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1.
《Chemical Geology》2006,225(3-4):230-243
Many minerals exhibit oscillatory zoning (OZ), whereby their chemical composition varies more or less regularly along a crystal core-to-rim profile. A well-known example of oscillatory zoning was previously obtained in the (Ba,Sr)SO4 solid solution system under controlled laboratory conditions. The OZ crystals were precipitated at room temperature from counter-diffusing aqueous solutions. In this contribution, we review a macroscopic model for the self-organized formation of oscillatory zoning in such binary solid solution. The model combines diffusive solute transport and an autocatalytic continuous crystal growth with a rate dependent on the mineral surface composition. Oscillatory solutions are obtained. It is also shown that fluctuations in the aqueous solution concentrations may contribute to the OZ formation by causing noise-induced transitions in the crystal growth regimes. We also present, for the first time, a cellular automata-based microscopic model, which considers diffusive motion and autocatalytic attachment kinetics of individual molecular units. OZ is obtained when the probability of attachment of a unit onto the crystal surface is calculated from the crystal composition averaged over the crystal surface (mean-field approach). However, OZ fails to develop when this probability is calculated from the local crystal composition at the attachment site in the absence of a lateral synchronization mechanism.  相似文献   

2.
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.  相似文献   

3.
Four types of oscillatory zoning patterns (OZP) produced by a dynamic model are described qualitatively and quantitatively and displayed as simulated cathodoluminescence images. The behavior of the dynamic model was investigated in terms of the parameter θ, which is the ratio of diffusivities of Ca2+ and H2CO3, and in terms of the parameter γ, which is the product of θ and the ratio of concentrations of Ca2+ and H2CO3 far away from the crystal surface. Qualitatively, the dynamics of the model has been characterized by a stable focus, an unstable focus changing to a stable node or to a stable limit cycle, and by periodical behavior with constant amplitude. Quantitative characteristics, including amplitude of oscillations and duration of oscillations, change between the patterns. It is shown that the process of forming oscillatory zoning in calcite with conditions corresponding to periodical behavior with constant amplitude is very slow in comparison to other OZPs. The oscillatory zoning pattern in a natural calcite crystal is described qualitatively in terms of four general OZPs produced by the dynamic model.  相似文献   

4.
The phase equilibrium and growth of mixed (Ba,Pb)(NO3)2 crystals in aqueous solutions were investigated. The microcrystallization method was adapted to systems with isomorphic components, where crystal composition deviates from the thermodynamic equilibrium even at a low supercooling. The solid phase is characterized by continuous miscibility of the components within the analyzed ranges of temperature and solution composition. The diagram is characterized by substantial nonlinearity of the solubility isotherms and nonuniformity of the solid isocomposite location. Quasiequilibrium paths change their slope depending on solution enrichment in Ba(NO3)2 and depletion in Pb(NO3)2. The area of curvature of quasiequilibrium paths is coordinated with the area of the changing isotherm slope, i.e., the area of “remarkable” points, where the behavior of the system changes drastically. Examples of theoretical zoning of a crystal approximated to a sphere were calculated for paths at a temperature decreasing from 50 to 15°C. The content of the Ba component decreases toward the periphery with the consecutive overgrowing of zones. Each zone corresponds to one gram of (Ba,Pb)(NO3)2. Crystals grown from different initial solutions consist of different numbers of zones owing to the nonlinearity of solubility isotherms. Specific features of mixed crystal formation should be taken into account in genetic interpretation of natural minerals of mixed composition.  相似文献   

5.
X‐ray composition maps and quantitative analyses for Mn, Ca and Cr have been made for six pelitic and calc‐pelitic garnet crystals and Al, Fe and Cr analyses maps have been made for two kyanite crystals, from lower and mid/upper amphibolite facies rocks from the Grenville Province of western Labrador, using an electron microprobe analyser and a laser ablation ICP‐MS. Garnet with spiral (‘snowball’) internal fabrics (Si) has spiral zoning in major elements, implying that growth was concentrated in discrete regions of the crystal at any one time (spiral zoning). Cr zoning is parallel to Si in low amphibolite facies garnet with both straight and spiral internal fabrics, indicating that the garnet overprinted a fabric defined by Cr‐rich (mica±chlorite±epidote) and Cr‐poor (quartz±plagioclase) layers during growth (overprint zoning) and that Cr was effectively immobile. In contrast, in mid/upper amphibolite facies garnet porphyroblasts lacking Si, Cr zoning is concentric, implying that Cr diffusion occurred. Cr zoning in kyanite porphyroblasts appears superficially similar to oscillatory zoning, with up to three or four annuli of Cr enrichment and/or depletion present in a single grain. However, the variable width, continuity, Cr concentration and local bifurcation of individual annuli suggest that an origin by overprint zoning may be more likely. The results of this study explain previously observed nonsystematic Cr zoning in garnet and irregular partitioning of Cr between coexisting metamorphic mineral pairs. In addition, this study points to the important role of crystal growth rate in determining the presence or absence of inclusions and the type of zoning exhibited by both major and trace elements. During fast growth, inclusions are preferentially incorporated into the growing porphyroblast and slow diffusing elements such as Cr are effectively immobile, whereas during slow growth, inclusions are not generally included in the porphyroblast and Cr zoning is concentric.  相似文献   

