首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 15 毫秒
1.
The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10−5 s−1. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ1 in the undeformed state to perpendicular to σ1 following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.  相似文献   

2.
A ‘soft’ carbon-based high-volatile bituminous (Ro max=0.68%) coal and a ‘hard’ carbon-based Pennsylvania anthracite (Ro max=5.27%) were deformed in the steady state at high temperatures and pressures in a series of coaxial and simple shear deformation experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Tests were carried out in a Griggs-t type solid (NaCl) medium apparatus at T=400–900°C, constant displacement rates of 10-5−10-6 s−1, at confining pressures of 0.6 GPa (coaxial) or 0.8 and 1.0 GPa (simple shear). Coaxial samples were shortened up to 50%, whereas shear strains up to 4.9 were attained in simple shear tests. Experiments lasted up to 118 h. Deformed, high-volatile bituminous coal was extensively coked and no correlation between strain and Ro max, bireflectance or coal texture was observed in any samples. With increasing temperature, Ro max and bireflectance increase in highly anisotropic, coarse mosaic units, but remain essentially constant in the fine granular mosaic, which becomes more abundant at higher temperatures. Graphite-like reflectances are observed locally only in highly reactive macerals and in pyrolytic carbon veins. The degree of molecular ordering attained in deformed bituminous coal samples appears to be determined by the heating-pressurization path rather than by subsequent deformation.Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T>700°C, with average bireflectance values increasing from 1.68% at 700°C to 6.36% at 900°C. Anisotropy is greatest in zones of high strain at all temperatures. In anthracite samples deformed in simple shear over the 600–900°C range at 1.0 GPa, the average Ro max values increase up to 11.9%, whereas average bireflectance values increase up to 10.7%. Bireflectance increases with progressive bedding rotation and, thus, with increasing shear strain. Graphitization occurs in several anthracite samples deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy of highly anisotropic material in one sample confirms the presence of graphite with d002=0.3363 nm. These data strongly suggest that shear strain, through its tendency to align basic structural units, is the factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 2200°C required in hydrostatic heating experiments at ambient pressure.  相似文献   

3.
Post-deformational annealing of calcite rocks   总被引:3,自引:3,他引:3  
The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10 4 s 1 to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.  相似文献   

4.
Non-steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake-driven episodic deformation in the middle crust. Three types of experiments were carried out using a modified Griggs-type solid medium deformation apparatus. All three start with high stress deformation at a temperature of 400 °C and a constant strain rate of 10− 4 s− 1 (type A), some are followed by annealing in the stability field of α-quartz for 14–15 h at zero nominal differential stress and temperatures of 800–1000 °C (type A + B), or by annealing for 15 h at 900 °C and at a residual stress (type A + C).The quartz samples reveal a very high strength > 2 GPa at a few percent of permanent strain. The microstructures after short-term high stress deformation (type A) record localized brittle and plastic deformation. Statisc annealing (type A + B) results in recrystallisation restricted to the highly damaged zones. The new grains aligned in strings and without crystallographic preferred orientation, indicate nucleation and growth. Annealing at non-hydrostatic conditions (type A + C) results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation. In this case, however, the recrystallised zone is several grain diameters wide, the grains are elongate, and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation. Quartz microstructures identical to those produced in type A + B experiments are observed in cores recovered from Long Valley Exploratory Well in the Quaternary Long Valley Caldera, California, with considerable seismic activity.The experiments demonstrate the behaviour of quartz at coseismic loading (type A) and subsequent static annealing (type A + B) or creep at decaying stress (type A + C) in the middle crust. The experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks.  相似文献   

