Microstructural and chemical transformations accompanying deformation of granite in a shear zone at Miéville,Switzerland; with implications for stress corrosion cracking and superplastic flow

At Miéville, in the Aiguilles-Rouges Massif, granitic rocks of the basement are deformed into mylonites within a major subvertical shear zone. The ambient temperature during translation is estimated at 250° C±30° C from fluid inclusion filling temperatures in syntectonic microveins, from ¹⁸O quartzilmenite of+15%, and from mineralogical criteria. Porphyroclasts of both oligoclase and orthoclase feldspar decrease from initial diameters of 20 mm and assume elliptical shapes during progressive deformation, due to recrystallisation of the margins to ultra-fine polygonal grains which extend out from the porphyroclasts in thin trails: the final stable grain size is <5 . The recrystallised feldspar has a composition of the parent porphyroclast,+albite, requiring relative gains of Na and losses of K+Ca compared to the precursor, and implying short range redistribution of the components during deformation. Decrease of free energy associated with the deformation catalysed change in feldspar composition, coupled with stored strain energy in the porphyroclasts may account for recrystallisation to a stable aggregate of ultrafine grain size. The long trails imply exceptionally high ductility, which, coupled with microstructural criteria, and admixture of quartz from neighbouring pure quartz aggregates by grain boundary sliding, is interpreted in terms of superplastic flow. Estimated temperatures of T/T _m0.2 for the inferred superplastic deformation is lower by a factor of 2 than previously recorded for this flow michanism in silicates. The feldspar and quartz probably accomodated grain boundary sliding by intercrystalline diffusion.Biotite responds to deformation by bendgliding, kinking, and recrystallisation in mantles. The reaction of high-Ti parent grains to low-Ti biotite+Fe-muscovite+ ilmenite+chlorite is catalysed at all of these microstructural sites. Progressive deformation of the fine-grained products in the mantles is coupled with steady reaction to low-Fe muscovite+epidote+ sphene+rutile resulting in exceptionally ductile trails, as for the case of feldspar.Biotite grains have pervasive networks of nondisplacive intragranular fractures. At the fracture tips increase of the stress intensity has catalysed the reaction of high-Ti parent grains to low-Ti biotite+muscovite+ ilmenite which occupy the fractures. The fractures propagate and coalesce resulting in mechanical breakdown of the parent grains: these microstructures are believed to be examples of natural stress corrosion cracking. These features are also abundant in feldspar porphyroclasts where at fracture tips orthoclasesecondary orthoclase+albite, and oligoclasesecondary oligoclase+albite. Stress corrosion cracking may be significant in the steady state deformation of crustal rocks at low temperatures when intracrystalline plasticity is not generally dominant.Two way mass balance calculations utilising major and selected trace element data, reveal that deformation of the granite was essentially isochemical, involving average additions of <1 % H₂O+CO₂, at approximately constant specific gravity. The parameters Fe²⁺/Fe and ¹⁸O_{whole rock} maintain relatively constant values across the shear zone, and this also implies limited participation of fluids in the deformation. Alkali elements and titanium display the largest percentage variation during progressive deformation, whereas SiO₂, Al₂O₃, and P₂O₅, together with V, Ni, Cr,Y,Zr, and Nb remain relatively constant. All variations decrease at increasing states of deformation and this is interpreted in terms of mechanical mixing of chemical inhomogeneities of the granite precursor within the shear zone. Constraints imposed by variations in abundance of the relatively immobile elements imply that volume changes accompanying deformation in the shear zone were less than ±10%.     

