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The mechanical behaviour of antigorite strongly influences the strength and deformation of the subduction interface. Although there is microstructural evidence elucidating the nature of brittle deformation at low pressures, there is often conflicting evidence regarding the potential for plastic deformation in the ductile regime at higher pressures. Here, we present a series of spherical nanoindentation experiments on aggregates of natural antigorite. These experiments effectively investigate the single-crystal mechanical behaviour because the volume of deformed material is significantly smaller than the grain size. Individual indents reveal elastic loading followed by yield and strain hardening. The magnitude of the yield stress is a function of crystal orientation, with lower values associated with indents parallel to the basal plane. Unloading paths reveal more strain recovery than expected for purely elastic unloading. The magnitude of inelastic strain recovery is highest for indents parallel to the basal plane. We also imposed indents with cyclical loading paths, and observed strain energy dissipation during unloading–loading cycles conducted up to a fixed maximum indentation load and depth. The magnitude of this dissipated strain energy was highest for indents parallel to the basal plane. Subsequent scanning electron microscopy revealed surface impressions accommodated by shear cracks and a general lack of dislocation-induced lattice misorientation. Based on these observations, we suggest that antigorite deformation at high pressures is dominated by sliding on shear cracks. We develop a microphysical model that is able to quantitatively explain Young’s modulus and dissipated strain energy data during cyclic loading experiments, based on either frictional or cohesive sliding of an array of cracks contained in the basal plane. This article is part of a discussion meeting issue ‘Serpentinite in the earth system’

The Cerro del Almirez massif (Spain) represents a unique fragment of serpentinized oceanic lithosphere that has been first equilibrated in the antigorite stability field (Atg-serpentinites) and then dehydrated into chlorite–olivine–orthopyroxene (Chl-harzburgites) at eclogite facies conditions during subduction. The massif preserves a dehydration front between Atg-serpentinites and Chl-harzburgites. It constitutes a suitable place to study redox changes in serpentinites and the nature of the released fluids during their dehydration. Relative to abyssal serpentinites, Atg-serpentinites display a low Fe 3+ /Fe Total(BR) (=0.55) and magnetite modal content (=2.8–4.3 wt%). Micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy measurements of serpentines at the Fe–K edge show that antigorite has a lower Fe 3+ /Fe Total ratio (=0.48) than oceanic lizardite/chrysotile assemblages. The onset of Atg-serpentinites dehydration is marked by the crystallization of a Fe 3+ -rich antigorite (Fe 3+ /Fe Total  = 0.6–0.75) in equilibrium with secondary olivine and by a decrease in magnetite amount (=1.6–2.2 wt%). This suggests a preferential partitioning of Fe 3+ into serpentine rather than into olivine. The Atg-breakdown is marked by a decrease in Fe 3+ /Fe Total(BR) (=0.34–0.41), the crystallization of Fe 2+ -rich phases and the quasi-disappearance of magnetite (=0.6–1.4 wt.%). The observation of Fe 3+ -rich hematite and ilmenite intergrowths suggests that the O 2 released by the crystallization of Fe 2+ -rich phases could promote hematite crystallization and a subsequent increase in f o 2 inside the portion of the subducted mantle. Serpentinite dehydration could thus produce highly oxidized fluids in subduction zones and contribute to the oxidization of the sub-arc mantle wedge.

Dehydration reactions in subducted slabs have long been correlated with embrittlement and intermediate‐depth earthquakes. However, the physical process of dehydration embrittlement remains unclear due to the complex and poorly constrained interactions between reaction progress, fluid pressure evolution, and deformation. Here we aim to quantify these interactions during antigorite dehydration with 2D hydro‐mechanical‐chemical numerical modeling and explore whether the reaction causes stress perturbations potentially leading to earthquakes. Negative total volume change during the reaction acts toward the relaxation of fluid overpressures, decreasing the chance of embrittlement. The reaction zone is the least likely to fracture due to reaction‐induced weakening and the locally larger increase of total pressure compared to fluid pressure. However, weakening also generates fluid overpressure zones and may induce strain localization/runaway processes potentially leading to brittle failure. Our results also imply that antigorite dehydration could be both the cause and effect of fast deformation in subducted slabs.

