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1.
A fundamental dichotomy exists between the low solubility of zircon in peraluminous melt predicted by experimental and geochemical studies and the large volume proportions of zircon overgrowths formed during high-temperature metamorphism and anatexis that are revealed by cathodoluminescence imaging. We investigate the potential of Ostwald ripening as a possible mechanism for overgrowth formation by presenting a numerical solution to the continuity equation governing open system, diffusion rate-limited Ostwald ripening in a zircon-saturated melt. Application of the model to a typical (log-normal) initial zircon crystal size distribution (CSD) suggests that despite uncertainties associated with the interfacial free energy of zircon, significant grain coarsening is possible via this mechanism under geological conditions and time scales relevant to high-grade metamorphism. Primary influences on the rate at which Ostwald ripening proceeds are (i) the temperature of the system, (ii) the duration of the time interval for which the system is above its solidus, and (iii) the nature of the initial (premelting) zircon CSD.To test the viability of the model, we examine zircon CSDs from three high-grade pelitic migmatites of the Tickalara Metamorphics (northwestern Australia), assuming that zircon crystals hosted by melanosome biotite were permanently occluded from the melt (and therefore approximate the premelting CSD). The model predicts that within 1 to 2 Ma, these biotite-hosted zircon CSDs will evolve into the observed leucosome-hosted zircon CSDs via melt-present Ostwald ripening, under geological conditions applicable to peak metamorphism.Although we have not conclusively demonstrated that Ostwald ripening contributed to changes in zircon CSDs during anatexis of the Tickalara metapelites, our results suggest that Ostwald ripening is a viable mechanism for zircon volume transfer in a zircon-saturated melt and capable of playing a significant role in overgrowth formation in rocks where the total volume of zircon overgrowths substantially exceeds the concentration of zircon dissolvable in the coexisting melt.  相似文献   

2.
The crystal size distributions (CSDs) of plagioclase and amphibolewere determined from andesites of the Soufrière Hillsvolcano, Montserrat. Plagioclase occurs as separate crystalsand as chadocrysts in large amphibole oikocrysts. The chadocrystsrepresent an earlier stage of textural development, preservedby growth of the oikocryst. Seventeen rock and eight chadocrystplagioclase CSDs are considered together as a series of samplesof textural development. All are curved, concave up, and coincident,differing only in their maximum crystal size. Three amphiboleCSDs have a similar shape and behaviour, but at a differentposition from the plagioclase CSDs. A dynamic model is proposedfor the origin of textures in these rocks. Crystallization ofplagioclase started following emplacement of andesite magmaat a depth of at least 5 km. A steep, straight CSD developedby nucleation and growth. This process was interrupted by theinjection of mafic magma into the chamber, or convective overturnof hotter magma. The magma temperature rose until it was buffered,initially by plagioclase solution and later by crystallization.During this period textural coarsening (Ostwald ripening) ofplagioclase and amphibole occurred: small crystals dissolvedsimultaneously with the growth of large crystals. The CSD becameless steep and extended to larger crystal sizes. Early stagesof this process are preserved in coarsened amphibole oikocrysts.Repetitions of this cycle generated the observed family of CSDs.Textural coarsening followed the ‘Communicating Neighbours’model. Hence, each crystal has its own, unique growth–solutionhistory, without appealing to mixing of magmas that crystallizedin different environments. KEY WORDS: Ostwald ripening; textural coarsening; oikocryst; CSD; texture  相似文献   

3.
An experimental study has been conducted to constrain how thermal history controls crystal size distribution (CSD) of plagioclase in cooling basalts. Data from all experiments are dominated by a log-linear segment of decreasing number density with increasing crystal size, consistent with observations in many natural rocks. The slope of the CSD is found to be a function of cooling rate, faster cooling leading to greater slopes. At constant cooling rate the CSDs flatten as temperature decreases, and are sometimes kinked, characteristics consistent with independent textural observations that crystal agglomeration contributes significantly to crystal “growth”. A downturn is observed toward small size, which suggests Ostwald ripening. Furthermore, we find that thermal history above the liquidus has a major influence on CSDs and on the temperatures of phase appearance. We conclude that near the liquidus heterogeneous nucleation typically dominates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

