Stress-induced amorphization triggers deformation in the lithospheric mantle.

The mechanical properties of olivine-rich rocks are key to determining the mechanical coupling between Earth's lithosphere and asthenosphere. In crystalline materials, the motion of crystal defects is fundamental to plastic flow1-4. However, because the main constituent of olivine-rich rocks does not have enough slip systems, additional deformation mechanisms are needed to satisfy strain conditions. Experimental studies have suggested a non-Newtonian, grain-size-sensitive mechanism in olivine involving grain-boundary sliding5,6. However, very few microstructural investigations have been conducted on grain-boundary sliding, and there is no consensus on whether a single or multiple physical mechanisms are at play. Most importantly, there are no theoretical frameworks for incorporating the mechanics of grain boundaries in polycrystalline plasticity models. Here we identify a mechanism for deformation at grain boundaries in olivine-rich rocks. We show that, in forsterite, amorphization takes place at grain boundaries under stress and that the onset of ductility of olivine-rich rocks is due to the activation of grain-boundary mobility in these amorphous layers. This mechanism could trigger plastic processes in the deep Earth, where high-stress conditions are encountered (for example, at the brittle-plastic transition). Our proposed mechanism is especially relevant at the lithosphere-asthenosphere boundary, where olivine reaches the glass transition temperature, triggering a decrease in its viscosity and thus promoting grain-boundary sliding.

Targeted census of lionfishes (Scorpaenidae) reveals high densities in their native range.

Indo-Pacific lionfishes generally exhibit cryptic behaviours and so can be missed when conducting non-targeted surveys. Here, the authors report the results from targeted surveys of lionfish at Moorea, French Polynesia. Lionfish from three species (Pterois antennata, Pterois radiata, Dendrochirus biocellatus) were observed at a mean density of 267 individuals ha-1 . This is substantially higher than previous estimates from the same area (Moorea) and represents the highest reported density of lionfishes from their Pacific range. Overall, this study highlights the importance of targeted survey techniques for detecting cryptic species on coral reefs.

Acoustic emission detection of micro-cracks under high pressure and high temperature in a deformation large-volume apparatus at the endstation P61B, PETRA III.

We successfully developed an in situ acoustic emission (AE) detection setup that allows recording of AE waveforms (triggered and streaming) and simultaneous x-ray diffraction and imaging on samples deformed at high pressure and high temperature (HPHT) conditions in the Aster-15 Large Volume Press at the synchrotron beamline station P61B. This high pressure AE detection system is a powerful tool to investigate AE phenomena from the HPHT chamber. Six commercial acoustic sensors, protected by a tungsten carbide support ring on each anvil of the same material, have excellent survivability throughout each successive experiment. By pulsing each sensor in succession, the average wave velocity through the anvils and cell assembly can be determined at any press load. The distance between the sensors is obtained by x-ray radiography and by logging the positions of each hydraulic ram. This provides a basis for accurately locating AE events in the sample. The feasibility of this AE detection setup was confirmed by compression and deformation test runs using several different self-designed AE sources in specialized assemblies. The present setup proves to be extremely efficient and accurate in measuring brittle processes in samples under HPHT. It is now available for applications for beam time and experiments without x rays at P61B. Combined with synchrotron x rays, in situ pressure, temperature, strain rate and stress, and phase changes can be monitored while recording AE activity. We provide a powerful tool to investigate the origin of earthquakes, for example, causing AE emissions due to brittle dehydration reactions or phase transformations in the Earth.

Plastic Deformation and Strengthening Mechanisms of Nanopolycrystalline Diamond.

