Androgynous {\{101‾2{10\overline{1}2}}\} twin in zinc

Under ambient conditions, Zn is a hexagonal metal with a large $c/a$ ratio of 1.856. Plastic deformation is predominantly accommodated by basal $⟨a⟩$ slip and compression twins on the $\{$${10\overline{1}2}$$\}$ planes. Increasing hydrostatic pressure drastically reduces the $c/a$ ratio of Zn and, when a critical threshold of /=√3 at about 10 GPa is crossed, the $\{$${10\overline{1}2}$$\}$ twins are predicted to change from compressive to tensile in nature. What happens at the transition point, when $c/a=\sqrt{3}$, remains unknown. Here, we strain-cycle a textured polycrystalline sample of pure Zn at uniform hydrostatic pressures ranging between 2 and 17 GPa, over which the / ratio crosses the /=√3 compressive-tensile transition for $\{$${10\overline{1}2}$$\}$ twins. During deformation, the state of the sample is monitored in situ through x-ray diffraction to extract texture and internal strain evolution. By comparing the experimental results with the predictions of an elastoviscoplastic polycrystal simulation, we confirm the androgynous nature of $\{$${10\overline{1}2}$$\}$ twin response at low and high pressures. When $c/a=\sqrt{3}$, polycrystalline Zn does not display any evidence of twinning and its plastic behavior is controlled by mostly basal and pyramidal $⟨c+a⟩$ slip activity, with a very small contribution of prismatic $⟨a⟩$ slip. Evidence for the activity of other $\{$${10\overline{1}n}$$\}$ twinning modes, which have been suggested for Zn under high pressure, are not observed

Stability and dynamics of serpentinite layer in subduction zone.

International audienc

High-pressure creep of serpentine, interseismic deformation, and initiation of subduction.

International audienc

PV Equation of State of Lizardite, Chrysotile, and Antigorite.

International audienc

Equation of State of Antigorite: stability field of serpentines and seismicity in subduction zones

International audienc

Elasticity of serpentines and extensive serpentinization in subduction zones

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The mafic layered complex of the Kaby´e massif (north Togo and north Benin): Evidence of a Pan-African granulitic continental arc root

International audienceBetween the predominantly Neoarchean–Paleoproterozoic West African Craton (WAC) and the Saharian Metacraton (SMC), the Dahomeyides suture zone represents a valuable witness of continental amalgamation during Pan-African times. In the Kaby´e massif (northern Togo and northern B´enin) mainly granulitic metagabbros, associated with Al-rich kyanite and garnet bearing felsic dykes, are exposed as tectonic lenses within the pre-Pan-African gneisses of the SMC. New geochemical data suggest that the high grade rocks (granulites) in the Kaby´e massif originated in a mature continental arc setting. AFC calculations constrain the amount of contamination of mantle wedge derived magmas by crustal etasediments to about 10%. Reconstruction of initial magmatic stratigraphy has been carried out using CIPW-norms for cumulitic sequences within the massif, indicating a normal igneous polarity from west to east. Published geochemical signatures along the Pan-African suture zone of the Dahomeyides, in Ghana, Togo and Benin, constrain the origin of mafic granulitic and eclogitic complexes. A distinction is made between bodies with mainly MORB signatures originated from the subducted WAC plate passive margin and those with magmatic arc signatures originated from the over-riding plate mantle wedge. This suggests that the closure of the oceanic domain between the WAC and the SMC from 640 to 610 Ma was mainly accommodated by oceanic subduction beneath the active continental margin, before Pan-African collision

Seismicity triggered by the olivine-spinel transition: New insights from combined XRD and acoustic emission monitoring during deformation experiments in Mg2GeO4