4.
ZEH  A. 《Journal of Petrology》2004,45(10):2101-2132
Crystal size distributions (CSD) and shapes of accessory apatite,titanite and allanite are investigated in three texturally distinctgarnet zones (Z1–Z3) and in the matrix (Z4) of a garnet–epidote–biotitegneiss from the Moine Supergroup. Additionally, textural relationshipsand interactions between the accessory minerals and surroundingrock-forming minerals are considered, results of numerical CSDmodelling are presented, and geochronological and geologicalconsequences of the inferred CSD evolutions are discussed. Texturesand CSDs indicate that the accessory minerals were in, or near,a stage of nucleation and initial growth immediately prior togarnet Z1 overgrowth, and formed within less than 20 000 years,either by a size-independent or size-dependent growth mechanism.Subsequently, the CSDs were modified by different growth mechanisms,as supported by several parameters including CSDs, grain numbers,grain sizes, specific volumes and others. The apatite CSD evolutionfrom Z1 to Z4 is consistent with open-system LPE (Law of ProportionateEffects) growth accompanied and followed by supply controlledrandom ripening, whereas transformation of the original titaniteCSD is more consistent with Ostwald ripening, temporarily accompaniedby positive or negative McCabe growth. The allanite CSDs alsopoint to Ostwald ripening between Z3 and Z4. The textural observationsindicate that the growth evolution of the accessory phases wasinfluenced by mineral reactions with surrounding rock-formingminerals, as well as by deformation and matrix coarsening, ina manner similar to that found in more simple ceramic systems.The observed textures require a successive temperature risethroughout the tectono-metamorphic evolution of the investigatedrock, in agreement with existing P–T data. Fast nucleationand initial growth of the accessory minerals during Z1 was perhapsinitiated by contact metamorphism, whereas subsequent growthand annealing (Z2–Z4) result from regional metamorphicevents. KEY WORDS: apatite; titanite; allanite; CSD; Moine Supergroup; textures; kinetics  相似文献   

5.
Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens, respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.  相似文献   

6.
We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An65)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250°C for plagioclase and 1,300°C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from ≈3 μm to only 8.7 μm in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t 1/3, from ≈3 μm to 23.2 μm in 496 h. A kinetic law in t 1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling ΔT (or supersaturation) involved in Ostwald ripening are essentially <1°C, the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers. We extrapolated our experimental data to geological time scales to estimate the effect of Ostwald ripening on the size of crystals in magmas. In the case of plagioclase, Ostwald ripening is only efficient for mean grain sizes of a few microns to 20 μm, even for a time scale of 105 years. It can, however, result in a significant decrease of the number of small crystals per unit volume, and contribute to the development of convex upwards CSDs. For olivine, the mean grain size increases from 2–3 μm to ≈70 μm in 1 year and 700 μm in 103 years; a mean grain size of 3 mm is reached in 105 years. Accordingly, the rate of grain size-dependent processes, such as compaction of olivine-rich cumulates or melt extraction from partially molten peridotites, may significantly be enhanced by textural coarsening.  相似文献   

7.
A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.
In regions where fluid flow was interpreted as pervasive', acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled', acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.
This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO3) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.
The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.  相似文献   