Bulk nanopolycrystalline diamond (NPD) samples were deformed plastically within the diamond stability field up to 14 GPa and above 1473 K. Macroscopic differential stress Δσ was determined on the basis of the distortion of the 111 Debye ring using synchrotron X-ray diffraction. Up to ∼5(2)% strain, Debye ring distortion can be satisfactorily described by lattice strain theories as an ellipse. Beyond ∼5(2)% strain, lattice spacing d111 along the Δσ direction becomes saturated and remains constant with further deformation. Transmission electron microscopy on as-synthesized NPD shows well-bonded grain boundaries with no free dislocations within the grains. Deformed samples also contain very few free dislocations, while density of {111} twins increases with plastic strain. Individual grains display complex contrast, exhibiting increasing misorientation with deformation according electron diffraction. Thus, NPD does not deform by dislocation slip, which is the dominated mechanism in conventional polycrystalline diamond composites (PCDCs, grain size >1 µm). The nonelliptical Debye ring distortion is modeled by nucleating 12⟨110⟩ dislocations or their dissociated 16⟨112⟩ partials gliding in the {111} planes to produce deformation twinning. With increasing strain up to ∼5(2)%, strength increases rapidly to ∼20(1) GPa, where d111 reaches saturation. Strength beyond the saturation shows a weak dependence on strain, reaching ∼22(1) GPa at >10% strain. Overall, the strength is ∼2-3 times that of conventional PCDCs. Combined with molecular dynamics simulations and lattice rotation theory, we conclude that the rapid rise of strength with strain is due to defect-source strengthening, whereas further deformation is dominated by nanotwinning and lattice rotation.

Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite.

Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet.

A laboratory nanoseismological study on deep-focus earthquake micromechanics.

Global earthquake occurring rate displays an exponential decay down to ~300 km and then peaks around 550 to 600 km before terminating abruptly near 700 km. How fractures initiate, nucleate, and propagate at these depths remains one of the greatest puzzles in earth science, as increasing pressure inhibits fracture propagation. We report nanoseismological analysis on high-resolution acoustic emission (AE) records obtained during ruptures triggered by partial transformation from olivine to spinel in Mg2GeO4, an analog to the dominant mineral (Mg,Fe)2SiO4 olivine in the upper mantle, using state-of-the-art seismological techniques, in the laboratory. AEs' focal mechanisms, as well as their distribution in both space and time during deformation, are carefully analyzed. Microstructure analysis shows that AEs are produced by the dynamic propagation of shear bands consisting of nanograined spinel. These nanoshear bands have a near constant thickness (~100 nm) but varying lengths and self-organize during deformation. This precursory seismic process leads to ultimate macroscopic failure of the samples. Several source parameters of AE events were extracted from the recorded waveforms, allowing close tracking of event initiation, clustering, and propagation throughout the deformation/transformation process. AEs follow the Gutenberg-Richter statistics with a well-defined b value of 1.5 over three orders of moment magnitudes, suggesting that laboratory failure processes are self-affine. The seismic relation between magnitude and rupture area correctly predicts AE magnitude at millimeter scales. A rupture propagation model based on strain localization theory is proposed. Future numerical analyses may help resolve scaling issues between laboratory AE events and deep-focus earthquakes.

Dehydration-driven stress transfer triggers intermediate-depth earthquakes.

Intermediate-depth earthquakes (30-300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.

Deep-focus earthquake analogs recorded at high pressure and temperature in the laboratory.

Phase transformations of metastable olivine might trigger deep-focus earthquakes (400 to 700 kilometers) in cold subducting lithosphere. To explore the feasibility of this mechanism, we performed laboratory deformation experiments on germanium olivine (Mg2GeO4) under differential stress at high pressure (P = 2 to 5 gigapascals) and within a narrow temperature range (T = 1000 to 1250 kelvin). We found that fractures nucleate at the onset of the olivine-to-spinel transition. These fractures propagate dynamically (at a nonnegligible fraction of the shear wave velocity) so that intense acoustic emissions are generated. Similar to deep-focus earthquakes, these acoustic emissions arise from pure shear sources and obey the Gutenberg-Richter law without following Omori's law. Microstructural observations prove that dynamic weakening likely involves superplasticity of the nanocrystalline spinel reaction product at seismic strain rates.