Serpentinization along subducting plates induces mechanical mixing of diverse rocks and interaction with compositionally distinct fluids, which is often accompanied by the formation of rare mineral species. In this study, newly discovered baddeleyites in the Higuchi serpentinite body (HSB), Japan, are described. The HSB occurs as a 15 × 8 m outcrop surrounded by high-P/T metapelite, and baddeleyite was collected from only one serpentinite block. The baddeleyite appear as aggregates exhibiting angular or subrounded shapes with sizes of up to 2 mm in length. The aggregates are composed of acicular baddeleyite surrounded by porous zircon rims. Both the baddeleyite and the zircon yielded U–Pb ages of ca. 96 Ma, corresponding to the peak metamorphic age of the region. Within the aggregates, Th-enriched areas with sizes of less than 20 μm were observed. The baddeleyite aggregates show enrichment of light rare earth elements with positive Eu anomalies. Based on thermodynamic stability relationships, the baddeleyite aggregates are inferred to have originated as zircon megacrysts, which were mechanically incorporated into the ultramafic rock and subsequently metamorphosed during serpentinization. Subsequent metasomatism associated with carbonation and pervasive silicification led to the formation of the zircon rim and trace-elemental maldistribution. This study demonstrates significant high field strength elements (HFSE) heterogeneity on scales ranging from millimeters to micrometers within serpentinite along subduction zones.

The unit-cell parameter of antigorite (usually expressed as the polysome value) has been determined as a function of temperature ( ) and pressure ( ) in the range of 600–650 °C, 25–45 kbar in weeklong piston-cylinder experiments. A well-characterized natural antigorite (with = 16 and less abundant = 15) was used as a starting material that coexisted with olivine, chlorite, Ti-humite, and aqueous fluid at run conditions. Transmission electron microscope (TEM) measurements on selected focused ion beam (FIB) wafers showed that antigorite values after the experiments varied between 14 and 22. More than 40 punctual analyses for each run condition were acquired to determine the range and the primary value. The most frequent antigorite -value decreased systematically from 17–19 at 600 °C to 15–16 at 650 °C. The spacing of the -isolines is getting narrower as the antigorite breakdown reaction is approached. The topology of the -isolines is similar to that previously characterized for the simple MgO-SiO -H O (MSH) system. However, the isolines are shifted to about 50–100 °C higher temperatures due to the incorporation of Al into antigorite. Powder samples and FIB wafers of natural antigorite from the Tianshan UHP belt (China) with peak metamorphic conditions of ~35 kbar, ~520 °C were also investigated with TEM. Low Al-antigorite formed at peak metamorphic conditions displays a peak value of 20–21, whereas high-Al antigorite formed during isothermal decompression displays a lower value of 19. Combination of our results with the published data of values from metamorphic antigorite that experienced various conditions allowed construction of a diagram that can be used in future studies to better constrain formation conditions of serpentinites. The decrease of values and the increase of Al in antigorite with increasing temperature result in small, continuous dehydration whereby the H O content of antigorite changes from 12.4 to 12.1 wt%. Therefore, it is expected that a pore fluid is present during the prograde deformation of serpentinites. TEM observations showed that antigorite adjusted its Al content by segregation of chlorite at the nanoscale. Together with the observation that multiple values are always present in a single sample, this result indicates that full equilibration of antigorite at the micrometer-scale is rare, with important implications for the interpretation of geochemical signatures obtained by in situ techniques.

Antigorite dehydration experiments were performed under ambient pressure using a non-isothermal thermogravimetric analysis. Antigorite, with a grain size of 5-10 µm, was analyzed using heating rates of 10, 15, 20, and 25 K/min at temperatures of up to 1260 K. The results show that the progress of the dehydration reaction varies with the heating rate, and the dehydration reaction of antigorite occurs within a temperature range of 800-1050 K. Several models were used to fit the dehydration results, and the double-Gaussian distribution activation energy model (2-DAEM) yielded the best fit to the experimental data. The dehydration kinetics of antigorite follow 2-DAEM, and there is a compensation effect between the pre-exponential factor and the average activation energy. The activation energy of the first step of antigorite dehydration stretches over a wide interval; the second step has a significantly higher activation energy, distributed over a narrower interval. We determined that the release rate of H2O is 8.0×10-5 and 2.1×10-3m3fluidm3rocks-1 at 893 and 973 K, respectively, which are near the onset temperature for the isothermal dehydration reaction. Our results indicate that antigorite dehydration is fast enough to induce mechanical instabilities that may trigger seismicity in the lower plane of the double seismic zone.

Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, southern Spain). At 630–660°C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680°C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al 2 O 3 ) and chromium contents (0.59 wt% Cr 2 O 3 ) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70°C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680–710°C and 1.6–1.9 GPa.

Shear‐sliding tests were conducted on serpentine (antigorite) gouge to understand the rheology of serpentine‐bearing faults. The experiments were carried out using a constant confining pressure (100 MPa), a constant pore water pressure (30 MPa), and a range of temperatures (from room temperature to 600°C). The transient response in frictional behavior following stepwise changes in the slip velocity were documented at each temperature. Slip rates varied between 0.0115 and 11.5 μm/s. Both the general level of frictional strength and the transient responses changed drastically at around 450°C. As the temperature increased from 400°C to 450°C, the strength of antigorite rose sharply. The transient response also indicated a change in the mode of deformation from flow‐type behavior at temperatures below 400°C to frictional behavior (stick‐slip) at temperatures above 450°C–500°C. Although only a limited volume of serpentine was involved in the dehydration reaction, X‐ray diffraction analyses and scanning electron microscopy observations showed that forsterite had nucleated in the experimental products at the higher temperatures that were associated with frictional behavior. Submicron‐sized, streaky forsterite masses in shear‐localized zones may be evidence of shear‐induced dehydration that caused strengthening and embrittlement of the gouge. Although antigorite rheology is complicated, the subsequent change in friction coefficient per order‐of‐magnitude change in sliding velocity increased with both increasing temperature and decreasing velocity, implying that a possible flow mechanism of intragranular deformation became activated. Key Points Dehydration that caused strengthening and embrittlement of the gouge Newly forsterite in shear‐localized zones means shear‐induced dehydration The subsequent change in friction implies a flow, intra‐grain deformation

We report on the presence of the serpentine-type antigorite in abyssal-serpentinized peridotite. At mid-ocean spreading ridges, antigorite crystallizes under retrograde metamorphic conditions during tectonic exhumation of the newly formed oceanic lithosphere. Using optical microscopy and micro-Raman spectroscopy, we identified antigorite in 49 samples drilled at the Hess Deep (East Pacific Rise) and the Atlantis Massif (Mid-Atlantic Ridge, 30°N), and dredged along the Southwest Indian Ridge (62°–65°E). Overall, antigorite is common, but occurs in limited modal amounts. SEM and TEM investigations reveal its frequent crystallization after lizardite and chrysotile via dissolution–recrystallization processes and a local association with olivine or talc. We explain antigorite crystallization by the interaction with seawater-derived hydrothermal fluids moderately enriched in silica (metasomatism). The origin of silica is attributed to alteration of mafic intrusions or pyroxenes. Antigorite can, therefore, be considered a marker of preferential fluid pathways under rock-dominated conditions during exhumation of a portion of the oceanic lithosphere. We also measured the in-situ major and trace-element composition of antigorite and the predating and postdating phases. Most of the elements are immobile during the mineralogical transitions. Other elements (Ni, Ca, Al, and Ti) evolve within the serpentine textures, including antigorite, as a result of chemical exchanges accompanying the development of the sequence of serpentine textures. A further category includes elements that are specifically enriched (Mn, Sn) or depleted (Fluid-Mobile Elements: B, Sr, As, U, Sb, and Cl) in antigorite compared to lizardite and chrysotile. These enrichments and depletions possibly reflect a change of the fluid physicochemical characteristics allowing a change in element mobility during the dissolution–recrystallization accommodating the lizardite/chrysotile-to-antigorite transition. Such depletion in FME is comparable to depletions described in studies of serpentinization and antigorite formation in subduction zone setting, which suggests that the origin of antigorite in some subducted samples could be reevaluated.