Nadège Hilairet

High-Pressure Creep of Serpentine, Interseismic Deformation, and Initiation of Subduction

The supposed low viscosity of serpentine may strongly influence subduction-zone dynamics at all time scales, but until now its role could not be quantified because measurements relevant to intermediate-depth settings were lacking. Deformation experiments on the serpentine antigorite at high pressures and temperatures (1 to 4 gigapascals, 200° to 500°C) showed that the viscosity of serpentine is much lower than that of the major mantle-forming minerals. Regardless of the temperature, low-viscosity serpentinized mantle at the slab surface can localize deformation, impede stress buildup, and limit the downdip propagation of large earthquakes at subduction zones. Antigorite enables viscous relaxation with characteristic times comparable to those of long-term postseismic deformations after large earthquakes and slow earthquakes. Antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation.

Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature in the Laboratory

Delineating Deep Faults Most large, damaging earthquakes initiate in Earths crust where friction and brittle fracture control the release of energy. Strong earthquakes can occur in the mantle too, but their rupture dynamics are difficult to determine because higher temperatures and pressures play a more important role. Ye et al. (p. 1380) analyzed seismic P waves generated by the 2013 Mw 8.3 Sea of Okhotsk earthquake—the largest deep earthquake recorded to date—and its associated aftershocks. The earthquake ruptured along a fault over 180-kilometer-long and structural heterogeneity resulted in a massive release of stress from the subducting slab. In a set of complementary laboratory deformation experiments, Schubnel et al. (p. 1377) simulated the nucleation of acoustic emission events that resemble deep earthquakes. These events are caused by an instantaneous phase transition from olivine to spinel, which would occur at the same depth and result in large stress releases observed for other deep earthquakes. Fractures generated by mineral phase transitions in the mantle produce acoustic emissions that resemble deep earthquakes. 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.

Hydrogen and minor element incorporation in synthetic rutile

Abstract The solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated. Experiments performed using different bulk Fe2O3 contents demonstrate that Fe3+ substitutes onto the main Ti site, charge-balanced by oxygen vacancies. Under more reducing conditions in Fe-poor systems, the concentration of Ti interstitials in rutile is increased, resulting in a decrease in H solubility. Variation in the solubility of different oxides in rutile as a function of ionic radius implies substitution onto the main Ti site, probably charge-balanced by oxygen vacancies. To a lesser degree, substitution of trivalent and divalent cations is locally charge-balanced by H incorporation. Variation in OH-stretching frequencies in infrared spectra as a function of composition implies that octahedral defects and structurally-incorporated H are coupled. However, in all samples, some of the H is also decoupled from substitutional impurities, as is evident from an OH-absorption band at 3279 cm-1. This band corresponds to the main OH band seen in spectra of many natural rutiles, implying that in most rutiles, H defects are decoupled from substitutional defects.

Deformation mechanisms and rheology of serpentines in experiments and in nature

Microstructures in serpentine samples recovered from deformation experiments performed at high pressure (1-8 GPa), high temperature (150-500 degrees C), and laboratory strain rates (4 10(-4)-10(-6)s(-1)) were studied using transmission electron microscopy on thin sections prepared by focused ion beam. Lizardite crystals deform by easy glide along the basal planes associated with microkink. This mechanism is associated with a plastic strength of similar to 100MPa that defines the upper bound of lizardite strength in natural conditions. Antigorite crystals deform essentially by conjugated slip along (101) and (10 (1) over bar) planes observed in sections close to (010). This conjugate system results in an apparent global slip system akin to [100](001). In all samples, delamination and comminution produce fine-grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von Mises criterion. The deformation laws of lizardite (plastic flow) and antigorite (strain rate dependent) differ because of their differing intracrystalline deformation mechanisms. Subgrain boundary geometry in natural samples and [100](001) crystal preferred orientations indicate the activation of slip systems similar to those observed in experimentally deformed samples, suggesting that the strain rate-dependent rheology of antigorite derived from experiments applies to subduction zone conditions. Delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones, suggesting plastic deformation of serpentinites at natural low strain rates and stresses is likely accompanied by recrystallization through dissolution-precipitation processes that act as a low-temperature equivalent of dynamic recrystallization through diffusion at high temperature.