5.
Previous experiments by Raleigh et al. (1971) have shown that at strain rates of 10−2.sec−1 to 10−7.sec−1 only slip occurs in dry enstatite at temperatures above 1300°C and 1000°C, respectively.The present experiments have been conducted on polycrystalline enstatite under wet conditions in this regime where enstatite only slips, polygonizes and recrystallizes. Slip occurs throughout the whole regime on the system (100)[001] and at strains greater than 40% the system (010)[001] is observed. Polygonization and intragranular recrystallization begin at about 1300°C and 10−4.sec−1 and the orientation of these neoblasts is host-controlled. At lower strain rates intergranular neoblasts develop and their fabric is one of [100] maximum parallel with σ1 and [010] and [001] girdles in the σ2 = σ3 plane, similar to those in natural enstatite tectonites.Dislocation substructures of experimentally deformed enstatite have been examined by transmission electron microscopy. The samples were deformed within the field in which slip polygonization and recrystallization are the dominant deformation mechanisms. Samples within this regime have microstructures that are characterized by stacking faults and partial dislocations. Under the conditions of steady-state flow in olivine, these microstructures inhibit the operation of recovery mechanisms in enstatite.Other samples deformed within the polygonization and recrystallization field have microstructures that confirm the optical observations of intragranular and intergranular growth of neoblasts. It is suggested that the former result from strain-induced tilt of subrains, whereas the latter may result from bulge nucleation into adjacent subgrains.Mechanical data from constant strain-rate experiments at steady state, stress relaxation and temperature-differential creep tests are best fit to a power-law creep equation with the stress exponent, n~3 and the apparent activation energy for creep, Q~65 kcal/mole. Extrapolation of this equation to a representative natural geologic strain rate of 10−4. sec−1, over the temperature interval 1000–2000°C, gives an effective viscosity range of 1020–1018 poise and stresses in the range of 7-0.1 bar, respectively. Comparison with corrected wet-olivine mechanical data (Carter, 1976) over the same environment indicates that olivine is consistently the weaker of the two minerals and will recrystallize whilst enstatite will only slip and kink, thus accounting for the different habits of olivine and enstatite in ultramafic tectonites.  相似文献   

6.
Plasticine and plasticine-like materials have been widely used as analogue materials for experimental deformation, but not many workers have conducted detailed investigations on their rheology. The physical properties of Beck's green and black plasticine, a modelling material made in Gomaringen, Germany, and plasticine/oil mixtures were investigated by means of uniaxial compression and relaxation tests. Beck's plasticine is a non-Newtonian fluid characterised by strain rate-dependent plastic yielding and strain hardening. Strain hardening is more pronounced at low strain rates leading to an increase of both stress exponent and viscosity. The addition of oil leads to an increase of the stress exponent and a decrease in viscosity. The strain dependence of viscosity decreases with increasing oil content. Compression tests on preflattened plasticine were also conducted in order to study possible ‘strain memory’ of the materials. Preflattened plasticine is characterised by a later onset of yielding and an increase in both stress exponent and viscosity. Our results suggest that Beck's green and black plasticine is a suitable analogue material for modelling rocks that deform by dislocation creep and exhibit pronounced strain hardening. Nevertheless, plane strain modelling of boudinage has verified analytical solutions for the dominant wavelength at viscosity contrasts of approximately 1.5 and 2.5.  相似文献   

7.
Finite-element folds of similar geometry   总被引:3,自引:0,他引:3  
Model folds of similar geometry have been produced by using the finite-element method and the constitutive relations of a layer of wet quartzite embedded in a marble matrix with an initially sinusoidal configuration and a 10° limb dip. The power law for steady-state flow of Yule Marble (Heard and Raleigh, 1972) is used for the matrix and our new law for Canyon Creek quartzite deformed in the presence of water is used for the layer. The equiv- alent viscosity of the wet quartzite is highly temperature-sensitive, giving rise to a strong temperature dependence of the quartzite: marble viscosity ratio which, at a strain rate of 10−14/sec, drops from 543 at 200° to 0.13 at 800°C. At 375°C (ηq/ηm = 10), concentric folds develop at all strains to 80% natural shortening and stress, finite strain and viscosity distributions are somewhat similar to those found previously. Raising the temperature to 550° C (ηq/ηm = 1), at any stage of prior amplification, causes the folds to flatten with increasing strain, accompanied by attenuation of limbs and thickening of hinges, leading to folds with similar geometries and isoclinal folds at extreme strains. The effects are more pronounced at higher temperatures and at 700° C (ηq/ηm = 0.3) limb attenuation is so severe as to give rise to unrealistic geometries. At temperatures below about 600° C (ηq/ηm = 2), similar folds do not form. It thus appears as if a viscosity contrast near unity is required to produce similar folds in rocks, under the conditions simulated and different temperature dependencies of viscosities of materials in layered sequences is one important means of reducing viscosity contrasts.  相似文献   

