimum V P corresponds to the pole to (001)atg, and the maximum V P occurs within the foliation for samples with a clearly defined foliation. Trench-parallel shear-wave splitting observations for subduction zones can best be explained by a combination of olivine B-type CPO and antigorite oriented with (001) parallel to the foliation; only a relatively thin (~20km) zone of hydrated mantle is required to explain the observed splitting times. [Copyright &y& Elsevier]

Outcrops of spinifex-textured metaperidotite in the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogitefacies (1.6-1.9 GPa, ≈680 °C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown in thin section, and microprobe analysis (carefully avoiding accidental inclusions) shows contents of Cr2O3 and TiO2 in the range 0 to 0.4 and 0 to 0.1 weight percent respectively. These amounts of Cr2O3 and TiO2 are matched by olivine in peridotite xenoliths from the mantle, komatiites, and pseudotachylytes. According to laboratory experiments, at 2 GPa and a likely redox state near QFM, these levels of Cr2O3 signify equilibration at temperatures in excess of 1600 °C. Curvature of the elongate olivines, and outcrop-scale alignment of olivine splays suggest a connection with komatiites. Two early studies at Cerro del Almirez emphasized this connection, but all papers from the late-1990s to the present time have advocated a metamorphic origin, analogous to jackstraw-olivine rocks. Some 5 to 6 weight percent or 13 to 15 volume percent of H2O was released at the isograd. We hypothesize that this dehydration water induced embrittlement of chlorite-metaharz-burgite, and frictional slip in shear zones raised the temperature by several hundred degrees C above the ambient, more than enough to melt wet or dry ultramafic rock and form spinifex texture on rapid cooling. The amounts of crystallized melt so formed are orders-of-magnitude larger than found in peridotite pseudotachylyte, but are perhaps not totally inconsistent with dehydration-induced seismic events repeated over a protracted period of time during Miocene subduction of an oceanic plate. The transient ultramafic melts yielded on rapid cooling up to 12 cm long olivines and interstitial aggregates of parallel to radiating orthopyroxene prisms, followed by crystallization from glass of assorted eclogite-facies minerals (chlorite, tremolite, talc). [ABSTRACT FROM AUTHOR]

Abstract: We have inferred the deformation mechanisms of antigorite by high-resolution EBSD mapping of samples from Cerro del Almirez ultramafic massif (Betic Cordillera, SE Spain). Textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600–630°C and 1.6–1.9GPa. Deformation was followed by static annealing of antigorite at ca. 680°C. The Crystal Preferred Orientation (CPO) of antigorite is characterised by a strong alignment of (001) poles normal to the foliation plane and a weaker, but clear, parallelism between [100] axes and the macroscopic lineation defined by the elongation of magnetite aggregates. Analysis of misorientations across subgrains shows predominance of [010] rotation axis, consistent with activation of the [100](001) slip system. However, tilt subgrain boundaries subparallel to (100), probably formed by edge dislocations of this system, are subsidiary. Most subgrain boundaries are subparallel to (001) planes. They are interpreted as (001) twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins. Modelling of the evolution of the antigorite CPO using a lower bound approach and considering different deformation regimes and sets of basal and non-basal slip systems has shown that intensities of [100] and [010] maxima reflect the relative strength of the antigorite [100](001) and [010](001) systems. Activation of other basal or non-basal slip systems does not change significantly the CPO patterns, but results in less concentrated CPO. 3D transpression models better reproduce the CPO of natural antigorite serpentinites. We propose that the widespread occurrence of strong CPO in high-pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on [hk0](001), together with the activation of twinning, implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge. [Copyright &y& Elsevier]

Abstract: An experimental study of serpentine decomposition at high pressure (4.5 GPa) was carried out to elucidate if water can be preserved in the system in the form other than structural admixtures in minerals. This problem is of interest because it is water that plays a leading role in the melting in a subducted slab and a mantle wedge. To estimate the possible content of an aqueous fluid in deep-seated rocks, a BARS pressless split-sphere apparatus was used in complex with thermobarogeochemistry and gas chromatography. It has been established that the serpentine decomposition is accompanied by the release of water, which concentrates in inclusions in the produced minerals (olivine and orthopyroxene) and their interstices. Chromatographic analysis with a stepwise heating of samples showed that most of the released water is localized in the interstices, and the rest is conserved in fluid inclusions in the minerals. The produced solid phases conserve 0.13 to 2.43 wt.% fluids as inclusions, with water amounting to 0.1–2.06 wt.%. The content of inclusions determined by microscopic examination falls in this region. Since the mobility of the fluid conserved as inclusions in the olivine and orthopyroxene is significantly lower than that in the interstices, this fluid might be better preserved in olivine-containing rocks subsided to depth. [Copyright &y& Elsevier]

