Nicolas P. Walte

Burgers vector determination in deformed perovskite and post-perovskite of CaIrO3 using thickness fringes in weak-beam dark-field images

The thickness-fringe method [Ishida et al., Philosophical Magazine 42 (1980) 453] for complete determination of the character of a dislocation Burgers vector has been performed in CaIrO(3) perovskite and post-perovskite deformed at high pressures and high temperatures. By selecting several main zone axes and determining the number of terminating thickness fringes at the extremity of a dislocation from a wedge-shaped thin-foil specimen in weak-beam dark-field transmission electron microscope (TEM) images, the Burgers vectors were unambiguously determined. The results demonstrate that [100] screw and edge dislocations on the (010) slip plane are dominant in the post-perovskite phase. Curved [100] and [010] dislocations and straight 110 screw dislocations on a potential (001) slip plane were identified in the perovskite phase as well as a high density of {110} twins. Low-angle tilt boundaries consisting of different groups of parallel edge dislocations on the {110} and (001) planes indicate diffusion-assisted climb in perovskite at high temperatures. The differences in dislocation microstructures could be due to activations of limited numbers of slip systems for post-perovskite and of a large number of multiple slip systems for perovskite, which may result in the strong crystallographic preferred orientation (CPO) in post-perovskite and the lack of CPO in deformed perovskite.

Disequilibrium melt distribution during static recrystallization

Melt migration and segregation, and the rheology of partially molten rocks in the upper mantle and lower crust, strongly depend on the grain-scale distribution of the melt. Current theory for monomineralic aggregates predicts a perfectly regular melt framework, but high-temperature experiments with rock-forming minerals + melt show considerable deviations from this predicted geometry. Disequilibrium features, such as fully wetted grain boundaries and large melt patches, have been described; these were mainly attributed to surface-energy anisotropy of the minerals. We present static analogue experiments with norcamphor + ethanol that allow continuous in situ observation of the evolving liquid distribution. The experiments show that all previously reported disequilibrium features can form during fluid-enhanced static recrystallization when small grains are consumed. There is no need to invoke surface-energy anisotropy, although this might enhance the effect. All disequilibrium features are transitory and evolve back toward equilibrium geometry. However, because the system undergoes continuous static recrystallization, disequilibrium features are always present in a partially molten polycrystalline aggregate and therefore control its properties.

Melt migration and melt-rock reactions in the deforming Earth’s upper mantle: Experiments at high pressure and temperature

Crucial to our understanding of melt migration and segregation from source rocks beneath mid-ocean ridges and volcanic provinces is the intimate relationship with ductile mantle deformation. For this reason, numerous experiments and theoretical studies have been performed to simulate the process of melt extraction from deforming mantle rocks, with the emerging view that segregation occurs into melt-rich bands that are inclined relative to the direction of deformation. This is seemingly in contrast, however, to observations made on mantle rocks exposed at the Earth’s surface, which display evidence for melt migration in bands that were originally parallel to the direction of deformation. Here, we present experimental evidence that reconciles these contradictory observations. Olivine aggregates containing 10 wt% of a melt that reacts with olivine to precipitate orthopyroxene were deformed at 2 GPa and 1150 °C in simple shear experiments. In agreement with previous studies, we observe the development of a melt-preferred orientation that is inclined (∼30°) with respect to the main compression axis σ 1 . However, the alignment of newly crystallized orthopyroxene aggregates defines a fabric that develops perpendicular to σ 1 and rotates toward the shear direction with increasing shear strain (γ of 0.3–2). This misalignment significantly changes the interpretation of evidence for melt channeling and transport in exposed upper mantle rocks: the fabric formed by the phases that have crystallized from a melt during deformation cannot be used directly as a marker for the melt migration direction.

A moissanite cell apparatus for optical in situ observation of crystallizing melts at high temperature

