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1.
We present the biostratigraphy (ammonites, brachiopods, foraminifers, and ostracodes), lithostratigraphy, sedimentology, sequence stratigraphy, magnetostratigraphy, and isotope stratigraphy of the Almonacid de la Cuba section located in the Iberian Range, central-eastern Spain. This section, which contains a continuous and expanded record of the Pliensbachian-Toarcian boundary (Early Jurassic), has been proposed as a complementary section for the Toarcian GSSP. An excellent ammonite-based biozonation has been obtained. Four ammonite assemblages characterized by the presence of Pleuroceras, Canavaria, Dactylioceras (Eodactylites), and Dactylioceras (Orthodactylites) have been distinguished. The base of the Toarcian is located at level CU35.2, based on the first occurrence of Dactylioceras. The occurrence of taxa from the NW European and the Mediterranean provinces is useful to improve the correlation between both provinces. Foraminiferal and ostracode assemblages are rich and diversified and no significant biostratigraphic events take place at the Pliensbachian-Toarcian boundary. The magnetostratigraphic data presented here are the most complete record of reversals of the earth magnetic field for the Pliensbachian-Toarcian boundary. A good record of the onset of the positive δ13C excursion reported in the Lower Toarcian of many European sections has been obtained. Average paleotemperatures measured at the latest Pliensbachian Spinatum Biochron of about 12.5°C, recorded a marked increase of the seawater temperature which started during the Toarcian, reaching average temperatures of 16.7°C at the Tenuicostatum Biochron. The obtained 87Sr/86Sr values fully agree with the LOWESS calibration curve.  相似文献   

2.
In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport along an interphase boundary depend on the product D ib K ib/s a, where D ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid along the interphase boundary at any time during cooling.  相似文献   

3.
Summary The different functions of cumulative probability of fracture that can be used in the Probabilistic Strength of Materials in the case of constant uniaxial compression are described. Sound fine-grained granite was used to study volume influence by fracturing rectangular prisms, and then no noticeable influence was observed. Since this is showing that all the fracture stresses are belonging to a single set they were included in a single group that exhibited two functions of specific risk of fracture. The population with the lesser fracture stress has no critical zone while the other population does have it and a critical zone in the order of 10–6 m3 is the minimum size exhibiting a complete fracture of the specimen when the same collapses. All the statistical functions were found to be acceptable according to theX 2 criterion.  相似文献   

4.
The contact between wave‐influenced foreshore and aeolian‐influenced backshore sediments (BA boundary) in raised spit deposits (Skagen Odde) is here used as a proxy for palaeo‐sea level over the past 7600 years. The elevation of the BA boundary was measured at 57 sample sites along the northwestern coast of the spit, and the age of these sites determined by optically stimulated luminescence (OSL) dating of quartz grains. The elevation of the BA boundary with age gives past variation in relative sea level; relative sea level rose between c. 7600 and c. 6250 years ago, when it reached a peak value around 12.5 m above present mean sea level (apmsl), followed by a slow sea‐level fall until c. 4600 years ago before it dropped rapidly to reach 2 m apmsl c. 2000 years ago. From the new data it is tentatively deduced that the land uplift rate declined from about 3 mm a−1 6000 years ago to about 1.5 mm a−1 2000 years ago (low estimate), or alternatively from 5 mm a−1 5000 years ago to 1.5 mm a−1 2000 years ago (extreme estimate). These data indicate that the long‐term average rate of vertical land movement during the past 5000 years was around 1.8 mm a−1 (low estimate) or around 2.5 mm a−1 (extreme estimate). These values seem reasonable compared with a modern value of about 1.6 to 1.7 mm a−1. The lack of an independent data set illustrating the isostatic uplift history with time, however, precludes the construction of a well‐constrained eustatic sea‐level curve.  相似文献   

