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1.
The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm−1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm−1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism.  相似文献   

2.
Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An20, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H2O = 2 zoisite + muscovite + 2 quartz + 15 albiteplagioclase+ 1 albiteflame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.  相似文献   

3.
The Fuscaldo assemblages show that in metabasites suitable for the production of glaucophane at higher pressures, amphibole poor(er) in Gl-molecule + albite + Al-rich chlorite is formed at lower pressure. Blue amphibole formed together with albite, chlorite and a Ca-silicate appears to have a fixed content of the Gl-molecule and of Ca2+, apart from the Fe2+/ R2+ ratio, which varies with host rock chemistry. The constant Gl-content indicates attainment of equilibrium, and is a function of T and especially P, so it may be used as a geobarometer. Glaucophane generally forms at the cost of albite+chlorite. In a P-T diagram the reaction is probably situated somewhat below the reaction albitejadeite+quartz, and has a smaller slope than the latter. The concomitant high-pressure character of glaucophane justifies reintroduction of Eskola's glaucophane-schist facies, of which glaucophane is critical.  相似文献   

4.
Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.  相似文献   

5.
Part I of this contribution (Gardés et al. in Contrib Mineral Petrol, 2010) reported time- and temperature-dependent experimental growth of polycrystalline forsterite-enstatite double layers between single crystals of periclase and quartz, and enstatite single layers between forsterite and quartz. Both double and single layers displayed growth rates decreasing with time and pronounced grain coarsening. Here, a model is presented for the growth of the layers that couples grain boundary diffusion and grain coarsening to interpret the drop of the growth rates. It results that the growth of the layers is such that (Δx)2 ∝ t 1−1/n , where Δx is the layer thickness and n the grain coarsening exponent, as experimentally observed. It is shown that component transport occurs mainly by grain boundary diffusion and that the contribution of volume diffusion is negligible. Assuming a value of 1 nm for the effective grain boundary width, the following Arrhenius laws for MgO grain boundary diffusion are derived: log D gb,0Fo (m2/s) = −2.71 ± 1.03 and E gbFo = 329 ± 30 kJ/mol in forsterite and log D gb,0En (m2/s) = 0.13 ± 1.31 and E gbEn = 417 ± 38 kJ/mol in enstatite. The different activation energies are responsible for the changes in the enstatite/forsterite thickness ratio with varying temperature. We show that significant biases are introduced if grain boundary diffusion-controlled rim growth is modelled assuming constant bulk diffusivities so that differences in activation energies of more than 100 kJ/mol may arise. It is thus important to consider grain coarsening when modelling layered reaction zones because they are usually polycrystalline and controlled by grain boundary transport.  相似文献   

6.
In the Tuoshi oilfield,located in the Cenozoic Jianghan Basin of southeastern China ,there have been found hydrocarbon reservoirs hosted in lacustrine sandstones of the Eogene Xingouzui Formation.The main diagenetic features identified in these sandstones include the dissolution of detrital K-feldspar and albite grains,the precipitation of quartz as overgrowths and /or cements ,and the precipitation and /or transformation of clay minerals.These diagenetic features were interpreted to have occurred in early,intermediate and late stages,based on the burial depth.The kinetics of fluid-mineral reactions and the concentrations of aqueous species au each stage of diagenesis were simulated numerically for these lacustrine sandstones,using a quasi-sta-tionary state approximation that incorporates simultaneous chemical reactions in a time-space continuum.During the early diagenetic stage,pore fluid was weakly acidic,which resulted in dissolution of K-feldspar and albite and,therefore,led to the release of K^ ,Na^ ,Al^3 and SiO2(aq) into the diagenetic fluid.The increased K^ ,Na^ ,Al^3 and SiO2(aq) concentrations in the diagenetic fluid caused the precipitation of quartz,kaolinite and illite.At the beginning of the intermediate diagenetic stage the concentration of H^ was built up due to the decomposition of organic matter,which was responsible for further dissolution of K-feldspar and albite and pre-cipitation of quartz,kaolinite,and illite.During the late diagenetic stage,the pore fluid was weakly alkaline,K-feldspar became stable and was precipitated with quartz and clay minerals.When the burial depth was greater than 3000 m,the pore fluids became supersaturated with respect to allbite,but undersaturated with respect to quartz,resulting in the precipitation of albite and the dissolution of quartz.The diagenetic reactions forecasted in the numerical modeling closely matched the diagenet-ic features identified by petrographic examination, and therefore,can help us to gain a better understanding of the diagenetic processes and associated porosity evolution in sandstone reservoirs.  相似文献   

