Jens M. Walter

Fluid-enhanced low-temperature plasticity of calcite marble: Microstructures and mechanisms

Postmetamorphic carbonate fault rocks from the Damara orogen, Namibia, show the following features: (1) obvious contrasts between centimeter-scale grains in the clasts and nanometer- to micrometer-scale grains in the matrix, and between macroscopic cataclastic and microscopic mylonitic microstructures; (2) coexistence of tangled dislocations and organized dislocation walls; (3) occurrence of subgrains along margins of clasts and their transition into dynamically recrystallized grains in the matrix; and (4) new grains in grain sizes of a few nanometers to 3 μm in diameter. The coexistence of brittle and ductile microstructures is attributed to comprehensive intragranular twinning, and kinking, fracturing, and subsequent dislocation remobilization or reorganization and recrystallization. Fracturing is triggered by dislocation pileups due to dynamic loading, twinning, and kinking. It also generates free dislocations and tangled dislocations. Fractures provide fluid paths, increase fluid-rock interfaces, and enhance the possibility of fluid-rock interaction. Fracturing is subsequently accommodated by low-temperature plasticity that is attributed to hydrolytic weakening, i.e., fluid-enhanced recovery and dynamic recrystallization due to the infiltration of fluids into the deforming grains. During hydrolytic weakening, remobilized free dislocations and tangled dislocations climb toward incoherent boundary-like fractures. The dislocations are reorganized into dislocation walls that commonly constitute parts of subgrains developing into new grains. We conclude that: (1) fluids may increase the rate of dislocation glide and dislocation climb and may also enhance the recovery of strain-hardened rocks to accommodate fracturing processes in calcite marbles at low temperatures; and (2) calcite marble may have low-temperature plasticity and may undergo crystal plastic deformation due to hydrolytic weakening at shallow crustal levels.

On the role and importance of orogen-parallel and -perpendicular extension, transcurrent shearing, and backthrusting in the Monte Rosa nappe and the Southern Steep Belt of the Alps (Penninic zone, Switzerland and Italy)

Abstract During Europe–Adria collision in Tertiary times, the Monte Rosa nappe was penetratively deformed in several stages after an eclogite-facies pressure peak: (1) top-to-the-NW thrust shearing (Mattmark phase, after 40 Ma); (2) orogen-parallel, top-to-the-SW extensional shearing and folding (Malfatta phase); (3) orogen-perpendicular, top-to-the-SE extensional shearing and folding (Mischabel phase, before 30 Ma); and (4) large-scale, upright, SE-vergent folding (Vanzone phase, c. 29–28 Ma). Structural analysis and neutron texture goniometry of quartz mylonites show that the Stellihorn shear zone in the Monte Rosa nappe accommodated a complex and multidirectional sequence of shearing movements during the Mattmark, Malfatta and Mischabel phases, and was folded in the Vanzone phase. In the tail-shaped eastward prolongation of the Monte Rosa nappe in the Southern Steep Belt of the Alps, both dextral and sinistral mylonites (Olino phase) were formed during and after the formation of the Vanzone fold, reflecting renewed orogen-parallel (SW–NE) extension contemporaneous with NW–SE shortening from c. 29 Ma onward. A similar sequence of deformation stages was identified in the Adula nappe at the eastern border of the Lepontine metamorphic dome. Important consequences arise for the Insubric fault at the southern border of the Lepontine dome: (1) the NW- to N-dipping orientation of the Insubric fault is not a primary feature but resulted from rotation of an originally SE-dipping shear zone after c. 30 Ma; and (2), the strong contrast in metamorphic grade across this fault (upper amphibolite facies to the north versus anchizone to the south) results from north-side-up faulting coupled with orogen-parallel extension of the northern block (Lepontine dome), while no such extension occurred in the southern block (Southern Alps). Extension in the northern block started in the Malfatta phase and continued in the Mischabel phase when the foliation in the area which later became the Southern Steep Belt still dipped towards south. During Vanzone/Olino deformation, further unroofing and uplift of the Lepontine dome relative to the South Alpine block took place while the Southern Steep Belt was progressively rotated into its present, overturned position, changing its character from a normal fault into a backthrust. Complex deformation paths in the Southern Steep Belt resulted from the combination of extension of the northern block with strike-slip motion along the Insubric fault.

Magnetic order in the double pyrochlore Tb2Ru2O7

Polycrystalline Tb(2)Ru(2)O(7) has been studied using dc susceptibility, specific heat and neutron scattering techniques. The high temperature paramagnetic state is dominated by the single ion character of Tb(3 + ) and very similar to that of the well-studied spin liquid Tb(2)Ti(2)O(7). However, both the Ru(4 + ) and Tb(3 + ) sublattices order, at about 110 K and 3.5 K, respectively. Although the Tb sublattice does not fully order until 3.5 K, it is polarized in the presence of the internal field generated by the Ru(4 + ) sublattice and possesses a significant moment at 7 K. Magnetic entropy measurements suggest that four levels exist in the first 30 K and inelastic neutron scattering investigations revealed two more levels at 10 and 14 meV. As the magnetic sublattices order, the excitations are perturbed from that measured in the paramagnetic state. These data are compared to data for other terbium based and double pyrochlores.

