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1.
We report observations from room temperature static recrystallization experiments (annealing times from minutes to year) of cold-pressed, synthetic, coarse-grained, wet sodium chloride, prepared by broad ion beam polishing and SEM observations at cryogenic temperature to observe directly the brine in grain boundaries. At all stages of annealing, the majority of the brine in the samples is connected in 2D sections along grain boundaries. Another part of the brine is in isolated brine inclusion arrays along grain boundaries and in brine inclusions left behind by migrating brine-filled grain boundaries. Most of these boundaries are mobile because the aggregate is coarsening. We interpret that the boundaries without observable brine films (<15 nm) and brine inclusion arrays are healed and immobile. Evolution of grain boundary structure involves three major processes. First, dissolution on one side of the grain boundary and precipitation on the other side, resulting in grain boundary migration. Second, the development of facets formed by low-index crystallographic planes of the grains bounding the grain boundary brine. When both sides of a grain boundary are able to develop low-index facets in a thick brine film, the resulting impingement boundary is interpreted to be immobile and may prevent the new grain from migrating into a deformed neighbor. When one side of a faceted boundary consists of low-index crystallographic planes and the other side passively follows this faceted shape along irrational surfaces, the boundary is mobile. Third, the healing of grain boundary brine films, producing solid–solid grain boundaries without resolvable brine films.  相似文献   

2.
The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.  相似文献   

3.
Microstructural study revealed that the ductile flow of intensely folded fine-grained salt exposed in an underground mine (Zechstein-Werra salt sequence, Neuhof mine, Germany) was accommodated by coupled activity of solution-precipitation (SP) creep and microcracking of the halite grains. The grain cores of the halite aggregates contain remnants of sedimentary microstructures with straight and chevron shaped fluid inclusion trails (FITs) and are surrounded by two concentric mantles reflecting different events of salt precipitation. Numerous intra-granular or transgranular microcracks originate at the tips of FITs and propagate preferentially along the interface between sedimentary cores and the surrounding mantle of reprecipitated halite. These microcracks are interpreted as tensional Griffith cracks. Microcracks starting at grain boundary triple junctions or grain boundary ledges form due to stress concentrations generated by grain boundary sliding (GBS). Solid or fluid inclusions frequently alter the course of the propagating microcracks or the cracks terminate at these inclusions. Because the inner mantle containing the microcracks is corroded and is surrounded by microcrack-free outer mantle, microcracking is interpreted to reflect transient failure of the aggregate. Microcracking is argued to play a fundamental role in the continuation and enhancement of the SP–GBS creep during halokinesis of the Werra salt, because the transgranular cracks (1) provide the ingress of additional fluid in the grain boundary network when cross-cutting the FITs and (2) decrease grain size by splitting the grains. More over, the ingress of additional fluids into grain boundaries is also provided by non-conservative grain boundary migration that advanced into FITs bearing cores of grains. Described readjustments of the microstructure and mechanical and chemical feedbacks for the grain boundary diffusion flow in halite-brine system are proposed to be comparable to other rock-fluid or rock-melt aggregates deforming by the grain boundary sliding (GBS) coupled deformation mechanisms.  相似文献   

4.
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   

5.
The microstructure of halite from the subhorizontal, bedded Main Röt Evaporite Member at Hengelo, The Netherlands (AKZO well 382, depth interval of 420–460 m), was studied by transmitted and reflected light microscopy of gamma-irradiation decorated samples. Primary microstructures compare favourably with those found in recent ephemeral salt pans. Large, blocky, fluid-inclusion-poor halite grains and elongated chevrons are interpreted to have formed in the saline lake stage, while void-filling clear halite is interpreted to have formed during the desiccation stage of the salt pan. In addition, in all layers the grains are rich in deformation-related substructures such as slip bands and subgrains indicating strains of a few percent. The study of gamma-irradiation decorated thin sections shows that the main recrystallization mechanism is grain boundary migration. Grain boundary migration removes primary fluid inclusions and produces clear, strain-free new grains. Differential stresses as determined by subgrain size piezometry were 0.45–0.97 MPa. The deformation of the salt layers is probably related to Cretaceous inversion in the area.  相似文献   

6.
The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H2O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.  相似文献   

7.
Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 m (D 0) to 65 m in 24 hours. Growth rates fit the equation, D n -D 0 n =Kt, where K is a constant and n2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.  相似文献   

