On the Ekman Spiral with an Anisotropic Eddy Viscosity     

A. Wirth 《Boundary-Layer Meteorology》2010,137(2):327-331

The eddy viscosity is a fourth-order tensor in three-dimensional space. When considering the viscous effects on the horizontal velocities in the vertical direction it is reduced to a second-order tensor in two-dimensional space, and is not necessarily horizontally isotropic. Anisotropic coherent structures (rolls) are a conspicuous feature of the planetary boundary layer. There is no reason to suppose that they should give rise to a horizontally isotropic eddy viscosity. The effects of an anisotropic constant eddy viscosity tensor on the Ekman layer dynamics is determined analytically. The shape of the Ekman spiral is modified. The magnitude of the bottom shear and the Ekman transport is changed in magnitude and rotated within an angle of 90°.  相似文献   

Synthetic near Σ5 (210)/[100] grain boundary in YAG fabricated by direct bonding: structure and stability     

Katharina Hartmann  Richard Wirth  Wilhelm Heinrich 《Physics and Chemistry of Minerals》2010,37(5):291-300

Several macroscopic physical and chemical properties, such as rheology, elasticity, or transport properties are governed by grain boundary processes. An improved understanding of the structure and evolution of grain boundaries has thus become a key challenge in geosciences and material sciences. Here, we report the structure of near Σ5 (210)/[100] grain boundaries in Y₃Al₅O₁₂ (YAG), which were synthesised by the wafer direct bonding method. The produced grain boundaries were annealed at different temperatures, ranging from 673 to 1,873 K. The grain boundaries annealed at different temperatures are not distinguishable based on their flatness and apparent cohesiveness in high resolution TEM (HRTEM) micrographs, but show a considerable step in their mechanical stability at around 1,273 K, a temperature that corresponds to roughly half the melting temperature of YAG. This study further focuses on the effect of a slight misorientation of the two crystals on the grain boundary structure and we discuss if the boundary can reach a state of minimum energy configuration during annealing. Along the grain boundaries, we observed a long-range strain contrast with a periodicity of 40 nm, which has not been reported for high-angle grain boundaries so far. We conclude that this contrast is caused by faceting along the grain boundary plane, which is needed to achieve minimum energy configuration of the grain boundary plane.  相似文献   

The behavior of protons during wet olivine deformation under the conditions of the kimberlite process     

N. R. Khisina  R. Wirth 《Geochemistry International》2010,48(4):338-345

This paper reports the results of optical and electron microscopic investigations of mantle olivine samples with H₂O contents of tens-hundreds ppm weight. Samples were obtained from the xenoliths and xenocrysts of the Udachnaya pipe. At the scale of optical microscope magnification, a peculiar banded microstructure was observed in thin sections prepared parallel to the olivine (010) plane. It is formed by cross-hatched bands parallel to four crystallographic directions of the olivine structure: [100], [001], [101], and [−101]. At the scale of electron optical magnifications, the banded microstructure is observed as nanometer-sized heterogeneities of various types which are related to olivine deformation: (a) planar defects parallel to (100) and (001) corresponding to the (100)[010] and (001)[100] dislocation glide systems, respectively; they are occasionally transformed into lamellae or decorated by nanoinclusions; and (b) nanometer-sized heterogeneities formed by nanoinclusion arrays not related to planar defects and oriented along the same directions of the olivine structure as the optically visible bands. The deformation structures are decorated by coupled point OH-bearing defects, which were initially present in the olivine. The crystallographically oriented arrays of nanoinclusions of high-pressure hydrous silicates are considered as a result of olivine deprotonization (elimination of OH-bearing defects from the olivine structure) in the zones of previous deformation compression in the crystal. Light refraction effects on the nanoinclusions make these zones optically visible and produce the banded microstructure, which is a consequence of previous deformation.  相似文献   

The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation   总被引：1，自引：0，他引：1  

Bernd Wunder  Fabien Deschamps  Anke Watenphul  Stéphane Guillot  Anette Meixner  Rolf L. Romer  Richard Wirth 《Contributions to Mineralogy and Petrology》2010,159(6):781-790

We determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite, and aqueous fluid in the P,T range 0.2–4.0 GPa, 200–500°C. For experiments in the systems lizardite-fluid and antigorite-fluid, ⁷Li preferentially partitioned into the fluid and Δ⁷Li values followed the T-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments, ⁷Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline Li isotope fractionation of octahedral Li^[6] with preference to ⁶Li and lithium within the channels (Li^[Ch]) of chrysotile, favoring ⁷Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements in serpentinized oceanic crust. This might explain higher Li abundances for low-T chrysotile-bearing serpentinites relative to high-T serpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during breakdown of serpentine phases will be depleted in ⁷Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites.  相似文献   

Antigorite-ophicarbonates: Phase relations in a portion of the system CaO-MgO-SiO2-H2O-CO2     