Evolution of an Early Permian extensional detachment fault from synintrusive, mylonitic flow to brittle faulting (Grassi Detachment Fault, Orobic Anticline, southern Alps, Italy)

Abstract Lower Permian volcanic and sedimentary rocks of the Collio Formation in the Orobic Anticline do not rest with a depositional contact on the Variscan basement but are separated from it by the subhorizontal Grassi Detachment Fault, consisting of a cataclasite layer underlain by mylonite. Field relations indicate that both the cataclasite and the mylonite are Early Permian in age. The mylonite formed in a continuous process before, during, and after the intrusion of the Val Biandino Quartz Diorite in the footwall of the detachment fault. Microstructure and quartz texture of the mylonite indicate top-to-the-southeast displacement. Quartz textures of mylonite close to the intrusive bodies are characterized by c-axis single maxima near the Y-direction of the finite strain, indicating prism glide as the dominant gliding system and hence high temperatures (above c. 500 °C) during mylonitization. This is explained by heat advection through the rising quartz diorite melt. During detachment faulting, the footwall of the Grassi Detachment Fault was bowed up to form a metamorphic core complex. The Ponteranica Conglomerate was deposited as a proximal, syntectonic fan-delta on the southeast side of the metamorphic core complex late in its evolution. The unconformity of the Verrucano Lombardo over the Collio Formation and the basement results from erosion of the topography created by detachment faulting, core complex updoming, and block tilting. These results indicate dramatic SE–NW stretching (in present-day coordinates) of the South-Alpine crust during the Early Permian. The return from the thickened, orogenic crust at the end of the Hercynian orogeny to the normal crustal thickness (c. 30 km) of Late Permian and Early Triassic times was accommodated to a large extent by crustal extension, at least in this part of the southern Alps.

On the thermo-elastic behaviour of kyanite: a neutron powder diffraction study up to 1200°C

Abstract The high-temperature (HT) behaviour of kyanite (Al2SiO5) was investigated by in situ neutron powder diffraction up to 1200°C. Within the investigated T range, no phase transition was observed. The axial and volume thermal expansion coefficient (αj = lj−1(∂lj/∂T), αV = V−1 (∂V/∂T)), calculated by weighted linear regression through the data points, are: αa = 5.5(2)×10−5, αb = 5.9(2)×10−5, αc = 5.18(8)×10−5, αV = 7.4(1)×10−3 °C−1, with αa:αb:αc = 1.06:1.14:1. All three angles of the kyanite lattice show a slight decrease with T, with ∂α/∂T = −2(2)×10−5, ∂β/∂T = −4(1)×10−5, ∂γ/∂T = −10(2)×10−5 °C. The magnitudes of the principal Lagrangian unit-strain coefficients (Ɛ1, Ɛ2, Ɛ3) and the orientations of the thermal strain-ellipsoids, between the ambient temperature and each measured T, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant with T. At T-T0 = 1177°C , Ɛ1^a = 76(2)°, Ɛ1^b = 70(2)°, Ɛ1^c = 38(3)°, Ɛ2^a = 49(3)°, Ɛ2^b = 66(3)°, Ɛ2^c = 127(4)°, Ɛ3^a = 135(3)°, Ɛ3^b = 31(3)°, Ɛ3^c = 91(2)° with Ɛ1:Ɛ2:Ɛ3 = 1.57:1.29:1. The structural refinements, performed at 23, 600, 650, 700, 750, 800, 900, 950, 1050 and 1200°C allowed the description of the structural evolution and the main T-induced deformation mechanisms, which are mainly represented by the polyhedral distortions of the AlO6 octahedra.

Correlation between structural and magnetic properties of La7/8Sr1/8Mn1−γO3+δ with controlled nonstoichiometry

The structural and magnetic properties of air sintered and Ar and O2 annealed La0.875Sr0.125Mn1−γO3+δ polycrystalline samples have been studied systematically. From the simultaneous refinement of room-temperature x-ray and neutron powder diffraction data, we have found that the crystal structure is orthorhombic (Pbnm, Z = 4; O) for the Ar annealed sample and rhombohedral (, Z = 2; R) for the air sintered and O2 annealed samples. At the O–R transition, the average Mn–O–Mn bond angle increases from 161.08(4)° (Ar) to 163.38(1)° (O2) and 163.64(1)° (air). Ar and O2 annealings cause a decrease of the Curie temperature Tc from 240(1) K (air) via 237(1) K (O2) to 192(1) K (Ar). The Tc is reduced by ~20% for the sample with O structure (Ar) as compared with that of the samples with R structure (air and O2). The decrease of Tc is explained by the decrease of total strength of magnetic interactions, due to the decreased bond angle and increased bond length, and by the decrease of number of nearest magnetic neighbours. The possible reasons for the decrease of the Mn site occupancy after Ar and O2 annealings and the sources of apparent excess oxygen are discussed. Thus we have shown that the actual distributions of cation vacancies in the A (La and Sr) and B (Mn) sublattices induced by the changes of Mn and oxygen contents through Ar and O2 annealings play an important role in the structural and magnetic properties. Comparing the Ar annealed with the air sintered sample, a decrease as small as ~2.2% of the relative oxygen content leads to a remarkably large (~20%) effect on Tc.

Evidence of extinction in strongly textured high-purity copper

In order to test the influence of extinction on texture measurements, pole figures of four copper samples were measured using neutron diffraction with three different wavelengths. The copper samples had previously been cold rolled to different extents and partially annealed. Therefore, three of the samples presented a relatively strong texture and the fourth sample a relatively weak texture. In this study, the results from equivalent pole figures were compared between the different samples. These investigations showed that, in general, except for the sample with weak texture, the 222 reflection showed higher pole density maxima than the 111 reflection. Since all measuring conditions were the same for both reflections, this difference was attributed to extinction. The pole density maxima of strong reflections also revealed a slight tendency to lower values with decreasing wavelengths. This phenomenon is attributed to the fact that more reflections exist at shorter wavelengths and therefore an increase in secondary extinction is measurable at these wavelengths.