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• 1995-1999  (47,477)
• 1999  (47,477)
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• 1
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Springer
Computational mechanics 22 (1999), S. 437-442
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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• 2
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Computational mechanics 22 (1999), S. 443-449
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract The interrelation of a certain class of finite elements based either on the Enhanced Assumed Strain (EAS) concept or the Hybrid Stress (HS) method is addressed. It is shown that both concepts lead to identical elements, if the material law is strongly satisfied in the Hu-Washizu principle. Conditions for the spaces of admissible functions are derived and the equivalence of the resulting weak formulations is proved. A recipe' for the selection of trial functions of corresponding elements is given, and a class of equivalent EAS- and HS-elements is presented.
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• 3
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Computational mechanics 23 (1999), S. 8-19
ISSN: 1432-0924
Keywords: Keywords: Material characteristic length ; strain-rate gradient ; dominant growth rate ; viscoplastic material ; stability
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract We study thermomechanical deformations of a viscoplastic body deformed in simple shear. The strain gradients are taken as independent kinematic variables and the corresponding higher order stresses are included in the balance laws, and the equation for the yield surface. Three different functional relationships, the power law, and those proposed by Wright and Batra, and Johnson and Cook are used to relate the effective strain rate to the effective stress and temperature. Effects of strain hardening of the material and elastic deformations are neglected. The homogeneous solution of the problem is perturbed and the stability of the problem linear in the perturbation variables is studied. Following Wright and Ockendon's postulate that the wavelength whose initial growth rate is maximum determines the minimum spacing between adjacent shear bands, the shear band spacing is computed. It is found that the minimum shear band spacing is very sensitive to the thermal softening coefficient/exponent, the material characteristic length and the nominal strain-rate. Approximate analytical expressions for the critical wave length for heat conducting nonpolar materials and locally adiabatic deformations of gradient dependent materials are also derived.
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• 4
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Computational mechanics 22 (1999), S. 425-428
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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• 5
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Computational mechanics 22 (1999), S. 429-436
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract This paper describes a continuum model, developed recently by R. Abeyaratne and the author, for the response of elastic solids capable of undergoing stress-induced phase transitions. Models of the kind sketched here and their generalizations are intended to apply to both quasi-static and dynamic experiments for shape memory alloys and to impact-induced phase changes in ceramics. The present discussion is confined to a purely mechanical theory, omitting thermal effects, so that the natural setting is the nonlinear theory of elasticity. The presentation below is limited to one space dimension.
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• 6
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Computational mechanics 23 (1999), S. 1-7
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract In this paper the Fourier series method is introduced to obtain the solution for the displacement response of rectangular plates subjected to a blast loading. Applying the Fourier series method, the von Karman nonlinear equations of plates are transformed into Duffing type nonlinear ordinary differential equations and then solved by adopting the Lindstedt-Poincar'e perturbation method. The complete analysis of square plates was carried out for all-clamped edge conditions. The results of deflection history and stress resultants which uses one perturbation term sufficiently correlated with results coming from both related literature as well as the finite element method.
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• 7
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Computational mechanics 23 (1999), S. 63-74
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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• 8
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Computational mechanics 23 (1999), S. 87-96
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract In the direct formulation of the boundary element method, body-force and thermal loads manifest themselves as additional volume integral terms in the boundary integral equation. The exact transformation of the volume integral associated with body-force loading into surface ones for two-dimensional elastostatics in general anisotropy, has only very recently been achieved. This paper extends the work to treat two-dimensional thermoelastic problems which, unlike in isotropic elasticity, pose additional complications in the formulation. The success of the exact volume-to-surface integral transformation and its implementation is illustrated with three examples. The present study restores the application of BEM to two-dimensional anisotropic elastostatics as a truly boundary solution technique even when thermal effects are involved.
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• 9
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Computational mechanics 24 (1999), S. 304-309
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract In this paper two techniques, dual reciprocity method (DRM) and direct integral method (DIM), are developed to transform domain integrals to boundary integrals for shear deformable plate bending formulation. The force term is approximated by a set of radial basis functions. To transform domain integrals to boundary integrals using the dual reciprocity method, particular solutions are employed for three radial basis functions. Direct integral method is also introduced in this paper to evaluate domain integrals. Three examples are presented to demonstrate the accuracy of the two methods. The numerical results obtained by using different particular solutions are compared with exact solutions.
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• 10
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Computational mechanics 24 (1999), S. 334-347
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract In this paper, the conventional moving least squares interpolation scheme is generalized, to incorporate the information concerning the derivative of the field variable into the interpolation scheme. By using this generalized moving least squares interpolation, along with the MLPG (Meshless Local Petrov–Galerkin) paradigm, a new numerical approach is proposed to deal with 4th order problems of thin beams. Through numerical examples, convergence tests are performed; and problems of thin beams under various loading and boundary conditions are analyzed by the proposed method, and the numerical results are compared with analytical solutions.
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• 11
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Computational mechanics 22 (1999), S. 450-462
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Summary In this paper the general hybrid-mixed finite element methods are investigated systematically in a framework of multi-field variational equations. The commonly accepted concept “saddle point problem” is argued in this paper. The existence, uniqueness, convergence, and stability properties of the solutions are proved undertaking the assumptions of Ker*-ellipticity and nested BB-conditions. The relation between discrete BB-condition and smallest singular value, and a so-called singular value principle are proposed for the practical applications using hybrid-mixed finite element methods.
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• 12
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Computational mechanics 23 (1999), S. 33-38
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A cyclic plasticity problem is numerically analyzed in [13], where a sub-optimal order error estimate is shown for a spatially discrete scheme. In this note, we prove an optimal order error estimate for the spatially discrete scheme under the same solution regularity condition. We also derive an error estimate for a fully discrete scheme for solving the plasticity problem.
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• 13
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Computational mechanics 23 (1999), S. 39-52
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A parameter study of the residual strength for a multiple site damaged (MSD) stiffened sheet is presented. The analysis is based on an elastic-plastic fracture analysis using the yield-strip model for interaction between a lead crack and the smaller MSD cracks. Two crack growth criteria, one with a pronounced crack growth resistance and one with no crack growth resistance and five different MSD crack patterns, are analysed for different sizes of the lead crack and the smaller MSD cracks. The analysis indicates that the residual strength reduction depends on all these parameters and that MSD may totally erode the crack arrest capability of a tear strap. Another important outcome is that for certain combinations also very small MSD cracks may induce a significant residual strength reduction.
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• 14
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Computational mechanics 23 (1999), S. 130-143
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract Advanced mesh generation and update methods for parallel 3D computation of complex flow problems are presented. The complexities of the class of problems targeted include complex geometries, unsteady behavior, and moving boundaries and interfaces, such as those encountered in fluid-object interactions. Parallel 3D simulation of 1000 spheres falling in a liquid-filled tube, and other computations, are presented in this paper to demonstrate the challenges involved in this class of flow problems and the methods developed to address these challenges.
