Oxygen fugacity dependence of Os solubility in haplobasaltic melt

Os equilibrium solubilities were determined at 1350 °C over a wide range of oxygen fugacities (−12 < log fO₂ < −7) applying the mechanically assisted equilibration technique (MAE) at 10⁵ Pa (= 1 bar). Os concentrations in the glass samples were analysed using ID-NTIMS. Additional LA-ICP-MS and SEM analyses were performed to detect, visualize and analyse the nature and chemistry of “nanonuggets.” Os solubilities determined range at a constant temperature of 1350 °C from 0.63 ± 0.04 to 37.4 ± 1.16 ppb depending on oxygen fugacity. At the highest oxygen fugacities, Os³⁺ can be confirmed as the main oxidation state of Os. At low oxygen fugacities (below log fO₂ = −8), samples are contaminated by nanonuggets which, despite the MAE technique, were still not removed entirely from the melt. However, the present results indicate that applying MAE technology does reduce the amount of nanonuggets present significantly, resulting in the lowest Os solubility results reported to date under these experimental conditions, and extending the experimentally accessible range of fO₂ for these studies to lower values. Calculated metal/silicate melt partition coefficients are therefore higher compared to previous studies, making Os more siderophile. Neglecting the as yet unknown temperature dependence of the Os metal/silicate melt partition coefficient, extrapolation of the obtained Os solubilities to conditions for core-mantle equilibrium, results in a , while metallic alloy/silicate melt partition coefficients range from 1.4 × 10⁶ to 8.6 × 10⁷, in agreement with earlier findings. Therefore remains too high by 2-4 orders of magnitude to explain the Os abundance in the Earth’s mantle as result of core-mantle equilibrium during core formation.     

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10⁻⁶ and 10⁻³ s⁻¹. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.     

Flow laws in polymineralic aggregates deformed by a combination of diffusion creep and dislocation creep

Microstructures in naturally deformed rocks in the upper crust demonstrate that creep strain in nature may be accommodated by a combination of dislocation creep, diffusion/dissolution processes and microcracking. A theoretical approach towards deriving an aggregate flow law is presented, where the strain in the constituent phases is assumed to occur by simultaneous operation of diffusive mass transfer and crystal plastic mechanisms (dislocation creep). Both uniform stress and uniform strain rate situations are considered.     

Cordierite-garnet-H2O equilibrium: A geological thermometer,barometer and water fugacity indicator

The high-grade assemblage Cd-Ga-Si-Qz can be thermodynamically modelled from available calorimetric data on the metastable reaction: (I) $$3 MgCd \rightleftarrows 2 Py + 4 Si + 5 Qz$$ naturalK _D ^Fe-Mg between garnet and cordierite and experimental results on cordierite hydration. In the system SiO₂-Al₂O₃-MgO-H₂O, reaction (I) becomes (II) $$3 MgCd \cdot nH_2 O \rightleftarrows 2 Py + 4 Si + 5 Qz + 3 nH_2 O$$ . However, hydrous cordierite is neither a hydrate nor a solid solution between water and anhydrous cordierite and when nH₂O (number of moles of H₂O in Cd) is plotted against \(P_{H_2 O} \) , the resulting isotherms are similar to adsorption isotherms characteristic of zeolitic minerals. Reaction (II) can thus be considered as a combination of reaction (I) with a physical equilibrium of the type nH₂O (in Cd)?nH₂O (in vapor phase). Starting from a point on equilibrium (I), introduction of H₂O into anhydrous Mg-cordierite lowers the chemical potential of MgCd and hence stabilizes this mineral to higher pressure relative to the right-hand assemblage in reaction (I). The pressure increment of stabilization,ΔP, above the pressure limit of anhydrous cordierite stability at constantT, has been calculated using the standard thermodynamics of adsorption isotherms. Cordierite is regarded as a mixture of Mg₂Al₄Si₅O₁₈ and H₂O. The activity of H₂O in the cordierite is evaluated relative to an hypothetical standard state extrapolated from infinite H₂O dilution, by using measured hydration data. The activity of Mg₂Al₄Si₅O₁₈ in the cordierite is then obtained by integration of the Gibbs-Duhem equation, and the pressure stabilization increment,ΔP, computed by means of the relation: $$\Delta V_s \Delta P \cong - RT\ln a_{MgCd}^{MgCd \cdot nH2O} \left( {\Delta V indepentdent of P and T} \right)$$ . Thus, one may contour theP-T plane in isopleths of nH₂O=constant within the area of Mg-cordierite stability allowed by the hydration data for \(P_{H_2 O} = P_{total} \) . The present model indicates greater stabilization of cordierite by H₂O than the model of Newton and Wood (1979) and the calculated curve for metastable breakdown of hydrous MgCd is consistent with experimental data on cordierite breakdown at \(P_{H_2 O} = P_{total} \) . Mg/(Mg+Fe) isopleths have been derived for cordierites of varying nH₂O in the SiO₂-Al₂O₃-MgO-FeO-H₂O system using the additional assumptions that (Fe, Mg) cordierite and (Fe, Mg) garnet behave as ideal solutions, and that typical values of the distribution coefficient of Fe and Mg between coexisting garnet and cordierite observed in natural parageneses can be applied to the calculations. The calculated stable breakdown curve of Fe-cordierite under conditions of \(P_{H_2 O} = P_{total} \) to almandine, sillimanite, quartz and vapor has a positive slope (dP/dT) apparently in contrast to the experimental negative slope. This may be explained by hydration kinetics, which could have allowed systematic breakdown of cordierites of metastable hydration states in the experiments. The bivariant Cd-Ga fields calibrated from the present model are, potentially, good petrogenetic indicators, as, given the iron-magnesium ratio of garnet and cordierite and the hydration number of cordierite, the temperature, pressure and water fugacity are uniquely determined. As this geothermobarometer is in part based on the water content of cordierite, it can be used only if there is some assurance that the actual hydration is inherited from high-grade metamorphic conditions. Such conditions could be realised if the slope of unloading-cooling retrograde metamorphism is more or less parallel to a cordierite isohydron.     

Effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt

The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940–1,220°C. The oxygen fugacity was measured using the H₂-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by ~3 log units per series. The range in oxygen fugacity investigated spans ~7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr–spinel and Fe–Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr–spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxide-free crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.     

The dependence of the partitioning of iron and europium between plagioclase and hydrous tonalitic melt on oxygen fugacity

The dependence of iron and europium partitioning between plagioclase and melt on oxygen fugacity was studied in the system SiO₂(Qz)—NaAlSi₃O₈(Ab)—CaAl₂Si₂O₈(An)—H₂O. Experiments were performed at 500 MPa and 850 °C/750 °C under water saturated conditions. The oxygen fugacity was varied in the log f _O2-range from −7.27 to −15.78. To work at the most reducing conditions the classical double-capsule technique was modified. The sample and a C—O—H bearing sensor capsule were placed next to each other within a BN jacket to minimise loss of hydrogen to the vessel atmosphere. By this setup redox conditions slightly more reducing than the FeO—Fe₃O₄ buffer could be maintained even in 96 h runs. Raman spectra showed that the BN was modified by reaction with hydrogen resulting in a low hydrogen permeability. The partition coefficients determined for Eu at 850 °C and 500 MPa vary from 0.095 at conditions of the Cu—Cu₂O buffer to 1.81 at the most reducing conditions (C—O—H sensor). In the same f _O2 interval the partition coefficient for Fe varies from 0.55 at oxidising conditions to 0.08 at the most reducing conditions. The partitioning of Sm, which was added as a reference for a trivalent REE, does not vary with the oxygen fugacity, yielding an average value for D = 0.07. Lowering the temperature to 750 °C for a given f _O2 decreases the partition coefficient of Eu and increases that of Fe. Comparison with published data at 1 atm and at higher temperatures shows that both temperature and composition of the melt have strong effects on the partitioning behaviour. As the change of the partition coefficients in the geologically relevant f _O2 range is quite strong, element partitioning of Eu and Fe might be used to estimate redox conditions for the genesis of igneous rocks. Furthermore, by modelling the partitioning data it is possible to extract information about the redox state of the melt. Resulting ferric-ferrous ratios show significant differences from those predicted by empirical models. Received: 14 October 1998 / Received: 5 March 1999     