石灰岩裂隙摩擦强度愈合的压力溶解和应力腐蚀模型研究

为模拟试验中观察到的石灰岩裂隙摩擦强度愈合现象,提出用压力溶解和应力腐蚀相结合的综合模型来模拟裂隙摩擦强度的愈合。综合模型可同时考虑压力溶解和应力腐蚀对裂隙接触面的改造作用,通过裂隙接触面积的变化表征摩擦强度的愈合。模拟结果表明：压力溶解模型低估了短控制时间内摩擦强度的愈合量,通过进一步的参数敏感性分析发现,随着控制时间的增长,温度的升高显著地提高了压力溶解模型的模拟值,而有效应力的增加对模拟值的影响相对较小;应力腐蚀模型则主要反映应力压缩引起的裂隙接触面积增长;压力溶解模型和应力腐蚀模型结合则能较好地模拟裂隙摩擦强度的愈合。因此,用压力溶解模型模拟裂隙摩擦强度愈合时,特别是在模拟短控制时间内和低温条件下力学占主导作用的愈合时,应考虑应力腐蚀引起的裂隙接触面积变化。     

一种新的断裂计算方法

利用单元中嵌合无厚度不连续面的方法 ,模拟断裂面的形成过程。用虚功原理建立包含断裂面影响的有限元平衡方程 ;利用余虚功原理推导了断裂面的矩阵的具体形式 ; 以第一类断裂问题 -开裂问题为例 ,建立了开裂面粘塑性开裂本构关系,进一步得到了以弹粘塑性形式表示的有限元计算格式。 通过简单算例证明了该方法的正确性。     

Mineral chemistry of Monte Vulture volcanics: petrological implications

Summary Two distinct periods of volcanic activity are distinguishable at Monte Vulture volcanic complex: the first activity started with tephritic and foiditic volcanics, documented by xenoliths in the phonotrachytic (Ph-T) ignimbrites, lava blocks in a basal explosion breccia deposit beneath the Ph-T ignimbrites, sandy lenses rich in volcanic components in fluviatile conglomerates, and ended with emplacement of phonotrachytic deposits (Ph-T ignimbrites and lava domes); whereas subsequent activity produced deposits of foiditic and tephritic composition which built up the central volcano. The volcanics of the two periods do not fit a simple evolutionary model on the basis of geochemical signatures.New whole-rock and microprobe analyses of mafic minerals from volcanics of the first period are presented here, together with the published data on M. Vulture volcanics, to give a more comprehensive picture of the magma evolution.Principal features can be summarized as follows: (i) melanitic garnet occurs as a stable phase only in the volcanics of the first period of activity; (ii) olivine and diopside occur only in the cone building deposits; (iii) pyroxenes from the volcanics of the first stage crystallized under different physico-chemical conditions with respect to those occurring in volcanics of the central edifice; (iv) the analyzed lavas occurring as blocks in the explosion breccia deposit of the first stage display diversities with respect to the lavas of the central volcano, and it is likely that diverse parental magmas fed the first and the subsequent activities. During the first stage, crystal/liquid fractionation processes determined the evolution from the mafic parental magmas towards phonotrachytic compositions, as with the foidite-tephrtte sequence of the second stage (cone building deposits). Mineralogical evidence of mixing processes is present in the volcanics of the sequences of the two stages.

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Die Mineralchemie der Monte Vulture Vulkanite: Petrologische Konsequenzen

