Paul Raterron

Pressure-induced slip-system transition in forsterite: Single-crystal rheological properties at mantle pressure and temperature

Abstract Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373.1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (ε) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the “puzzling” seismic-anisotropy anomalies commonly observed in subduction zones

High‐temperature deformation of diopside single crystal: 1. Mechanical data

Laboratory deformation experiments were carried out on diopside single crystals. Creep tests were made in a dead load apparatus at temperatures T = 1020 to 1320°C, axial compressive stresses σ = 50 to 170 MPa with strain rates ( e˙) ranging from 2×10−9 s−1 to 4×10−7 s−1. The specimens were oriented such that mechanical twinning was not possible. The experiments were designed to activate {110}1/2〈a±b〉 slip systems. Our results demonstrate that they are the major operative systems for T > 1000°C and appear to be predominant over the (100)[c] slip system. The mechanical data are fitted to a power law e˙ = A σn exp(−E*/RT). An inversion method was used to determine the parameter In (A), the activation energy E*, and the stress exponent n. Below a critical temperature Tc ≃ 1130–1140°C, E* is found to be 440 ± 30 kJ/mol associated with the {110}1/2〈a±b〉 slip systems, activated symmetrically without any contribution from the (100)[c] system (orientation [2]); E* is found equal to 740±30 kJ/mol if all three systems have the same resolved shear stresses (orientation [1]). For T > Tc, values of E* drop to 50±15 and 85±30 kJ/mol for orientations [2] and [1], respectively, so that the creep law of diopside becomes nearly temperature independent. The law derived from the orientation [2] crystals could be consistent with natural deformations. For this orientation and within the range of temperatures covered, n is 6.5±0.4. The strong decrease in the activation energy at Tc may be ascribed unambiguously to the occurrence of partial melting leading to microdroplets which pin the mobile dislocations (Ingrin et al., this issue). The high value found for the stress exponent (n) in the creep law suggests that clinopyroxene is softer than olivine at high stresses (at T = 900°C, stress σ ≥ 20 MPa and e˙≥10−15 s−1), while olivine is softer at lower stresses. This competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle.

High‐temperature deformation of diopside crystal: 3. Influences of pO2 and SiO2 precipitation

Single crystals of gem quality diopside (with Fe/(Ca+Mg+Fe) ≃ 0.02) were deformed in a dead load apparatus under controlled oxygen partial pressure (pO2), in the range 8×10−14–2×10−9 MPa, at two temperatures T1 = 1100°C and T2 = 1200°C. The aim of these experiments was to investigate the sensitivity of diopside creep rate to pO2 at these two temperatures. T1 and T2 are on both sides of a critical temperature Tc ≃ 1130°–1140°C at which the activation energy E* of the creep rate decreases (from 442 to 48 kJ/mol) with rising temperature (Raterron and Jaoul, 1991) when siliceous microdroplets (≃0.1 μm in size) form (Ingrin et al., 1991). Specimens were deformed with axial compressive stress σ (110–143 MPa) along [010]; with this setting, the {110}1/2〈a±b〉 slip systems are symetrically activated, and strain rates e˙ are in the range 2×10−8–2×10−7 s−1. At T1 and under low pO2, we find e˙∝pO2-0.200±0.033 for samples that lack SiO2-rich precipitates in the host. At T2 and at the highest pO2 explored, e˙ becomes insensitive to pO2 for samples that contain SiO2-rich precipitates in the matrix. Electrical conductivity σe shows similar sensitivities to pO2 (Huebner and Voigt, 1988). We propose a point defect model based on the chemistry of nonstoichiometric compounds with cationic vacancies and ferric iron Fe3+ as majority point defects. The model predicts the critical values of Tc and pO2c beyond which the increasing abundance of the majority point defects promotes SiO2 precipitation. Tc and pO2c values are interdependent; they are also functions of Fe content in diopside and of its initial nonstoichiometry. This model offers an explanation of the pO2 dependencies of point defects concentrations as well. A comparison with experimental e˙ and σe sensitivities to pO2 suggests that interstitial divalent cations, which are minority defects, control electrical transport and diffusion-assisted dislocation glide. The model also shows that the occurrence of SiO2 precipitation does not necessarily imply a supersilicic starting material.