8.
Aggregates composed of olivine and magnesiowüstite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning, phase segregation and possible localization of deformation in a polyphase material. Samples with 20 vol.% of natural olivine and 80 vol.% of (Mg0.7Fe0.3)O were synthesized and deformed in a gas-medium torsion apparatus at temperatures of 1127 °C and 1250 °C, a confining pressure of 300 MPa and constant angular displacement rates equivalent to constant shear strain rates of 1–3.3 × 10− 4 s− 1. The samples deformed homogeneously to total shear strains of up to γ  15. During constant strain rate measurements the flow stress remained approximately stable at 1250 °C while it progressively decreased after the initial yield stress at the lower temperature. Mechanical data, microstructures and textures indicate that both phases were deforming in the dislocation creep regime. The weaker component, magnesiowüstite, controlled the rheological behavior of the bulk material and accommodated most of the strain. Deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowüstite and may have developed due to the presence of the second phase. At 1127 °C, olivine grains behaved as semi-rigid inclusions rotating in a viscous matrix. At 1250 °C, some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a-slip on (010) resulted in a weak foliation and a texture typical for pure dry olivine aggregates. Both a-slip and c-slip on (010) were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions.  相似文献   

9.
Glide systems of hematite single crystals in deformation experiments   总被引:1,自引:0,他引:1  
The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}<m>-slip in the temperature range 25 °C to 400 °C, at 400 MPa confining pressure, and a strain rate of 10− 5 s− 1 by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 °C and 400 °C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133–150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 °C at strain rates of 10− 5 s− 1 and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}<m>-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)<a>-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 °C to  5 MPa at 800 °C and for {a}<m>-slip from > 560 MPa at 25 °C to  40 MPa at 700 °C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 °C  60 MPa. The data indicate that the CRSSs above 200 °C seem to be between the values for r-twinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600 °C could be evaluated to  60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.  相似文献   

10.
The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (dp=18°, dm=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (dp=5.3°, dm=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (dp=2.4°, dm=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.  相似文献   

11.
High pressure deformation in two-phase aggregates   总被引:1,自引:0,他引:1  
We investigate the rheological behavior of multi-phase aggregates at high pressure and high temperature. Using synchrotron X-ray radiation as the probing tool, we are able to quantify the stress state of individual phases within the aggregates. This method provides fundamental information in interpreting the behavior of two phase/multi-phase mixtures, which contribute to our understanding of the deformation process at deep earth conditions. We choose MgAl2O4 spinel and MgO periclase as our model materials. Mixtures of various volume proportions were deformed in a multi-anvil high pressure deformation apparatus at pressure of 5 GPa and elevated temperatures. Stress is determined from X-ray diffraction, providing a measure of stress in each individual phase of the mixture in situ during the deformation. Macroscopic strain is determined from X-ray imaging. We compare the steady state strength of various mixtures at 1000 °C and 800 °C and at the strain rate in the range of 1.8 to 8.8 × 10− 5 s− 1. Our data indicate that the weak phase (MgO) is responsible for most of the accumulated strains while the strong phase (spinel) is supporting most of the stress when the volume proportion is 75% spinel and 25% MgO. The intermediate compositions (40/60) are much weaker than either of the end members, while the grain sizes for the intermediate compositions (submicrons) are much smaller than the end members (5–10 μm). We conclude that a change in flow mechanism resulting from these smaller grains is responsible for the low strength of the intermediate composition mixtures. This study demonstrates an approach of using synchrotron X-rays to study the deformation behaviors of multi-phase aggregates at high pressure and high temperature.  相似文献   