Zusammenfassung Zwei Stadien vulkanischer Aktivität sind im Monte Vulture Komplex zu unterscheiden. Das erste Stadium ist durch tephritische und foiditische Vulkanite, die als Xenolithe in den phonotrachytischen (Ph-T) Ignimbriten erhalten sind, durch Lava Blöcke in einer basalen Explosionsbrekkzie im Liegenden der Ph-T Ignimbrite und durch Sandlinsen in fluviatilen Konglomeraten, die reich an vulkanogenen Komponenten sind, gekennzeichnet. Es endet mit der Eruption der Phontrachyte (Ph-T Ignimbrite und Lava Kuppen). Während des zweiten Stadiums wurden vulkanogene Ablagerungen foiditischer und tephritischer Zusammensetzung, die heute den Zentralteil des Vulkans aufbauen, eruptiert. Die Geochemie der Vulkanite läßt sich nicht mit Hilfe eines einfachen Bildungsmodelles erklären. Neue Gesamtgesteins- und Mikrosondenanalysen von Vulkaniten des Erststadiums und deren mafischer Minerale werden gemeinsam mit bereits publizierten Daten über die Vulkanite des Monte Vulture präsentiert, um ein zusammenfassendes Bild seiner Magmenentwicklung zu geben.Grundsätzlich kann folgendes festgehalten werden: (i) Granat tritt als stabile Phase nur in den Vulkaniten des ersten Stadiums auf. (ii) Olivin und Diopsid sind auf die Vulkanschlote beschränkt. (iii) Die Pyroxene der Vulkanite des Frühstadiums kristallisierten unter anderem physikalisch-chemischen Bedingungen, als die im Zentralteil des Vulkans. (iv) Die untersuchten Laven, die als Blöcke in der basalen Explosionsbrekkzie auftreten, unterscheiden sich ebenfalls von denen des zentralen Teiles. Wahrscheinlich sind sie von unterschiedlichen Ausgangsmagmen abzuleiten. Durch Fraktionierung entstanden während des ersten Stadiums aus einem ursprünglich mafischen ein phonotrachytisches Magma und während des zweiten die foiditisch-tephritische Abfolge der Vulkanschlote. Hinweise auf Mischungsprozesse liegen vor.

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With 6 Figures     

应力腐蚀和压力溶解引起的裂隙开度-刚度变化对THMM耦合过程影响的数值模拟

在使用Yasuhara等建立的裂隙开度的应力腐蚀和压力溶解模型的基础上,考虑裂隙闭合量对裂隙刚度的影响,针对一个假设的位于非饱和双重孔隙-裂隙岩体中且有核素泄漏的高放废物地质处置模型,拟定两种计算工况：（1）裂隙的刚度系数是裂隙闭合量的线性函数;（2）裂隙刚度为常数,进行热-水-应力-迁移耦合的二维有限元数值模拟,考察了岩体中的温度、裂隙开度的闭合速率、闭合量、孔（裂）隙水压力、地下水流速、核素浓度、裂隙刚度和正应力的变化、分布等情况。结果表明,两种工况岩体中的温度场、孔（裂）隙水压力、地下水流速、核素浓度无明显差别;裂隙闭合基本由应力腐蚀产生;在相同计算时间内两种工况的裂隙闭合量较为接近,工况1略大;工况1中离玻璃固化体越近,裂隙刚度值越高,并且在玻璃固化体附近的应力值较大,且集中程度较高。     

Trace-element distributions in silicates during prograde metamorphic reactions: implications for monazite formation

To assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism, REE were measured across coexisting zoned silicates in garnet through kyanite‐grade pelitic schists from the Great Smoky Mountains, western Blue Ridge terrane, southern Appalachians, to establish REE concentrations and distributions before and after the monazite‐in isograd, and to identify the role major silicates play in the formation of monazite. Results indicate significant scavenging of light rare‐earth elements (LREE) from silicates during the monazite‐in isograd reaction; however, the absolute concentration of LREE hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists. Monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated LREE (possibly adsorbed onto grain boundaries), even though direct evidence for allanite is lacking in a majority of the samples. Laser‐ablation ICP‐MS analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates. Allanite breakdown initially formed monazite, and monazite production drew LREE liberated from allanite, major silicates and possibly from crystal boundaries. In many rocks the reaction was further promoted by the staurolite‐in reaction, allowing for rapid, isogradic monazite growth.     

The effect of stress boundary conditions on fluid-driven fracture propagation in porous media using a phase-field modeling approach

A phase-field approach for fluid-driven fracture propagation in porous media with varying constant compatible stress boundary conditions is discussed and implemented. Since crack opening displacement, fracture path, and stress values near the fracture are highly dependent on the given boundary conditions, it is crucial to take into account the impact of in situ stresses on fracturing propagation for realistic applications. We illustrate several numerical examples that include the effects of different boundary conditions on the fracture propagation. In addition, an example using realistic boundary conditions from a reservoir simulator is included to show the capabilities of our computational framework.     