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• 15
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Computational mechanics 23 (1999), S. 151-157
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract Recently, the Enhanced-Discretization Interface-Capturing Technique (EDICT) was introduced for simulation of unsteady flow problems with interfaces such as two-fluid and free-surface flows. The EDICT yields increased accuracy in representing the interface. Here we extend the EDICT to simulation of unsteady viscous compressible flows with boundary/shear layers and shock/expansion waves. The purpose is to increase the accuracy in selected regions of the computational domain. An error indicator is used to identify these regions that need enhanced discretization. Stabilized finite-element formulations are employed to solve the Navier-Stokes equations in their conservation law form. The finite element functions corresponding to enhanced discretization are designed to have two components, with each component coming from a different level of mesh refinement over the same computational domain. The primary component comes from a base mesh. A subset of the elements in this base mesh are identified for enhanced discretization by utilizing the error indicator. A secondary, more refined, mesh is constructed by patching together the second-level meshes generated over this subset of elements, and the second component of the functions comes from this mesh. The subset of elements in the base mesh that form the secondary mesh may change from one time level to other depending on the distribution of the error in the computations. Using a parallel implementation of this EDICT-based method, we apply it to test problems with shocks and boundary layers, and demonstrate that this method can be used very effectively to increase the accuracy of the base finite element formulation.
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• 16
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Computational mechanics 23 (1999), S. 339-352
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract Some Characteristic behavior of the T * ɛ (Atluri, Nishioka and Nakagaki (1984)) is identified in this paper through an extensive numerical study. T * ɛ is a near tip contour integral and has been known to measure the magnitude of singular deformation field at crack tip for arbitrary material models. In this paper, T * ɛ is found to behave quite differently for different choices of near tip integral contours. If the integral contour moves with advancing crack tip (moving contour), then T * ɛ measures primarily the energy release rate at the crack tip. It is very small for metallic materials, and tends to zero in the limit as Δa→0 for low hardening materials. Thus, T * ɛ evaluated on a moving contour tends to zero as ɛ→0 and Δa→0, for low hardening materials. If the integral contour elongates as crack extends (elongating contour), then T * ɛ measures total energy inside the volume enclosed by Γɛ [i.e., the energy dissipated in the extending wake] plus the energy release at the crack tip. Furthermore, the difference in the behavior of CTOA and T * ɛ, when the applied load is slightly perturbed, is identified. The CTOA is found to be quite insensitive to applied load change. T * ɛ is found to be roughly proportional to the square of the applied load. The functional shape of T * ɛ in terms of the size ɛ of integral contour (for the elongating contour case), is identified, using the crack tip asymptotic formula of Rice (1982). Also, the behaviors of CTOA and T * ɛ are discussed from the view point of Rice's asymptotic solution. It is recommended that as a crack tip parameter for ductile materials, T * ɛ with elongating path be used. CTOA is sometimes not very sensitive to the applied load change, therefore it may create some numerical problems in application phase crack propagation analysis.
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• 17
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Computational mechanics 23 (1999), S. 397-403
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A new full approximation storage multigrid method has been developed for Euler equations. Instead of the usual approach of using frozen τ (the relative truncation error between fine and coarse grid levels), the relative truncation error is distributed over coarse grids based on the solution of a set of model equations at every time step. This allows for more number of sweeps at coarse grid level. As a result, the present multigrid method is able to accelerate the solution at much faster rate than the conventional multigrid method. A first order Steger and Warming flux vector splitting strategy has been used here for solving Euler equations as well as the model equations for τ. Results are presented to demonstrate the ability of the present multigrid method.
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• 18
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Computational mechanics 23 (1999), S. 420-429
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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• 19
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Computational mechanics 23 (1999), S. 482-487
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract  Based upon legitimate variational principles, one microscopic-macroscopic finite element formulation for linear dynamics is presented by Hybrid Stress Finite Element Method. The microscopic application of Geometric Perturbation introduced by Pian and the introduction of infinitesimal limit core element (Baby Element) have been consistently combined according to the flexible and inherent interpretation of the legitimate variational principles initially originated by Pian and Tong. The conceptual development based upon Hybrid Finite Element Method is extended to linear dynamics with the introduction of physically meaningful higher modes.
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Computational mechanics 23 (1999), S. 502-509
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A general transfer matrix method (GTMM) is developed to derive the characteristic equation for the multi-material junction and wedge problems. Using complex variable techniques, the boundary conditions can be represented by a matrix equation. The matrix itself can be simplified to be a square matrix of order 4, for both the junction and wedge problems. In the case of a uniform temperature field, non-homogeneous terms occur on the interface of different kinds of materials. Including the logarithmic terms of potential for temperature, the GTMM can also be applied in this case. Since the derivation of the method involves only a matrix of order 4 or less, the GTMM is much more convenient in the implementation of computer programs.
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Computational mechanics 23 (1999), S. 524-532
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A novel filtering strategy is proposed to be utilized in conjunction with the Generalized Integral Transform Technique (GITT), in the solution of nonlinear diffusion problems. The aim is to optimize convergence enhancement, yielding computationally efficient eigenfunction expansions. The proposed filters include space and time dependence, extracted from linearized versions of the original partial differential system. The scheme automatically updates the filter along the time integration march, as the required truncation orders for the user requested accuracy begin to exceed a prescribed maximum system size. A fully nonlinear heat conduction example is selected to illustrate the computational performance of the filtering strategy, against the classical single-filter solution behavior.
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Computational mechanics 24 (1999), S. 41-51
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract The complex-valued dual BEM has been employed by Chen and Chen (1998) to solve the acoustic modes of a cavity with or without a thin partition. A novel method using only the real part of the complex-valued dual BEM was found by Chen (1998) to be equivalent to the dual MRM. However, spurious eigenvalues occur. In this paper, we propose the singular value decomposition technique to filter out spurious eigenvalues and to determine the multiplicity of true eigenvalues by combining the real-part dual equations. Also, the role of the real-part dual BEM for problems with a degenerate boundary is examined. Four examples, including a square cavity with multiple eigenvalues, a rectangular cavity, a rectangular cavity with a zero thickness partition and a rectangular cavity with a partition with finite thickness, are presented to demonstrate the validity of the proposed method. Also, the analytical solution if available, the FEM results obtained by Petyt et al. and obtained using ABAQUS and the experimental measurements are compared with those of the proposed method, and it is found that agreement between them is very good.
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Computational mechanics 24 (1999), S. 29-40
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract In this paper, the dual BEM in conjunction with the domain partition technique is employed to solve both natural frequencies and natural modes of a rod. In this new approach, there exists no spurious eigenvalue using the complex-valued singular or hypersingular equation alone. In the derivation of the singular and hypersingular integral equations, if only the real parts of the kernel functions are chosen, the resulting eigenequations have spurious eigenvalues. Such spurious eigenvalues stem from adding the dummy links into the interior structures considered. Although the spurious eigenvalues exist in this approach which uses the real-valued kernel functions, the possible indeterminacy of eigenmodes using the conventional real-valued singular or real-valued hypersingular equations disappears when the domain partition technique is adopted. The conventional real-valued multiple reciprocity BEM results in spurious eigenvalues for the mixed boundary conditions and indeterminacy of eigenmodes owing to insufficient rank of the leading coefficient matrix for the Dirichlet and Neumann boundary conditions. Such problems can be solved by combining the singular and hypersingular equations together; however, they also can be treated by using the real-valued singular or hypersingular equation alone if the domain partition technique is adopted. Three examples including the Dirichlet, Neumann and mixed type boundary conditions are investigated to show the validity of current approach.
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Computational mechanics 24 (1999), S. 319-333
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract Limit analysis of cracked structures using mathematical programming and finite element method is presented. This direct algorithm is based on a proposition of the modified Markov variational principle. The obtained upper bound formula is applicable with any kinematic admissible finite elements. The regularization of plastic dissipation function is performed to overcome the indetermination of the objective gradient in the rigid region and to realize a smooth transition of the objective function between the plastified and non-plastified regions. In order to simulate the singularity of crack-tip strain field for an ideal plastic model, the separate-point degenerated finite elements are used around the crack tips. This improves the precision of limit solutions. Numerical results of plane and axisymmetric cracked structures are extensively compared with analytic ones. The application of limit analysis in fracture mechanics is illustrated by an example.
