Oxide perovskites: pressure derivatives of the bulk and shear moduli

The pressure derivatives of elastic moduli (∂M/∂P; M=K_S and G) for a suite of polycrystalline oxide perovskites (2 titanates, 1 stannate and 2 aluminates) have been measured up to 3 GPa using the ultrasonic interferometry method combined with a buffer rod technique. Two empirical systematic relationships (∂G/∂P vs K_S/G and ∂K_S/∂P vs K_S (/ρ)^1/3) have been used to investigate the elasticity systematics of this suite of perovskites and to estimate ∂M/∂P of MgSiO₃ perovskite. The pressure derivatives ∂G/∂P and ∂K_S/∂P for this suite of perovskites scatter between well-defined linear trends for the rutile, rocksalt and spinel structures. The more diffuse trends observed for the perovskites might reflect greater flexibility in the response of its corner-connected octahedral framework structure to changing pressure. The pressure derivatives of the elastic moduli for MgSiO₃ perovskite estimated by the “perovskite bands” are ∂G/∂P=1.6–2.2 and ∂K_S/∂P=3.9–4.2. Received: 13 November 1997 / Revised, accepted: 31 August 1998     

Pressure and temperature dependence of the single crystal elastic moduli of the cubic perovskite KMgF3

The elastic moduli (c) of single crystal KMgF₃ have been determined by the ultrasonic pulse superposition technique as a function of temperature from T=298?550 K, and as a function of pressure from P=1 bar?2.5 kbar. Room temperature values of the elastic moduli and their temperature derivatives are consistent with Reshchikova's (1969) values. Comparison with the data for SrTiO₃ indicates that, for most of the moduli, 1/c(?c/?T) _P and (?c/?P) _T are very similar for the fluoride-oxide analogue pair, KMgF₃-SrTiO₃. Values of (?c/?P) _T for KMgF₃ are calculated from a simple central force model using parameters determined for KF and are in good agreement with the measured values. The bulk sound velocity-mean atomic weight relationship, v _ф M ^1/2=constant, is well obeyed by the fluoroperovskites; comparison with the perovskite oxide data on a log-log plot of v _ф versus M leads to a value of 70% for the relative effective charge of the oxides with respect to the fluorides.     

Molecular dynamics study of the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2

Molecular dynamics (MD) simulations have been used to calculate the structures and bulk moduli of crystals in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) using an interatomic potential model (CMAS94), which is composed of pairwise additive Coulomb, van der Waals, and repulsive interactions. The crystals studied, total of 27, include oxides, Mg meta- and ortho-silicates, Al garnets, and various Ca or Al bearing silicates, with the coordination number of cations ranging 6 to 12 for Ca, 4 to 12 for Mg, 4 to 6 for Al, and 4 and 6 for Si. In spite of the simplicity of the CMAS94 potential and the diversity of the structural types treated, MD simulations are quite satisfactory in reproducing well the observed structural data, including the crystal symmetries, lattice parameters, and average and individual nearest neighbour Ca-O, Mg-O, Al-O, and Si-O distances. In addition MD simulated bulk moduli of crystals in the CMAS system compare well with the observed values.     

Thermal expansion,Debye temperature and Grüneisen parameter of synthetic (Fe,Mg)SiO3 orthopyroxenes

Thermal expansion properties of synthetic orthopyroxenes (Fe_0.20Mg_0.80)SiO₃, (Fe_0.40Mg_0.60)SiO₃, (Fe_0.50Mg_0.50)SiO₃, (Fe_0.75Mg_0.25)SiO₃ and (Fe_0.83Mg_0.17)SiO₃ were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO₃) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO₃) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α _a and α _c both increase as temperature is elevated, with α _c increasing faster, while α _b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α _b >α _c >α _a between 300 and 500 K, but at higher temperatures, the order changes to α _c >α _b >α _a for Mg-rich orthopyroxenes and α _c >α _a >α _b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α _b is mainly attributed to the changes in the Fe₂₊ population at the M2 site and the relative stiffness of the M2(Fe₂₊)-O bonds compared to the M2(Mg₂₊)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10^?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α _V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO₃) to 561 K (FeSiO₃), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK₀/dP) are also composition-dependent and decrease from 11.2 (MgSiO₃) to 8.77 (FeSiO₃). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.     

A dimensional analysis to quantify the thermal budget around lithospheric‐scale shear zones

The thermal evolution around shear zones is controlled by three major thermal processes: diffusion, advection and shear heating. We present a dimensional analysis to quantify, to first‐order, the relative contributions of these three processes to the thermal evolution around lithospheric‐scale shear zones. We consider 11 parameters that control the kinematics, the three‐dimensional (3‐D) geometry, the initial thermal structure and the average strength of the shear zone. Three dimensionless parameters are presented to quantify the relative contributions of the three thermal processes. We validate the dimensional analysis with 2‐D thermo‐kinematic numerical models. The applicability of the dimensional analysis to any kind of shear zone (i.e. thrust, normal‐slip and strike‐slip shear zones) makes it a useful tool that is complementary to previous numerical and analytical studies. Finally, thrust‐type shear zones are used to illustrate how the three thermal processes control the thermal evolution.     

