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1.
Simple one-dimensional numerical models are presented for coupled advection-hydrodynamic dispersion and kinetically controlled oxidatioin-reduction reactions in graphite-free porous media containing magnetite coexisting with silicate assemblages. Fluid-solid interactions involving either OH (O2-H2O-H2) or COH (O2-H2O-H2-CO2-CO-CH4) fluids are considered at ∼500 ∘C and 5 kbar. The major implications of the modeling are as follows: (1) Regional (km scale) reduction of typical magnetite-bearing rocks originally at f O2 near NNO may be possible during long-term metamorphic fluid flow if the infiltrating fluids have sufficiently low f O2 and sufficiently large concentrations of CH4 and/or H2. Regional oxidation of such rocks by highly oxidized OH or COH fluids appears to be difficult to achieve. (2) Nearly identical mineral assemblages and modes may be produced by very different kinetic reaction pathways. The model implies that “equilibrium” assemblages preserved in rocks may not always reflect the true kinetic reaction path that evolved during fluid flow, and highlights the need for quantitative measurements of metamorphic reaction rates. (3) Preservation of sharp lithologic contacts between rocks of very different redox states containing accessory amounts of oxides may be unlikely if fluid-rock interaction times exceed 103104 years. Substantial contact disruption over these times scales is predicted even for oxide-rich rocks if redox contrasts between layers are large. Flow across lithologic contacts may produce asymmetric patterns of metasomatic mineral zonation that may prove useful for mapping flow directions in metamorphic sequences. (4) For fluid flow in typical T gradients through originally homogeneous rock, significant major element metasomatism (e.g., K, Na, Ca) may be possible without producing large changes in oxide abundances. Received:12 November 1997 / Accepted: 9 March 1998  相似文献   

2.
Rate and mechanism in prograde metamorphism   总被引：5，自引：3，他引：5  
For a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature (disequilibrium) depends on the slowest of three sequential steps: (a) surface detachment of reactant minerals, (b) transport of material to the site of mineral growth, and (c) nucleation and growth of the product mineral. We have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism. The dissolution and growth steps are linear functions of the driving force (– G) of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself.While little is known about heterogeneous nucleation, we argue from field observations that the rate of nucleation is not in general rate limiting. Additionally, if a fluid phase is present true grain boundary diffusion cannot be the mechanism which transports material over the mm to cm distances observed between reactant and product minerals.Simple models of contact (200° C temperature rise in 10,000 years) and regional (10° C per million years) metamorphic events lead to several conclusions concerning the rate determining step. Firstly, growth and dissolution are extremely rapid, dehydration reactions at 500° C going to completion in 2×102 years (contact) and 1×104 years (regional), if all solutes are readily transported. Secondly, the effect of substantial fracture flow of fluid is to divert the transporting medium away from the grain boundary region and hence to retard the transport step. Under most such circumstances it appears that diffusive transport of aqueous SiO2 or Mg species will be rate controlling. Despite this retardation of reaction rates, the extent of disequilibrium is rarely more than a few degrees C. Extensive disequilibrium (40° C) can only occur for reactions such as the andalusite sillimanite transformation which have very small entropy changes and which occur in rapid metamorphic events.  相似文献   

3.
Equilibria for several reactions in the system CaO-Al2O3-SiO2-CO2-H2O have been calculated from the reactions calcite+quartz=wollastonite+CO2 (5) and calcite+Al2SiO5+quartz=anorthite+CO2 (19) and other published experimental studies of equilibria in the systems Al2O3-SiO2-H2O and CaO-Al2O3-SiO2-H2O.The calculations indicate that the reactions laumontite+CO2=calcite+kaolinite+2 quartz+2H2O (1) and laumontite+calcite=prehnite+quartz+3H2O+CO2 (3) in the system CaO-Al2O3-SiO2-CO2-H2O, are in equilibrium with an H2O-CO2 fluid phase having -0.0075 for P fluid=P total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO2.Using the above reactions as model equilibria, several probelms of p CO2 in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).  相似文献   

4.
A petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in X H2O/X CO2 during regional metamorphism. The common association of microcline+diopside ± zoisite formed from biotite +calcite+quartz may be produced by continuous (Fe-Mg) exchange reactions, despite the implied metastability in the CaO-K2O-MgO-Al2O3-SiO2-H2O-CO2 system. Different assemblages are produced in a reaction zone between carbonate and pelite beds of varying thickness. This illustrates the importance of exhaustion of fluid buffering assemblages due to mineral reactions proceeding on a local scale. Siliceous dolomites at the same metamorphic grade have produced significantly different assemblages to the interlayered carbonatepelite sequence.  相似文献   