6.
Zoning of phosphorus in igneous olivine   总被引：2，自引：2，他引：0  
We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P2O5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.  相似文献   

7.
The microstructures of microcline from some granitic rocks and pegmatites   总被引：1，自引：0，他引：1  
Numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents. The simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased, and that the water content in the melt has little influence on this result. Comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees C can be rapidly imposed on plutonic phenocrysts. Large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals. The observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth; the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates. Even at large undercooling, there is no significant increase in temperature at the interface caused by the release of heat of crystallization.Simulations are presented to illustrate how disequilibrium growth processes due to undercooling can modify the normal zoning profiles expected from fractionation. Assuming that an undercooling is necessary to cause nucleation, normal zoning can result if crystal growth takes place at constant or increasing undercooling, but reverse zoning can occur at decreasing undercooling. Undercooling during growth is controlled by the relative rate of cooling and the rate at which the liquidus temperature is decreased by the accumulation of residual components and volatiles in the melt. Consequently, normal zoning should be promoted by rapid cooling, contemporaneous crystallization of other phases, and absence of volatiles, while reverse zoning should be expected in phenocrysts grown in slowly-cooled melts or in melts where volatiles are concentrated. The zoning patterns found in many plutonic plagioclase crystals suggest that their compositions are in significant disequilibrium with the melt; consequently, they are unsuitable for use in geothermometers.Approximate calculations suggest that the amount of water concentrated at the surface of growing phenocrysts in plutons can promote local convection. Comparison of simulated and observed oscillatory zoning profiles suggests that oscillatory zoning is not explained by a re-nucleationdiffusion model (Harloff 1927), but is readily explained by periodic local convection.  相似文献   

8.
Eclogite inclusions from kimberlitic diatremes on the Colorado Plateau contain intricately zoned garnet and pyroxene and unusual textures. Detailed electron microprobe traverses for a clinopyroxene-garnet-phengite-lawsonite-rutile assemblage show garnet zoning from Alm69Gr21Py10 (core) to Alm61Gr13Py26 (rim) and pyroxene zoning from Jd50 (core) through Jd77 to Jd55 (rim). Pyroxene cores are Cr-rich in another rock. Sharp compositional discontinuities and zoning reversals are preserved in garnet and pyroxene. Oscillatory zoning occurs in both phases on a 10–20 m scale, with variations of up to 6% Py in garnet and 15% Jd in pyroxene. Phengite is unzoned and contains 74% celadonite endmember.Skeletal, pyroxene-filled garnet crystals are common in some rocks, and garnets in other rocks clearly began growth as shell-like crystals. Some rocks contain domains of coarse, prismatic pyroxene with very fine-grained, interstitial magnesium silicates. The texture appears to have resulted from crystallization in the presence of a fluid phase, and water pressure is inferred to have equalled total pressure during crystallization. Eclogite formation at high water pressure may reflect subcrustal crystallization.An analysis of error propagation shows that ferrous iron calculations from electron probe data are not meaningful for these jadeitic pyroxenes, and temperature differences between core and rim crystallization cannot be documented. The garnet textures and oscillatory zoning are unusual for metamorphic rocks, and they suggest disequilibrium crystallization after overstepping of reaction boundaries. All data fit a model of eclogite formation during cooling and metasomatism of basaltic dikes intruded into a cool upper mantle, but the results here do not preclude other origins, such as subduction zone metamorphism.  相似文献   