Polycrystalline Mg2GeO4-olivine has been deformed (strain rates from 2.10−4/s to 10−5/s) in the deformation-DIA in 13-BM-D at GSECARS (Advanced Photon Source) at ca. 2 GPa confining pressure for temperatures between 973 and 1573 K (i.e., in the Mg2GeO4-ringwoodite field). Stress, advancement of transformation, and strain were measured in-situ using X-ray diffraction (XRD) and imaging, and acoustic emissions (AE) full wave-form were recorded simultaneously. When differential stress is applied (ca. 1- to 2 GPa GPa) and temperature is increased, the very beginning of the transformation to the ringwoodite structure (as evidenced by in situ XRD) is accompanied by AE bursts which locate within the sample. At high strain rates (> 10−4/s) and low temperatures (800-900 degrees C), the number of AEs is comparable, if not larger, to that observed during the cold compression of quartz grains. The largest events always occur at a temperature slightly below that of appearance of the ringwoodite-structure phase on the XRD images patterns. This suggests that AEs are generated while the transition is still nucleation controlled (pseudo-martensitic stage). During stress-relaxation periods, the rate of AE triggering decreases, but does not completely vanish. The AE production rate increases again as soon as deformation is started again. Importantly, we still observed very large AEs at strain rates as low as approx. 10−5/s. At these early stages of the transformation, the samples did not show any macroscopic rheological weakening. Focal mechanism analysis of the largest AEs showed that they are all of shear type, some being even pure double couple. They radiate about the same amount of energy as typically recorded during fast crack propagation in amorphous glass material. This suggests that they cannot only originate from the martensintic nucleation of oriented spinel-lamellae within a single germanium olivine crystal. Microstructural analysis (SEM, EBSD and TEM) high- lights the presence of thin transformation bands made of incoherent spinel micro-grains which, possibly, run across germanium-olivine grain boundaries. These bands are all oriented near perpendicular to the principal compressive stress. Our observations point out that under high deviatoric stress, the olivine - spinel transition is a source of instability which produces micro-seismicity (no AEs are were recorded when in a similar experiment are performed hydrostatically). These instabilities might eventually be precursor to brittle fracturing as observed by Burnley et al. (1990) in their deformation experiments on very similar samples. Both types of study emphasize the potential of phase transitions (with negative volume variations?) in radiating acoustic energy and triggering brittle failure. Obviously, this has important consequences for the understanding of deep-focus earthquakes occurring in cold and metastable olivine within the transition zone

Seismicity triggered by the olivine-spinel transition: New insights from combined XRD and acoustic emission monitoring during deformation experiments in Mg2GeO4

Polycrystalline Mg2GeO4-olivine has been deformed (strain rates from 2.10−4/s to 10−5/s) in the deformation-DIA in 13-BM-D at GSECARS (Advanced Photon Source) at ca. 2 GPa confining pressure for temperatures between 973 and 1573 K (i.e., in the Mg2GeO4-ringwoodite field). Stress, advancement of transformation, and strain were measured in-situ using X-ray diffraction (XRD) and imaging, and acoustic emissions (AE) full wave-form were recorded simultaneously. When differential stress is applied (ca. 1- to 2 GPa GPa) and temperature is increased, the very beginning of the transformation to the ringwoodite structure (as evidenced by in situ XRD) is accompanied by AE bursts which locate within the sample. At high strain rates (> 10−4/s) and low temperatures (800-900 degrees C), the number of AEs is comparable, if not larger, to that observed during the cold compression of quartz grains. The largest events always occur at a temperature slightly below that of appearance of the ringwoodite-structure phase on the XRD images patterns. This suggests that AEs are generated while the transition is still nucleation controlled (pseudo-martensitic stage). During stress-relaxation periods, the rate of AE triggering decreases, but does not completely vanish. The AE production rate increases again as soon as deformation is started again. Importantly, we still observed very large AEs at strain rates as low as approx. 10−5/s. At these early stages of the transformation, the samples did not show any macroscopic rheological weakening. Focal mechanism analysis of the largest AEs showed that they are all of shear type, some being even pure double couple. They radiate about the same amount of energy as typically recorded during fast crack propagation in amorphous glass material. This suggests that they cannot only originate from the martensintic nucleation of oriented spinel-lamellae within a single germanium olivine crystal. Microstructural analysis (SEM, EBSD and TEM) high- lights the presence of thin transformation bands made of incoherent spinel micro-grains which, possibly, run across germanium-olivine grain boundaries. These bands are all oriented near perpendicular to the principal compressive stress. Our observations point out that under high deviatoric stress, the olivine - spinel transition is a source of instability which produces micro-seismicity (no AEs are were recorded when in a similar experiment are performed hydrostatically). These instabilities might eventually be precursor to brittle fracturing as observed by Burnley et al. (1990) in their deformation experiments on very similar samples. Both types of study emphasize the potential of phase transitions (with negative volume variations?) in radiating acoustic energy and triggering brittle failure. Obviously, this has important consequences for the understanding of deep-focus earthquakes occurring in cold and metastable olivine within the transition zone

Creep of phyllosilicates at the onset of plate tectonics

International audienc