12.
Long-term creep tests of gabbro which have been performed with a maximum bending stress (20 bar) under a high confining pressure (1 kbar) and various temperatures, are described. Methods and techniques used in the experiment are mainly similar to those reported previously by the same authors (Itô and Sasajima, 1980) except for the application of high pressure and temperature. The techniques include the bending system, size and preparation of the sample, and the determination of its deformation by use of interference fringes of Na-D light. In order to measure a very small deformation of creep, intermittent breaks of the application of loading, confining pressure and temperature are necessary, and the creep curve is constructed from the intermittent advance of permanent deformation.The experiment has revealed two strange phenomena : one is a sinuous progress of the creep curve, and the other is that the deformation recovery shows strange behavior after the unloading. These results are discussed in close connection with the mechanism of the “turn back of creep” denoted by Itô and Sasajima (1980). The mean creep curves, at 25°C. 95°C and 150°C, obtained so far lead to viscosities of 1.6 · 1020, 1.9 · 1019 and 4.8 · 1018 poise, respectively and the maximum strain rates employed in the samples were 4.2 · 10−14, 3.6 · 10−13 and 1.4 · 10−12/sec, respectively, which cover the geological strain rate. Although we have only three data points, the logarithm of viscosity is linearly related to the reciprocal of absolute temperature (see Fig. 7), and an activation energy for creep of gabbro is found to be 7.6 kcal/mol. It should be noted that viscosities obtained are considerably smaller than those estimated for the crust and mantle, and that the activation energy is surprisingly smaller than those obtained by high-pressure experiments of rock deformation, which have been carried out under a strain rate larger than 10−8/sec.  相似文献   

13.
Torsion experiments were performed on the Al2SiO5 polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation. The experiments resulted in extensive transformation of andalusite and kyanite to sillimanite. No transformation occurred during the hot-press (no deformation) stage of sample preparation, which was carried out at similar PT conditions and duration as the torsion experiments. Experiments were conducted on fine-grained (< 15 µm) aggregates of natural andalusite, kyanite and sillimanite at 1250 °C, 300 MPa, and a constant shear strain rate of 2 × 10− 4/s to a maximum shear strain of 400%. Electron back-scattered diffraction (EBSD) analysis of the experiments revealed development of lattice-preferred orientations, with alignment of sillimanite and andalusite [001] slightly oblique to the shear plane. The kyanite experiment could not be analyzed using EBSD because of near complete transformation to sillimanite. Very little strain ( 30%) is required to produce widespread transformation in kyanite and andalusite. Polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 µm wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub-µm (100–150 nm) fibrolitic sillimanite. Shear bands are observed across the entire strain field preserved in the torsion samples. Scanning transmission electron microscope imaging shows evidence for transformation away from shear bands; e.g. fibrolitic rims on relict andalusite or kyanite. Relict grains typically have an asymmetry that is consistent with shear direction. These experimental results show that sillimanite is by far the weakest of the polymorphs, but no distinction can yet be made on the relative strengths of kyanite and andalusite. These observations also suggest that attaining high bulk strain energy in strong materials such as the Al2SiO5 polymorphs is not necessary for triggering transformation. Strain energy is concentrated along grain boundaries, and transformation occurs by a dynamic recrystallization type process. These experiments also illustrate the importance of grain-size sensitive creep at high strains in a system with simultaneous reaction and deformation.  相似文献   

