Alexandre Dimanov

Grain boundary diffusion creep of synthetic anorthite aggregates: The effect of water

To investigate the effect of trace amounts of water on plastic deformation of feldspar, we fabricated synthetic polycrystalline aggregates of pure anorthite from a glass. The glass powder was either densified and crystallized at 1473 K and 0.1 MPa or hot isostatically pressed at 1443 K and 300 MPa confining pressure. Fourier transform infrared spectrometry indicates a water content of 0.002–0.0035 wt % (300–550 H per 106 Si) for specimens prepared at atmospheric pressure. Hot-isostatically pressed samples contain 0.05 wt % to 0.1 wt % (8000–15000 H per 106 Si), depending on whether they were crystallized from glass powder predried at 1073 K for 2–3 days or from glass powder as received. In the wet samples, <1 vol % glass was found. Transmission electron microscopy showed that two-grain boundaries are glass-free to a resolution of 5 nm. We performed creep experiments at 0.1 MPa pressure with temperatures ranging from 1373 to 1573 K and stresses ranging from 1 to 100 MPa. The mechanical data indicate grain boundary diffusion controlled creep with a stress exponent n = 1.0±0.1 and a grain size exponent m = −2.9±0.2. The activation energy for creep is 585±45 kJ mol−1 or 377±38 kJ mol−1 for dry or wet specimens, respectively. If extrapolated to natural temperatures (≅973–923 K), our mechanical data indicate 3–4 orders of magnitude difference in strength between dry samples and specimens containing trace amounts of water.

High‐temperature creep of partially molten plagioclase aggregates

We have investigated the high-temperature creep of synthetic labradorite (An60Ab40) between 1323 K and 1523 K at atmospheric pressure and low stresses (2–65 MPa). Average grain size varies from 12 to 16 μm. Samples contained up to 12 vol % melt (An60Ab40 glass). Fourier transform infrared measurements indicate that trace amounts of water (∼0.1–0.15 wt % H2O) were incorporated during crystallization. This water was only partly released at high temperature and atmospheric pressure. At low stresses and temperatures, deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n∼1 and an activation energy of 365±25 kJ/mol (regime I). With increasing stress and temperature, mechanical data and microstructural observations indicate a transition to power law creep with n∼3 (regime II). This transition occurs in both partially molten and melt-free aggregates. In regime I the strain rate of samples containing as much as 12 vol % melt is between 3 and 5 times higher than in fully crystalline specimens. The activation energy is only slightly affected by the melt content between 0 and 10±2 vol %. Transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets. Grain boundaries are not wetted except surrounding large melt pockets. The melt distribution of undeformed and deformed samples is similar. The strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water. Both plagioclase compositions are slightly weaker than synthetic diopside.

“Average” interdiffusion of (Fe,Mn)-Mg in natural diopside

(Fe,Mn)-Mg interdiffusion has been investigated in single crystals of natural diopside at ambient pressure, high temperatures (900°C to 1240°C) and low oxygen fugacity (pO 2 ≈ 10 −18 atm to 10 −13 atm). Interdiffusion couples were made with polycrystalline ferro-johannsenite thin film (∼ 250 A to ∼ 500 A) as the iron and manganese reservoir (Fe/(Fe + Mn + Mg) ∼ 48 at. %) and with diopside single crystals as the semi-infinite calcium- and magnesium-rich medium. The diopside specimens contain Fe as major impurity (Fe/(Fe + Mg) ≈ 6 at. %). Diffusion profiles of Fe and Mn ranging from ∼ 250 A to ∼ 3500 A were analysed by Rutherford back-scattering spectrometry (RBS). This technique is well adapted to measurements of concentration profiles of heavy elements in silicates, but it can not resolve the signals from elements with nearly same mass (Fe and Mn). Therefore Fe and Mn concentration profiles overlap, but the characteristic shapes of the convoluted diffusion profiles indicate that Fe and Mn diffuse at similar rates. This is also confirmed by secondary-ion mass spectroscopy (SIMS), which allows one to obtain separately Fe and Mn concentration profiles. We obtained an “average” inter-diffusion coefficient characterised by an apparent activation energy of 406 kJ/mol and a pre-exponential factor of 0.955 cm 2 /s. Such a result is surprisingly low for a diffusion process involving Fe and Mg exchanges.