5.
Chromatographic and electrochemical measurements, combined with computer simulation of the natural mineral parageneses and estimation of the stability field of muscovite-bearing assemblages, yielded a consistent model of the fluid regime for the amphibolite-facies metamorphism of the Dzhugdzhur-Stanovoy fold area (DSFA). The model allows the fluid differentiation into “internal” and “external” fluids. The “internal” fluid is formed by the volatiles of the rock, while the “external” fluid arrived from an outer source: the mantle or other reservoir. It is established that the chromatographic and electrochemical measurements refer to the “external” fluid, whereas the redox state estimated from the mineral equilibria is related to the “internal” fluid, whose composition is buffered by the equilibrium mineral assemblage. The “external” fluid trapped by rocks preserves its own redox state only at the regtrograde stage, when the solid-phase reactions slacken and the buffer role of the mineral assemblages is minimized. This aspect explains the contradiction between the wide variations in the oxidation state of the mineral equilibria (log fO2 from ?15 to ?20), on the one hand, and the persistent oxidation state of the external fluid established by the chromatographic and electrochemical methods, on the other hand. The main reason for the wide development of hornblende-bearing assemblages in the amphibolite-facies metamorphic rocks of the Dzhugdzhur-Stanovoy fold system is the high H2O pressure in the “external” fluid. According to the obtained data, the composition of the “external” fluid is determined by the conditions $ P_{H_2 O} Chromatographic and electrochemical measurements, combined with computer simulation of the natural mineral parageneses and estimation of the stability field of muscovite-bearing assemblages, yielded a consistent model of the fluid regime for the amphibolite-facies metamorphism of the Dzhugdzhur-Stanovoy fold area (DSFA). The model allows the fluid differentiation into “internal” and “external” fluids. The “internal” fluid is formed by the volatiles of the rock, while the “external” fluid arrived from an outer source: the mantle or other reservoir. It is established that the chromatographic and electrochemical measurements refer to the “external” fluid, whereas the redox state estimated from the mineral equilibria is related to the “internal” fluid, whose composition is buffered by the equilibrium mineral assemblage. The “external” fluid trapped by rocks preserves its own redox state only at the regtrograde stage, when the solid-phase reactions slacken and the buffer role of the mineral assemblages is minimized. This aspect explains the contradiction between the wide variations in the oxidation state of the mineral equilibria (log fO2 from −15 to −20), on the one hand, and the persistent oxidation state of the external fluid established by the chromatographic and electrochemical methods, on the other hand. The main reason for the wide development of hornblende-bearing assemblages in the amphibolite-facies metamorphic rocks of the Dzhugdzhur-Stanovoy fold system is the high H2O pressure in the “external” fluid. According to the obtained data, the composition of the “external” fluid is determined by the conditions ≥ 0.7 PS and = 0.01–0.3. The oxidation potential of the “external” fluid is close to that of the H2O-C system under carbon-saturated vapor conditions. Original Russian Text ? O.V. Avchenko, I.A. Aleksandrov, V.O. Khudolozhkin, M.A. Mishkin, 2009, published in Tikhookeanskaya Geologiya, 2009, Vol. 28, No. 4, pp. 3–15.  相似文献   

6.
Several different metamorphic events — an early or prevariscian regional, a variscian contact and the alpine regional — on marbles from the Schlegeistal (Western Tauern Area, Tyrol, Austria) have resulted in a great variety of mineral assemblages. These assemblages include calcite, dolomite, tremolite, diopside, forsterite, clinohumite-titanianclinohumite-chondrodite, chlorite-serpentine, brucite, and boron minerals karlite and ludwigite.Microprobe analysis fo the minerals indicate that three different generations of chlorite minerals exist (clinochlor, penninite, Al-serpentine). The occurence of these chlorites is explained by formation of serpentine component during the last (alpine) regional metamorphism from the breakdown of forsterite, humite-minerals and diopside. The phase relations are described in the system CaO-MgO-SiO2-H2O-CO2-HF and a petrogenetic grid for the low low X F mineral region is given. The reactions are typical for ophicarbonate rocks, but include clinohumite and chlorite, due to the presence of F and minor amounts of Al2O3.  相似文献   