8.
晶体粒度分布（CSD）是定量化分析火成岩和变质岩结构的重要手段。在变质岩中测量的CSD提供了有关变质过程中晶体成核和生长速率、生长时间的定量信息。文章选取内蒙古霍各乞二号矿床含矿富石榴石岩样品中石榴子石晶体为研究对象，在GIS软件和R语言的支持下，应用空间点模式分析、CSD分析、空间最邻近分析、空间多距离分析、Fry分析等综合方法，探讨了晶体空间数据和点空间数据表征的微结构变化特征，将CSD曲线分段变化记录的信息与地质演化历史联系起来。分段的CSD曲线记录了变质事件的叠加。不同形式的晶体粒度分布直接反映了区域变质岩与接触变质岩演化历史的差异。由于接触变质作用的高温阶段持续时间较短，因此产生的CSD曲线是线性的，不受退火的影响。而区域变质作用涉及长时间升温及其之后的冷却阶段，所以最初的线性CSD后来被退火改造为钟形曲线。含矿富石榴石岩样品的核密度和CSD分析结果显示了两个晶体群密度。分析认为一类晶体群可能与造山过程中的区域变质活动有关，另一类晶体群可能与大面积区域变质期后发生在特定位置的岩体侵入迅速升温的接触热变质事件有关。   相似文献   

9.
Serial sectioning and imaging with a flatbed scanner yielded the three-dimensional size and spatial distribution of garnet porphyroblasts in two garnet schists and one staurolite-bearing schist from the Everett Formation, north-west Connecticut. The dominant garnet-producing reaction in all samples was chlorite+quartz=garnet+H2O. The appearance of staurolite, and additional garnet growth in the staurolite-bearing sample, was due to the reaction chloritoid=garnet+staurolite+chlorite. Statistical measures of garnet spatial distributions, using the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm. Such clustered nucleation may reflect minor bulk compositional variations. Covariance measures between garnet size and nearest-neighbour distance, using the mark covariance function (MCF), suggest a very weak correlation between crystal size and nearest-neighbour distance for length scales of 2 mm or less. These statistical data suggest that if diffusional gradients were present around growing garnet crystals, they did not influence nucleation and growth patterns at length scales greater than c. 2 mm. Compositional maps, through the garnet centres, show that the smaller crystals have lower Mn core compositions relative to larger crystals, consistent with progressive nucleation during pro-grade metamorphism. Radius-rate plots calculated from compositional X-ray maps show similar growth rates for garnet crystals of different size, consistent with an interface-controlled growth model for garnet. The presence of minor diffusional gradients around growing garnet cannot be entirely dismissed, but the lack of observable reaction rims, the clustered spatial distribution and the radius-rate data are most consistent with an interface-controlled garnet growth model.  相似文献   

10.
Crystal-size in crystalline rocks is a fundamental measure of growth rate and age. And if nucleation spawns crystals over a span of time, a broad range of crystal sizes is possible during crystallization. A population balance based on the number density of crystals of each size generally predicts a log-linear distribution with increasing size. The negative slope of such a distribution is a measure of the product of overall population growth rate and mean age and the zero size intercept is nucleation density. Crystal size distributions (CSDs) observed for many lavas are smooth and regular, if not actually linear, when so plotted and can be interpreted using the theory of CSDs developed in chemical engineering by Randolph and Larson (1971). Nucleation density, nucleation and growth rates, and orders of kinetic reactions can be estimated from such data, and physical processes affecting the CSD (e.g. crystal fractionation and accumulation, mixing of populations, annealing in metamorphic and plutonic rocks, and nuclei destruction) can be gauged through analytical modeling. CSD theory provides a formalism for the macroscopic study of kinetic and physical processes affecting crystallization, within which the explicit affect of chemical and physical processes on the CSD can be analytically tested. It is a means by which petrographic information can be quantitatively linked to the kinetics of crystallization, and on these grounds CSDs furnish essential information supplemental to laboratory kinetic studies. In this three part series of papers, Part I provides the general CSD theory in a geological context, while applications to igneous and metamorphic rocks are given, respectively, in Parts II and III.  相似文献   