8.
A dense (~3.34 g cm–3) garnet–sillimanite-rich metamorphic rock from the suevite breccia of the Ries impact crater was studied by scanning-electron microscopy and Raman microprobe spectroscopy. In the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves. Kyanite aggregates were found as thin (~0.3–2.0 m) seams along grain boundaries between, and fractures within, sillimanite grains. Within these seams kyanite c-axes are oriented perpendicular to original grain boundaries and fractures. In addition, larger (up to 10 m) isolated kyanite grains were rarely found within host sillimanite. Filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease (~10%). Locally, broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub-micrometer-sized euhedral crystals. The silica-rich glass suggests incongruent melting of sillimanite at local post-shock temperatures significantly higher than 1,300°C. The edges of adjacent sillimanite grains are thermally and chemically altered. The local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts.  相似文献   

9.
The structure of grain boundaries in a granite-origin ultramylonite, composed mainly of fine-grained feldspar and quartz, was studied by high-resolution electron microscopy (HREM). At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in adjacent grains meet at the interface with no other appreciable phases. In these boundaries, some of the straight segments correspond to a low-index plane of one of the connected grains. Boundaries containing voids, with a spheroidal shape elongated along the boundaries, were observed only between quartz grains. It is suggested that these boundaries were formed by healing of microcracks. The structural width of major boundaries, deduced from lattice-fringe imaging, is less than about 0.5 nm. Received: 15 September 1998 / Revised, accepted: 8 April 1999  相似文献   

10.
Grain boundary migration between strained, substructured grains and newly appearing, strain free grains has been observed during static in-situ annealing of pre-deformed rocksalt in the SEM. With increasing temperature (T) the migration velocity increases and the character of grain boundary migration changes. As temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates. In addition, the frequency of migrating boundaries that form traces of a {100} boundary plane of at least one of the crystals involved increases, and moving grain boundaries between new and old grains change from highly irregular to smooth, straight boundaries. At the same time there is a decrease in the influence of the substructure of pre-existing strained grains on the grain boundary movement. Resultant microstructures reflect these changes. At  325–350 °C, the deformed-then-annealed microstructure is characterized by very irregular grain boundaries, a high abundance of 5–50 m scale remnants of old, substructured grains within new grains, giving a poikilitic microstructure. At  350–400 °C, grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old, strained grains are located at former grain boundaries. At > 400 °C, grain boundaries between new and old, strained grains are straight to smoothly curved.The grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations. Additionally, at low annealing temperature, impurity (solute) drag and driving-force variations are influential, while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important. Transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks, thereby making it is possible to relate microstructural characteristics to the pre-annealing and post-deformational annealing history.  相似文献   

11.
Intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains. Intragranular recrystallized grains within large, twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study. Lattice distortions, low angle boundaries and some high angle boundaries (>15°) in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb-accommodated dislocation creep involving subgrain rotation recrystallization. Changes in the ratio of host and twin domain, as the deformation zone is entered, show that twin boundary migration also occurred. Recrystallized grains have similar sizes (10–60 μm) to subgrains, suggesting that they formed by subgrain rotation. However, within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15° and misorientation axes are randomly oriented. Moreover recrystallized grain orientations average around the porphyroclast orientation. We suggest that the recrystallized grains, once formed, are able to deform partly by diffusion accommodated grain boundary sliding, which is consistent with predictions made from lab flow laws.  相似文献   

12.
Dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition, driven by strain energy in the form of dislocations. Driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals, which form solid solutions such as feldspar, amphiboles and pyroxenes, if they change their composition during recrystallization. To estimate the relative importance of these driving potentials, the chemical composition of porphyroclasts and recrystallized grains of plagioclase, clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the Ivrea Zone, Italy. The plagioclases show two different recrystallization microstructures: bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing. Deformation took place under amphibolite facies conditions on a retrograde P,T-path. Porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their Or-content and yield a ΔG between mean host grain and mean recrystallized grain composition at fixed P,T-conditions of approximately 5 Joules/10−4 m3. Extreme compositional variations yield approximately 60 J/10−4 m3. The increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 Joules/10−4 m3 for high values of dislocation densities of 1014 m−2. Thus, the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences. In the other type of microstructure, porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content. The maximum ΔG induced by the compositional disequilibrium is on the order of 100 J/10−4 m3. This maximum value is of the same magnitude as the ΔG derived from high dislocation densities of 1014 m−2. The resulting combined ΔG is approximately twice as high as for deformational ΔG alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures. The recrystallization mechanism depends on the nature of the driving potential. Grain boundary migration (GBM) and heterogeneous nucleation can release Gibbs free energy induced by compositional disequilibrium, whereas this is not likely for subgrain rotation. Therefore, only GBM and heterogeneous nucleation may link metamorphism and deformation, so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction. Received: 8 July 1996 / Accepted: 17 November 1997  相似文献   