14.
Strain measurements and tectonics of New Zealand   总被引:1,自引:0,他引:1  
Measurements of shear strain from triangulation data have been made at 30 locations in New Zealand. The standard error of measurement in terms of strain rate is about ±1 · 10−7 y−1 and values of up to 7 · 10−7 y−1 are observed. Together with 22 fault-plane solutions for crustal earthquakes the measurements indicate broad-scale patterns of deformation. Between the Hikurangi and Flordland active margins is a 100-km-wide belt, the axial tectonic belt, with shear strain rate averaging 5 ± 1 · 10−7y−1 and an azimuth of the principal axis of compression of 114 ± 8°. The rate of movement (45 mm y−1) and direction (085°) between the Pacific and Indian plates from the Minster et al. pole can be accounted for by the measured strain in the axial tectonic belt through simple shear parallel to, and compression normal to, the belt. The similarity in the rates determined from triangulation data averaged over 20–100 years and from plate movement averaged over 5 m.y. indicates plate movement to be uniform in time. West of the axial tectonic belt in Nelson and Fiordland are two zones in which movement is highly oblique to plate movement, and can be explained by slip line deformation analogous to the deformation of Asia. The azimuth of the principal axis of compression in the Taupo rift and East Cape region is NE—SW, perpendicular to its direction in the axial tectonic belt, suggesting extension in the rift and East Cape region normal to the subduction zone.  相似文献   

15.
Plastic-deformati on mechanisms in quartz: The effect of water   总被引:1,自引:0,他引:1  
High-temperature and pressure-deformation experiments indicate that small amounts of water strongly affect the plastic-deformation mechanisms that operate in quartz. Inherently dry natural quartz crystals exhibit a transition from T = (0001), to , t = [0001] glide at approximately 700° C at a strain rate of 8 · 10−6 sec−1. The transition temperature decreases with decreasing strain rate. When water is diffused into dry natural crystals at high temperature, duplex slip occurs on the systems (0001), 112&#x0304;0and either {112&#x0304;0}, [0001] or {101&#x0304;0}, [0001] depending on which of the latter systems is most highly stressed.Hydrothermally grown synthetic quartz crystals contain relatively large amounts of structurally bound water. In these crystals, a transition occurs from (0001), 112&#x0304;0 to dominantly pencil glide in the [0001] and possibly 112&#x0304;3 directions. The transition temperature is strongly dependent on the water content of the crystals, and coincides with the hydrolytic weakening temperature defined by Griggs and Blacic for these crystals. Inhomogeneous distribution of water in the synthetic crystals controls the distribution of plastic strain and may, in part, control the orientation of subbasal deformation lamellae in these.It is suggested that anisotropic diffusion of H-OH in quartz may be responsible for the operation of the [0001] pencil-glide system in the hydrolytic weakening regime. Crystal structure anisotropy may also play a role in the slip-system transition through its influence on the Peierls stress, and this is most likely the dominant factor in the dry crystals.  相似文献   

16.
A relatively undeformed quartzite sample from the Weverton formation was experimentally deformed in plane strain at a temperature of 700° C, a confining pressure of 15 kb and a constant strain rate of 10−6/sec, in a modified Griggs apparatus. A comparison of the known experimental strain for the sample with that measured from deformed rutile needles within the quartz grains shows fairly close agreement between the two values. This confirms the validity of using the needles as intracrystalline strain markers. A comparison has been made of the microstructures and preferred orientations in the experimentally deformed sample and a naturally deformed sample of the same quartzite which has undergone the same strain. The experimentally deformed sample exhibits more inhomogeneous intragranular deformation and a “double funnel” pattern of c axes, while the naturally deformed sample exhibits more homogeneous intragranular deformation and a broad great circle girdle of c axes normal to the foliation and lineation.  相似文献   