5.
Yb-Y inter-diffusion along a single grain boundary of a synthetic yttrium aluminium garnet (YAG) bicrystal has been studied using analytical transmission electron microscopy (ATEM). To investigate the diffusion, a thin-film containing Yb as the diffusant was deposited perpendicular to the bicrystal grain boundary by pulsed laser deposition (PLD). Structural properties and their change with time in both the diffusant source and the grain boundary are reported. The diffusion profiles are incorporated in a numerical diffusion model, which is applied to determine the grain boundary diffusion coefficient, D gb , at 1.723 K it is equal to 3 × 10−15 m2/s. We find that grain boundary diffusion is 4.85 orders of magnitude faster than volume diffusion, which was determined from the same diffusion experiment. This result is discussed in the context of special versus general grain boundaries. Finally, we successfully tested the capability of synchrotron-based nano-X-ray fluorescence analysis to map micro-chemical patterns in two dimensions with sub-micrometre resolution.  相似文献   

6.
The demarcation of the Lower–Middle Triassic boundary is a disputed problem in global stratigraphic research. Lower–Middle Triassic strata of different types, from platform to basin facies, are well developed in Southwest China. This is favorable for the study of the Olenekian–Anisian boundary and establishing a stratotype for the Qingyan Stage. Based on research at the Ganheqiao section in Wangmo county and the Qingyan section in Guiyang city, Guizhou province, six conodont zones have been recognized, which can be correlated with those in other regions, in ascending order as follows: 1, Neospathodus cristagalli Interval-Zone; 2, Neospathodus pakistanensis Interval-Zone; 3, Neospathodus waageni Interval-Zone; 4, Neospathodus homeri-N. triangularis Assemblage-Zone; 5, Chiosella timorensis Interval-Zone; and 6, Neogongdolella regalis Range-Zone. An evolutionary series of the Early–Middle Triassic conodont genera Neospathodus-Chiosella-Neogongdolella discovered in the Ganheqiao and Qingyan sections has an intermediate type named Neospathodus qingyanensis that appears between Neospathodus homeri and Chiosella timorensis in the upper part of the Neospathodus homeri-N. triangularis Zone, showing an excellent evolutionary relationship of conodonts near the Lower–Middle Triassic boundary. The Lower–Middle Triassic boundary is located at 1.5 m below the top of the Ziyun Formation, where Chiosella timorensis Zone first appears in the Qingyan section, whereas this boundary is located 0.5 m below the top of the Ziyun Formation, where Chiosella timorensis Zone first appears in the Ganheqiao section. There exists one nearly 6-m thick vitric tuff bed at the bottom of the Xinyuan Formation in the Ganheqiao section, which is usually regarded as a lithologic symbol of the Lower–Middle Triassic boundary in South China. Based on the analysis of high-precision and high-sensitivity Secondary Ion Mass Spectrum data, the zircon age of this tuff has a weighted mean 206Pb/238U age of 239.0±2.9Ma (2s), which is a directly measured zircon U-Pb age of the Lower–Middle Triassic boundary. The Ganheqiao section in Wangmo county can therefore provide an excellent section through the Lower–Middle Triassic because it is continuous, the evolution of the conodonts is distinctive and the regionally stable distributed vitric tuff near the Lower–Middle Triassic boundary can be regarded as a regional key isochronal layer. This section can be regarded not only as a standard section for the establishment of the Qingyan Stage in China, but also as a reference section for the GSSP of the Lower–Middle Triassic boundary.  相似文献   

7.
The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of well-preserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution.In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. 87Sr/86Sr ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and δ13C and δ18O values, were measured with multiple collector-inductively coupled plasma–mass spectrometric techniques.The chemostratigraphy of 87Sr/86Sr ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of δ13C and 87Sr/86Sr indicates that the 87Sr/86Sr excursions preceded the δ13C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO2, caused biological depression.  相似文献   