7.
The lower Triassic/Bunter sandstone and lower Jurassic/Rhät formations of the Northern Germany sedimentary basin constitute feasible reservoirs for the storage of CO2 from combustion of fossil fuels or industrial production processes. This study presents analyses of geochemical interactions between CO2, formation fluid and rock of these potential reservoirs using geochemical modelling in order to assess the short and long term impact of CO2 sequestration. Batch equilibrium modelling was performed first for assessing the consistency of fluid and mineralogy field data and for identifying potential secondary minerals under the influence of injected CO2. Inclusion of reaction kinetics in the batch models allowed an observation of reaction paths and to estimate the time frame of geochemical reactions. Finally, one-dimensional equilibrium reactive transport modelling was used in order to investigate the direction of reactions under conditions of fluid flow and mass transport and to quantify feedbacks of reactions on transport processes.Results of the simulations performed show that dawsonite may act as the main CO2 storage mineral in both formations, while the carbonates calcite and dolomite dissolve over time. Also, changes in porosity and permeability are significant in the equilibrium reactive transport simulations. The time scale of kinetically controlled reactions observed in the kinetic batch modeling, however, suggests that CO2 mineral trapping in both formations requires very long time frames, and hence other mechanisms such as structural or solubility trapping seem to be more relevant within the injection or early post-injection phase for the studied formations.  相似文献   

8.
The preservation of premetamorphic, whole-rock oxygen isotope ratios in Adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived, hot CO2-H2O fluids during granulite facies metamorphism. This conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations. The data suggest that these rocks were not an open system during metamorphism; that fluid/rock ratios were in many instances between 0.0 and 0.1; that externally derived fluids, as well as fluids derived by metamorphic volatilization, rose along localized channels and were not pervasive; and thus that no single generalization can be applied to metamorphic fluid conditions in the Adirondacks.Analyses of 3 to 4 coexisting minerals from Adirondack marbles show that isotopic equilibrium was attained at the peak of granulite and upper amphibolite facies metamorphism. Thus the isotopic compositions of metamorphic fluids can be inferred from analyses of carbonates and fluid budgets can be constructed.Carbonates from the granulite facies are on average, isotopically similar to those from lower grade or unmetamorphosed limestones of the same age showing that no large isotopic shifts accompanied high grade metamorphism. Equilibrium calculations indicate that small decreases in 18O, averaging 1 permil, result from volatilization reactions for Adirondack rock compositions. Additional small differences between amphibolite and granulite facies marbles are due to systematic lithologie differences.The range of Adirondack carbonate 18O values (12.3 to 27.2) can be explained by the highly variable isotopic compositions of unmetamorphosed limestones in conjunction with minor 18O and 13C depletions caused by metamorphic volatilization suggesting that many (and possibly most) marbles have closely preserved their premetamorphic isotopic compositions. Such preservation is particularly evident in instances of high 18O calcites (25.0 to 27.2), low 18O wollastonites (–1.3 to 3.5), and sharp gradients in 18O (18 permil/15m between marble and anorthosite, 8 permil/25 m in metasediments, and 6 permil/1 m in skarn).Isotopic exchange is seen across marble-anorthosite and marble-granite contacts only at the scale of a few meters. Small (<5 m) marble xenoliths are in approximate exchange equilibrium with their hosts, but for larger xenoliths and layers of marble there is no evidence of exchange at distances greater than 10 m from meta-igneous contacts.  相似文献   

9.
Abstract The Siluro-Devonian Waits River Formation of north-east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite-albite, (2) ankerite-oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite-albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite-oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06. Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole-rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O-rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O-rich fluid drove mineral reactions during metamorphism. Average time-integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite-oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation. The mapped pattern of time-integrated fluxes indicates that the Strafford-Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O-rich fluids given off by the crystallizing magmas, and CO2-H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton-driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system.  相似文献   

10.
Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2‐poor fluid in the outer aureole. Metamorphic CO2‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐XCO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO2‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2‐rich fluid dominates in the inner aureole during early heating, whereas CO2‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO2‐rich fluid, and high‐grade rocks reflect the presence of CO2‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.  相似文献   