11.
Garnets, up to 1.2 cm across, from a metapelite in the Acadian metamorphic terrain of eastern Vermont have been analysed for major elements and segregated into different fractions for isotopic analysis. The garnets preserve abundant inclusions of minerals present during garnet growth which allow a nearly complete reaction history to be established. The Sm-Nd and U-Pb isotopic analyses yield concordant ages of 380 Ma for the rim of one garnet and this is interpreted as the formation age. The changing mineral assemblages through garnet growth, their evolving compositions and the thermodynamic dataset of Holland and Powell (1990) are used to put constraints on the P-T evolution during growth. These imply growth during heating from 540 to 635° C and decompression from 9.7 to 7.2 kbar, representing a temperature increase of 95° C and an uplift of 7 km during growth of the garnet. While growth during heating and decompression is consistent with both field evidence and analysis of garnet microstructures and is predicted by theoretical models of regional metamorphism, the extent of the temperature increase requires either very slow uplift (0.15 mm a-1) or an additional magmatic heat input. Slow uplift is precluded by existing constraints on both the duration of the uplift event and that of garnet growth and it is concluded that an external magmatic heat input is required. Comparison with published data on the timing of metamorphism in other parts of the terrain suggests that the peak occurred earlier in lower grade regions, a conclusion that is again supported by theoretical studies. Following the peak, cooling and uplift occurred at a modest rate consistent with simple isostatic recovery.  相似文献   

12.
Rocks from the Micaschist–Marble Complex of the Wölz Tauern, which are part of the middle Austroalpine unit, contain large (up to 2 cm) garnet crystals that show clear evidence of multistage growth. Isotopic dating indicates that a Variscan (～270 Ma) garnet core was overgrown by new garnet formed during Eo-Alpine metamorphism at Cretaceous times. P-T paths for the Eo-Alpine metamorphism were obtained using the method of pseudosections (Powell and Holland in Metam Geol 16:309–343, 1998) and are consistent with earlier results from independent thermobarometry. Due to the large size of the garnets, growth zoning was preserved during amphibolite facies metamorphism at both Variscan and Alpine times. Full multicomponent diffusion modeling of compositional zoning at the interface of the Alpine and Pre-Alpine garnets in conjunction with the retrieved P-T paths allow average subduction/exhumation as well as heating/cooling rates to be retrieved. The modeling suggests that a minimum subduction/exhumation rate of ～4 cm/a and heating/cooling rates on the order of 100–260°C/Ma for a 60°C subduction angle are required to preserve the observed compositional zoning overall while modifying the zoning at the interface between two garnets to the extent observed. Such rapid rates of burial/exhumation are consistent with the results of direct GPS measurements of convergence rates at several orogenic belts as well as with inferred rates from modeling in the Alps and other areas. In combination, this indicates that such rapid rates are commonplace during metamorphism in collisional orogens and places important constraints on the rheological behavior of crustal blocks in such orogens.  相似文献   

13.
MOCK  A.; JERRAM  D. A. 《Journal of Petrology》2005,46(8):1525-1541
Growth histories and residence times of crystals in magmaticsystems can be revealed by studying crystal sizes, size distributionsand shapes. In this contribution, serial sectioning has beenemployed on a sample of porphyritic rhyolite from a Permo-Carboniferouslaccolith from the Halle Volcanic Complex, Germany, to reconstructthe distribution of felsic phenocrysts in three dimensions inorder to determine their true shapes, sizes and three-dimensionalsize distributions. A model of all three phenocryst phases (quartz,plagioclase, K-feldspar) with 217 crystals, and a larger modelcontaining 1599 K-feldspar crystals was reconstructed in threedimensions. The first model revealed a non-touching frameworkof crystals in three dimensions, suggesting that individualcrystals grew freely in the melt prior to quenching of the texture.However, crystal shapes are complex and show large variationon a Zingg diagram (intermediate over long axis plotted againstshort over intermediate axis). They often do not resemble thecrystallographic shapes expected for phenocrysts growing unhinderedfrom a melt, indicating complex growth histories. In contrast,the three-dimensional size distribution is a simple straightline with a negative slope. Stereologically corrected size distributionsfrom individual sections compare well with stereologically correctedsize distributions obtained previously from the same sample.However, crystal size distribution (CSD) data from individualsections scatter considerably. It is shown that CSDs can berobustly reproduced with a sampling size of greater than 200crystals. The kind of shape assumed in stereological correctionof CSDs, however, has a large influence on the calculation andestimation of crystal residence times. KEY WORDS: 3D reconstruction; crystal shapes; CSD; porphyritic rhyolite; quantitative petrography  相似文献   