8.
Wohl's model and Kohler's model are two empirical excess free energy of mixing models which have been formulated for ternary solutions. The two models are identical when the binary systems are regular solution model systems. When it is assumed that the binary systems are subregular model systems the two ternary models differ. This difference is examined using the subregular model parameter values suggested by Saxena to approximate the experimental work of Seck on coexisting feldspars in the system Anorthite-Albite-Orthoclase at 900° C and 0.5 Kb. For these conditions Wohl's model is closer to the experimental data than Kohler's model in generating the position of the solvus isotherm and is better in matching the shape of the albite partitioning curve. With regard to the slope of the tie-lines, Kohler's model provides a better fit to the experimental results than Wohl's model. Experimentally determined activities for this system are not yet available so that there is no absolute way of selecting the model which would provide the most realistic activities.A ternary solvus can be completely displayed on one diagram when two sets of contours are overlain on a ternary plot. One set of contours consists of isotherms while the other set consists of icophases which are usually at a reasonable angle to the isotherms. Not only are icophases a clear way of coding tie-line information, they also assist in the positioning of the consolute or critical line. The simple new activity matching algorithm that is used in the comparative calculations has the ability to produce icophases directly.Symbols N mole fraction - a activity - T temperature °K - R gas constant, 1.987 cals/mole - G x excess free energy of mixing - u i x excess chemical potential of mixing of component i - f activity coefficient - E AC , W AC characteristic parameter (cals/mole) for binary AC in a solution model  相似文献   

9.
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10.
Empiricaldecline curve forecasts of future production rates and cumulative production to be expected from certain classes of petroleum reservoirs has been a methodology practiced and relied on by petroleum geologists for many years. New ways to consider the subject are described in this paper. Thereby it is seen that these forecasts sometimes can be made even when some of the record of the past performance history is sparse (say, because it has been lost, or never recorded).SI Units D, D i initial reference production rate,s –1 - n dimensionless constant [cf. Eq. (1)] - N cumulative production, m3 - Q, Q i production rate att and initially, m3/s - R dimensionless time variable [cf. Eq. (9)] - t time, s  相似文献   

11.
Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion‐controlled. From the rate constants, the D′δ‐values of the rate‐limiting species were derived, i.e. the product of grain boundary diffusion coefficient D′ and the effective grain boundary width, δ. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D′CaOδ)/(D′SiO2δ)≥1.33, and that SiO2 diffusion controls rim growth. D′SiO2δ increases from about 10?25 to 10?23 m3 s?1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol?1. In runs at 1 GPa, performed in a piston‐cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion‐advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate‐limiting. D′δ increases from about 10?21 to 10?20 m3 s?1 as temperature increases from 1100 to 1200 °C. D′SiO2δ or D′CaOδ in rims on calcite is c. 10 times larger than D′SiO2δ in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact‐metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine‐infiltrated metacherts to water‐bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact‐metamorphism as compared to our experimental results.  相似文献   

12.
The phenomenon of normal grain growth in pure single phase systems is modeled with the Monte Carlo technique and a series of simulations are performed in 2- and 3-dimensions. The results are compared with natural and experimental monomineralic rock samples. In these simulations various lattice models with different anisotropic features in grain boundary energy are examined in order to check the universality of the simulation results. The obtained microstructure varies with the artificial parameter T ′ in each lattice model, where T ′ means scaled temperature and controls thermal fluctuation on grain boundary motion. As T ′ (thermal fluctuation) increases, the boundary energy anisotropy characterizing each lattice model becomes less important for the evolution of the microstructure. As a result the difference in the grain size distribution among the lattice models, which is significantly large for low T ′, is reduced with increasing T ′. The distribution independent of both the lattice model and the dimension is obtained at sufficiently high T ′ and is very close to the normal distribution when carrying out the weighting procedure with a weight of the square of each grain radius. A comparison of the planar grain size distribution of the natural and experimental rock samples with the 3-D simulation results reveals that the simulations reproduce very well the distributions observed in the real rock samples. Although various factors such as the presence of secondary minerals and a fluid phase, which are not included in the simulation modeling, are generally considered to influence the real grain growth behavior, the good agreement of the distribution indicates that the overall grain growth behavior in real rocks may still be described by the simplified model used in the present simulations. Thus the grain size distribution obtained from the present simulations is possessed of the universal form characterizing real normal grain growth of which the driving force is essentially grain boundary energy reduction through grain boundary migration. Received: 7 January 1997 / Accepted: 25 August 1997  相似文献   