7.
Milke et al. (Contrib Mineral Petrol 142:15–26, 2001) studied the diffusion of Si, Mg and O in synthetic polycrystalline enstatite reaction rims. The reaction rims were grown at 1,000°C and 1 GPa at the contacts between forsterite grains with normal isotopic compositions and a quartz matrix extremely enriched in 18O and 29Si. The enstatite reaction rim grew from the original quartz-forsterite interface in both directions producing an inner portion, which replaced forsterite and an outer portion, which replaced quartz. Here we present new support for this statement, as the two portions of the rim are clearly distinguished based on crystal orientation mapping using electron backscatter diffraction (EBSD). Milke et al. (Contrib Mineral Petrol 142:15–26, 2001) used the formalism of LeClaire (J Appl Phys 14:351–356, 1963) to derive the coefficient of silicon grain boundary diffusion from stable isotope profiles across the reaction rims. LeClaires formalism is designed for grain boundary tracer diffusion into an infinite half space with fixed geometry. A fixed geometry is an undesired limitation in the context of rim growth. We suggest an alternative model, which accounts for simultaneous layer growth and superimposed silicon and oxygen self diffusion. The effective silicon bulk diffusivity obtained from our model is approximately equal within both portions of the enstatite reaction rim: D Si,En eff =1.0–4.3×10–16 m2 s–1. The effective oxygen diffusion is relatively slow in the inner portion of the reaction rim, D O,En eff =0.8–1.4×10–16 m2 s–1, and comparatively fast, D O,En eff =5.9–11.6×10–16 m2 s–1, in its outer portion. Microstructural evidence suggests that transient porosity and small amounts of fluid were concentrated at the quartz-enstatite interface during rim growth. This leads us to suspect that the presence of an aqueous fluid accelerated oxygen diffusion in the outer portion of the reaction rim. In contrast, silica diffusion does not appear to have been affected by the spatial variation in the availability of an aqueous fluid.
  相似文献   

8.
Naturally deformed feldspars from foliated granites in a shear zone in Newfoundland exhibit transitional brittle-ductile behaviour. Brittle failure is subordinate to dynamic recrystallization, microcracking, strain enhanced diffusion and reaction enhanced ductility during the deformation. Both plagioclase (An28) and K-feldspar are transformed to albite with increasing strain. Interaction of metamorphic and structural processes at the grain scale is emphasised. This is illustrated with examples of quartz-filled veins (segregation bands) in plagioclase and recrystallized polycrystalline aggregates in plagioclase and K-feldspar. The role of microcracking in plagioclase and of pre-existing internal growth structures in the formation of initially coarse grained recrystallized aggregates from large single crystals is suggested.  相似文献   

9.
The zonal structure of prograde garnet in pelitic schists from the medium-grade garnet zone and the higher-grade albite-biotite zone was examined to investigate the evolution of prograde P–T paths of the Sanbagawa metamorphism. The garnet studied shows a bell-shaped chemical zoning of the spessartine component, which decreases in abundance from the core towards the rim. Almandine and pyrope contents and XMg [=Mg/(Mg+Fe2+)] increase monotonously outwards. The general scheme of the zonal structure for grossular content [XGrs=Ca/(Fe2++Mn+Mg+Ca)] can be summarized as: (1) XGrs increases outwards (inner segment) and reaches a maximum at an intermediate position between the crystal core and the rim, then decreases towards the outermost rim (outer segment) (2) the inner segment of garnet in the garnet zone samples tends to have a higher XGrs/XSps values for a given XSps than those in the albite–biotite zone samples (3) average XSps at the maximum XGrs position in the albite–biotite zone samples ranges from 0.02 to 0.12 and is lower than that in the garnet zone samples (0.13–0.32) (4) the maximum XGrs in the albite–biotite zone samples (0.34–0.39 on average) tends to be higher than that in the garnet zone samples (0.26–0.36), and (5) differences of XGrs between the maximum and rim in the albite–biotite zone samples are between 0.10 and 0.14 and higher than those in the garnet zone samples (< 0.11). These facts imply that albite–biotite zone materials (a) were recrystallized under lower dP/dT conditions at an early stage of the prograde metamorphism (b) began their exhumation under higher P–T conditions and (c) have been continuously heated during exhumation for a longer duration than the garnet zone materials. The systematic changes of prograde P–T paths can be interpreted as documenting the evolution of the Sanbagawa subduction zone.  相似文献   