Abstract An experimental apparatus is described that allows for optical in situ observation of samples at 1 bar and temperatures up to 1250 °C for durations up to several days. The apparatus resembles a Bassett-type externally heated diamond cell, where the diamond anvils are replaced by cylinders of synthetic moissanite (SiC). The sample is placed inside a gasket of glassy carbon between the two moissanite windows. The moissanite cell allows for the direct and continuous observation of the sample with an optical resolution in the range of a few micrometers. Partially molten systems can be investigated during cooling-heating cycles within a single experimental run. Texture evolution can be observed continuously through time. The technique also allows for direct measurements of crystal growth and dissolution rates at defined cooling and heating rates. The method was tested by observing the crystallization of KNO3-LiNO3 and NaCl-KCl salt melts as well as of a basaltic-trachyandesitic melt. Dendrite formation, static grain growth, and dissolution were observed in the salt melts. During textural coarsening (static grain growth) of rock salt in the presence of melt, grain coalescence was observed. Both nitrate and rock salt grains growing in contact with melt generally showed curved boundaries and roundish shapes, rather than facets. In one experiment, the final texture did not contain any traces of the previous textural evolution anymore. In a basaltic trachyandesite at temperatures near the liquidus several phenomena, in particular the interactions between crystals, magmatic flow and bubbles during nucleation, growth and dissolution of crystals, and crystal-clusters were observed simultaneously. Crystal growth rates and dissolution rates were also measured

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

The development of shape- and crystallographic-preferred orientation in CaPtO3 post-perovskite deformed in pure shear

Abstract Knowledge of the deformation mechanism of post-perovskite is important for interpreting observed seismic anisotropy in terms of mantle flow. Experiments on post-perovskite MgSiO3 and the low-pressure analog material CaIrO3 yield different textures, leaving the interpretation of the observed seismic signatures unclear. Here we present results of deformation experiments on CaPtO3 post-perovskite that may be a better analog to MgSiO3. Post-perovskite CaPtO3 deforms by glide of [100] dislocations on the (010) plane, consistent with previous experimental results on CaIrO3. In addition, samples containing a weak minority phase also display shape-preferred orientation with grains elongated in the crystallographic a-direction forming a planar fabric perpendicular to the compression direction. This shape-preferred orientation strengthens the observed crystallographic-preferred orientation and results in a rapid development of texture during deformation. This observation supports the recent suggestion that the D″ reflector might be due to a rapid generation of texture in post-perovskite. Furthermore, the role of shape-preferred orientation in generating seismic anisotropy in multi-phase assemblages should be considered for the D″ assemblage.

Experimental deformation of ordered natural omphacite: a study by transmission electron microscopy

The Ca–Na clinopyroxene omphacite and garnet are the defining minerals and main phases of eclogites. The rheological properties of eclogites are dominated by the mechanical behaviour of omphacite. Omphacite can occur in an ordered structure with the space group P 2/ n and, above 800 °C, in a disordered structure with the space group C 2/ c . Most eclogites found in the Earth’s crust contain ordered omphacites with the space group P 2/ n . While there is an increasing number of studies by transmission electron microscopy (TEM) of deformation effects in omphacites from eclogites, corresponding TEM studies of experimentally deformed P 2/ n -omphacite are lacking. Natural omphacites from Tianshan and Weissenstein with space group P 2/ n were deformed at temperatures from 500 to 800 °C and strain rates from ca. 10 −4 to 10 −6 s −1 in a deformation-DIA (D-DIA) apparatus at usually 2 GPa confining pressure. In seven experiments, the specimens were coaxially shortened; one specimen was deformed by simple shear. TEM investigations of the experimentally deformed specimens revealed deformation twin lamellae on (100) and (001), traces of failed twin attempts on (100), dislocations, faults parallel to (010) and {110} and non-crystallographic faults. These crystal defects were not observed in all the specimens; however, a correlation between the deformation effects and the experimental conditions was not apparent. Most of the experimentally produced crystal defects were also found in naturally deformed omphacites. The experiments show that there are sufficient mechanisms for deformation of P 2/ n -omphacite at lower temperatures, which can compensate for the loss of perfect dislocations with the Burgers vector 1/2〈110〉, which are available only in C 2/ c -omphacite.

Dislocations and plasticity of experimentally deformed coesite

Dislocation microstructures have been characterized by transmission electron microscopy in polycrystalline coesite deformed experimentally at 4 GPa, 1200 °C. Burgers vectors have been determined by large-angle convergent-beam electron diffraction. Sample orientation was assisted by precession electron diffraction to overcome difficulties arising from pseudo-hexagonal symmetry. The results are explained by using a pseudo-hexagonal setting. We found that most dislocations observed are of the ⅓〈2110〉 type. No clear glide plane was identified, suggesting that climb is activated under these conditions. This conclusion is supported by the observation of numerous subgrain boundaries. We have also observed some [0001] dislocations. Finally, the C 12 /c 1 space group to which coesite belongs being centred, an additional slip system is observed: ⅙ [1213] (0111) (½ [110] (110) in the monoclinic setting).