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Computational mechanics 24 (1999), S. 127-137
ISSN: 1432-0924
Source: Springer Online Journal Archives 1860-2000
Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes: Abstract A triangular flat finite element for the analysis of thin shells which undergo large displacements is proposed. It is based upon the geometrically nonlinear theory of von Kármán for thin plates and the total Lagrangian approach. It has a total of only twelve degrees of freedom, namely, three translations at each vertex and one rotation at each mid-side. The stiffness matrix and the tangent stiffness matrix are derived explicitly. The element is tested against nonlinear patch test solutions and its performance is evaluated by solving several standard problems. The directional derivatives of the potential energy function required for the stability analysis are also provided.
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Constructive approximation 15 (1999), S. 109-134
ISSN: 1432-0940
Keywords: Key words. Numerical conformal mapping, Quadrilaterals, Conformal modules, Domain decomposition. AMS Classification. 30C30, 65E05, 30E10, 30C35.
Source: Springer Online Journal Archives 1860-2000
Topics: Mathematics
Notes: Abstract. We consider the conformal mapping of strip-like'' domains and derive a number of asymptotic results for computing the conformal modules of an associated class of quadrilaterals. These results are then used for the following two purposes: (a) to estimate the error of certain engineering formulas for measuring resistance values of integrated circuit networks; and (b) to compute the modules of complicated quadrilaterals of the type that occur frequently in engineering applications.
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Constructive approximation 15 (1999), S. 135-151
ISSN: 1432-0940
Keywords: Key words. Orthogonal matrix polynomials, Matrix moment problem. AMS Classification. 42C05, 44A60.
Source: Springer Online Journal Archives 1860-2000
Topics: Mathematics
Notes: Abstract. We describe the image through the Stieltjes transform of the set of solutions V of a matrix moment problem. We extend Riesz's theorem to the matrix setting, proving that those matrices of measures of V for which the matrix polynomials are dense in the corresponding ${\cal L}$ 2 space are precisely those whose Stieltjes transform is an extremal point (in the sense of convexity) of the image set.
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ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: In the Navier-Stokes equations the removal of the turbulent fluctuating velocities with a frequency above a certain fixed threshold, employed in the Large Eddy Simulation (LES), causes the appearance of a turbulent stress tensor that requires a number of closure assumptions. In this paper insufficiencies are demonstrated for those closure models which are based on a scalar eddy viscosity coefficient. A new model, based on a tensorial eddy viscosity, is therefore proposed; it employs the Germano identity [1] and allows dynamical evaluation of the single required input coefficient. The tensorial expression for the eddy viscosity is deduced by removing the widely used scalar assumption of the high-frequency viscous dissipation and replacing it by its tensorial counterpart arising in the balance of the Reynolds stress tensor. The numerical simulations performed for a lid driven cavity flow show that the proposed model allows to overcome the drawbacks encountered by the scalar eddy viscosity models.
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ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: Ice shelves consist of two layers, an upper layer of meteoric ice nourished by the flow from the connected inland ice and precipitation, and a lower layer of marine ice that is built by the melting and freezing processes at the ice-ocean interface and the accretion of frazil ice from the underlying ocean. The governing thermomechanical equations in the two layers are formulated as are the boundary and transition conditions that apply at the free surface, the material interface between the meteoric and the marine ice and the ice-ocean interface. The equations comprise in the bulk mass balances for the ice and the salt water (in marine ice), momentum balance and energy balance equations, and at the boundaries kinematic equations as well as jump conditions of mass, momentum and energy. The side boundary conditions involve a prescription of the mass flow along the grounding line from the inland ice and a kinematic law describing the mass loss by calving along the floating ice-shelf front. An appropriate scaling, in which the shallowness of the ice shelves is used, gives rise to the development of a perturbation scheme for the solution of the three-dimensional equations. Its lowest-order approximation – the shallow-shelf approximation (SSA) – shows the ice flow to be predominantly horizontal with a velocity field independent of depth, but strongly depth-dependent temperature and stress distributions. This zeroth order shallow-shelf approximation excludes the treatment of ice rumples, ice rises and the vicinity of the grounding line, but higher-order equations may to within second-order accuracy in the perturbation parameter accommodate for these more complicated effects. The scaling introduced finally leads to a vertical integrated system of non-linear partial integro-differential equations describing the ice flow and evolution equation for temperature and the free surfaces.
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ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: Relativistic Extended Thermodynamics is a very important scientific achievement of the last decades, and has inspired many subsequent authors to apply its methodology to lots of other possible applications. In short it furnishes field equations which are “closed” by imposing the relativity principle and the entropy principle up to second order, with respect to equilibrium; the linear closure is explicitly reported. Here these principles are imposed up to fourth order; the second order closure is explicitly reported, while the subsequent ones are reported as implicit functions. It is also proved that no internal inconsistencies are generated by the theory. In fact, in the process of computations many complicated conditions on the lower order terms are deduced, but they turn out to be identically satisfied.
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ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: We consider general finite deformation thermoviscoplasticity models and focus attention to the thermoelasticity laws involved in the constitutive theory. The goal is to show how such constitutive relations having forms as simple as possible may be defined with respect to both the plastic intermediate configuration and the actual configuration. In particular, using two families of strain and associated stress tensors, we arrive at two different thermoelasticity laws formulated with respect to the actual configuration. These are extensions to nonisothermal deformations of corresponding hyperelasticity laws introduced previously by Simo [1].
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ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: We evaluate how the curvature dependence of surface tension affects the shape of electrically charged interfaces between a perfectly conducting fluid and its vapour. We consider two cases: i) spherical droplets in equilibrium with their vapour; ii) menisci pending in a capillary tube in presence of a conducting plate at given electric potential drop. Tolman-like dependence of surface tension on curvature becomes important when the “nucleation radius” is comparable with the interface curvature radius. In case i) we prove existence of the equilibrium minimal radius and estimate its dependence on the electric fields and Tolman-like curvature effects. In case ii) the menisci are subject to the gravitational force, surface tension and electrostatic fields. We determine the unknown surface of the menisci to which the potential is assigned using an iterative numerical method and show that Tolman-like corrections imply: 1) a variation of the height (up to 10% in some cases) of the tip of the menisci; 2) a decrease of the maximum electrical potential applicable to the menisci before their break-down amounting to 40V over 800V in the considered cases. We conjecture that these effects could be used in new experiments based on electric measurements to determine the dependence of the equilibrium surface tension on curvature.
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Continuum mechanics and thermodynamics 11 (1999), S. 107-111
ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: We consider the system of moments associated with the relativistic Boltzmann-Chernikov equation. Using the particular symmetric form obtained by the closure procedure of Extended Thermodynamics we deduce a lower bound for the maximum velocity of wave propagation in terms of the number of moments for a non-degenerate gas. When the number of moments increases this velocity tends to the speed of light. We also give the lower bound estimate in the limit cases of ultrarelativistic fluids and in the non relativistic approximation.