10.
The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm63Prp26Grs10)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm50Prp14Grs35)–jadeite (Jd80–97Di0–4Hd0–8Acm0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/PT. The inferred limiting a(H2O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.  相似文献   

11.
The growth rates of albite and pyroxene (enstatite + diopside + spinel) reaction rims were measured at 1000°C and ˜700 MPa and found to be parabolic indicating diffusion-controlled growth. The parabolic rate constants for the pyroxene (+ spinel) rims in samples with 0.5 wt% H2O added or initially vacuum dried at 25°C and 250°C are 1.68 ± 0.09, 0.54 ± 0.05 and 0.25 ± 0.06 μm2/h, respectively. The values for albite rim growth in samples initially dried at 60°C and with 0.1 wt% H2O added are 0.25 ± 0.04 and 0.33 ± 0.03 μm2/h, respectively. The latter values were used to derive the product of the grain boundary diffusion coefficient D′A, where A = SiO2, NaAlO2, or NaAlSi−1, and the grain boundary thickness δ in albite. The calculated D′SIO2δ in the albite aggregate for the situations of two different water contents are about 9.9 × 10−23 and 1.4 × 10−22 m3 s−1, respectively. Both the rate constants and the calculated D′Aδ demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic pyroxene (+ spinel) aggregates is small, consistent with recent studies of monomineralic enstatite and forsterite rims. Received: 1 July 1995 / Accepted: 1 August 1996  相似文献   

12.
This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ～20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe2+, Fe3+ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe2+ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ～103–104 years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 105–106 and 106–107 years respectively.  相似文献   

13.
 The melting reaction: albite(solid)+ H2O(fluid) =albite-H2O(melt) has been determined in the presence of H2O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H2O–CO2 fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X (H2O) ) melting relations in the two systems are almost coincident. By contrast, H2O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 °C higher in H2O–NaCl brines than in H2O–CO2 fluids for X (H2O) =0.8 and ∼100 °C higher for X (H2O) =0.5. The melting temperature of albite in H2O–NaCl fluids of X (H2O)=0.8 is ∼100 °C higher than in pure water. The P–T curves for albite melting at constant H2O–NaCl show a temperature minimum at about 5 kbar. Water activities in H2O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H2O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a (H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996  相似文献   

14.
A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz = orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this. These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures >500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet, orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade, low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for the Mafic Formation; and that this dehydration event did not involve partial melting. Received: 15 February 2000 / Accepted: 26 June 2000  相似文献   

15.
K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ～7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ～150–400°C but the release of 39Ar from irradiated K-feldspar during furnace step-heating experiments in vacuo yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ～500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of 40Ar/39Ar ages with U–Pb zircon ages at ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity.  相似文献   

16.
Study of micro-area chemical compositions indicates that phengite in albite gneiss from hole ZK2304 of the Donghai region has evident compositional zoning. SiO2 and tetrahedrally coordinated Si contents decrease, and Al2O3, AlIV and AlVI contents increase gradually from core to rim. However, K2O, MgO and FeO contents basically remain unchanged from core to rim. According to P-T estimates obtained from geothermometers and barometers, combined with previous experimental data, the core belt (micro-area I) of phengite was formed at T=637-672℃ and P=1.55-1.73 GPa, and the transitional belt (micro-area II) of the phengite were formed at r=594-654℃ and P=1.35-1.45 GPa. Towards the rim belt (micro-area III), the temperature decreased slightly, but the pressure decreased rapidly with r=542-630℃ and P=1.12-1.19 GPa. The P-T evolution path recorded by the compositional zoning of phengite is characterized by significant near-isothermal decompression, revealing that the gneiss has. undergone high-pressure-ultrahi  相似文献   