13.
The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.  相似文献   

14.
A possible model for the pulsar PSR J1852+0040 associated with the supernova remnant Kes 79 and detected in place of a central compact object in this remnant is discussed. The main observational properties of the pulsar can be understood as consequences of its weak surface magnetic field (B s < 3 × 1011 G) and short rotational period (P ～ 0.1 s). Its X-ray emission is thermal, and is generated in a small region near the surface of the neutron star due to cooling of the surface as the surface accretes matter from a relict disk surrounding the pulsar. The radio emission is generated in the outer layers of the pulsar magnetosphere by the synchrotron (cyclotron) mechanism. The optical luminosity of J1852+0040 is estimated to be L opt < 1028 erg/s. If the spectral features in another central compact object, 1E 1207.4+5209, are interpreted as electron cyclotron lines, this provides evidence for a weak surface magnetic field for this neutron star as well (B < 6 × 1010 G). The hypothesis that all central compact objects have weak surface fields makes it possible to explain the number of detected central compact objects, the absence of pulsar-wind nebulae associated with these objects, and the fact that no pulsar has yet been detected at the position of SN 1987a. We suggest that, after the supernova remnant has dissipated, the central compact object becomes a weak X-ray source (XDINS), whose weak emission is also due to the weakness of its magnetic field.  相似文献   

15.
The purpose of this paper is to present a simple, unified critical state constitutive model for both clay and sand. The model, called CASM (Clay And Sand Model), is formulated in terms of the state parameter that is defined as the vertical distance between current state (v, p′) and the critical state line in v–ln p′ space. The paper first shows that the standard Cam-clay models (i.e. the original and modified Cam-clay models) can be reformulated in terms of the state parameter. Although the standard Cam-clay models prove to be successful in modelling normally consolidated clays, it is well known that they cannot predict many important features of the behavior of sands and overconsolidated clays. By adopting a general stress ratio-state parameter relation to describe the state boundary surface of soils, it is shown that a simple, unified constitutive model (CASM) can be developed for both clay and sand. It is also demonstrated that the standard Cam-clay yield surfaces can be either recovered or approximated as special cases of the yield locus assumed in CASM. The main feature of the proposed model is that a single set of yield and plastic potential functions has been used to model the behaviour of clay and sand under both drained and undrained loading conditions. In addition, it is shown that the behaviour of overconsolidated clays can also be satisfactorily modelled. Simplicity is a major advantage of the present state parameter model, as only two new material constants need to be introduced when compared with the standard Cam-clay models. © 1998 John Wiley & Sons, Ltd.  相似文献   

16.
Fast diffusion along mobile grain boundaries in calcite   总被引：1，自引：0，他引：1  
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.  相似文献   

17.
The present paper investigates bifurcation analysis based on the second‐order work criterion, in the framework of rate‐independent constitutive models and rate‐independent boundary‐value problems. The approach applies mainly to nonassociated materials such as soils, rocks, and concretes. The bifurcation analysis usually performed at the material point level is extended to quasi‐static boundary‐value problems, by considering the stiffness matrix arising from finite element discretization. Lyapunov's definition of stability (Annales de la faculté des sciences de Toulouse 1907; 9 :203–274), as well as definitions of bifurcation criteria (Rice's localization criterion (Theoretical and Applied Mechanics. Fourteenth IUTAM Congress, Amsterdam, 1976; 207–220) and the plasticity limit criterion are revived in order to clarify the application field of the second‐order work criterion and to contrast these criteria. The first part of this paper analyses the second‐order work criterion at the material point level. The bifurcation domain is presented in the 3D stress space as well as 3D cones of unstable loading directions for an incrementally nonlinear constitutive model. The relevance of this criterion, when the nonlinear constitutive model is expressed in the classical form (dσ = Mdε) or in the dual form (dε = Ndσ), is discussed. In the second part, the analysis is extended to the boundary‐value problems in quasi‐static conditions. Nonlinear finite element computations are performed and the global tangent stiffness matrix is analyzed. For several examples, the eigenvector associated with the first vanishing eigenvalue of the symmetrical part of the stiffness matrix gives an accurate estimation of the failure mode in the homogeneous and nonhomogeneous boundary‐value problem. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