14.
The spatial disposition, compositional zoning profiles, and size distributions of garnet crystals in 11 specimens of pelitic schist from the Picuris Range of New Mexico (USA) demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks. An ordered disposition of garnet centers and a significant correlation between crystal radius and near-neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre-existing crystals. Compositional zoning profiles require diffusionally controlled growth, the rate of which increases exponentially as temperature increases with time; an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen. Crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process, but decline in the later stages because of suppression effects in the vicinity of earlier-formed nuclei. Simulations of porphyroblast crystallization, based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics, quantitatively replicate textural relations in the rocks. The simulations employ only two variable parameters, which are evaluated by fitting of crystal size distributions. Both have physical significance. The first is an acceleration factor for nucleation, with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock. The second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it. Crystal size distributions for the Picuris Range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals. The same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet, phlogopite, sphene, and pyroxene in rocks from both regional and contact metamorphic occurrences. These commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments.  相似文献   

15.
Cation diffusion rates at 690 ± 30 °C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles in 40 garnets from two kyanite-bearing metapelite samples from the High Himalayan Crystalline Series, Zanskar, northwest India. Knowledge of the initial growth profile of Mn in garnet is a pre-requisite for this technique. Following previous workers we model Mn partitioning into growing garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mngarnet:whole rock partition coefficient is 60–100. Three-dimensional zonation profiles were obtained by successively grinding and polishing ∼1 cm slabs of each sample at 0.1–0.2 mm intervals and analysing the garnets at each stage, thus ensuring that core sections were measured. The diffusion model assumes that garnet has a spherical geometry and behaves as a closed system, and simulates diffusive modification of the hypothetical Mn Rayleigh growth profile for each garnet. The derived measure of the time-integrated diffusion history for each garnet is then combined with radiometric and field-relation constraints for the duration of the Himalayan metamorphic event to calculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets in the two samples examined is (6 ± 3.2) × 10−23 m2s−1. This interdiffusion rate pertains to a temperature of 690 ± 30 °C, which is 0.97 × T PEAK, the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques. Garnet compositions are Py2–17Alm65–77Gro6–16Sp1–17. These new diffusion data are consistent with, and more precise than, existing high-temperature (>1000 °C) experimentally determined diffusion data, although some uncertainties remain difficult to constrain. Qualitative comparison between diffusively modified Mn growth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of metamorphism affecting the Dalradian garnets was 10–20 times longer than that endured by the Himalayan garnets. Received: 5 June 1996 / Accepted: 29 January 1997  相似文献   

16.
Three-dimensional quantitative textural analysis coupled with numerical modelling has been used to assess the dominant mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments. In every case, spatial dispositions, crystal size distributions, and compositional zoning patterns of porphyroblasts indicate the dominance of diffusion-controlled nucleation and growth mechanisms.
Nine samples from three geological areas were studied: a suite of semi-pelitic rocks from the Picuris Mountains, New Mexico (USA); a suite of mafic samples from the Llano Uplift, Texas (USA); and a kyanite schist from Mica Dam, British Columbia (Canada). The semi-pelitic suite exhibits post-deformational garnet growth, whereas garnet in the mafic suite and in the kyanite schist grew synkinematically in rocks displaying weak and strong penetrative fabrics, respectively.
For each sample, the centres and radii of thousands of garnet crystals were located and measured in three dimensions, using images produced by high-resolution computed X-ray tomography. Statistical measures of the degree of ordering and clustering of nucleation sites, and estimates of crystal isolation for each porphyroblast, were then computed from the measured spatial dispositions. These measures can be reproduced in simple numerical models only by diffusion-controlled nucleation and growth mechanisms. Normalized radius-rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion-controlled growth, providing independent confirmation of the conclusions based on textural analysis. The unexpected similarity of results from all samples indicates that diffusion-controlled nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes.  相似文献   