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Continuum mechanics and thermodynamics 11 (1999), S. 113-139
ISSN: 1432-0959
Source: Springer Online Journal Archives 1860-2000
Topics: Physics
Notes: In the extrusion of polymeric melts at high flow rates, often flow instabilities observed as surface distortions of the extrudate occur. The flow instability ‘spurt’ in piston-driven flows is accompanied by persistent oscillations in the pressure. In this paper spurt is explained in terms of constitutive instabilities (mechanical failure of the polymeric fluid itself), while the no-slip boundary condition at the wall of the die is maintained. The influence of compression on the onset of spurt is investigated. The polymeric melt is modelled as a JSO-fluid, but two other constitutive models are also considered. Numerical computations disclose that persistent oscillations in the pressure as well as in the volumetric flow rate occur for a bounded range of prescribed plunger speeds. The occurrence of the persistent oscillations is also explained by a linearized stability analysis. The frequency of the persistent oscillations is determined both from the linearized stability theory as well as by means of a Fourier spectral analysis. In conclusion, the theory is validated by a qualitative comparison with experimental results.
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Continuum mechanics and thermodynamics 11 (1999), S. 181-191
ISSN: 1432-0959
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Topics: Physics
Notes: A model with phase change for material convection in a saturated porous medium with a frozen region is formulated as a Darcy-Stefan problem. We propose a new generalized formulation for this Stefan-type problem with convection governed by Darcy's law. This approach, which is valid for irregular geometries with irregular subregions, has the advantage of not requiring the smoothness of the temperature, that restricted previous mathematical works to two-dimensional particular cases. We show existence of generalized solutions, passing to the limit in suitable approximated problems, which in principle can be solved numerically by the finite element method.
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Continuum mechanics and thermodynamics 11 (1999), S. 141-172
ISSN: 1432-0959
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Topics: Physics
Notes: This article is concerned with the nonlinear analysis of the stability of thick elastic bodies subjected to finite elastic deformations. The analysis is based on the theory of small elastic deformations superimposed on a finite elastic deformation. Attention is drawn to methods developed in the stability analysis of fluids and of thin shells and plates which are readily applicable to the present circumstances. The state of development of the nonlinear stability analysis of thick elastic bodies is summarized in order to provide a basis for subsequent studies, and some new results relating to the stability of an elastic plate subjected to a pre-stress associated either with uniaxial thrust or with simple shear in the presence of all-round pressure are discussed. Near-critical modes in the neighbourhood of so-called critical configurations are considered to depend on, for example, a slow time variable, and nonlinear evolution equations for the mode amplitudes are derived both in the case of a monochromatic mode and for a resonant triad of modes. The crucial role of the ‘nonlinear coefficient’ in such an equation in the analysis of stability, imperfection sensitivity and localization is highlighted. An efficient (virtual work) method for the determination of this coefficient is described together with an alternative method based on the calculation of the total energy of a monochromatic near-critical mode. The influence of the boundary conditions and of the form of the pre-stress is examined and explicit calculation of the nonlinear coefficient is provided for the two representative pre-stress conditions mentioned in the above paragraph. It is shown, in particular, that a resonant triad of modes has an effect similar to that generated by the presence of a geometrical imperfection. The Appendices gather together for reference certain expressions which are used in the body of the article. These include expressions, not given previously in the literature, for the components of the tensor of third-order instantaneous elastic moduli in terms of the principal stretches of the deformation in respect of a general form of incompressible isotropic elastic strain-energy function.
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Continuum mechanics and thermodynamics 11 (1999), S. 173-179
ISSN: 1432-0959
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Topics: Physics
Notes: A novel Liapunov functional was used in previous work to establish nonlinear stability of certain nontrivial equilibrium states; essentially the context was that of pure nonlinear diffusion. This paper uses the same Liapunov functional to derive a nonlinear stability criterion in the context of a highly nonlinear system of p.d.e.'s involving nonlinear diffusion as an element. The context is that of convection of a thermofluid (i) conforming to Darcy's law and the Boussinesq approximation, (ii) with temperature dependent thermal diffusivity and viscosity, in an infinite vertical slab of porous material. The vertical faces are held at different constant temperatures, a steady state is identified, and is shown to be nonlinearly stable provided that the Rayleigh number does not exceed a quantity which reflects the temperature dependence of the pertinent physical properties. It may be that the versatility of the Liapunov functional thus exhibited may extend to other nonlinear systems involving nonlinear diffusion.
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Continuum mechanics and thermodynamics 11 (1999), S. 193-216
ISSN: 1432-0959
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Topics: Physics
Notes: New constitutive relations are derived for amorphous glassy polymers based on the concept of cooperative relaxation. A polymer is treated as a system of rearranging regions (flow units) embedded into a homogeneous elastic matrix. The viscoelastic (time-dependent) response of a medium is explained by rearrangements of segments of long chains in relaxing regions which occur at random instants. The kinetics of rearrangement is described in the framework of the Eyring concept of thermally activated processes, whereas the energy of any flow unit is assumed to randomly change at the instant of its reformation. Based on experimental data, phenomenological formulas are proposed for material functions. Adjustable parameters are found by fitting observations for mixtures of nylon with lithium halides in isothermal tensile relaxation tests. The thermoviscoelastic response in other tests is studied numerically. It is demonstrated that the material behavior predicted by the constitutive model in non-isothermal tests substantially differs from that predicted by conventional models whose adjustable parameters are determined by using the same experimental data.
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Continuum mechanics and thermodynamics 11 (1999), S. 217-225
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Topics: Physics
Notes: In this paper we make use of 26 field equations derived by the moment method to describe the stationary plane Couette flow for dense and rarefied gases. In such high moment theories one needs more boundary conditions than can be controlled experimentally. We employ a new minimax principle for the entropy production to determine the remaining boundary conditions. There results a heat flux in the direction of the flow, proportional to the curvature of the velocity field. Such a heat flux confirms results from molecular dynamics, it vanishes in a dense gas.
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Continuum mechanics and thermodynamics 11 (1999), S. 247-262
ISSN: 1432-0959
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Topics: Physics
Notes: This work deals with the thermodynamic formulation of a model for a class of materials containing microstructure which evolves or changes relative to\/ the (global) bulk material. The approach taken here is based on a generalization of the total energy, total energy flux, and total energy supply to take into account the corresponding additional degrees of freedom involved. Restricting attention for simplicity to thermoelastic materials with scalar-valued such degrees of freedom, the thermodynamically-consistent forms of the remaining balance and corresponding constitutive relations for this material class are obtained in the context of the Müller-Liu entropy principle. In particular, the thermodynamically-consistent form of the evolution relation for the additional scalar-valued degrees of freedom obtained in this fashion contains in part\/ the well-known generalized Euler-Lagrange or Ginzburg-Landau relations established in many previous work, with the remaining terms accounting for effects associated with microinertia, or with non-equilibrium processes.
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Continuum mechanics and thermodynamics 11 (1999), S. 227-245
ISSN: 1432-0959
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Topics: Physics
Notes: This is a study on the initial and boundary value problem of a symmetric hyperbolic system which is related to the conduction of heat in solids at low temperatures. The nonlinear system consists of a conservation equation for the energy density e and a balance equation for the heat flux $Q_i$ , where e and $Q_i$ are the four basic fields of the theory. The initial and boundary value problem that uses exclusively prescribed boundary data for the energy density e is solved by a new kinetic approach that was introduced and evaluated by Dreyer and Kunik in [1], [2] and Pertame [3]. This method includes the formation of shock fronts and the broadening of heat pulses. These effects cannot be observed in the linearized theory, as it is described in [4]. The kinetic representations of the initial and boundary value problem reveal a peculiar phenomenon. To the solution there contribute integrals containing the initial fields $e_0 (x), Q_0 (x)$ as well as integrals that need knowledge on energy and heat flux at a boundary. However, only one of these quantities can be controlled in an experiment. When this ambiguity is removed by continuity conditions, it turns out that after some very short time the energy density and heat flux are related to the initial data according to the Rankine Hugoniot relation.