Volkmar Trommsdorff  Bernard W. Evans 《Contributions to Mineralogy and Petrology》1977,60(1):39-56

The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO₂-H₂O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has 0.8$$ " align="middle" border="0"> , and there are indications that during their progressive metamorphism is approximately equal to P _total.  相似文献   

On phase equilibria in metamorphosed siliceous dolomites     

Paul Metz  Volkmar Trommsdorff 《Contributions to Mineralogy and Petrology》1968,18(4):305-309

Basing on calculations and experiments the equilibrium curves of 15 reactions occuring during metamorphism of siliceous dolomites are presented. In a T—X_{CO 2} diagram these curves intersect at four isobaric invariant points. This is consistent with the fact, that different reaction sequences are known from the nature.  相似文献   

Strain softening and microstructural evolution of anorthite aggregates and quartz-anorthite layered composites deformed in torsion   总被引：1，自引：0，他引：1  

Shaocheng Ji  Zhenting Jiang  Richard Wirth  Bin Xia 《Earth and Planetary Science Letters》2004,222(2):377-390

Torsion experiments of anorthite (An) aggregates and layered composites with equal volume fractions of quartz (Qtz) and An were performed in a gas-medium apparatus at a confining pressure of 400 MPa, temperatures from 1373 to 1473 K, and twist rates from 1.0×10⁻⁴ to 3.0×10⁻⁴ rad/s. Dense specimens were fabricated from An glass and Qtz crystalline powder using hot isostatic pressing (HIP) techniques. Both An aggregates and Qtz-An layered composites show a continuous strain weakening from a peak stress at γ=0.2-0.3 to γ=3.2, and steady-state flow has not reached under the experimental conditions. The weakening is even more pronounced in the layered composites than the monolithic aggregates, suggesting channeling or localization of flow into the weak material between strong layers. The sheared An specimens developed pervasively C-S-C′ structures which are similar to those observed in natural ductile shear zones. TEM and electron backscattering diffraction (EBSD) fabric analyses suggest that grain boundary migration recrystallization-accommodated dislocation creep with (010)[100] as the dominant slip system was operating in the An. The strain softening may be due to the development of crystallographic preferred orientation (CPO), the operation of dynamic recrystallization and the formation of extremely fine-grained recrystallized material in the narrow C′ shear bands.  相似文献   

Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits   总被引：1，自引：0，他引：1  

Anne-Magali Seydoux-Guillaume  Richard Wirth  Urs Schärer 《Geochimica et cosmochimica acta》2004,68(11):2517-2527

The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO₂, and ∼0.94 wt. % UO₂. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation.  相似文献   

The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid   总被引：8，自引：0，他引：8  

Bernd Wunder  Anette Meixner  Rolf L. Romer  Richard Wirth  Wilhelm Heinrich 《Lithos》2005,84(3-4):206-216

The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ¹¹B_(mica-fluid) = δ¹¹B_(mica) − δ¹¹B_(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ¹¹B_{(mica,clay,melt–neutral fluid)} = − 10.69 · (1000/T [K]) + 3.88; R² = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ¹¹B_{(mica–fluid)} of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and ¹¹B/¹⁰B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.  相似文献   

Magnetic anisotropy of calcite at room-temperature     

Volkmar Schmidt  Detlef Günther  Ann M. Hirt 《Tectonophysics》2006,418(1-2):63

The intrinsic room temperature magnetic properties of pure calcite were determined from a series of natural crystals, and they were found to be highly dependent on the chemical composition. In general, dia-, para-, and ferromagnetic components contribute to the magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). With a combination of magnetic measurements and chemical analysis these three contributions were determined and related to their mineralogical sources. The intrinsic diamagnetic susceptibility of pure calcite is − 4.46 ± 0.16 × 10^− 9 m³/kg (− 12.09 ± 0.5 × 10^− 6 SI) and the susceptibility difference is 4.06 ± 0.03 × 10^− 10 m³/kg (1.10 ± 0.01 × 10^− 6 SI). These diamagnetic properties are easily dominated by other components. The paramagnetic contribution is due to paramagnetic ions in the crystal lattice that substitute for calcium; these are mainly iron and manganese. The measured paramagnetic susceptibility agrees with the values calculated from the known concentration of paramagnetic ions in the crystals according to the Curie law of paramagnetic susceptibility. Substituted iron leads to an increase in the AMS. The paramagnetic susceptibility difference was found to correlate linearly with the iron content for concentrations between 500 and 10,000 ppm. An empirical relation was determined: (k₁ − k₃)^para (kg/m³) = Fe-content (ppm) × (1 ± 0.1) × 10^− 12 (kg/m³/ppm). The maximum susceptibility difference (Δk = k₁ − k₃) was found to be unaffected by iron contents below 100 ppm. Ferromagnetic contributions due to inclusions of ferromagnetic minerals can dominate the susceptibility. They were detected by acquisition of isothermal remanent magnetization (IRM) and their contribution to the AMS was separated by high-field measurements.  相似文献