17.
Strain rates from snowball garnet   总被引：3，自引：0，他引：3  
Spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals, during syn‐metamorphic deformation. Thus, they contain information on the strain rate of the rock. Strain rates may be interpreted from such inclusion trails if two functions are known: (1) The relationship between rotation rate and shear strain rate; (2) the growth rate of the crystal. We have investigated details of both functions using a garnetiferous mica schist from the eastern European Alps as an example. The rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock. The growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals, thermodynamic pseudosections and information on the grain size distribution. For the largest porphyroblast size fraction (size L=12 mm) we constrain a growth interval between 540 and 590 °C during the prograde evolution of the rock. Assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of =6.6 × 10?14 s?1. These estimates are consistent with independent estimates for the strain rates during the evolution of this part of the Alpine orogen.  相似文献   

18.
Mt. Merapi in Central Java is one of the most active stratovolcanoes on Earth and is underlain by a multistage plumbing system. Crystal size distribution analyses (CSD) were carried out on recent Merapi basaltic-andesites and co-eruptive magmatic and plutonic inclusions to characterise the crystallisation processes that operate during storage and ascent and to obtain information on respective time scales. The basaltic-andesites exhibit log-linear, kinked-upwards CSD curves for plagioclase and clinopyroxene that can be separated into two main textural populations. Large plagioclase phenocrysts (≥1.6 mm) make up one population, but correspond to crystals with variable geochemical composition and reflect a period of crystal growth at deep to mid-crustal levels. This population was subsequently influenced by crystal accumulation and the onset of crustal assimilation, including the incorporation of high-Ca skarn-derived xenocrysts. Textural re-equilibration is required for these crystals to form a single population in CSD. A second episode of crystal growth at shallower levels is represented by chemically homogenous plagioclase crystals <1.6 mm in size. Crustal assimilation is indicated by, for example, oxygen isotopes and based on the CSD data, crystallisation combined with contamination is likely semi-continuous in these upper crustal storage chambers. The CSD data observed in the basaltic-andesite samples are remarkably consistent and require a large-volume steady state magmatic system beneath Merapi in which late textural equilibration plays a significant role. Plagioclase CSDs of co-eruptive magmatic and plutonic inclusions may contain a third crystal population (<1 mm) not found in the lavas. This third population has probably formed from enhanced degassing of portions of basaltic-andesite magma at shallow crustal levels which resulted in increased crystallinity and basaltic-andesite mush inclusions. A suite of coarse plutonic inclusions is also present that reflects crystallisation and accumulation of crystals in the deep Merapi plumbing system, as deduced from CSD patterns and mineral assemblages.  相似文献   

19.
In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67 –0.13 +0.19 mm/million years and 0.88 –0.19 +0.34 mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7 –0.7 +1.2 ×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4 –2 +3 mm/year and 2±1 mm/year, respectively.  相似文献   

20.
王河锦 《矿物学报》1991,11(3):258-266
本文运用晶体大小分布理论对陕西丹凤秦岭群石榴子石进行了变质结晶动力学的研究,通过对石榴子石在岩石中出现的频率及其大小值的系统测定,有关变质结晶动力学的分析计算,最终得出了丹凤秦岭群石榴子石的变质结晶初始成核密度、成核速度、晶体生长速度、生长时间及冷却过程中体积转移量等诸动力学要素,并从晶体大小分布研究中获得了多期复杂变质这一与地质事实一致的重要地质信息。  相似文献   

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