Abstract: We studied a unique chrysotile–antigorite serpentinite, drilled on Deep Sea Drilling Project Leg 84 (Site 566) in the Guatemala forearc. Our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction. Chrysotile of the studied serpentinite, formed by the hydration of an upper mantle peridotite precursor, is partially replaced by antigorite (alone) which also occurs in 0.5mm wide unoriented veins crosscutting the rock. Based on textural relationships and the P–T–X stability of the rock forming phases, the replacement of chrysotile by antigorite occurred at T<300°C, due to interaction between the chrysotile–serpentinite and an aqueous fluid. A comparison of the chemical compositions of reactant and product phases reveals that about 90% of the Cl, more than 80% of the B and about 50% of the Sr hosted originally by chrysotile was lost during fluid-assisted chrysotile-to-antigorite transformation and accompanying partial dehydration, and documents the much lower affinity of antigorite for trace element uptake than that of chrysotile. The fluid-assisted chrysotile-to-antigorite transformation and associated trace element loss documented here can occur in the shallow (<30km) region of subduction zones. This transformation decreases notably the Cl and B inventory of subducting serpentinites, which are regarded as one of the most important carriers of these elements into subduction zones. The evolution of serpentinites during initial subduction stages thus appears to be critical in the recycling of specific trace elements such as B or Cl from forearc to subarc depths. [Copyright &y& Elsevier]

Antigorite peridotite, Cr-magnetite dunite, chlorite harzburgite, and other ultramafic rock fragments included in the Green Knobs diatreme in NW New Mexico are used to investigate the evolution of the mantle wedge during low-angle subduction. The diatreme, part of the Navajo Volcanic Field, has a matrix of serpentinized ultramafic microbreccia (SUM). The meta-peridotite inclusions have been characterized by petrographic, electron microprobe, and bulk-rock analysis. The assemblage antigorite–diopside–olivine formed and was stable in the mantle. In a Cr-magnetite dunite, olivine compositions span the range Fa9–3, and chlorite-rich pockets contain extraordinarily pure pyrope (Py94·4Alm5·5Gr0·1). The Cr-magnetite dunite is a product of prograde metamorphism of a brucite-bearing serpentinite. Second generations of olivine in that dunite and other samples formed during a sharp temperature increase caused by intrusion of the magma that triggered the eruption. Histories of lawsonite eclogite, garnetite, and other inclusions in SUM diatremes have been interpreted to provide context: some are from the lithospheric part of the mantle wedge, hydrated near the trench, tectonically eroded and transported some 700 km in a mélange, and emplaced below the Colorado Plateau during low-angle subduction of the Farallon plate. The Cr-magnetite dunite was also derived from that mélange, whereas some other inclusions represent mantle hydrated in place above the slab. Tectonic erosion of the Colorado Plateau mantle lithosphere, or serpentinite diapirism into that lithosphere, or both, accompanied the low-angle subduction. Serpentinization far from the trench and low-angle transport of parts of the lithospheric mantle wedge are consistent with models of the Laramide orogeny and may be common during low-angle subduction. [ABSTRACT FROM PUBLISHER]

Abstract: Antigorite dehydration has been studied in situ using time-resolved X-ray diffraction (XRD) in the 1.1–5.5 GPa pressure range. The sequence of XRD spectra reveals that the high-temperature stability limit of antigorite is decreased by 50–100 °C under H2O-unsaturated conditions. The decomposition of antigorite into the high-temperature products, forsterite+enstatite, proceeds via an intermediate assemblage of forsterite and a “talc-like” phase, observed within a temperature interval of 130±20 °C. The analysis of the transformation-time data using the Avrami model suggests that the breakdown of antigorite and the “talc-like” phase is kinetically controlled by surface growth processes at the edges of grains. The overall transformation rates are 10 to 100 times faster than those observed in water-saturated experiments, and show that the H2O activity is a strong driving force for dehydration. The breakdown of antigorite in subducting oceanic mantle with such kinetics would result in a fluid discharge rate of the order of 10−6 to 10−8 s−1. This is faster than the viscous relaxation of serpentinites, and could lead to brittle failure or weakening of pre-existing faults. This provides an explanation for the seismicity in the lower plane of double seismic zones. [Copyright &y& Elsevier]