A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling

 The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward geobarometric formulation in the absence of direct X-ray analysis is: P(kbar)=698.443+4.985⋅Al_T−26.826⋅Fe²⁺ _M1−3.764⋅Fe³⁺ +53.989⋅Al_M1+3.948⋅Ti+14.651⋅Cr −700.431⋅Ca−666.629⋅Na−682.848⋅Mg_M2−691.138⋅Fe²⁺ _M2−688.384⋅Mn−6.267⋅(Mg_M2)²−4.144⋅(Fe²⁺ _M2 where: (Fe²⁺ _M1⋅Mg_M2)/(Fe²⁺ _M2⋅Mg_M1)=e^**(0.238⋅R³⁺+0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R³⁺=Al_M1+Fe³⁺+Ti+Cr, with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev. ≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al₂O₃ basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pressure. Within the considered compositional space at given P-T, a _CaO and a _SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al₂O₃ (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer. Received: 3 April 1994 / Accepted: 23 December 1995     

Crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep

The crystallographic preferred orientations of a series of experimentally deformed fine-grained albite aggregates were measured by synchrotron source X-ray diffraction. Most samples were deformed and extensively recrystallized by low-temperature recrystallization-accommodated dislocation creep. In axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on {001}<100>; the sheared samples have a marked asymmetry of the <100> maxima with respect to the shear zone boundaries. One sample was axially compressed by solution precipitation creep; it developed a somewhat different but equally strong preferred orientation, perhaps reflecting crystallographic anisotropy in rates of dissolution and growth.     

Experimental determination of Ni diffusion coefficients in olivine and their dependence on temperature, composition, oxygen fugacity, and crystallographic orientation

Diffusion couple experiments were carried out with San Carlos olivine (Fo₉₀) and NiFe alloys (Ni₁₀₀, Ni₉₇Fe₃, Ni₉₀Fe₁₀) or other olivine compositions (Fo₁₀₀, Fo₂₅) in order to determine the dependence on temperature, oxygen fugacity, composition and crystallographic orientation of Ni diffusion coefficient (D_Ni) in olivine. Experiments at 1 atmosphere total pressure cover a temperature range of 900-1445°C with run durations from 48 to 2155 h at different oxygen fugacities. In an Arrhenius plot the best fit for all data for Fo₉₀ yields an activation energy (E_D) of 220 ± 14 kJ/mol and an fO₂ dependence of (1/4.25)·Δ log fO₂ = Δ log D_Ni. The relationship between diffusion coefficients along different crystallographic axes at 1200°C is given by D_[001] ≈ 6·D_[100] ≈ 6·D_[010]. D_Ni depends strongly on the major element (i.e. Fe/Mg) composition of olivine and decreases by about 1 order of magnitude as the olivine composition changes from Fo₃₅ to Fo₉₀. Thus, experimental investigations in Fe-free systems cannot be applied to natural samples. For calculation of residence times or cooling rates the present Ni data yield shorter timescales compared to those obtained using diffusion data published until now.In addition to Ni diffusion coefficients, Fe-Mg, Mn and Ca diffusion data were obtained from some of the same diffusion couples (Fo₉₀-Fo₁₀₀). It is found that the activation energies, E_D[Ni] ≅ E_D[Fe-Mg] ≅ E_D[Mn] ≤ E_D[Ca]. All diffusion coefficients are strongly dependent on the major element composition of olivine.     

The effect of oxygen fugacity on ionic conductivity in olivine

The oxygen fugacity(f_O2) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe²⁺ to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to f_O2 under relatively reduced conditions(f_O2 below Re-ReO₂ buffer), whereas it has a clear f_O2-dependence under relatively oxidized conditions(f_O2 around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions.     

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ～8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.     

Study on the compositional dependence of the apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene solid solution

Compositional dependence of apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene from Mt. Higasiakaisi is studied by means of a multicomponent regular solution model. It is shown that garnet and clinopyroxene solid solutions are positively non-ideal, and the non-ideal parameters according to the symmetric regular solution model are 2.58 kcal and 2.39 kcal, respectively, assuming the equilibration temperature of the mass to be 550° C.Notations a _i ^h activity of component i in phase h - _ij interaction parameter of component i and j in a solid solution - _i activity coefficient of component i - X _i mole fraction of component i - K partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - K apparent partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - G ⁰ difference in free energy of the partition reaction - H ⁰ difference in enthalpy of the partition reaction - S ⁰ difference in entropy of the partition reaction - R gas constant - G garnet - Alm almandine component - Py pyrope component - Gr grossular component - Sp spessartine component - CPx clinopyroxene - Hd hedenbergite component - Di diopside component - Jd jadeite component - Ts Tschermac's molecule component Deceased on April 17, 1974.     

Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and clinopyroxene

An experimental study initiated to calibrate the distribution coefficient \(K_D = \frac{{({\text{FeO}}/{\text{MgO}})_{{\text{ga}}} }}{{{\text{(FeO}}/{\text{MgO)}}_{{\text{cpx}}} }}\) in eclogites as a geothermometer has been done on (a) a mineral mis, (b) a glass of the typical tholeiite composition and (c) a series of glasses of tholeiite compositions with \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 93.\) The mineral mix was found to be unsuitable as reactant due to incomplete equilibration but the minimum K _D of the mineral mix and the K _D from glass of tholeiite composition are identical within experimental uncertainty. These data constitute a reversal of the garnet/clinopyroxene partition relationship and provide justification of the use of glass as a reactant for the project. To eliminate any uncertainty in interpretation of mineral analyses due to possible variation in Fe⁺⁺⁺/Fe⁺⁺ between runs, experiments were carried out in iron capsules on the nine tholeiite glasses, thus maintaining iron as Fe⁺⁺. Microprobe analytical techniques yielded mineral analyses of comparable accuracy to analyses of natural phases for experiments within the temperature range from 600° C to 1500° C and a pressure range from 20 kb to 40 kb. It has been shown that for \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 85\) , the bulk chemical composition does not perceptibly affect the K _D value. At 30 kb the K _D value ranges from 18.0 at 600° C to 1.45 at 1400° C, defining the linear relationship in a ln K _D vs 1/T(°K) plot. The pressure dependence of the K _D -value has been shown to be greater than previously predicted. There is a straight line relationship in ln K _D vs Pressure (Kb) between 20 and 40 kb at constant temperature (1100°C). This enables us to determine K _D =fn (T, P) and \(T(^\circ {\text{K}}) = \frac{{3686 + 28.35 \times P({\text{Kb}})}}{{\ln K_D + 2.33}}\) . This expression uniquely determines the temperature of equilibration of natural eclogites of basaltic bulk composition when the K _D ^ga,cpx is known and a pressure estimate can be given.     

The temperature dependence of water solubility in enstatite

The solubility of water in pure enstatite was measured on samples synthesized under water-saturated conditions at 15 kbar and temperatures ranging from 700 to 1,100°C. Polarized FTIR measurements on millimetre-sized, clear crystals showed that water solubility increases strongly with temperature, from 101 ppm by weight at 700°C to 269 ppm by weight at 1,100°C. The position and shape of the infrared bands hardly changes with temperature, with one notable exception: a band close to 3,380 cm^–1 is present in samples synthesized between 700 and 1,000°C, while this band is absent from samples synthesized at 1,100°C. This effect appears to be very reproducible and points towards a slight change in the crystal structure of enstatite between 1,000 and 1,100°C at 15 kbar. The water solubility data of this study as well as those of Rauch and Keppler (Contrib Mineral Petrol 143:525–536, 2002) can be reproduced by the equation where K is water solubility, is water fugacity, A is 0.01354 ppm/bar, V^solid=12.1 cm³/mol is the volume change of enstatite during incorporation of water, and H^{1 bar}=-4,563 J/mol is the reaction enthalpy at 1 bar. This equation predicts the following behaviour of water solubility in enstatite as a function of pressure and temperature: (1) water solubility increases with pressure up to a maximum around 80 kbar; (2) water solubility decreases with temperature at 1 bar; and (3) water solubility increases with temperature between 10 and 100 kbar. If the observed temperature dependence for enstatite were representative for other upper mantle minerals as well, it would have the following implications: (1) Lateral temperature gradients in the upper mantle could cause major variations in water contents at the same depth; in particular, hot mantle plumes may scavenge water from the surrounding shallow upper mantle. (2) The scavenging of water by hot plumes could be a major factor in increasing the mobility of plumes. (3) The predicted temperature dependence of water solubility at the base of the upper mantle may allow plumes to bypass the transition zone water filter postulated by Bercovici and Karato (Nature 425:39–44, 2003).     