11.
A model system for mineral facies in pelitic schists   总被引：5，自引：0，他引：5  
The system Na2O-K2O-Al2O2-SiO2-H2O contains many mineral phases of major importance in the diagenesis and metamorphism of shales and sandstones, as well as in felsic igneous rocks and their metamorphic derivatives. It is thus a useful model-system containing many of the key equilibria of concern in the genesis of such rocks. It is also a system for which extensive experimental and thermodynamic data are available.The discontinuous reactions among the phases quartz, albite, potassic feldspar, muscovite, paragonite, pyrophyllite, kaolinite, kyanite, andalusite, sillimanite, jadeite and analcime have therefore been used to construct a model system for mineral facies in pelitic rocks. There appear to be fiftynine possible facies types, separated by forty-one discontinuities, only thirtysix of which are readily observed in the field. The continuous reactions, involving rotations of tie-line and displacements of three-phase triangles in the NaAlO2-KAlO2-Al2O3 projection from SiO2-H2O, may be formulated using either an Na or K end-member reaction together with Na-K exchange reactions between coexisting white micas, alkali feldspars and analcimes. The general stoichiometric coefficients for all likely discontinuous reactions have been evaluated in terms of mol fractions of end-members. Available experimental data have been used to calibrate the discontinuous equilibria for the limiting conditions of = 0 and = 1. The (Na-K) facies types may be correlated with (Fe-Mg) facies types in pelitic rocks or with assemblages in mafic rocks.This paper is a revised and extended version of a paper entitled Mineral Facies in Pelitic Schists by J. B. Thompson, Jr., published in 1961 as part of a testimonial volume in honor of D. S. Korzhinskii (originally published in Russian with an English summary)  相似文献   

12.
Petrological consequences of variations in metamorphic reaction affinity   总被引：3，自引：0，他引：3  
The extent to which kinetic barriers to nucleation and growth delay the onset of prograde metamorphic reaction, commonly known as overstepping, is related to the macroscopic driving force for reaction, termed reaction affinity. Reaction affinity is defined in the context of overstepping as the Gibbs free‐energy difference between the thermodynamically stable, but not‐yet‐crystallized, products and the metastable reactants. Mineral reactions which release large quantities of H2O, such as chlorite‐consuming reactions, have a higher entropy/volume change, and therefore a higher reaction affinity per unit of temperature/pressure overstep, than those which release little or no H2O. The former are expected to be overstepped in temperature or pressure less than the latter. Different methods of calculating reaction affinity are discussed. Reaction affinity ‘maps’ are calculated that graphically portray variations in reaction affinity on equilibrium phase diagrams, allowing predictions to be made about expected degrees of overstepping. Petrological consequences of variations in reaction affinity include: (i) metamorphic reaction intervals may be discrete rather than continuous, especially in broad multivariant domains across which reaction affinity builds slowly; (ii) reaction intervals may not correspond in a simple way to reaction boundaries and domains in an equilibrium phase diagram, and may involve metastable reactions; (iii) overstepping can lead to a ‘cascade effect’, in which several stable and metastable reactions involving the same reactant phases proceed simultaneously; (iv) fluid generation, and possibly fluid presence in general, may be episodic rather than continuous, corresponding to discrete intervals of reaction; (v) overstepping related to slowly building reaction affinity in multivariant reaction intervals may account for the commonly abrupt development in the field of certain index mineral isograds; and (vi) P–T estimation based on combined use of phase diagram sections and mineral modes/compositions on the one hand, and classical thermobarometry methods on the other, may not agree even if the same thermodynamic data are used. Natural examples of the above, both contact and regional, are provided. The success of the metamorphic facies principle suggests that these kinetic effects are second‐order features that operate within a broadly equilibrium approach to metamorphism. However, it may be that the close approach to equilibrium occurs primarily at the boundaries between the metamorphic facies, corresponding to discrete intervals of high entropy, dehydration reaction involving consumption of hydrous phases like chlorite (greenschist–amphibolite facies boundary) and mica (amphibolite–granulite facies boundary), and less so within the facies themselves. The results of this study suggest that it is important to consider the possibility of reactions removed from equilibrium when inferring the P–T–t evolution of metamorphic rocks.  相似文献   