基于非线性规划法的滑动面抗剪强度参数优化

实际工程中,由于土质的不均匀性、试验条件限制、取样和运输过程产生扰动等因素的制约,使得岩土抗剪强度参数的试验值与实际相差太大。为了取得可靠的计算参数,有必要对滑动带土的抗剪强度参数C,φ进行优化,本文根据滑坡的变形发展阶段确定了合理的稳定系数后,利用Microsoft Excel非线性规划求解器的既约梯度法,对边坡的抗剪强度参数进行了优化,得到了可靠的滑坡稳定系数和抗剪强度参数。     

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ¹³C and δ¹⁸O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ¹³C and δ¹⁸O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ¹³C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ¹³C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.     

Numerical modeling and neural networks to identify model parameters from piezocone tests: II. Multi‐parameter identification from piezocone data

This paper completes the study presented in the accompanying paper, and demonstrates a numerical algorithm for parameter prediction from the piezocone test (CPTU) data. This part deals with a development of neural network (NN) models which are able to map multi‐variable input data onto typical geotechnical characteristics and constitutive parameters of the modified Cam clay model, which has been applied in this study. The identification procedure is designed for the coupled hydro‐mechanical boundary value problem in normally‐and lightly overconsolidated clayey soils including partially drained conditions that may appear during cone penetration. The NN models are trained with pseudo‐experimental measurements derived with the aid of the numerical model of the piezocone test, presented in the accompanying paper. Different input configurations containing CPTU measurements and some complementary data are studied with respect to the accuracy of predicted parameter values. Finally, the performance of the developed NN predictors is tested with field CPTU data which are derived from three well‐documented characterization sites, and the obtained predictions are compared with benchmark laboratory results. Copyright © 2011 John Wiley & Sons, Ltd.     

A determination of the bulk rotational deformation resulting from displacements in discrete shear zones in the Hercynian Fold Belt of South Ireland

In the Hercynian Fold Belt of South Ireland, near the town of Ardmore, County Waterford, units of openly folded Upper Palaeozoic sandstone and siltstone are traversed by closely spaced arrays of en échelon quartz veins. These arrays mark the sites of shear zones which are bounded by weakly deformed wall rocks. We used the attitude of cleavage and the sigmoidal shapes of veins to calculate the cumulative shear strain across each zone, and then determined the bulk deformation suffered by the outcrop by tensor analysis, with the aid of an off-axis Mohr circle construction. We conclude that, as a consequence of vein formation, the rocks at this outcrop suffered significant bulk deformation with an 8–14° clockwise rotation.     

干旱半干旱地区包气带热参数模型研究

在干旱半干旱地区,包气带垂向温度梯度对其中水分运动的影响不可忽略。欲定量研究温度变化引起的包气带水分运移,需要建立水热耦合模型,而模型中热参数的确定成为模型求解的关键和难点。本文以实测数据为基础,对数据进行深入分析,提出一种以含水率为自变量推求导热系数的模型,模型中提出饱和导热系数和残余导热系数的概念,并对其可靠性、求解方法及敏感性进行了分析和验证,结果认为该模型能够方便求解水热耦合模型中的导热系数,并且可用多种方法求解,拟合精度高,实用性强。     

A molecular dynamics study of bulk and shear viscosity of liquid iron using embedded-atom potential

Molecular dynamics simulations employing a many-body embedding potential model have been conducted to calculate both bulk and shear viscosity of pure liquid iron at the Earth's outer core conditions. Liquid iron shear viscosity thus obtained is in the order of 10⁻² Pa · s and is in close agreement with previous estimates. In contrast, liquid iron bulk viscosity is in the order of 10⁻³ to 10⁻⁴ Pa · s and is much smaller than previous estimates. Consequently, the ratio of bulk to shear viscosity is close to 0.1. This value disagrees with both the common speculation that bulk and shear viscosities are equal at ambient pressure, and the previous inference that bulk viscosity of liquid iron is much larger than shear viscosity at outer core conditions. Potential implications of present data are also briefly given for the dynamic state of the outer core. Received: 9 April 1999 / Revised, accepted: 7 June 1999     

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones ₁) to steep inclination (shear zones ₂). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.     

Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Garnets from recrystallized, staurolite- and kyanite-bearing mica schists from the central Saualpe basement, representing the host rocks of the type-locality eclogites, give concordant Sm–Nd garnet–whole-rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre-sized, mostly inclusion-free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re-equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20  kbar and 680  °C and similar to those recorded by the eclogites. The garnet rims record about 575  °C/7  kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole-rock Sm–Nd age of 94.0±2.7 Ma.
Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.
The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high- P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c . 600  °C for this isotopic system in garnet.     