17.
It is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress. However, it is still open to discussion if theoretical models that underpin recrystallized grain size–stress relations offer a satisfactorily microphysical basis. The temperature dependence of recrystallized grain size, predicted by most of these models, is rarely observed, possibly because it is usually not systematically investigated. In this study, samples of wet halite containing >10 ppm water (by weight) were deformed in axial compression at 50 MPa confining pressure. The evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0.07, 0.12 and 0.25 at a strain rate of 5×10−7 s−1 and a temperature of 125 °C. The stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0.29–0.46 (and one to a strain of 0.68) at constant strain rates of 5×10−7–1×10−4 s−1 and temperatures of 75–240 °C, yielding stresses of 7–22 MPa. The microstructures and full grain size distributions of all samples were analyzed. The results showed that deformation occurred by a combination of dislocation creep and solution-precipitation creep. Dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration. During deformation, grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection (i.e. moving boundaries that crosscut or consume parts of neighbouring grains). At steady state, grain growth and grain size reduction processes balance, yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature. Evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the (grain size sensitive) solution-precipitation and (grain size insensitive) dislocation creep fields. Application of a piezometer, calibrated using the recrystallized grain size data, to natural halite rock revealed that paleostresses can vary significantly with temperature (up to a factor of 2.5 for T=50–200 °C) and that the existing temperature independent recrystallized grain size–stress piezometer may significantly underestimate flow stresses in natural halite rock.  相似文献   

18.
The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na3AlF6] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions.Microthermometry revealed three different types of inclusions: (1) pure CO2, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid.Stable isotope compositions of CO2 and H2O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ13C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ18O of CO2 (+ 21 to + 42‰ VSMOW) and δ18O of H2O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H2O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite.Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.  相似文献   

19.
The Spanish Central System (SCS) has been subjected to repeated deformation and fluid flow events which have produced both sulphide-bearing and barren vein systems. Although several hydrothermal episodes took place between 300 and 100 Ma, fluid circulation during the Permian was especially important, giving rise to a range of different types of deposits. This study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the As–(Ag) mineralised stockwork of Mónica mine (Bustaviejo, Madrid). Fluid inclusion data indicate the presence of two different fluids. An initial ore stage (I) formed from a low- to moderate salinity (3–8 wt.% eq. NaCl) H2O–NaCl–CO2–CH4 fluid, at minimum trapping temperature of 350±15 °C and 0.3 kbar. A second H2O–NaCl fluid is found in three types of fluid inclusions: a high temperature and low salinity type (340±20 °C; 0.8–3.1 wt.% eq. NaCl) also associated to ore stage I, a moderate temperature and very low salinity type (160–255 °C; 0–1.5 wt.% eq. NaCl) represented in ore stage III, and a very low temperature and hypersaline type (60–70 °C; 30–35 wt.% NaCl), unrelated to the mineralising stages and clearly postdating the previous types. 40Ar–39Ar dating on muscovite from the early As–Fe stage (I) has provided an age of 286±4 Ma, synchronous with the late emplacement phases of La Cabrera plutonic massif (288±5 Ma) and with other Permian hydrothermal events like Sn–W skarns and W–(Sn) sulphide veins. δ18O of water in equilibrium with stage I quartz (5.3–7.7‰), δD of water in equilibrium with coexisting muscovite (−16.0‰ to −2.0‰), and sulphide δ34S (1.5–3.6‰) values are compatible with waters that leached metamorphic rocks. The dominant mechanism of the As–(Ag) deposition was mixing and dilution processes between aqueous–carbonic and aqueous fluids for stage I (As–Fe), and nearly isobaric cooling processes for stages II (Zn–Cu–Sn) and III (Pb–Ag). The origin and hydrothermal evolution of As–(Ag) veins is comparable to other hydrothermal Permian events in the Spanish Central System.  相似文献   

20.
The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Araçuaí Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (~ 750 °C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530–535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at ~ 570–580 Ma, i.e., an HT deformation > 35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from 40Ar–39Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, 40Ar–39Ar mineral ages suggest a very low cooling rate: < 3 °C/My between 570 and ~ 500 Ma and ~ 5 °C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling “hot orogens” that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号