13.
Metamorphosed pelitic rocks from Mica Creek, British Columbia contain sillimanite, kyanite with minor fibrolite and andalusite-bearing quartz pods. Mineral equilibria were used to infer peak P-T conditions and fluid compositions in equilibrium with the solid phases. Fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions. In samples from two localities, fluid inclusions from schist and quartz-rich segregations have densities appropriate to the peak metamorphic conditions. The observed compositions for these fluids (low salinity with 12 mole % dissolved CO2) agree with calculated values of 0.84 to 0.85, based upon paragonite-quartz-albite-Al2SiO5 equilibria. The fluids unmixed as the schists were uplifted and cooled; fluid inclusions trapped during this stage outline a solvus in the CO2-H2O-NaCl system. A later influx of fluids containing CH4 and N2 accompanied formation of andalusite-bearing plagioclaserich segregations. The restricted association of andalusite-bearing pods and low density fluids suggest a localized but pervasive fluid influx during uplift. Preservation of high density fluid inclusions during uplift and erosion, coupled with evidence for unmixing of H2O- and CO2-rich fluids on the solvus, provide constraints on the P-T uplift path.  相似文献   

14.
Summary Regional metamorphic petrogenetic theory is currently dominated by the hypothesis that the principal grade indicator silicates and their related minerals arise by a variety of chemical reactions. It is postulated that, with rise (or fall) in regional crustal temperature and pressure, original materials become destabilized, break down, and are dissolved in the intergranular metamorphic fluid. The resulting ions then diffuse within, and are transported by, this fluid, in due course reacting with each other to form new minerals in equilibrium with each other and the associated fluid under the new conditions and in accordance with the Phase Rule. A second mechanism, widely accepted for such as metamorphic oxide, carbonate, sulphate and sulphide minerals, but not for the principal metamorphic silicates, is that the latter form largely by in situ solid:solid transformation plus or minus SiO2 plus or minus H2O, rather than by the serial process indicated above. It is now proposed that a test of the reaction theory is whether or not chemical equilibrium has been established on a fine (grain) scale in materials metamorphosed to high grades. On the basis of some simple – but very precise – electron microprobe measurements on some high grade, but still finely bedded, exhalative metasedimentary rocks, it appears to fail the test.  相似文献   

15.
Low temperature metamorphosed clastic rocks of northern Apennines (Verrucano) contain detrital and metamorphic muscovite along with aggregates of interleaved phyllosilicate grains. The extent of celadonite substitution in the metamorphic muscovite progressively increases from Al-rich pyrophyllite-bearing samples to Al-poor K-feldspar-bearing samples and appears to be compatible with the phase relation in the AKF and AKNa diagrams. Some detrital muscovite grains, not equilibrated with the present metamorphic mineral assemblages, seem to be unreacted grains retaining their premetamorphic composition. The chemistry of the re-equilibrated detrital muscovite is mainly controlled by the activity of Al2O3 that is implied by mineral assemblages in the host rocks and is independent of the original composition of detrital mica. Four types of phyllosilicate associations in the interleaved phyllosilicate grains were recognized: 1) muscovite- pyrophyllite-sudoite; 2) muscovite-pyrophyllite-chlorite; 3) muscovite-paragonite-chlorite; 4) muscovite-chlorite. A microstructural and petrological model is proposed for the origin of interleaved phyllosilicate grains in the Verrucano rocks. The model supports the idea that the interleaved phyllosilicate grains are the result of the trend towards equilibrium between detrital muscovite, metamorphic mineral assemblages and the fluid phase.  相似文献   

16.
The kinetics of chemical reactions at mineral surfaces and the rates of diffusion of species in an aqueous phase are coupled in many geochemical systems. Analytical solutions to equations describing coupled mineral dissolution/growth and solute transport in both transient and steady-state systems are used to delimit regimes of pure reaction control, pure transport control and mixed kinetic control of mass-transfer rates. The relative significance of the two processes depends on the magnitudes of the diffusion coefficients and rate constants as functions of temperature, and the degree of disequilibrium in the system. In addition, the system geometry, the ratio of mineral surface area to diffusion cross-section, and the porosity and tortuosity of the medium through which aqueous species diffuse affect reaction vs. diffusion control. In general, diffusion control increases with increasing temperature and increasing distance over which diffusion occurs. Calculations for the mixed kinetic regime in transient systems demonstrate that the relative significance of diffusion and surface reaction varies with reaction progress, and approaches a limiting value as equilibrium is approached. This limiting value may be appropriate to natural water-rock interactions that occur at conditions that are close to equilibrium. This result permits extension of simple models for irreversible mass transfer in homogeneous systems to systems in which mass-transfer kinetics are controlled by coupled surface reactions and mass transport. Criteria are established for time and length scales and fluid velocity limits on the validity of the continuum hypothesis and the local equilibrium assumption in mass-transport modeling.  相似文献   