(Fe,Mn)-Mg interdiffusion in natural diopside: effect of pO 2

We investigated (Fe,Mn)-Mg interdiffusion along the [001] axis in single crystals of natural diopside at ambient pressure, at temperatures between 1000°C to 1200°C and at oxygen fugacities between 10 −18.5 atm to 10 −6 atm. Thin layers (~350–550 A) of ferro-johannsenite deposited by RF plasma sputtering provided iron and manganese for exchange with magnesium from the diopside substrates. Interdiffusion profiles of Fe-Mg and Mn-Mg were analysed by secondary ion mass spectrometry in depth profiling mode. We found that the (Fe,Mn)-Mg interdiffusion coefficient is strongly pO 2 sensitive. At 1100°C and between 10 −7 atm and 10 −15 atm the interdiffusion coefficient is proportional to ~(pO 2 ) m , indicative of an extrinsic point defects regime. We obtained m = 0.22 ± 0.02, indicating a vacancy mechanism. The data at 1100°C further suggest that at pO 2 × 10 −15 atm and at pO 2 > 10 −7 atm the interdiffusion coefficient is pO 2 -independent, possibly in relation with the appearance of Early Partial Melting (EPM, Jaoul & Raterron, 1994). At constant oxygen fugacity and without EPM the interdiffusion coefficient has an activation energy of 297 ± 31 kJ/mol. Our data significantly improve the constraints on the activation energy beyond the preliminary results reported earlier (Dimanov & Sautter, 2000). This work contributes towards a better understanding of the point defects chemistry and diffusion mechanisms in Fe-bearing diopside.

Brittle and semibrittle creep of Tavel limestone deformed at room temperature

Deformation and failure mode of carbonate rocks depend on the confining pressure. In this study, the mechanical behaviour of a limestone with an initial porosity of 14.7 % is investigated at constant stress. At confining pressures below 55 MPa, dilatancy associated with micro-fracturing occurs during constant stress steps, ultimately leading to failure, similar to creep in other brittle media. At confining pressures higher than 55 MPa, depending on applied differential stress, inelastic compaction occurs, accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities, or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile-ups, ultimately causing failure. Strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity, in agreement with elastic wave velocity evolution and microstructural observations.

Three-dimensional full-field X-ray orientation microscopy.

A previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. Grain by grain tomographic reconstructions using convex optimization and prior knowledge are carried out in a six-dimensional representation of position-orientation space, used for modelling the inverse problem of X-ray orientation imaging. From the 6D reconstruction output we derive 3D orientation maps, which are then assembled into a common sample volume. The obtained 3D orientation map is compared to an EBSD surface map and local misorientations, as well as remaining discrepancies in grain boundary positions are quantified. The new approach replaces the single orientation reconstruction scheme behind X-ray diffraction contrast tomography and extends the applicability of this diffraction imaging technique to material micro-structures exhibiting sub-grains and/or intra-granular orientation spreads of up to a few degrees. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature.

Newtonian flow of heterogeneous synthetic gabbros at high strain: Grain sliding, ductile failure, and contrasting local mechanisms and interactions

We combined experimental and numerical approaches, in order to constrain the rheology of ultramylonitic rocks characteristic of gabbroic high-temperature shear zones. Synthetic samples containing 70 vol% fine-grained anorthite matrix (<5 μm) and 30 vol% coarse diopside inclusions (<55 μm) were deformed in torsion at T = 1150 °C and P = 370 MPa. At modest shear stresses (<40 MPa) the data evidenced linear-viscous flow with a stress exponent n ≈ 1. The outermost rim of the samples exhibited substantial cavitation coalescence leading to ductile damage and ultimately to ductile failure at shear strains between γ = 2–3.5. The mechanical data and the microstructures indicate dominant grain boundary sliding (GBS) and diffusional mass transfer. However, extensive crystal slip plasticity (CSP), dislocation creep and dynamic recrystallization operated within and in the vicinity of the diopside inclusions. For instance, the inclusions presented an extremely fine-grained interfacial layer, resulting from their intense peripheral recrystallization. The latter crystal plasticity mechanism require differential stresses exceeding by far the overall flow stress supported by our specimens, which indicates substantial local stress enhancement related to the heterogeneous nature of the two-phase rock. In order to characterize the local mechanical responses we performed finite element numerical modelling of the shear deformation process, considering elastoviscoplastic behavior based on the constitutive laws for GBS and CSP of the constituent phases. We emphasized the effects of inclusion shapes and interactions. We show that even at relatively low concentrations (25 vol%) and for overall Newtonian flow, the important strength contrasts and the interactions between neighbouring and irregular inclusions rapidly induce significant local stress amplifications, allowing for twinning and dislocation creep. On the other hand, the simulations suggest that the presence of fine-grained recrystallized interfacial layers deforming by GBS allows for very efficient local stress relaxation. As a result, we suggest that whilst the overall material sustains fairly low stresses and presents GBS related Newtonian flow, locally the diopside inclusions and the surrounding matrix experience highly fluctuating stresses in relation with cycles of dislocation creep, peripheral recrystallization and interfacial GBS. On the one hand, from a stochastic point of view, the cyclic nature of the process may ensure a stable overall flow stress. On the other hand, at the local scale the sequential mechanism allows for continuous size reduction of the stronger inclusions, a process that we call “ductile abrasion”.