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.

Multifit/Polydefix: a framework for the analysis of polycrystal deformation using X-rays

Multifit/Polydefix is an open source IDL software package for the efficient processing of diffraction data obtained in deformation apparatuses at synchrotron beamlines. Multifit allows users to decompose two-dimensional diffraction images into azimuthal slices, fit peak positions, shapes and intensities, and propagate the results to other azimuths and images. Polydefix is for analysis of deformation experiments. Starting from output files created in Multifit or other packages, it will extract elastic lattice strains, evaluate sample pressure and differential stress, and prepare input files for further texture analysis. The Multifit/Polydefix package is designed to make the tedious data analysis of synchrotron-based plasticity, rheology or other time-dependent experiments very straightforward and accessible to a wider community.

In situ monitoring of phase transformation microstructures at Earth's mantle pressure and temperature using multi-grain XRD

Microstructures govern the mechanical properties of materials and change dramatically during phase transformations. A detailed understanding of microstructures at different stages of a transformation is important for the design of new materials and for constraining geophysical processes. However, experimental studies of transformation microstructures at the grain scale have been mostly based on ex situ observations of quenched products, which are difficult to correlate with bulk sample properties and transformation kinetics. Here, it is shown how multi-grain crystallography on polycrystalline samples, combined with a resistively heated diamond anvil cell, can be applied to investigate the microstructural properties of a material undergoing a phase transition in situ at high pressure and high temperature. This approach allows the extraction of the crystallographic parameters and orientations of several hundreds of grains inside a transforming sample. Important bulk information on grain size distributions and orientation relations between the parent and the newly formed phase at the different stages of the transformation can be monitored. These data can be used to elucidate transformation mechanisms (e.g. coherent versus incoherent growth), growth rates and orientation-dependent growth of individual grains. The methodology is demonstrated on the α–γ phase transitions in hydrous Mg2SiO4·H2O up to 22 GPa and 940 K. This transformation most likely occurs in the most abundant mineral of the Earths upper mantle (Mg0.8Fe0.2SiO4) in deep cold subducted slabs and plays an important role in their subduction behaviour.

A laboratory nanoseismological study on deep-focus earthquake micromechanics

Nanoseismological analyses on labquakes under controlled conditions shed new lights on mechanisms of deep-focus earthquakes. 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.

Elasto-viscoplastic self consistent modeling of the ambient temperature plastic behavior of periclase deformed up to 5.4 GPa

Anisotropy has a crucial effect on the mechanical response of polycrystalline materials. Polycrystal anisotropy is a consequence of single crystal anisotropy and texture (crystallographic preferred orientation) development, which can result from plastic deformation by dislocation glide. The plastic behavior of polycrystals is different under varying hydrostatic pressure conditions, and understanding the effect of hydrostatic pressure on plasticity is of general interest. Moreover, in the case of geological materials, it is useful for understanding material behavior in the deep earth and for the interpretation of seismic data. Periclase is a good material to test because of its simple and stable crystal structure (B1), and it is of interest to geosciences, as (Mg,Fe)O is the second most abundant phase in Earths lower mantle. In this study, a polycrystalline sintered sample of periclase is deformed at ∼5.4 GPa and ambient temperature, to a total strain of 37% at average strain rates of 2.26 × 10−5/s and 4....

Rheology at High Pressures and High Temperatures

We review recent progress in studying deformation at simultaneous high pressure and high temperature using in-situ diffraction and imaging in the large volume press (LVP). Current state-of-the-art deformation devices and diffraction and imaging techniques are introduced, along with theoretical background for stress measurements based on diffraction. Examples are given to illustrate the kind of information that can be extracted using these techniques. Future prospects are discussed.