孔隙水压力对锯齿状结构面剪切蠕变特性的影响

为探究孔隙水压对岩体结构面剪切蠕变特性的影响,自主研制了结构面一体化制作模具和多功能剪切流变仪,开展了孔隙水压力下锯齿状结构面的剪切蠕变试验,分析了孔隙水压对结构面蠕变变形、蠕变速率和长期强度的影响。试验结果表明：不同孔隙水压力下的结构面先后经历了瞬时变形阶段、减速蠕变阶段和稳定蠕变阶段,并且孔隙水压力的增大促进了结构面非线性特征的发展;随着孔隙水压力的增大,结构面瞬时位移、蠕变位移和稳态蠕变速率逐渐增大,而蠕变时长、破坏应力和长期强度均呈现明显降低的趋势。根据试验结果,考虑孔隙水压力对模型参数的影响,将蠕变模型中的瞬时剪切模量、黏性剪切模量以及黏性系数替换为孔隙水压力的函数,构建了能够反映孔隙水压力影响的结构面蠕变模型,并对模型参数进行辨识,将试验曲线和理论模型曲线进行对比,验证了模型的正确性和适用性。研究成果对富水区岩体长期稳定性分析提供一定的理论指导。     

层状含水页岩蠕变特性试验研究

为分析层理和水对页岩的蠕变特性影响,室内制取0°、30°、60°、90°和不含层理5种页岩,每种层理页岩均处理成干燥、自然、不饱水、饱水4种含水状态,对其进行单轴压缩下的蠕变试验。试验结果表明,（1）层理角度和含水率对页岩的蠕变特性均有较大影响。同一含水率下随层理角度的增大,页岩的初始瞬时弹性呈指数式减小,瞬时弹性模量成指数式增加。相同层理角度下,随含水率升高,页岩的初始瞬时弹性应变线性增长,瞬时弹性模量线性下降;（2）采用含有弹性元件、黏性元件及开尔文体的“伯格斯”蠕变模型对页岩的蠕变过程进行描述,并对模型的参数进行了反演和验证,表明该模型能够较好地反映页岩的蠕变特征。根据试验所得层理和水对页岩蠕变变形的影响规律,通过瞬时弹性模量和黏性系数与层理角度和含水率的拟合关系,将蠕变模型中的瞬时弹性模量和黏性系数替换为层理倾角和含水率的函数,构建了能够反映页岩层理和含水率的蠕变模型。     

A unified criterion for initiation of sediment motion and inception of sheet flow under water waves

A unified criterion is developed for initiation of non‐cohesive sediment motion and inception of sheet flow under water waves over a horizontal bed of sediment based on presently available experimental data. The unified threshold criterion is of the single form, U_o = 2πC[1 + 5(T_R/T)²]^?1/4, where U_o is the onset velocity of sediment motion or sheet flow, T is wave period, and C and T_R are the coefficients. It is found that for a given sediment, U_o initially increases sharply with wave period, then gradually approaches the maximum onset velocity U_o = 2πC and becomes independent of T when T is larger. The unified criterion can also be extended to define sediment initial motion and sheet flow under irregular waves provided the significant wave orbital velocity and period of irregular waves are introduced in this unified criterion.     

Microstructural evidence for the transition from dislocation creep to dislocation-accommodated grain boundary sliding in naturally deformed plagioclase

We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.     

Grain size effect on the electrical conductivity of clinopyroxene

Complex impedance spectra of polycrystalline samples (with grain size fractions ~5–63, 63–160 and 160–250 μm) and a single crystal sample (with orientation parallel to b), prepared from a natural megacryst augite, were measured in a piston cylinder apparatus at 10 kbar and 500–1,000°C and with a Solartron 1260 Impedance/Gain Phase analyzer over a frequency range of 0.1–10⁶ Hz. The main charge carriers are attributed to small polarons, and the activation enthalpy is 83 ± 3 to 90 ± 3 kJ/mol. The measured electrical conductivity shows no difference between the polycrystalline and single crystal samples, suggesting independence of electrical conductivity on grain size given a change above ~5 μm. The electrical conductivity of augite is much higher than that of olivine, indicating that, if regionally enriched, augites may lead to zones of high electrical conductivity and electrical anisotropy in the deep lithosphere.     