13.
Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth’s interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo90) samples that had been previously deformed at 1200 °C and 0.3 GPa to shear strains up to γ?=?7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700–1400 °C. We observed that (1) the electrical conductivity of samples with a small grain size (3–6 µm) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of ~?4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.  相似文献   

14.
Plastically deformed quartzites from the Betic Movement Zone (Betic Cordilleras, Spain) exhibit microstructures indicative of crystal plasticity on a mineral grain scale. Quartzites with dynamically recrystallized grain sizes larger than 10 μm have strong crystallographic preferred orientations, narrow grain boundaries, little creep damage, and an inverse proportionality of dislocation density and grain size. Mylonites with grain sizes smaller than 10 μm have low crystallographic preferred orientations, wide grain boundaries (up to 1000 Å), abundant creep damage, and decreasing dislocation density with diminishing grain size. This is thought to reflect a clear-cut shift in deformational regimes from dislocation creep to superplastic flow at 10 μm grain size. Superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 μm during an initial dislocation creep stage. Dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain-boundary sliding in the mylonites.It seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens. In the mylonites, dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms, and an estimate for late stage stresses is provided by unbound dislocation densities. In both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization.  相似文献   

15.
Recrystallization of perthites in granulite facies (T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with diffusion creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during diffusion creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive creep, resulting in strain localization.  相似文献   

16.
Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second-phase minerals, such as mica, whereas that of large irregular grains is unaffected by second-phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz ( D ) and coexisting mica ( d ) in the duplex microstructure are systematically related to the volume fraction of mica ( f ) by D = 0.728 d (1/ f )0.629.  相似文献   

17.
The morphology and internal structure of individual olivine grains from ultramafic rocks in the Guli and Gal’moenan dunite massifs differing in origin are considered. To restore the ontogeny of mineral aggregates, traces of elastic deformation retained in mineral grains have been used. Comparison of anatomy of olivine grains from these two massifs showed that the mechanism of accommodation of rocks to changing geological settings is expressed as the response of the mineral aggregate structure and variation in the anatomy of individual mineral grains. At the level of individual grains, this is annihilation of older defects and origination of younger dislocations; refinement of the crystal lattice; exsolution; formation and transformation of new mineral phases; and creep and migration of subboundaries within grains. At the aggregate level, this is rotation and migration creep of the internal boundaries of rock; formation of new boundaries of mineral intergrowths; reorientation of boundaries; and variation in their extent, density, and grain dimensions. The prehistory of massifs controls the manifestation and abundance of various elastic deformations and related types of recrystallization of olivine grain boundaries and subboundaries in aggregates. New conditions and accommodation of mineral aggregates to these conditions have instigated specific schemes of recrystallization, which bear information on the history of rocks and their massifs.  相似文献   

18.
Samples of olivine mixed with small amounts of tholeiitic basalt which were hot-pressed above the solidus temperature were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. Two sets of samples were compared. One set was hot-pressed for approximately 1 h near 1,300° C at 0.2 GPa, and the other set was held for approximately 200 h near 1,250° C at 1.0 GPa. SEM observations reveal that, in samples which were hot-pressed for 1 h, the glass phase occurs in irregular pockets surrounded by four or five olivine grains as well as in triple junctions. The crystal-glass interfaces show both positive and negative curvature. These observations and the presence of voids suggest that the microstructure is far from textural equilibrium. In contrast, in samples which were hot-pressed for 200 h, glass is largely confined to triple junctions of uniform size and the crystal-glass interfaces have uniform curvature indicating a much greater degree of textural equilibrium. TEM images reveal layers of glass 10–50 nm thick along most of the grain boundaries in the samples annealed for short times. However, within the limit of resolution, 2 nm, virtually all of the grain boundaries in the samples hot-pressed for long times appear to contain no glass. These observations indicate that segregation of melt from grain boundaries to triple junctions is an integral part of the process of textural equilibration.  相似文献   

19.
Fast diffusion along mobile grain boundaries in calcite   总被引：1，自引：0，他引：1  
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.  相似文献   

20.
Experimental observations are reported of weakening of sediment-like aggregates by addition of hard particles. Sieved mixtures of calcite and halite grains are experimentally compacted in drained pressure cells in the presence of a saturated aqueous solution. The individual halite grains deform easily by pressure solution creep whereas calcite grains act as hard objects and resist compaction. The fastest rate of compaction of the mixed aggregate is not obtained for a 100% halite aggregate but for a content of halite grains between 45% and 75%. We propose that this unusual compaction behavior reflects the competition between two mechanisms at the grain scale: intergranular pressure solution at grain contacts and grain boundary healing between halite grains that prevent further compaction.  相似文献   

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