Crack propagation trajectories for rocks under mixed mode I-II fracture

Propagation of a crack in engineering materials including rocks can cause failure. Knowledge of the stress state under which a crack can propagate, and the trajectory it may follow during its growth are thus very important for the stability of rock masses/materials and for the safe design of structures in/on rocks. In this paper, the crack initiation angle and subsequent crack propagation path are experimentally investigated for limestone rock specimens. This investigation was conducted under various mixed mode I-II loading conditions, including pure mode-I and pure mode-II.This study includes conducting diametrical compression tests on notched Brazilian disk specimens. Moreover, the effect of confining pressure and temperature on crack initiation and propagation were also studied. The experimental results were compared with theoretical predictions of crack initiation angle. The results showed that limestone behaves in brittle fashion, and the effects of confining pressure and temperature on failure trajectories were not significant. Generally, the crack initiation angle can be predicted by the maximum tangential stress criterion. However, for notched Brazilian disk with high value of crack orientation with respect to loading direction, crack does not propagate from the tip of the crack. This important observation indicated that the tensile-strength failure can become more critical than the fracture-toughness failure.     

Stress corrosion cracking of Lac du Bonnet granite in tension and compression

Rocks subjected to long-term loading have been known to suffer microcracking. The rate of cracking is sensitive to the type of the applied stress (tensile or compressive), and the magnitude of the stress relative to the instantaneous strength. In addition, crack growth is influenced by the environment (pressure and temperature) including the presence or absence of moisture.For tensile loading, the sensitivity of granite to time-dependent cracking is demonstrated through a fracture mechanics test known as double torsion. The crack velocity versus stress intensity function is established for two environments, room temperature and humidity and room temperature and 100 percent humidity.For compressive loading, time dependent cracking is evaluated from creep tests conducted in uniaxial compression in the same two environments. The rate of cracking is defined by finding the functional relationship between the rate of crack growth, expressed as the rate of crack volume strain, and uniaxial compressive stress.A variety of mathematical functions has been fitted to the obtained data. The traditionally-used power and exponential relationships give good correlation for both crack velocity and crack volume strain rate.The crack volume strain rate versus stress function can be integrated to obtain a lifetime estimate for Lac du Bonnet granite. After 1 000 years of loading in uniaxial compression at room temperature and 100 percent humidity, the strength of this granite could reduce from 225 MPa to 90–100 MPa.     

A numerical procedure for simulation of hydraulically-driven fracture propagation in poroelastic media

A procedure for numerical approximation to two-dimensional, hydraulically-driven fracture propagation in a poroelastic material is described. The method uses a partitioned solution procedurè to solve a finite element approximation to problems described by the theory of poroelasticity, in conjunction with a finite difference approximation for modelling fluid flow along the fracture. An equilibrium fracture model based on a generalized, Dugdale–Barenblatt concept is used to determine the fracture dimensions. An important feature is that the fracture length is a natural product of the solution algorithm. Two example problems verify the accuracy of the numerical procedure and a third example illustrates a fully-coupled simulation of fracture propagation. Photographs taken from a high-performance engineering workstation provide insight into the nature of the coupling among the physical phenomena.     