17.
Orthopyroxene and olivine exposed along the rim of a harzburgite xenolith from La Palma (Canary Islands) show polycrystalline selvages and diffusion zones that result from contact with mafic, alkaline, silica-undersaturated melts during at least 10-100 years before eruption. The zoned selvages consist of a fine-grained reaction rim towards the xenolith and a coarser grained, cumulate-like layer towards the melt contact. The diffusion zones are characterized by decreasing magnesium number from about 89-91 in the xenolith interior to 79-85 at the rims, and clearly result from Fe-Mg exchange with surrounding mafic melt. The width of the diffusion zones is 80-200 µm in orthopyroxene and 1,020-1,730 µm in olivine. Orthopyroxene also shows decreasing Al2O3 and Cr2O3 and increasing MnO and TiO2 towards the reaction rims. Textural relations and comparisons with dissolution experiments suggest that orthopyroxene dissolution by silica-undersaturated melt essentially ceased after days to weeks of melt contact, possibly because of decreasing temperature and formation of the reaction rims. The short dissolution phase was followed by prolonged growth of diffusion zones through cation exchange between xenolith minerals and melt across the reaction rims, and by the growth of cumulus crystals. The observations indicate that orthopyroxene xenocrysts and harzburgite xenoliths can survive in mafic, silica-undersaturated, subliquidus magmas at 1,050-1,200 °C and 200-800 MPa for tens of years. Modeling and comparison of the diffusion zones indicate that the average Fe-Mg interdiffusion coefficient DFeMg in orthopyroxene is 2 log units lower than that in olivine; at 1,130 °C and QFM-buffered oxygen fugacity, DFeMgopx = 3 ×10 - 19  m2  s- 1D_{FeMg}^{opx} = 3 \times 10^{ - 19} \,{\rm m}^2 \,{\rm s}^{{\rm - 1}} . The new data overlap well with recently published data for DFeMg in diopside, and indicate that DFeMg opxD_{FeMg\,}^{opx} (as predicted by previous authors) may be extrapolated to higher temperatures and oxygen fugacities. It is suggested that DFeMg opx D_{FeMg\,}^{opx} and DFeMg in Mn-poor ferromagnesian garnet are similar within 0.5 log units at temperatures between 1,050 and 1,200 °C.  相似文献   

18.
As part of a study of ion migration mechanisms in feldspars, the dynamical behaviour of the alkali metal cations ions in albite and K-feldspar has been investigated using a combination of dielectric spectroscopy and atomistic computer simulation techniques. The low-frequency dielectric properties of these minerals have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectra was observed for both albite and K-feldspar; the activation energy for these processes was determined to be 1.33 eV in both albite and K–feldspar. Atomistic simulation techniques were used to elucidate the mechanism and energetics of the cation migration processes. Mechanisms involving the conventional hopping of Na+ and K+ ions between cation sites in the (010) plane were found to give calculated energy barriers in good agreement with the experimentally determined activation energies. These results assist in understanding the nature of the processes responsible for the observed dielectric behaviour.  相似文献   

19.
ABSTRACT Two styles of feldspar alteration – carbonatization and albitization – were investigated using a cathodoluminescence (CL) technique. The nature of the alteration depends on the composition of the fluids. The infiltration of CO2-rich fluids causes decomposition of An-rich zones in plagioclase followed by the formation of secondary calcite, albite and white mica. K-feldspar is more resistant to CO2-induced alteration. The circulation of aqueous fluids results in decomposition of primary oligoclase into albite and clinozoisite. Secondary K-feldspar exsolved as small independent grains on the rim of the primary oligoclase, if the primary plagioclase was enriched in the orthoclase component. The fluids easily penetrate the crystals using, crystallographic plains, e.g., twinning or cleavage or simply along cracks. These migration pathways enable the fluids to enter the inner parts of the grain, which would otherwise not be affected by grain-surface alteration.  相似文献   

20.
 The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition, major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar megacrysts differing in tetrahedral Al-content (2t1) and exsolution microtextures: 1. Alkali feldspar megacrysts (Or70An2Ab28) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved K-feldspar host is monoclinic (2t1=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite. 2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering (2t1=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones. Received: 12 June 1995 / Accepted: 29 January 1996  相似文献   

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