9.
The relative growth rates of zircon crystal faces are recorded by growth zoning, which is recognizable in cathodoluminescence photographs of oriented crystal sections. The kinematics of zircon growth is graphically presented by the slopes of Grabahnen between crystal sectors. The relative velocities of advance of crystal faces correlate to the pattern of growth zoning. Widely spaced zoning, interrupted by surfaces of dissolution, on the one hand, and narrowly spaced uninterrupted oscillatory zoning, on the other hand, are interpreted as markers of low and high zircon-supersaturation of the melt, respectively. The following model is deduced from this correlation. The prism {110} is that crystal form, the growth rate of which reacts most sensitively to the zircon-supersaturation of the melt. The growthrate of the steep pyramid {211} is not primarily controlled by zircon-supersaturation. Instead, the growth of {211} is delayed by the adsorption of foreign atoms on its faces. The kinematics of zircon growth in anatectic, plutonic and meta-rhyolitic host rocks in the pre-Mesozoic basement of the Tauern Window (Eastern Alps) is explained by this model. Results show that the kinematics of zircon growth carry a much greater petrogenetic significance than has been previously suggested for the final shape of crystals alone.  相似文献   

10.
Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved granites and aplites which are encountered mostly at the plutons roof and margins. Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization. The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the differentiates. The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation of mineral/melt partition coefficients (K d s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models constructed using the K d s thus obtained along with modal abundances from petrographic estimates. Lamont-Doherty Geological Observatory Contribution Number 3701  相似文献   

11.
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12.
《Chemical Geology》2006,225(3-4):266-277
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13.
The mechanism of barite growth has been investigated in a fluid cell of an Atomic Force Microscope by passing solutions of constant supersaturation (Ω) but variable ion activity ratio (r=aBa/aSO4) over a barite substrate.The observed dependence of step-spreading velocity on solution stoichiometry can be explained by considering non-equivalent attachment frequency factors for the cation and anion. We show that the potential for two-dimensional nucleation changes under a constant thermodynamic driving force due to the kinetics of barium integration into the surface, and that the growth mode changes from preexisting step advancement to island spreading as the cation/anion activity ratio increases. Scanning electron microscopy studies of crystals grown in bulk solutions support our findings that matching the ion ratio in the fluid to that of the crystal lattice does not result in maximum growth and nucleation rates. Significantly more rapid rates correspond to solution stoichiometries where [Ba2+] is in excess with respect to [].Experiments performed in dilute aqueous solutions of methanol show that even 0.02 molar fraction of organic cosolvent in the growth solution significantly accelerates step growth velocity and nucleation rates (while keeping Ω the same as in the reference solution in water). Our observations suggest that the effect of methanol on barite growth results first of all from reduction of the barrier that prevents the Ba2+ from reaching the surface and corroborate the hypothesis that desolvation of the cation and of the surface is the rate limiting kinetic process for two-dimensional nucleation and for crystal growth.  相似文献   

14.
The effect of the real structure of solutions on crystallization is one of the basic issues of crystallogenesis, which is also important for resolving problems of genetic mineralogy. The study of the NaNO3-H2O and KNO3-H2O model systems yielded new data on anomalous characteristics of crystal-forming systems, including morphological and kinetic properties of crystals, crystal-solution equilibrium, and physical properties of solutions (light scattering, thermal properties, IR parameters, pH), providing information on the structure of solutions. The internally consistent data confirm the previously suggested variations in structural heterogeneity of solutions related to minor (2–4%) variations in their composition, which result in numerous disturbances of monotonicity (thermal-concentration oscillations) in the liquidus curves of salts. It is shown that these variations can be caused by variable size and composition of crystal hydrate clusters. The experimental data indicate that the effect of the real solution structure on crystal morphology and crystal-solution equilibrium is enhanced in multicomponent systems, including natural crystal-forming systems. Anomalous faceting and habit, zoning, a sectorial structure of crystals, and nonuniform entrapment of admixtures cannot be ruled out in these systems.  相似文献   

15.
Rhythmic zoning is ubiquitous in igneous rocks. Based on polymerization and regular solution models, a non-ideal, disequilibrium and non-linear interface equation has been established to describe the process of crystal growth:f = 1/[1 + (β/x s )(1 -x s )exp(-W / RT)(1 - 2f)] wheref and xs represent respectively the mole fraction of a component in the crystal and melt at the interface;W the total exchange energy;R the gas constant; andT the temperature; β=k B /k A , withk A andk B representing respectively the rate constants of components A and B. Results of numerical simulation of the equation demonstrate that a domain of triple valuedness exists ifW / RT < -2. This model, together with the mass balance equation, explains reasonably the rhythmic phenomena in silicate solid solutions, indicating that self-organization is responsible for this process during mineral growth.  相似文献   