水-岩化学作用等效裂纹扩展细观力学模型

从化学腐蚀下岩石细观结构的变化出发,研究水化学溶液对岩石中裂纹的腐蚀作用。运用地球化学矿物-水反应的溶解动力学,考查水-岩反应化学溶液中不同时间段离子浓度的变化,从理论上探讨化学腐蚀下等效裂纹扩展的定量化分析方法。通过质量守恒定律计算出裂纹在迹长及隙宽方向的变化,将水化腐蚀等效成有效裂纹长度,建立了水-岩化学作用下等效裂纹扩展的计算公式并计算应力强度因子。通过对砂岩的研究结果表明,水-岩化学作用对岩石裂纹的扩展有显著影响,并且具有时间效应。同时,水化学溶液的离子浓度及pH值大小对裂纹的扩展存在影响,其中pH值的变化对岩石的断裂力学效应更加显著。当溶液的酸性越强或碱性越强,腐蚀越大,裂纹扩展越快;pH值为中性时,腐蚀作用得到较大缓解,裂纹发展缓慢。为定量研究裂隙岩体在水岩化学作用下的细观机理提供了一种新的思路。     

闭合裂纹断裂的有效剪应力准则

将闭合裂纹表面的有效剪应力引入裂尖应力强度因子的计算,获得了无限大板和有限宽板含中心闭合裂纹在不同裂纹长度、倾角以及摩擦系数下裂纹尖端的应力强度因子值。引入等径向剪应力线 这一概念,建立了闭合裂纹断裂的有效剪应力准则：（1）岩板内闭合裂纹将沿着等 线上双剪应力的和最小的方向扩展;（2）裂纹尖端的应力强度因子KⅡ达到材料的临界值KⅡC,裂纹将开始扩展。该准则成功预测了闭合裂纹的临界起裂角 ,与各种经典复合型断裂准则计算Ⅱ型裂纹起裂角比较,结果较为接近。将其应用于闭合裂纹的断裂判定是安全的。     

Mineral matter in coals and their reactions during coking

Degradation of coke in the blast furnace is influenced by its inherent mineral matter, the formation of which is itself dependent upon the nature of the coal mineral matter. To date few studies have been made of coke mineralogy and its relationship to the mineralogy of the parent coal. In this study the effect of carbonisation on coal mineral matter has been investigated by a detailed quantitative mineralogical examination of nine cokes and their parent coals. The quantitative analysis was performed on X-ray diffraction patterns of the mineral matter of cokes and coals, using SIROQUANT^™. Coke mineralogy and its composition varied strongly between cokes, more strongly than variations in elemental composition of the ash. The mineral matter in the studied cokes consisted of crystalline mineral phases and also significant levels of amorphous phase (ranging between 44 and 75%). Decomposition of clays such as kaolinite, montmorillonite, illite and chamosite produced the amorphous phase and some of the crystalline mineral phases such as mullite, γ-alumina, spinel, cristobalite and leucite. The type of association of mineral matter in coals had an important role in how the clays decomposed. For example, association of kaolinite with silica-bearing minerals in intimate intermixture favoured formation of mullite over γ-alumina. Akermanite and diopside result from reaction of kaolinite with associated calcium bearing minerals (calcite, dolomite or ankerite). Quartz, fluorapatite and the three polymorphs of TiO₂ (anatase, brookite and rutile) were the coal minerals that were least affected during carbonisation, as they were also found in the cokes, yet even they were affected in some cases.     

Heating rate dependency of petroleum-forming reactions: implications for compositional kinetic predictions

The generation of bulk petroleum, liquid and gaseous hydrocarbons from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at four different heating rates (0.013, 0.1, 0.7 and 5.0 K/min). Using the established parallel reaction kinetic model, temperature and compositional predictions were tested to be suitable for geological conditions by comparing the laboratory results with natural changes in source bitumens and reservoir oil maturity sequences from the Duvernay Formation. In the case of bulk liquid and gaseous hydrocarbons, the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of paraffins, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates, the compositional predictions are consistent with the bulk composition of natural products in the Duvernay-petroleum system showing an increase of paraffinicity and hydrogen content. In contrast to that, the “hump” decreases with decreasing heating rate, a trend which is confirmed by the low amounts of unresolved compounds in natural high maturity products. Because of these heating-rate dependent compositional changes, geological predictions of natural molecular composition by the commonly used kinetic models are not suitable.     

