Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

Different types of deformation experiments were conducted on specimens of Carrara marble at a temperature of 1000 K, 300MPa confining pressure, and 3 × 10−4s−1 shear strain rate. Microstructural examinations were carried out using light and electron microscopy. Fabric analysis was performed through electron backscattered diffraction (EBSD) measurements. The stress-strain relationships from the experiments indicate that the mechanical response of Carrara marble depends on the deformation history. After a first ductile deformation event the marble becomes permanently softened and is able to undergo plastic strain at relatively lower stresses compared to previously undeformed marble. After shear strain reversal of γ=2 or less the original microstructure is restored; in highly re-strained samples (γ = ±3,4 and 5) dynamic recrystallization mechanisms create an evident foliation whose angle in respect to the shear plane defines a shear sense criterion which is in agreement with the reversed sense of shearing. After a minimum shear strain reversal of γ=3 the pre-existing foliation is totally overprinted. The fabric produced during strain reversal experiments shows features, which are comparable with those of single-stage experiments; but the strength of the crystallographic preferred orientation (CPO) is significantly lower, indicating complex microstructural interactions during plastic reactivation. Our tests indicate that the strain weakening in Carrara marble is predominantly caused by grain size refinement through dynamic recrystallization. The development of a crystallographic preferred orientation contributes towards only about one third of the total weakening observe

Coal characterization for ECBM recovery: Gas sorption under dry and humid conditions, and its effect on displacement dynamics

AbstractEnhanced Coal Bed Methane (ECBM) recovery is a technique under investigation as a possible approach to the geological storage of CO2 in a carbon dioxide capture and storage (CCS) system. This technology allows enhancing the recovery of coal bed methane by injecting CO2 in the coal seam at supercritical conditions. Through an in situ sorption/desorption process the displaced methane is produced and the adsorbed CO2 is permanently stored. In the case of coal, the uptake of CO2, CH4 and N2 is a combination of adsorption on its surface and penetration (absorption) into its solid matrix, the latter resulting in coal’s swelling. These two processes act simultaneously, making the coal a challenging material to be studied, in particular with respect to the understanding of the fundamental aspects of gas adsorption. High pressure sorption data of CO2, CH4 and N2 on a coal sample from Australia are presented; the interpretation of the experimental data takes into account the dual nature of the sorption process and a Langmuir-like model is applied to the sorption data, by fitting the isotherm parameters to the experimental values. The results confirm that this equation is a valuable option to describe gas sorption on coal. Moreover, a one-dimensional mathematical model previously derived is used to perform numerical simulations on the performance of ECBM recovery in coal beds. Important insights are obtained regarding the gas flow dynamics during displacement and the effects of gas sorption on the ECBM operation

Measurement of calcite crystallographic-preferred orientations by magnetic anisotropy and comparison to diffraction methods

The anisotropy of magnetic susceptibility (AMS) of rocks reflects the preferred orientations of minerals. Therefore AMS is a quick and easy way to characterize rock fabrics (Hrouda 1982,Borradaile 1988); the obtained result is also called the magnetic fabric of the rock. The method has been often used to measure the orientation of ferromagnetic minerals, mainly magnetite, but in recent studies it has been increasingly used to measure textures of paramagnetic minerals as phyllosilicates (Lüneburg et al. 1999, Cifelli et al. 2004). A further application is the measurement of diamagnetic textures, especially calcite textures. Calcite is suitable for the AMS method, because it has a high magnetic anisotropy with the minimum susceptibility along the crystallographic c-axis. Therefore a preferred orientation of the c-axes, which can be induced by deformation, generates a magnetic fabric...conferenc

Reaction-induced strain localization: torsion experiments on dolomite

We investigated the mechanical behaviour and microstructural evolution of a dolomite marble from Mt. Frerone (Adamello, N-Italy) during decarbonation to calcite and periclase in torsion experiments. Tests were performed in a Paterson gas-medium apparatus on cylindrical samples of 10 mm diameter and 10 mm length. Experiments were conducted at 800 °C, 300 MPa confining pressure under both vented and non-vented conditions, up to a maximum bulk shear strain of about γ = 1.8, at different strain rates (3 × 10− 5 s− 1 up to 3 × 10− 4 s− 1). Under hydrostatic conditions the nominal equilibrium P(CO₂) should be around 100 MPa, but in the vented experiments the CO₂ was free to escape, causing the breakdown of dolomite.During the decomposition, deformation was systematically localized at the ends of the specimens, near the porous spacers into a fine-grained mixture of calcite and periclase.Due to the low permeability of the marble, pore fluid could not escape from the central part of the sample building up CO₂ pressure which suppressed the decarbonation reaction. The fluid pressure embrittled the material and caused the development of en-echelon tension fractures, inclined opposite to the sense of shear.We conclude that decarbonation produced a weak polyphase matrix composed of submicron sized reaction products. Such a small grain size induced strain localization which was probably promoted by a switch from grain-size insensitive to grain-size sensitive deformation mechanism

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W–E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust–mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km

Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

ISSN:1661-8734ISSN:1661-872

Effect of partial melting on Vp and Vs in crustal enclaves from Mazarr\uf3n (SE Spain)

The combination of compressional and shear wave velocity is an important tool in discriminating rock types and identifying residing melts within the continental crust. Here we report the laboratory measurements for Vp and Vs obtained in varying conditions up to those exceeding the beginning of melting (950 degrees C at 500 MPa confining pressure) on two residual garnet-biotite-sillimanite-cordierite-spinel metapelitic enclaves recovered from the Neogene dacites of Mazarron (SE Spain). The enclaves preserve widespread interstitial rhyolitic glass as evidence of primary melt extraction. At 500 MPa, the experimental Vp ranges from 721 divided by 7.46 km s(-1) at room temperature to 5.44 km/s at 950 degrees C The mean Vs is 4.5 km/s at room temperature with shear-wave splitting of 0.25 0.3 km/s, measured along three mutually orthogonal directions. When temperature increases, the Vs evidences a reversible slope change at about 650 degrees C, and the shear-wave splitting reduces to zero (isotropic material) at 850-950 degrees C, where the sample Vs is similar to 3.0 km/s. The Vp anisotropy is 7-10% up to 700 degrees C increasing to similar to 20% at 950 degrees C, while the Vs anisotropy continuously decreases with temperature from 5% to 7% at room temperature to zero at 950 degrees C. No mineral reactions are observed up to 650-700 degrees C; however, microstructure equilibrates due to the relaxation of the primary glass at the glass transition temperature. Between 850 degrees C and 950 degrees C, a new melting reaction is observed producing biotite + spinel + ilmenite + plagioclase + melt. At melting, the rock becomes isotropic for Vs, and Poisson's ratio increases to 0.30. Our seismic data show that the seismic signature of high grade metapelitic rocks is similar to that of mafic materials. The evolution of Vp and Vs at melting conditions is compatible with the geophysical data of the area, supporting the hypothesis of the current existence of anatectic melts at intermediate depths of the Alboran domain

Post-deformational annealing of calcite rocks

International audienceThe evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 ?m), Solnhofen limestone (96%, 5 ?m) and synthetic calcite aggregates (99%, 7 ?m). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10? 4 s? 1 to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD). All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing

Sound velocity measurements of polycrystalline clinohumite and phase A at high pressure

ISSN:0016-7037ISSN:1872-953