18.
The Upper Cretaceous La Cova limestones (southern Pyrenees, Spain) host a rich and diverse larger foraminiferal fauna, which represents the first diversification of K-strategists after the mass extinction at the Cenomanian–Turonian boundary.The stratigraphic distribution of the main taxa of larger foraminifera defines two assemblages. The first assemblage is characterised by the first appearance of lacazinids (Pseudolacazina loeblichi) and meandropsinids (Eofallotia simplex), by the large agglutinated Montsechiana montsechiensis, and by several species of complex rotalids (Rotorbinella campaniola, Iberorotalia reicheli, Orbitokhatina wondersmitti and Calcarinella schaubi). The second assemblage is defined by the appearance of Lacazina pyrenaica, Palandrosina taxyae and Martiguesia cyclamminiformis.A late Coniacian-early Santonian age was so far accepted for the La Cova limestones, based on indirect correlation with deep-water facies bearing planktic foraminifers of the Dicarinella concavata zone. Strontium isotope stratigraphy, based on many samples of pristine biotic calcite of rudists and ostreids, indicates that the La Cova limestones span from the early Coniacian to the early-middle Santonian boundary. The first assemblage of larger foraminifera appears very close to the early-middle Coniacian boundary and reaches its full diversity by the middle Coniacian. The originations defining the second assemblage are dated as earliest Santonian: they represent important bioevents to define the Coniacian-Santonian boundary in the shallow-water facies of the South Pyrenean province.By means of the calibration of strontium isotope stratigraphy to the Geological Time Scale, the larger foraminiferal assemblages of the La Cova limestones can be correlated to the standard biozonal scheme of ammonites, planktonic foraminifers and calcareous nannoplankton. This correlation is a first step toward a larger foraminifera standard biozonation for Upper Cretaceous carbonate platform facies.  相似文献   

19.
A solution is developed for a point dislocation traversing a slab of saturated porous material under prescribed upper and lower hydraulic boundary conditions as an analogue to penetration in a layer of finite thickness. Pressure response is conditioned by geometrical parameters and those of dimensionless penetration rate UD, dimensionless time following penetration initiation tD, and dimensionless time following penetration arrest t′D. The extended set of dimensionless parameters controlling the response makes parameter determination problematic and questionably non-unique. Pressure response in the proximity of a lower permeable or impermeable boundary is indistinguishable from the homogeneous case for coefficients of consolidation c in excess of 2 cm2/s. Below this threshold, penetration-generated pore pressures are visibly modified in the presence of a discrete boundary. In situ parameters inferred directly from pressure magnitudes, without due consideration for the influence of layering, may therefore be in considerable error. In the hydraulically visible range, the influence of layering on the generated tip pressures is apparent at a separation of the order of 1·5 cm for standard penetration. Although absolute pressure magnitudes are strongly modified in the presence of boundaries, dissipation rates remain relatively unaffected and are consistent with those recorded in the absence of boundaries. The monitoring of dissipation rates, post-arrest, is suggested as the most reliable and accurate method of extricating parameters, in situ.  相似文献   

20.
Part I of this contribution (Gardés et al. in Contrib Mineral Petrol, 2010) reported time- and temperature-dependent experimental growth of polycrystalline forsterite-enstatite double layers between single crystals of periclase and quartz, and enstatite single layers between forsterite and quartz. Both double and single layers displayed growth rates decreasing with time and pronounced grain coarsening. Here, a model is presented for the growth of the layers that couples grain boundary diffusion and grain coarsening to interpret the drop of the growth rates. It results that the growth of the layers is such that (Δx)2 ∝ t 1−1/n , where Δx is the layer thickness and n the grain coarsening exponent, as experimentally observed. It is shown that component transport occurs mainly by grain boundary diffusion and that the contribution of volume diffusion is negligible. Assuming a value of 1 nm for the effective grain boundary width, the following Arrhenius laws for MgO grain boundary diffusion are derived: log D gb,0Fo (m2/s) = −2.71 ± 1.03 and E gbFo = 329 ± 30 kJ/mol in forsterite and log D gb,0En (m2/s) = 0.13 ± 1.31 and E gbEn = 417 ± 38 kJ/mol in enstatite. The different activation energies are responsible for the changes in the enstatite/forsterite thickness ratio with varying temperature. We show that significant biases are introduced if grain boundary diffusion-controlled rim growth is modelled assuming constant bulk diffusivities so that differences in activation energies of more than 100 kJ/mol may arise. It is thus important to consider grain coarsening when modelling layered reaction zones because they are usually polycrystalline and controlled by grain boundary transport.  相似文献   

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