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Topics: Geosciences
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Notes: Abstract Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth differentiation. SHRIMP zircon U-Th-Pb dating has identified, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites and a metagranodiorite that have igneous ages of 4002 ± 4, 4012 ± 6 and 4031 ± 3 Ga respectively. These are the first identified Priscoan terrestrial rocks. A record of metamorphic events at ∼3.75, ∼3.6 and ∼1.7 Ga also is preserved. These discoveries approximately double, to ∼40 km2, the area over which ∼4.0 Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as old as 4.06 Ga might yet be found in the region. As early as 4.03 Ga, terrestrial differentiation was already producing tonalitic magmas, probably by partial melting of pre-existing, less differentiated crust.
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Notes: Abstract A relatively simple petrogenetic grid for partial melting of pelitic rocks in the NCKFMASH system is presented based on the assumption that the only H2O available for melting is through dehydration reactions. The grid includes both discontinuous and continuous Fe-Mg reactions; contours of Fe/(Fe+Mg) for continuous reactions define P-T vectors along which continuous melting will occur. For biotite-bearing assemblages (garnet+biotite + sillimanite + K-feldspar + liquid and garnet + biotite + cordierite + K-feldspar + liquid), Fe/(Fe+Mg) contours have negative slopes and melting will occur with increasing temperature or pressure. For biotite-absent assemblages (garnet + cordierite + sillimanite + K-feldspar + liquid or garnet + cordierite + orthopyroxene + K-feldspar + liquid) Fe/(Fe + Mg) contours have flat slopes and melting will occur only with increasing pressure. The grid predicts that abundant matrix K-feldspar should only be observed if rocks are heated at P 〈 3.8 kbar, that abundant retrograde muscovite should only be observed if rocks are cooled at P 〉 3.8 kbar, and that generation of late biotite + sillimanite replacing garnet, cordierite, or as selvages around leu- cosomes should be common in rocks in which melt is not removed. There is also a predicted field for dehydration melting of staurolite between 5 and 12 kbar. Textures in migmatites from New Hampshire, USA, suggest that prograde dehydration melting reactions are very nearly completely reversible during cooling and crystallization in rocks in which melt is not removed. Therefore, many reaction textures in “low grade” migmatites may represent retrograde rather than prograde reactions.
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Notes: Abstract Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last ∼400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Arc (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as “adakites” derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust.
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Notes: Abstract The oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss (biotite gneiss and granitic gneiss) associated with ultrahigh pressure (UHP) eclogites in the Shuanghe district of the eastern Dabie Mountains. There are significant differences in δ18O between the two gneisses: the UHP biotite gneiss varying from −4.3‰ to 10.6‰ similar to the associated eclogites, whereas the non-UHP granitic gneiss ranges only from −3.8‰ to 1.2‰. The δD values are similar in the two gneisses with −37 to −64‰ for epidote/zoisite, −92 to −83‰ for amphibole, and −63 to −109‰ for biotite/phengite. Hydrogen isotope disequilibrium among the coexisting hydroxyl-bearing minerals is ascribed to retrograde exchange subsequent to amphibolite-facies metamorphism. Oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock δ18O. Oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals. The metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss. The isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite-facies metamorphism. However, high temperatures of 550 to 650 °C are obtained from those minerals resistant to retrograde oxygen isotope exchange, implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers. The 18O-depletion of both gneisses is interpreted to result from meteoric-hydrothermal exchange before/during plate subduction. Therefore, the measured δ18O values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the UHP metamorphism. It is inferred that the UHP unit is in foreign contact with the non-UHP unit like a tectonic melange, but both of them experienced the two common stages of geodynamic evolution: (1) 18O-depletion prior to the UHP metamorphism, (2) uplifting since the amphibolite-facies metamorphism.
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Topics: Geosciences
Notes: Abstract The high-pressure response of the cell parameters of calcite, CaCO3, has been investigated by single crystal X-ray diffraction. The unit cell parameters have been refined from 0 to 1.435 GPa, and the linear and volume compressibilities have been measured as β a =2.62(2) × 10−3 GPa−1,β c =7.94(7) × 10−3 GPa−1, β v =13.12 × 10−3 GPa−1. The bulk modulus has been obtained from a fit to the Birch-Murnaghan equation of state, giving K 0=73.46 ± 0.27 GPa and V 0=367.789 ± 0.004 Å3 with K′=4. Combined with earlier data for magnesite, ankerite and dolomite, these data suggest that K 0 V 0 is a constant for the Ca-Mg rhombohedral carbonates.
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Notes: Abstract To decipher the geodynamic significance of Cenozoic magmatism along the Red River shear belt, geochemical analyses, U-Pb and Rb-Sr dating, and Pb-Sr-Nd isotope tracing were undertaken. Zircon, monazite, titanite, and a Ti-U-oxide from foliated granitoid intrusions in the shear belt gneisses yield U-Pb emplacement ages of 33.1 ± 0.2 (2σ), 31.9 ± 0.3, 25.8 ± 0.2 and 24.7 ± 0.2 Ma, and an age of 35.0 ± 0.3 Ma was obtained for the roughly 100 km long, adjacent Jinping (Phan Si Pang) alkali granite. Together with our previous data the new ages suggest that magmatism and left-lateral strike-slip movements occurred coevally during latest Eocene–Oligocene times from 33 to 22 Ma. The Rb-Sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 °C occurred at 52.6 ± 1.1 Ma, pre-dating the onset of magmatism, whereas further cooling to 300 °C took place at 28.9 ± 0.6. This shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone. Geochemical data substantiate two types of magmas: (1) amphibole-bearing intrusions of alkaline trend which are derived from sources with Isr: 0.7065–0.7089 and i Nd: −3.7 to −6.6; (2) leucogranitic layers and bodies having Isr: 0.7084–0.7354 and i Nd: −3.3 to −13.4. The former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks, whereas leucogranites are restricted to the shear belt gneisses. Source signatures of the alkaline intrusions lie adjacent to the those of OIB, plotting at the lower end of the Mantle Array. Contamination of these melts by continental material seems to be very limited. On the other hand, the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures, requiring melting of crust different from the actually exposed gneisses. Magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains, most likely present in the lithosphere underneath the region. Since lithospheric thickening or subduction can be ruled out to produce both types of magmas, the presence of an important thermal anomaly is required, which is coevally active with left-lateral strike-slip shear. Adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas, which successively migrate through the crust to induce anatectic melting at 20–15 km crustal depth. Alkaline magmas largely dominate the volume of magmatism along the belt, being continuously present in the shear zone for millions of years. Such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the Red River belt, possibly also playing a rôle for where and when zones of plate-scale lateral extrusion develop.
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Notes: Abstract The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE; low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption.