Plastic flow of pyrope at mantle pressure and temperature

Abstract Despite the abundance of garnet in deforming regions of the Earth, such as subduction zones, its rheological properties are not well defined by laboratory measurements. Here we report measurements of steady-state plastic properties of pyrope in its stability field (temperature up to 1573 K, pressure up to 6.8 GPa, strain rate ~10-5 s-1) using a Deformation-DIA apparatus (D-DIA) coupled with synchrotron radiation. Synthetic pyrope (Py100) and natural pyrope (Py70Alm16Gr14) are both studied in a dry environment. Transmission electron microscopy (TEM) investigation of the run products indicates that dislocation glide, assisted by climb within grains and dynamic recrystallization for grain-boundary strain accommodation, is the dominant deformation process in pyrope. Both synthetic-and naturalpyropes . stress and strain-rate data, as measured in situ by X-ray diffraction and imaging, are best fitted with the single flow law: where ε̇ is the strain rate, σ = | σ1-σ3 | is the differential stress, R is the gas constant and T the absolute temperature. Synthetic forsterite and synthetic pyrope were stacked along the uniaxial compression direction in the same cell assembly during deformation to compare their strength at mantle condition. Forsterite is observed to be stronger than pyrope, deforming at a rate about 10% slower than the pyrope at 5.2 GPa and 1573 K. San Carlos olivine and natural pyrope were compared in a similar fashion at 6.8 GPa and 1473 K. In this case, San Carlos olivine deformed 2~3 times faster than natural pyrope. The experimental data suggest that pyrope is stronger (by more than a factor of 4) than the dominant mineral (olivine) in the upper mantle when temperatures exceed 1273 K.

TEM investigation of dislocation microstructure of experimentally deformed silicate garnet

Deformation experiments have been carded out on single crystals of garnet (Py25 A167 Sp2 Gr6) under high confining pressure (P = 6.5 GPa) and temperature (T = 1440°C) in a multi-anvil apparatus. The high pressure sample assembly was designed so as to generate high differential stress. In one experiment, the differential stress was limited by a single crystal of San Carlos olivine (oriented along [010]) added on top of the specimen. Garnet crystals have been plastically deformed which shows that under our experimental conditions, garnet is ductile. The dislocation microstructure, analysed by transmission electron microscopy (TEM), suggests that both ½(111){110} and (100){110} glide systems have been activated. Some dislocations appear to be dissociated. The observations of dislocation junctions and subgrain structures indicate that dislocation climb is enhanced under our experimental conditions.

Activation volume of silicon diffusion in San Carlos olivine

The activation volume of silicon diffusion (Vsi) in olivine is reported for the first time. Specimens of San Carlos olivine single crystal were annealed at 1763 K and at pressures to 9 GPa in an uniaxial split-sphere apparatus. 30Si was used as tracer and the diffusion profiles were analyzed using the resonant nuclear reaction 30Si(p,γ). We obtain Vsi = (−1.9 ± 2.4)×10−6 m³/mol with a more probable value close to zero.

In situ Rheological Measurements at Extreme Pressure and Temperature using Synchrotron X-ray Diffraction and Radiography

Dramatic technical progress seen over the past decade now allows the plastic properties of materials to be investigated under extreme pressure and temperature conditions. Coupling of high-pressure apparatuses with synchrotron radiation significantly improves the quantification of differential stress and specimen textures from X-ray diffraction data, as well as specimen strains and strain rates by radiography. This contribution briefly reviews the recent developments in the field and describes state-of-the-art extreme-pressure deformation devices and analytical techniques available today. The focus here is on apparatuses promoting deformation at pressures largely in excess of 3 GPa, namely the diamond anvil cell, the deformation-DIA apparatus and the rotational Drickamer apparatus, as well as on the methods used to carry out controlled deformation experiments while quantifying X-ray data in terms of materials rheological parameters. It is shown that these new techniques open the new field of in situ investigation of materials rheology at extreme conditions, which already finds multiple fundamental applications in the understanding of the dynamics of Earth-like planet interior.