17.
Zusammenfassung Die wichtigsten Merkmale der Ichnofaunen des mittleren und oberen Buntsandsteins, der Mittleren Trias (wesentlich der Fährtensandstein du Lyonnais), des Oberen Trias werden hier betrachtet. Die Verschiedenheiten beziehen sich auf ihre Zusammensetzung, die Morphologie der Eindrücke und die Häufigkeiten der Arten oder Gattungen. Für die Vergleiche wurden nicht nur charakteristische Artenverbindungen sondern auch spezielle lokale Arten benutzt. Unter der Bedingung, die Ichnofaunen sind repräsentativ, erlauben die vorhergehenden Punkte und die Kenntnis der Archosaurierextremitäten Entwicklung während der Trias, die bezügliche stratigraphische Lagen der verschiedenen Fundschichten zu präzisieren.
The main characters of the Ichnofaunas of 1) the middle and upper Buntsandstein, 2) the Middle Triassic (essentially the Fährtensandstein du Lyonnais) and 3) the upper Triassic are here considered. The difference between these Ichnofaunas relate to their composition, the morphology of the footprints and the frequencies of the ichnogenera or the ichnospecies. For the comparisons are utilized not only ichnospecies associations but also the local or peculiar ichnospecies.On condition that these Ichnofaunas are representative, the results of the studies described above and the knowledge of the evolution of the Archosaur appendicular skeletons during the Triassic, allow to settle the relative stratigraphical positions of the different layers.
Résumé Les caractères principaux des Ichnofaunes du Buntsandstein moyen et supérieur, du Trias moyen (essentiellement le Fährtensandstein du Lyonnais) et du Trias supérieur sont mis en évidence. Les différences entre ces Ichnofaunes portent sur leur composition, la morphologie des empreintes et les fréquences des ichnogenres ou des ichnoespèces. Pour les comparaisons sont utilisées non seulement les associations d'ichnoespèces mais également les ichnoespèces locales ou particulières.Sous la condition que ces ichnofaunes soient représentatives, les points évoqués cidessus et la connaissance de l'évolution des autopodes des Archosauriens pendant le Trias permettent d'établir les positions stratigraphiques relatives des différents gisements.
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18.
In this paper we consider crystallization of solid solutions and formation of growth zoning in minerals. To ascertain the role of various mechanisms producing zoning we have constructed kinetic models of nonsteady solid solution crystal growth. The equations obtained describe the temporal evolution of the solute and crystal composition. Since these equations are not solvable analytically we have solved them numerically by a fourth-order Runge–Kutta method. On the basis of this solution we can compute the zoning profiles for different crystallization modes and conditions. The constructed models have been used for study of mechanisms of zoning formation in metamorphic garnets. We conclude that the main mechanism of production of growth zoning is fractionation. The role of change of distribution coefficient in equilibrium crystallization is negligible. The modelling of zoning profiles reveals that simple arc-shaped profiles originate from crystallization in a closed system while complicated nonmonotonic profiles appear with crystallization in open systems under fluid flow. The duration of metamorphic garnet crystallization is estimated.  相似文献   

19.
The Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO2/(CO2+H2O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.  相似文献   

20.
ABSTRACT One-dimensional fluid advection-dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up- and down-temperature metamorphic fluid flow that may, in theory, be used to determine the fluid flow direction with respect to the palaeotemperature gradient. Under equilibrium conditions, down-temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages. In contrast, up-temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts. However, during contact metamorphism, mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down-temperature fluid flow. In addition, uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions. Equilibrium down-temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp δ18O fronts, whereas up-temperature flow of fluids derived by metamorphic devolatilization may produce gradational δ18O vs. distance profiles. However, if fluids are channelled, significant kinematic dispersion occurs, or isotopic equilibrium is not maintained, the patterns of isotopic resetting may be difficult to interpret. The one-dimensional models provide a framework in which to study fluid-rock interaction; however, when some of the complexities inherent in fluid flow systems are taken into account, they may not uniquely distinguish between up- and down-temperature fluid flow. It is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required.  相似文献   

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