16.
《Chemical Geology》2006,225(3-4):322-335
This work deals with the growth behaviour of calcite {101¯4} surfaces in contact with multicomponent aqueous solutions containing divalent cations (Ba2+, Sr2+, Mn2+, Cd2+, or Mg2+). The result is the formation of solid solutions, with calcite or aragonite as one of the end-members. In situ atomic force microscopy has revealed a wide variety of surface phenomena occurring during the formation of these solid solutions. Among them are: (1) the thickening of growth steps and the subsequent dissolution of surfaces followed by the nucleation of secondary three-dimensional nuclei on calcite surfaces, (2) the transition between growth mechanisms, (3) the formation of an epitaxial layer that armours the substrate from further dissolution and (4) the inhibitory effect of the newly formed surface on the subsequent growth (template effect). The two last phenomena can considerably limit coprecipitation as an effective mechanism for divalent metal uptake. All the phenomena described are a consequence of the interplay between thermodynamics, supersaturation of the aqueous solution with respect to the possible solid solutions and the crystallographic control of the surfaces on the cation incorporation, and indicates that there are many differences between the crystal growth of solid solutions and phases with fixed composition.  相似文献   

17.
The character of oscillatory zoning within a zircon crystal from the syenite Kipawa Complex, Quebec, varies with scale of observation. Analysis of an scanning electron microscopy (SEM) back-scatter gray-scale traverse at a resolution of one pixel = 2.43 μm revealed 145 zones over 5130 μm, whereas a detailed high-resolution (one pixel = 0.195 μm) section near the crystal rim revealed 225 zones over 795 μm. In order to mathematically characterize the zoning pattern, wavelet, Fourier, and nonlinear analysis techniques were used on profiles of the SEM gray-scale data, and a series constructed was from the zone widths. Results demonstrate that the zircon oscillatory zoning preserves nonlinear and periodic components. Secondary ion mass spectrometry, electron microprobe, and SEM analyses of trace elements show the SEM back-scatter bright zones are enriched in U, Th, and rare earth elements (REE) in comparison to the darker zones. REE patterns are sharply heavy REE enriched and have negative Eu anomalies and prominent positive Ce anomalies. We model the oscillatory zoning, including a measure of its chemical variation, by use of a periodically forced nonlinear system. Results of this data-driven model are quantitatively similar to the natural data. We envisage that the small-scale oscillatory zoning was the result of a nonlinear feedback process wherein the crystal growth modified the adjacent melt, which in turn affected the crystal composition. The large-scale harmonic zones likely reflect changes in the bulk geochemistry of the system from which the zircon grew.  相似文献   

18.
Many of the observed features of zoning in magmatic phenocrysts may be due to the orientation of the section rather than inherent properties of the crystals. An ideal section for the studying of zoning in magmatic crystals has two characteristics: it goes through the center of the crystal, and is perpendicular to one or more crystal faces. Using a model zoned olivine crystal, it is possible to construct accurate zoning profiles for different types of section (centered, symmetrical and skewed). The probability of obtaining a random section which passes within x% of the center of a crystal is shown to be P=0.0x, while the probability that a random section will be within A degrees of perpendicular to a given plane is P=sin(A). A systematic approach to the study of zoned crystals is outlined. In particular, it is suggested that composition be plotted against distance cubed, in order to correct for the volume versus size problem. A method of determining if a given section goes through (or near) the center of a zoned crystal is also presented. The reasoning in this work applies to other types of magmatic crystals such as pyroxenes and plagioclase.  相似文献   

19.
Apatite (Ca10[PO4]6[OH]2) precipitation from aqueous solutions involves multiple steps, such as nucleation of a precursor calcium phosphate phase, cluster aggregation, crystal growth, and transformation to apatite, which are affected by the presence of other ions in solution. I report the mechanisms by which two ions common in natural solutions, Mg2+ and H+, affect heterogeneous calcium phosphate nucleation on the amorphous silica surface.  相似文献   

20.
Sector zoned aegirine crystals occur in the interstices of peralkaline nepheline syenites in Ilímaussaq. The crystals have grass-green [001] sectors enriched in Ca and Fe2+ (as CaFeSi2O6), Mn and Zr; pale green {010} sectors enriched in Al (as NaAlSi2O6); blue-green {110} sectors enriched in Ti (as NaTi0.5Fe 0.5 2+ Si2O6); and light green {100} sectors enriched in Fe3+ (as NaFe3+ Si2O6).The crystals grew in the liquid with a rate that did not exceed the diffusion rate of most elements in the liquid. However. Fe3+ seems to have had diffusion rates lower than the crystal growth rate, and this probably caused the development of some sectors enriched in acmite and others enriched in the hedenbergite component. For Al, Ti and Zr a crystal structural control is envisaged although a recent structure-based model for sector zoning fails to explain the efficient separation of these elements into different sectors.Three more occurrences of sector zoned aegirine are noted, all from peralkaline nepheline syenites. The phenomenon is probably more widespread than hitherto realised.Contribution to the mineralogy of Ilímaussaq no. 62  相似文献   

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