Constitutive Law and Flow Mechanism in Diamond Deformation

Constitutive laws and crystal plasticity in diamond deformation have been the subjects of substantial interest since synthetic diamond was made in 1950s. To date, however, little is known quantitatively regarding its brittle-ductile properties and yield strength at high temperatures. Here we report, for the first time, the strain-stress constitutive relations and experimental demonstration of deformation mechanisms under confined high pressure. The deformation at room temperature is essentially brittle, cataclastic, and mostly accommodated by fracturing on {111} plane with no plastic yielding at uniaxial strains up to 15%. At elevated temperatures of 1000°C and 1200°C diamond crystals exhibit significant ductile flow with corresponding yield strength of 7.9 and 6.3 GPa, indicating that diamond starts to weaken when temperature is over 1000°C. At high temperature the plastic deformation and ductile flow is meditated by the <110>{111} dislocation glide and a very active {111} micro-twinning.

Early partial melting in the upper mantle: an A.E.M. study of a lherzolite experimentally annealed at hypersolidus conditions

Abstract A natural spinel lherzolite (60% olivine, 25% enstatite, 13% Cr-diopside, and 2% Cr-spinel) was annealed and deformed at 1 GPa and 900–1000°C in H2O-saturated conditions and at fo2 roughly corresponding to the fayalite-magnetite-quartz buffer. The topology and textural development of the glass (i.e. quenched melt) was investigated by analytical transmission electron microscopy. In addition to the large glass slots (> 10 μm) previously observed using scanning electron microscopy (Bussod, G.Y. and Christie, J.M., 1991, Textural development and melt topology in spinel lherzolite experimentally deformed at hypersolidus conditions, J. Petrol., special lherzolite issue, pp. 17–39), we detected intracrystalline glass droplets of 0.1–0.2 μm within pyroxenes and olivine grains and intergranular isolated glass pockets 1 to 3 μm wide at grain boundaries. X-ray microanalysis shows that the intracrystalline glass droplets are highly enriched in silica (≈70 wt% SiO2), and depleted in MgO. Their shape and composition are similar in both pyroxenes and olivine grains. These droplets are characteristic of the phenomenon of early partial melting (EPM) previously observed in pyroxenes. The intergranular pockets are also SiO2-rich (54 to 66 wt%); their Mg content increases with the size of the pocket (from 0 to 7.6 wt% MgO). These observations provide a plausible scenario for the very first stage of melt formation in the upper mantle.

Deformation of Periclase Single Crystals at High Pressure and Temperature: Quantification of the Effect of Pressure on Slip-system Activities

In order to investigate the effect of pressure on periclase (MgO) dislocation slip-system activities, creep experiments have been carried out on MgO single crystals, at T and P, respectively, ranging from 1000 °C to 1200 °C and 4 to 9 GPa, in a deformation-DIA apparatus coupled with x-ray synchrotron radiation. Crystals were deformed in compression along either [100], [100], or [111] directions. These orientations were chosen to activate, respectively, either 1/2〈1-10〉{110} dislocation slip systems, 1/2〈1-10〉{100} systems, or simultaneously 1/2〈1-10〉{110} and 1/2〈1-10〉{100} systems. Experiments are carried out in a temperature range of 1000 °C to –1200 °C and a pressure range up to 8 GPa. Experimental results indicate that pressure influences differently the activities of these slip systems, which should yield a transition of dominant slip systems from 1/2〈1-10〉{110} at low pressure to 1/2〈1-10〉{100}. This pressure induced transition is expected to occur at 23 GPa, which would correspond to a pressure in ...