We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine–orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine–orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine–orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources.Laser ablation ICP-MS analyses of fluid inclusions in the olivine–orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4–2 wt.% NaClequiv) as internal standards, yielding maximum average fluid/rockDB∼5 and fluid/rockDLi∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4–8 wt.% NaClequiv. The antigorite breakdown produces less saline fluids (0.4–2 wt.% NaClequiv) and is characterized by lower partition coefficients for Cl (25–60) and B (12–30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. fluid/rockDB/fluid/rockDCl<1 (∼0.5) indicates that Cl preferentially partitions into the evolved fluids relative to B and that the B/Cl of fluids progressively increases with increasing depths and temperatures.Despite light element release in fluids, appreciable B, Cl and Li are still retained in chlorite, olivine and Ti-clinohumite beyond the antigorite stability field. This permits a bulk storage of about 10 ppm B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in mantle reservoirs. Chlorite is the Cl repository and its stability controls the Cl and H2O budget beyond the antigorite stability; B and Li are bound in olivine and clinohumite. The subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources, potentially introducing anomalies in the upper mantle. [Copyright &y& Elsevier]

• Isua mantle peridotite 3.7 Ga UHP assemblages resemble modern subduction rocks. • UHP assemblages overprinted variably-serpentinised abyssal peridotites. • Incompletely serpentinised rocks preserve relict Fo 91-92 mantle olivine. • Total serpentinisation gave early serpentine + magnetite assemblages. • <4.0 δ18O of the Fo 97-98 olivine demonstrates recycling of volatiles into the deep Earth. The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens 'A' variant is dominated by an aggregate of Fo 91-92 olivine with δ18O VSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo 90.5. The northern lens 'B' contains coarser-grained Fo 96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo 96-98 olivine has δ18O VSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals. We interpret that the Fo 90-92 versus the Fo 96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo 90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18O VSMOW + 5.4‰ olivine. The Fo 96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18O VSMOW Fo 96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism. In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo 96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth's history. [ABSTRACT FROM AUTHOR]

The Monte Maggiore ophiolitic unit of Alpine Corsica consists of an intact to variably hydrated spinel lherzolite intruded by Jurassic gabbro dykes. A widespread serpentinization, marked by low-pressure serpentine polymorphs, results from sea water interaction with mantle rocks during Jurassic sea-floor spreading of the Piemonte-Liguria ocean. A second serpentinization event, marked by the presence of antigorite, is related to the Cretaceous to Paleogene subduction of the Piemonte-Liguria oceanic lithosphere. Tectonic pseudotachylyte veins locally crosscutting the unit, display several original characteristics. (1) They are hosted by serpentinite, showing that seismic ruptures can propagate through hydrated mantle rocks. (2) The host serpentinite in contact with pseudotachylyte veins shows a thin (500 μm or less) rim of secondary olivine newly crystallized at the expense of serpentine. The heat necessary for serpentine dehydration is likely provided by the frictional melt. Antigorite-bearing clasts (themselves partly dehydrated along their boundaries) reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability. This indicates that frictional meting occurred at pressures between 0.7 and 0.85 GPa, that is, at ~20 to ~30 km depths in the subducting Piemonte-Liguria oceanic slab. Since the dehydration of serpentine into olivine is only observed at the host-rock selvages of pseudotachylyte veins, it cannot be related to the crossing, by the subducting slab, of the regional 610 ± 100 °C isotherm of the subduction zone, temperature at which antigorite starts to dehydrate into olivine. An estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake, the average amount is about 1.36 L/m2 or 1.36 × 105 m3 for the entire rupture surface (assumed to be 100 km2). If the released water is not incorporated in the nearby frictional melt, it can flow away from the slip zone and contribute to the various fluid-rock interactions active in subduction zones. Alternatively, if the water cannot escape from the slip zone and if the pore pressure is locally high enough, it could trigger aftershocks following the initial seismic rupture. • Seismic rupture can propagate through serpentinized peridotite in subducting slabs at 20 to 30 km depths. • Co-seismic frictional melting can locally dehydrate antigorite into olivine. • Co-seismic dehydration appears as a possible source of water in subducting slabs. [ABSTRACT FROM AUTHOR]