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Notes: Abstract The trace element distribution in three selected olivine-bearing gabbros from the Northern Apennine ophiolites has been determined. These rocks consist of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyroxene. Fe-Ti-oxides, titanian pargasite, orthopyroxene and apatite occur as interstitial accessory minerals. Plagioclase, clinopyroxene and accessory minerals were analysed for rare earth (REE) and selected trace elements by secondary ion mass spectrometry. Both plagioclase and clinopyroxene are compositionally zoned. The plagioclase rims have slightly lower anorthite component and higher light REE (LREE), Ba and K than the cores. Likewise, the clinopyroxene rims show a slight Mg and Cr decrease, and a marked increase in Zr, REE and Y relative to the core. The rims of plagioclase and clinopyroxene, Fe-Ti-oxides, apatite and titanian pargasite most likely formed through post-cumulus fractional crystallization of interstitial liquid. It is argued that such interstitial liquid had an exotic component, probably related to the infiltration of highly evolved, slightly LREE enriched liquid in the cumulate pile. On the basis of mass balance calculations, we show that Fe-Ti-oxides play an important role in the Ti budget of the whole rock, as does apatite for LREE.
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Notes: Abstract The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.
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Notes: Abstract Ion-microprobe analysis of zircons from an andalusite-bearing orthogneiss within the major Alpujárride nappe complex in the central part of the Betic Cordilleras has yielded a Hercynian age of 285 ± 5 (2σ) Ma for euhedral rims, interpreted as the magmatic age of the andalusite-bearing biotite granite parent rock for the gneisses. Zircon age zoning systematics suggest a Paleozoic sedimentation age for the parent material for the anatectic source rock. Zircon cores represent several groups of ages: (1) Archean, c. 2.7 Ga; (2) Early Proterozoic, 2.2–2.0 Ga; (3) Middle Proterozoic, 1.1–0.9 Ga; (4) Pan-African, 0.8–0.5 Ga; including a well-defined event at 612 ± 13 (2σ) Ma. Paragenetic and textural relations indicate that gneissification took place during a high-P (12–13 kbar) low-T (450–500 °C) collisional event during which the primary Alpine nappe pile was produced. The second and final Alpine tectono-metamorphic event led to reorganization of the primary nappe pile by extensional tetonics with coeval very fast rock uplift and cooling (from c. 8? to c. 1 kbar and c. 600 to 100 °C within the period 19.5–18.5 Ma). The fast uplift/cooling stage was triggered by slab break-off which is thought to have induced diapiric underplating by high-T asthenospheric material. This may have heated the collisional wedge, causing thermal weakening which might have advanced the late stage fast uplift/cooling. The Alpine events did not leave a zircon crystallization record. The inherited, Archean–Pan-African zircon age pattern corresponds to that established for the Hercynian basement in central and southern Europe which is considered as reworked Gondwana crust. Deeper levels of core complexes within the Betic-Rif belt thus belong to the pre-Triassic basement of the Tethyan realm (Betic-Ligurian lithosphere) and represent reworked material ultimately derived from Archean and Proterozoic rock complexes from the Gondwana crustal domain. This study implies that thermobarometry of rock complexes which went through several phases of tectono-metamorphic reworking may render ambiguous results if based upon field observations and petrography alone. Zircon ion-microprobe dating may provide additional constraints required to arrive at a feasible tectono-metamorphic history, that is P-T-t trajectory, for such rock complexes.
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Notes: Abstract The petrology and U-Pb geochronology of pelitic migmatite and calc-silicate gneiss reveal a detailed prograde to post-peak metamorphic thermal history for a single outcrop of Paleoproterozoic supracrustal rocks in the eastern part of the Grand Canyon. Metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 Ma and continued to grow until about 1697 Ma. The U-Pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting, which involved the breakdown of muscovite to sillimanite, commenced at about 1702 Ma, prior to the metamorphic peak. Partial melting continued until about 1690 Ma, the youngest U-Pb date from magmatic monazite in the leucosomes. Field and petrographic evidence, as well as inheritance patterns in monazites from the leucosomes, suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region. Between 1702 and 1690 Ma, the migmatite package heated to peak metamorphic conditions of about 720 °C and 6 kbar, cooled to about 675 °C at a cooling rate 〉30 °C/million years, and decompressed to about 4 kbar. The U-Pb geochronological data for metamorphic titanite from a calc-silicate gneiss exhibit a clear relationship between grain size and the 207Pb/206Pb date indicating that the titanite crystals record cooling ages. These data, combined with the titanite Pb diffusion data of Cherniak (1993), yield a cooling rate of 5.4−0.9 +1.7 °C/million years, integrated over the interval 1690 to 1676 Ma and suggest that by 1675 Ma, the cooling rate slowed to less than 2 °C/million years. The rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large-scale tectonic processes associated with the accretion of juvenile crust to the margin of Laurentia. Juvenile arc crust appears to have been assembled, accreted and stabilized into Laurentian lithosphere in less than 30 million years.
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Notes: Abstract The “forbidden” assemblage (ferropericlase + enstatite) as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle. Magnesite is probably the only stable carbonate at depths greater than ∼220 km. Like dehydration reactions, the reaction boundary for the decarbonation of magnesite has a positive dT/dP slope at lower pressures, which becomes negative at higher pressures, if no other phase intervenes. This reaction boundary probably intersects the geotherm between ∼900 and ∼1100 km, below which magnesite decomposes into an assemblage periclase + diamond + oxygen. Thus, ferropericlase is the most likely inclusion in diamond formed in the lower mantle. The high frequency of sole occurrence of ferropericlase in diamonds from Sao Luiz, Brazil seems to substantiate the present speculation.
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Notes: Abstract The major platinum-group elements (PGE) concentrations in layered intrusions are typically associated with zones in which the sulfide abundance begins to increase. In a number of layered intrusions, there is also a distinct stratigraphic separation in the peak concentrations of the PGE from those of the base metals, gold and sulfur through these zones. These stratigraphic “offsets” are characterized by a lower, typically S-poor, Pt- and Pd-enriched zone overlain by a zone enriched in the base metals, S and Au. The separations amount to a few decimeters to several tens of meters. In some instances, the high Pt and Pd concentrations are associated with trivial amounts of sulfide. Theoretical considerations suggest that these offsets can be modeled as chromatographic peaks that develop during an infiltration/reaction process. Using Pd as a typical PGE and Cu as a typical base metal, a numeric model is developed that illustrates how metal separations can develop in a vapor-refining zone as fluid evolved during solidification of a cumulus pile leaches sulfide and redeposits it higher in the crystal pile. The solidification/degassing ore-element transport is coupled with a compaction model for the crystal pile. Solidification resulting from conductive cooling through the base of the compacting column leads to an increasing volatile concentration in the intercumulus liquid until it reaches fluid saturation. Separation and upward migration of this fluid lead to an upward-migrating zone of increasingly higher bulk water contents as water degassed from underlying cumulates enriches overlying, fluid-undersaturated interstitial liquids. Sulfide is resorbed from the degassing regions and is reprecipitated in these vapor-undersaturated interstitial liquids, producing a zone of relatively high modal sulfide that also migrates upward with time. Owing to its strong preference for sulfide, Pd is not significantly mobile until all sulfide is resorbed. The result is a zone of increasing PGE enrichment that follows the sulfide resorption front as solidification/degassing continues. In detail, the highest Pd concentrations occur stratigraphically below the peak in S and base metals. The high Pd/S ratio mimics values conventionally interpreted as the result of high (silicate liquid)/(sulfide liquid) mass ratios (“R” values). However, in this case, the high Pd/S ratio is the result of a chromatographic/reaction front enrichment and not a magmatic sulfide-saturation event.