西安咸阳地下热水氘过量参数研究

通过分析西安、咸阳地区地下热水的δ18O和δD同位素数据,研究了氘过量参数d的特征及水岩交换程度对d值分布的影响。结果表明:西安、咸阳地区浅层地下热水水岩作用不明显,深层地下热水水岩交换作用显著,发生氧漂移,咸阳部分水点发生较为明显的2H交换;含水层封闭性越好,地质环境越还原,水岩作用就越强,d值就越小;氧漂移愈偏离大气降水线,d值越小,埋深愈大,滞留时间愈长,矿化度愈大,地下热水温度越高。     

The thermal history of the Acapulco meteorite and its parent body deduced from U/Pb systematics in mineral separates and bulk rock fragments

U/Pb systematics of the Acapulco meteorite have been determined on phosphate and feldspar separates and on grain size fractions of bulk material. The latter show an enrichment of U and Th with respect to CI chondrites and a low (∼1) Th/U ratio. This is consistent with the model that the majority of U and Th was added early by a low temperature melt to the Acapulco precursor. The feldspar exhibits a Pb isotope composition that is close to the primordial Pb composition. Mineral separates and bulk fractions define a ²⁰⁷Pb/²⁰⁶Pb isochron. The age corresponds to 4555.9 ± 0.6 Ma. This age anchors the thermal evolution of the Acapulco parent body into an absolute time scale. Evaluation of the Hf/W and U/Pb records with the cooling rates deduced from mineralogical investigations confirms the idea that the Acapulco parent body was fragmented during its cooling. The U/Pb system precisely dates this break-up at 4556 ± 1 Ma.     

3D seismic study of a ductile shear zone from laboratory and petrofabric data (Saint Barthélémy Massif, Northern Pyrenees, France)

In order to constrain interpretations of seismic reflection records more effectively, the seismic properties of a middle crustal section exposed in the Saint Barthélémy Massif have been determined. The massif, transected by a 200 m thick shear zone has been systematically sampled for density measurements and modal analysis has been performed in order to define the spatial variations of physical properties. Seismic velocities ( V _p, V _S, shear wave birefringence), have been measured on five representative samples to 600 MPa and 600°C simultaneously in the three structural directions (X, Y and Z). For two samples, the experimental data have been compared with calculated values, based on petrofabric analyses. The Lattice Preferred Orientation (LPO) is determined using universal stage, electron channelling microscopy and neutron diffraction goniometry. Using the experimental and calculated velocity data, we establish a lateral homogeneous anisotropic model.     

Crustal thermal regime of Ikogosi warm spring, Nigeria inferred from aeromagnetic data

Spectral analysis method was applied to aeromagnetic data obtained for Ikogosi warm spring (IWS) area of southwestern Nigeria. This was done with the objective of determining the bottom of the magnetized crust called Curie point depth (CDP) and understand the nature and extent of the local geothermal system at depth beneath IWS. The depth to the centroid, Z _o, of the deepest distribution of the magnetic dipoles was obtained by computing least-squares fit to the lowest-frequency segment of the azimuthally averaged log power spectrum. The average depth to the top of the deepest crustal block was computed as the depth to the top, Z _t, of the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest magnetic dipoles, the inferred Curie point depth, was then calculated from Z _b?=?2Z _o???Z _t. The Curie depth estimates for IWS range between 4.68 and 11.38 km (below sea level). We also estimate the heat flow and Curie temperature using a one-dimensional conductive heat transport model. The average heat flow, 42 mW m^?2, and geothermal gradient, 32°C/km, obtained suggest a low enthalpy thermal regime. The Curie temperature for the region varies between 153°C and 350°C. Also, an inverse linear relationship between heat flow and Curie depths was determined. Good agreement between the Curie point depths derived from heat flow data and magnetic data suggests that the Curie point depth analysis is useful to estimate the regional thermal structure and the tectonic settings.     

The structure of the Tuapse shear zone according to the field tectonophysical data

The results of field structural studies of the Tuapse shear zone in the Northwest Caucasus are presented. This zone is characterized by shear displacements of various scales with a dominant horizontal shear, viz., a geodynamic type of the stress state, which leads to the formation of faults with mostly lateral displacement of wings, i.e., along the strike of the fault surface. The quantitative characteristics of the local stress conditions in the shear zone (the positions of principal axes and the Lode–Nadai coefficient) are determined on the basis of cataclastic analysis and geological indicators of the paleostresses. The differences between these characteristics are considered for the large tectonic zones. Significant spatial (areal) variations in orientations of the axes of major normal stresses in the shear zone and their local weak gentle variations are evidence of a consistent general stress direction during the formation of faults during the Late Eocene–Miocene deformation epoch.