Serpentinite from the North Qaidam ultrahigh pressure metamorphic belt in northern Tibet is studied to provide insight into petro-geochemical evolution of serpentinite from seafloor hydration to subduction zone metamorphism. The North Qaidam serpentinite can be divided into undeformed lizardite serpentinite that was not severely overprinted during subduction and deformed antigorite serpentinite that was well recrystallized during subduction. Petrological and geochemical analyses demonstrate that the serpentinite was originally melt-percolated refractory abyssal harzburgite. Considering the local geodynamic setting, it is inferred that the serpentinite probably originated from the oceanic lithosphere that subducted before continental subduction. Relatively uniform low δ18O (4.0‰–4.5‰) of the antigorite serpentinite indicates high temperature hydrothermal alteration of protolith harzburgite by seawater. In contrast, much lower δ18O (0.6‰–2.7‰) of the lizardite serpentinite is ascribed to exchange with meteoric water at the Earth's surface. Relict serpentinization textures are well preserved in the lizardite serpentinite, with reactions of olivine to lizardite and magnetite, pyroxenes to talc, tremolite, and lizardite, and spinel to chromite and chlorite. The replacement of lizardite by antigorite during subduction mostly initiates along grain boundaries and interconnecting veinlets, implying fluid-assisted transformation of lizardite into antigorite. Partial decomposition of antigorite produces magnesian secondary olivine in the antigorite serpentinite, while direct breakdown of metastable lizardite generates ferroan secondary olivine in the lizardite serpentinite. The serpentinite is enriched in fluid-mobile elements, with U primarily accumulated during seafloor alteration and alkalis notably introduced by sedimentary fluids at bending faults or in accretionary wedge. Infiltration by fluids equilibrated with sediments is also supported by highly elevated 87Sr/86Sr of the serpentinite. Moreover, 87Sr/86Sr of the antigorite serpentinite (0.710649–0.713996) is higher than that of the lizardite serpentinite (0.707184–0.708502), which implies more intense interaction of sedimentary fluids with the former than the latter. However, the lizardite serpentinite contains more alkalis and less U than the antigorite serpentinite, which indicates that large proportions of alkalis were lost during partial dehydration of serpentinite, while U was not released. • Subducted serpentinite preserves relict textures of hydration and metamorphism. • Antigorite and lizardite breakdown produce Mg- and Fe-rich olivine, respectively. • Fluid-mobile elements in serpentinite are stepwise enriched and released. [ABSTRACT FROM AUTHOR]

Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from serpentinites during the lizardite–antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for serpentinite thickness of the metamorphosed slab (500 m), antigorite serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle. [ABSTRACT FROM AUTHOR]

Phase relations for a natural serpentinite containing 5 wt% of magnetite have been investigated using a multi-anvil apparatus between 6.5 and 11 GPa and 400-850 °C. Post-antigorite hydrous phase assemblages comprise the dense hydrous magnesium silicates (DHMSs) phase A (11.3 wt% H2O) and the aluminous phase E (Al-PhE, 11.9 wt% H2O). In addition, a ferromagnesian hydrous silicate (11.1 wt% H2O) identified as balangeroite (Mg,Fe)42Si16O54(OH)40, typically described in low pressure natural serpentinite, was found coexisting with Al-PhE between 650 and 700 °C at 8 GPa. In the natural antigorite system, phase E stability is extended to lower pressures (8 GPa) than previously reported in simple chemical systems. The reaction Al-phase E = garnet + olivine + H2O is constrained between 750 and 800 °C between 8 and 11 GPa as the terminal boundary between hydrous mineral assemblages and nominally anhydrous assemblages, hence restricting water transfer into the deep mantle to the coldest slabs. The water storage capacity of the assemblage Al-PhE + enstatite (high-clinoenstatite) + olivine, relevant for realistic hydrated slab composition along a relatively cold temperature path is estimated to be ca. 2 wt% H2O. Attempts to mass balance run products emphasizes the role of magnetite in phase equilibria, and suggests the importance of ferric iron in the stabilization of hydrous phases such as balangeroite and aluminous phase E. [ABSTRACT FROM AUTHOR]