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Notes: Abstract In the absence of an externally applied stress, the segregation of small amounts of granitic or tonalitic melts from their residual mafic crystals is possible only if the melt forms an interconnected network phase. Accordingly, this research focuses on melt connectivity at low melt fraction (〈4 wt% or 5 vol.%). Connectivity of granitic and tonalitic melts in amphibole-rich rock was assessed by performing two types of piston-cylinder experiments at 1 GPa and 800 °C. The first involved annealing samples that consisted of either alternating layers or homogeneous mixtures of calcic amphibole and metaluminous obsidian powder. The second type of experiment involved creating diffusion couples. Here, an upper cylinder of amphibole-saturated granitic or tonalitic melt was placed against a lower cylinder consisting of an amphibole-rich rock containing zero or a small melt (granitic or tonalitic) fraction. The upper part of the diffusion couple was doped with β emitter (151Sm or 14C) and functioned as an infinite melt reservoir. The lower part of the diffusion couple was considered to be the host rock. The experiments approached textural equilibrium which allowed us to characterize the wetting behaviour of the calcic amphibole by the hydrous silicic melt (granitic or tonalitic). These particular experiments also provided information concerning diffusive transport, because the β emitter could diffuse through the connected melt (liquid) in the amphibole-rich rock. The dihedral angle measurements show that melt connectivity was achieved. This conclusion is based on the fact that the dihedral angles, θ, consistently yielded median apparent values of 53°〈θ〈58° for an amphibole-rich rock/granitic melt system, and 46°〈θ〈48° for an amphibole-rich rock/tonalitic melt system. However, the frequency distribution of θ angles is found to be relatively broad. The results of the diffusion-couple experiments, assessed using the β radiographic technique, complement the dihedral (wetting) angle measurements by showing that melt connectivity is achieved at a melt fraction less than 4wt% (5 vol.%).
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Topics: Geosciences
Notes: Abstract Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France) were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence (CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes. Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon. The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected.
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Notes: Abstract  A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation.
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Notes: Abstract Average concentrations of incompatible elements and isotopes of radiogenic Sr, Nd and Pb in a 350 km long belt of Central European volcanics (CECV) resemble those in OIB in general and in OIB from the N Atlantic in particular. This similarity allows to infer comparable sources for CECV and OIB which might have been located in the lower mantle according to seismic tomography and chemistry, with the latter unlike a MORB source. The incompatible element contribution of lower mantle origin can be modelled from primitive mantle minus continental crust and upper mantle inventories. Alkali basaltic magmas from the modelled source are close in composition to CECV and OIB. The continental crust contains almost half of the silicate Earth's content of Rb, K, Pb, Ba, Th and U, which were mobilized to a major extent through dehydration of subducted ocean crust. Related losses from the lower mantle had to be replaced by deep subductions of oceanic lithosphere recognized from their isotopic imprint. From a balance based on Nd isotopes it can be concluded that average CECV contains 60% matter from residual primitive mantle and 40% from deeply subducted lithosphere (including some young upper mantle materials). Plume products from separate CECV regions developed, within 45 Ma, from rather depleted to more primitive isotopic signatures. Four periods of volcanism from Eocene to late Quaternary time are explained as four pulses of an almost stationary ultrafast plume uprise as modelled by Larsen and Yuen (1997). Magma production has increased from the first to the third pulse with the peak during Miocene time in the Vogelsberg region. The final pulse produced the Quaternary Eifel volcanoes. Tectonism from the Alpine orogen has probably triggered the synchronous volcanism of CECV, Massif Central etc. The European lithospheric plate has moved under the control of the opening Atlantic almost in an eastern direction with a velocity of 1 cm per year and has shifted extinct volcanoes off their source channels.
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Notes: Abstract Experiments to study the partitioning of Ni between olivine and garnet at natural abundances were performed at 1200 °C – 3 GPa, 1300 °C – 3, 5 and 7 GPa and 1500 °C – 5 GPa using piston-cylinder and multi-anvil apparatus. The experiments were intended to provide a further test of the Ni-in-garnet geothermometer calibrated previously using enhanced Ni levels. The run products were analysed by electron microprobe using special operating conditions to enhance counting statistics. Ni-Mg and Fe-Mg exchange between olivine and garnet was tightly reversed. Results of the experiments suggest that Ni substitution in garnet behaves according to Henry's Law up to levels of ∼3000 ppm. The Ni-in-garnet geothermometer derived from reversed experiments in this study is consistent (within error) with that derived from previous experiments at enhanced levels of Ni; neither of these experimental calibrations is consistent with any empirically derived Ni-in-garnet geothermometer. Sources of the discrepancy between the empirical and experimental calibrations of the Ni-in-garnet geothermometer are examined in detail and it is concluded that the main reason for the differences involves errors in the calculation of temperatures in the empirical version. These errors are clearly demonstrated using a recently published data set for garnet peridotite xenoliths in which both Ni-in-garnet and more conventional multi-equilibrium thermobarometry was employed.
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Notes: Abstract Phase equilibria in the ternary system H2O-CO2-NaCl were studied at 800 °C and 9 kbar in internally heated gas pressure vessels using a modified synthetic fluid inclusion technique. The low rate of quartz overgrowth along the b' and a' axes of quartz crystals was used to avoid fluid inclusion formation during heating, prior to attainment of equilibrium run conditions. The density of CO2 in the synthetic fluid inclusions was calibrated using inclusions in the binary H2O-CO2 system synthesised by the same method and measured on the same heating-freezing stage. In the two-phase field, two types of fluid inclusions with different densities of CO2 were observed. Using mass balance calculations, these inclusions are used to constrain the miscibility gap and the orientation of two-phase tie-lines in the H2O-CO2-NaCl system at 800 °C and 9 kbar. The equation of state of Duan et al. (1995) approximately describes the P-T section of the ternary system up to about 40 wt% of NaCl. At higher NaCl concentrations the measured solubility of CO2 in the brine is much smaller than predicted by the EOS. A “salting out” effect must be added to the equation of state to include coulomb interaction in the model of Anderko and Pitzer (1993) and Pitzer and Jiang (1996). The new experimental data together with published data up to 5 kbar (Shmulovich et al. 1995) encompass practically all subsolidus crustal P-T conditions. A feature of the new experimental results is the large compositional range in the H2O-CO2-NaCl system occupied by the stability fields of halite + CO2-rich fluid ± H2O-NaCl brine. The prediction of halite stability in equilibrium with CO2-rich fluid in deep-crustal rocks is supported by recent petrological and fluid inclusion studies of granulites.
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Notes: Abstract Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (deformation enhanced ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif.
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Notes: Abstract Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D1). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional event (D2). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma. The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma) is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during the first deformation event (D1) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D2 pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about 500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics (two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt.
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Notes: Abstract We have studied the Sr isotopic composition of partial melts of biotite granite generated experimentally and by intrusion of basalt into the Sierra Nevada Batholith. The experiments employed large, 3-cm cubes of granite to duplicate natural grain-boundary textures and were performed in air over the temperature interval 1000–1250 °C, to simulate basalt-induced wall rock and xenolith melting in the shallow crust. In both the experimental and natural analogs, fusion of plagioclase + alkali-feldspar ± quartz and biotite + plagioclase ± quartz results in the formation of colorless and brown melt (quenched to glass) respectively. Accordant with this melting behavior, brown glasses are enriched in radiogenic Sr and MgO, FeO, CaO, and TiO2 relative to colorless glasses. These results support recent studies indicating that the isotopic compositions of crustal melts can reflect the relative contributions of mineral phases entering the melt, rather than the isotopic composition of the bulk source rock. In addition, we show that at shallow-crustal conditions preferential breakdown of biotite leads to initial high-87Sr/86Sr, low-Sr concentration melts. However, as the degree of melting increases, melts become less radiogenic yet are more enriched in elemental Sr due to loss of biotite from the restite and increased consumption of feldspars. Our results therefore suggest, if partial melts of granitic crust segregate rapidly during episodic magmatic underplating, successive melt batches can evolve from high-87Sr/86Sr to low-87Sr/86Sr liquids as melting progresses.