The clinopyroxene/plagioclase symplectite in retrograde eclogites: A potential geothermobarometer

The retrograde P-T-t path of intermediate and high-temperature eclogites (T 500 ± 50°C) is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae. Often, this symplectite shows a two stage microstructural evolution, i.e. a coarse lamellar width symplectite, »S1«, followed by a thinner lamellar width symplectite, »S2«. Each symplectite may also evolve independently into a final globular microstructure, »SG«.According to metallurgical concepts, the phase transition »omphacite clinopyroxene + plagioclase« is classified as a discontinuous precipitation reaction. In discontinuous precipitation reactions, the lamellar spacing (L) depends on temperature (T) according to the growth law log (L) = A-B/T, in which A and B are constants. Therefore thin symplectites characterize low temperatures, where-as coarse symplectites characterize higher temperatures.In order of specify the thermal evolution of continental collision zones, the reliability of the discontinuous precipitation growth law has been tested on eclogites showing contrasted retrograde P-T-t histories in different geological environments, i.e. Alpine, Caledonian and Variscan Belts. Indeed, our investigations reveal that the clinopyroxene/plagioclase symplectite can be used as a potential geothermobarometer in eclogites recording temperatures above 500 ± 50°C. The temperature is obtained by the lamellar spacing and pressure by the Jadeite content of the symplectite clinopyroxene in the presence of plagioclase and quartz.

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Zusammenfassung Der retrograde P-T-t Pfad von intermediären und hochtemperierten Eklogiten (T 500 ±50°C) ist charakterisiert durch die Umformung von Omphazit in symplektitische Verwachsungen von Klinopyroxen und Plagioklas-Lamellen. Häufig zeigt dieser Symplektit eine zweiphasige mikrostrukturelle Entwicklung, das heißt, ein Symplektit mit breiten Lamellen »S1« wird gefolgt von einem Symplektit mit dünneren Lamellen »2«. Jeder Symplektittyp kann sich schließlich selbständig zu einer kugelförmigen MikroStruktur »SG« entwikkeln.Auf Grund von metallurgischen Vorstellungen wir die Phasenumwandlung Omphazit zu Klinopyroxen + Plagioklas als diskontinuierliche Ausfällungsreaktion klassifiziert. In diskontinuierlichen Ausfällungsreaktionen hängt der Lamellenabstand (L) von der Temperatur (T) nach dem Wachstumsgesetz log (L) = A-B/T ab, in dem A und B Konstanten sind. Daher zeigen dünne Symplektite niedrige Temperaturen an, während breite Symplektite höhere Temperaturen anzeigen.Um die thermische Entwicklung von kontinentalen Kollisionszonen zu spezifizieren mußte die Zuververlässigkeit des diskontinuierlichen Ausfällungswachstumsgesetz an Eklogiten getestet werden, die unterschiedlich retrograde P-T-t Geschichten aus verschieden Umgebungen zeigen; das heißt aus alpinen, kaledonischen und variszischen Gebirgszügen. Tatsächlich offenbaren unsere Bemühungen, daß der Klinopyroxen/Plagioklas Symplektit als potentielles Geothermobarometer in Eklogiten benutzt werden kann, die Temperaturen über 500°C ± 50°C erfahren haben. Die Temperatur kann man aus dem Lamellenabstand erhalten; den Druck durch den Jadeitgehalt des symplektitischen Pyroxens, in Anwesenheit von Plagioklas und Quarz.

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Résumé La déstabilisation de l'omphacite, en une symplectite à alternance de lamelles de clinopyroxène et de plagioclase, caractérise l'initiation du trajet rétrograde P-T-t pour les éclogites de température 500 ± 50°C. Cette symplectite présente parfois une évolution microstructurale depuis une symplectite grossière à alternance de lamelles larges, »S1«, relayée par une symplectite à alternance de lamelles fines, »S2«. Chaque symplectite peut aussi évoluer indépendemment en une microstructure globulaire tardive, »Sg«.En accord avec les concepts établis par les métallurgistes, la transformation »omphacite clinopyroxène + plagioclase« est une précipitation discontinue. Pour ce type de transformation, l'espace interlamellaire (L) dépend de la température (T) selon la loi de croissance Log (L) =A-B/T (*), où A et B sont des constantes. Ainsi, une symplectite fine (de type S2) caractérise des températures relativement plus basses qu'une symplectite grossière (de type S1).Afin de préciser l'évolution thermique des zones de collision continentale, nous avons testé la loi de croissance (*) sur des éclogites échantillonnées dans divers contextes géologiques (chaînes alpine, calédonide et varisque), présentant des trajets rétrogrades P-T-t contrastés. Les résultats montrent que la symplectite à clinopyroxène/plagioclase représente un géothermobaromètre potentiel de la rétromorphose des éclogites: la taille de l'espace interlamellaire dépend de la température et la teneur en jadéite des lamelles de clinopyroxène, en présence de quartz et à l'équilibre avec le plagioclase, dépend de la pression.

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