Geometric nature of hydraulic fracture propagation in naturally-fractured reservoirs

The geometry of hydraulic fracture propagation in the absence and presence of natural fractures in reservoirs was studied. The results revealed a marked influence of natural fractures on the symmetry of fracture propagation observed for rock free of natural fractures. Natural fracture properties such as stiffness and approaching angle, and the distance from wellbore to the natural fracture were found to influence the hydraulic fracture geometry. Furthermore, the location of the wellbore with respect to the natural fractures in a reservoir having a system of natural fractures displayed a significant influence on the resulting geometry of hydraulic fracture propagation.     

围岩作用下锚杆砂浆锈胀开裂过程分析

从砂浆锚杆锚固体自身的特点出发,借鉴钢筋混凝土结构锈胀开裂研究成果,对锚杆砂浆锈胀开裂过程进行了分析。考虑锚杆锈蚀体积膨胀过程,以填充系数表示砂浆开裂过程中锚杆锈蚀产物的损失,建立了锚杆锈蚀后砂浆内部位移模型,以弹性力学为基础,引入围岩参数影响系数概念,对锚杆锈蚀导致砂浆保护层开裂过程以及围岩参数对开裂过程的影响程度等进行了分析研究,并采用有限元分析方法对其进行了验证。理论分析结果表明,因锚杆锈蚀而导致的砂浆保护层开裂以及裂纹的扩展情况均与围岩特性密切相关,砂浆开裂后裂纹会经历一段稳定扩展过程,裂纹的最大稳定扩展半径随围岩参数影响系数的增加而增大,在合适的围岩参数条件下,裂纹可以一直稳定扩展致整个砂浆保护层外壁。有限元分析结论与理论结果一致。     

Quasistatic propagation of a normal fault: A fracture mechanics model

We have carried out boundary element calculations to simulate quasistatic propagation of a normal fault in the earth's crust under a horizontal tensile loading. Byerlee's frictional law is employed to describe the mechanical behavior of the fault surface. We hypothesize that in order for a normal fault to grow quasistatically, the mixed-mode effective shear stress intensity factor must exceed a threshold value (fracture toughness), a crustal material property. We suggest that the fault grows in a direction of local maximum shear stress. The direction of fault propagation thus depends on the ratio of tensile and shear stress intensity factors. A listric normal fault is likely to form in crustal material with a small shear fracture toughness. A listric normal fault is also more likely to form in crustal material with a high degree of plasticity.The propagation trajectory of an incrementally growing normal fault is examined. As the normal fault extends to a greater depth, the shear stress intensity factor drops, owing to an increase in fault surface friction. The equilibrium depth to which a normal fault will grow is controlled by the far field loading and the fracture mechanical property of the crustal material. The decrease of shear stress intensity with fault length also stabilizes the fault growth.     

The relationship between regional stress field,fracture orientation and depth of weathering and implications for groundwater prospecting in crystalline rocks

In a uniform granite gneiss study area in central Zimbabwe, lineaments oriented parallel to the maximum regional compressive stress orientation exhibit the thickest regolith development, while lineaments oriented perpendicular to the maximum compressive stress show the shallowest development of weathered regolith. The principal fracture set orientations were mapped using aerial imagery. The regional stress field, estimated from global stress maps, was used to determine the stresses acting on each principal lineament orientation. Multi-electrode resistivity profiling was carried out across fractures with different orientations to determine their subsurface regolith conditions. The results indicate that the 360 and 060° lineaments, which are sub-parallel to the principal compressive stress orientation (σ₁) exhibit maximum development of the regolith, while 130° lineaments perpendicular to σ₁ do not exhibit significant regolith development. Since regolith thickness has been positively correlated with groundwater resources, it is suggested that fractures with orientations sub-parallel to the principal compressive stress direction constitute favourable groundwater targets. Knowledge of the regional stress field and fracture set orientations can be used as an effective low cost tool for locating potentially higher yielding boreholes in crystalline rock terrains.