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Notes: Abstract The equilibrium intracrystalline distribution coefficient, k D *, of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k D * as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔS XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.
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Notes: Abstract Subhorizontally layered pegmatite-aplite bodies are characterized by fine-grained, sodic to granitic aplite that is usually juxtaposed abruptly above by much coarser-grained, commonly graphic potassic pegmatite. Although well studied, there currently is little concensus as to how such dikes form. The Little Three dike near Ramona, California, is representative of such zoned bodies in this and other regions, and contains discontinuous miarolitic pockets near the base of the graphic pegmatite zone. Tourmaline, garnet, biotite, and muscovite show no changes in major- or minor-element compositions indicative of progressive magmatic fractionation until the immediate vicinity of the main miarolitic zone, where they record abrupt and extreme enrichments in Li, F, and Mn. There is no correlation of chemical changes in the dike with the appearance of small miarolitic vugs well below the main miarolitic zone, nor is there any indication that the aplite, graphic pegmatite, or miarolitic pockets represent separate magma injections. The chemistries of tourmaline, garnet, and micas, however, preclude conventional models of Rayleigh fractionation or traditional zone refining. Textural features and modeled cooling histories indicate that the dike cooled quickly and might have solidified partially or totally to glass before crystallization commenced. Geothermometry based on the compositions of coexisting plagioclase and homogeneous, nonperthitic K-feldspar indicates inward crystallization of the dike, from ∼400–435 °C at the margins to ∼350–390 °C within 20–30 cm of the pocket horizon, then a sharp decrease to 240–275 °C in the pockets where K-feldspar is perthitic. We interpret the feldspar geothermometry (except perhaps in the miarolitic cavities) to reflect the temperatures at crystallization fronts that advanced into the pegmatite, first from the foot wall and eventually joined by a similar front downward from the hanging wall. Crystallization down from the hanging wall may have commenced after ∼70–80% of the foot wall aplite had crystallized. The very abrupt increases of Li, Mn, and F in tourmaline and garnet near the miarolitic zone appear to be explained best by the process of constitutional zone refining, in which a fluxed crystallization front sweeps an incompatible element-enriched boundary layer through a solid or semi-solid. After these two highly fluxed boundary layers merged near the main miarolitic zone, compositional evolution could have proceeded by crystal-melt fractionation.
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Notes: Abstract Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures. The δD values of hydrous silicates vary from −63 to −105‰. Most δ18O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ18O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central Andean Volcanic Zone (δ18O = 7.0–7.4‰). Only one sample in this study (δ18O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile.
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Notes: Abstract Small tectonic slices of undeformed eclogites and ultrahigh-pressure granulites occur in three tectonic units of the Śnieżnik Mts. (SW Poland). Ultrahigh-pressure granulite/eclogite transitions with peak metamorphic conditions between 21 and 28 kbar at 800 to 1000 °C occur only in the Złote unit. Conventional U-Pb multigrain analyses of zircons from a mafic granulite provided 207Pb/206Pb ages between 360 to 369 Ma which are interpreted to approximate timing of original crystallisation from a melt. Diffusion kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite, although some bulk-chemical differences were also recognised. The ultrahigh-pressure granulites from the Złote unit exclusively contain H2O-rich inclusions with variable salinities which distinguishes them from high-temperature (HT)-granulites world-wide. This is also in contrast to the fluid regime (H2O-N2-CO2) recognised in the lower-temperature eclogites (600–800 °C) from the closely associated Międzygórze and Śnieżnik units. The variation in fluid composition between the lower-temperature eclogites and ultrahigh-pressure granulites on the one hand and ultrahigh-pressure granulites and HT-granulites on the other hand probably indicates contrasting P-T-t paths as a result of different tectonic environments.
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Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract The diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers. Initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase-melt partition coefficient. Trace element diffusion will subsequently act to modify this initial profile and – given enough time – produce a profile that is in equilibrium with the anorthite variations within the crystal. We argue that the trace element partition coefficient Da/b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal-liquid partition coefficients, and that the equilibrium profile of the crystal can be calculated. The time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase. Strontium plagioclase-melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions. A one-dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir. The model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the Kameni Island dacites, Santorini, and from the 1979 Soufriere andesite, St. Vincent. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase. Maximum crystal residence times range from 100 to 450 years. This is in good agreement with estimates from crystal size distribution and from Ra-Th disequilibrium studies for the Kameni Islands. For Soufriere, however, such short residence times are incompatible with U-Th mineral errorchron data that suggest residence times of 〉40 ka in a thermally buffered magma reservoir. To reconcile these apparently different ages, we invoke a more complicated magmatic history for Soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption.
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• 70
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract Petrographic observations on quartz crystals from the Mole Granite (Australia) and other localities shed new light on the mechanisms of post-entrapment modification of fluid inclusions. These modifications include migration away from pseudosecondary trails, changes in fluid salinity and density, shape distortion and the formation of “sweat-haloes” around strongly deformed inclusions. Increases in fluid salinity, which usually are associated with inclusion migration, indicate water-losses of up to 50%. However, LA-ICP-MS-analysis of unmobilized and mobilized inclusions of the same trail reveals basically unchanged ratios of major – and trace element cations, with the exception of Li, which seems to be incorporated into the crystal lattice during migration. Despite the fact that all these modifications are closely related to deformation processes, they occur not only in mechanically deformed quartz, but also in free-standing crystals. In the latter samples, stress has been generated internally as a result of brazil-twinned growth and compositional zonation. These observations and their interpretation leads to a list of practical criteria that should help in differentiating between reliable and suspect fluid inclusions in other samples.
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• 71
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract. The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively.
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• 72
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.
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• 73
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at [8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of [8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the [8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE.
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• 74
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract The stability relations of phase egg, AlSiO3(OH), have been investigated at pressures from 7 to 20 GPa, and temperatures from 900 to 1700 °C in a multi-anvil apparatus. At the lower pressures phase egg breaks down according to the univariant reaction, phase egg = stishovite + topaz-OH, which extends from 1100 °C at 11 GPa to 1400 °C at 13 GPa where it terminates at an invariant point involving corundum. At pressures above the invariant point, the stability of phase egg is limited by the breakdown reaction, phase egg = stishovite + corundum + fluid, which extends from the invariant point to 1700 °C at 20 GPa. Stishovite crystallized in the Al2O3-SiO2-H2O system contains Al2O3, and the amount of Al2O3 increases with increasing temperature. It is inferred that the Al2O3 content is controlled by the charge-balanced substitution of Si4+ by Al3+ and H+. Aluminum-bearing stishovite coexisting with an H2O-rich fluid may contain a certain amount of water. Therefore, phase egg and stishovite in a subducting slab could transport some H2O into the deep Earth.
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• 75
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Springer
ISSN: 1432-0967
Source: Springer Online Journal Archives 1860-2000
Topics: Geosciences
Notes: Abstract The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO2 3.5 wt% K2O, 〈4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo79-Fo2), augite (X Fe = 0.18−0.95), feldspars (An80Or1-An1Or62), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo18–28 Fs41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO2 (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures.
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