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1.
Distribution coefficients (K D·Fe ++ –Mg) calculated for orthopyroxene-clinopyroxene pairs from 12 basic granulites of the Quairading district, Western Australia, range from 1.87 for magnesian pyroxenes (Opx Mg value=78.1) to 1.70 for iron-rich varieties (Opx Mg value 37.7). Field and petrographic evidence indicates that these pyroxenes have probably reached equilibrium within a narrow temperature range. In order to account for the observed variation of K D values it is suggested that one (or both) of the pyroxene structures is not the ideal Fe++-Mg solution proposed in the thermodynamic model of the pyroxene equilibrium exchange. After consideration of the geometry of the pyroxene cation sites, the relative bond energies of each site (especially crystal field stabilization energy) and structural ordering a model is proposed to explain the non-ideal behaviour of Fe++-Mg in the pyroxene system. The distribution pattern in low-iron pyroxenes will probably show Fe++ favouring the M 2·Opx site; competing unfavourably with Mg++ for the M 1·Cpxsite; and probably excluded by Mg++ from the M 1·Opxsite. As the iron content of the system increases the M 2·Opxsite will begin to become saturated with Fe++ and this ion will enter the M 1·Opx site. Further increases in the iron content of the system will cause the Fe++-Mg distribution to depend increasingly on the relative attraction of the M 1 sites of both pyroxenes. Of these sites Fe++ should show preference to the more distorted M 1·Cpxsite. The distribution coefficient reflects this swing towards a relative enrichment of Fe++ in the clinopyroxene by decreasing regularly with increasing iron content. It is likely that this downward trend will not become evident until the iron content of the M 2·Opx site reaches saturation. This would explain why the K D values for the magnesian pyroxenes remain practically unchanged until the orthopyroxene Mg value is approximately 60; from here on the iron-rich pyroxenes show a rapid decrease in K D value with increasing iron content.The Ca content of the pyroxenes is also significant since the Quairading pyroxenes show a marked increase in mutual solubility with increasing iron content. Calcium taken into the orthopyroxene structure will enter the M 2 site ahead of Fe++ so that this site will reach Fe++ saturation at a lower iron content than when the orthopyroxene is Ca-free.The application of K D values to the regional study of basic granulites, particularly when establishing relative temperature zones on the basis of K D variation, should only be attempted when pyroxenes which extend over a wide range of Fe-Mg content are available.  相似文献   

2.
OH in zoned amphiboles of eclogite from the western Tianshan,NW-China   总被引：1，自引：0，他引：1  
Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( B Na2 C M2+ 3 M3+ 2 T Si8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( A 0.5 B Ca1.5Na0.5 C M 2+ 4.5 M 0.53+ T Si7.5Al0.5(OH)2) (A = Na and/or K; M 2+ = Mg and Fe2+; M 3+ = Fe3+ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm−1) which can be attributed to a local O(3)-H dipole surrounded by M(1) M(3)Mg3, M(1) M(3)Mg2Fe2+ and M(1) M(3) Fe2+ 3, respectively, an empty A site and T Si8 environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm−1) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.  相似文献   

3.
The separate distributions for MgSiO3 and FeSiO3 in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe2+ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe2+ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO3 content but the corresponding Mg site occupancy appears largely independent of MgSiO3. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe2+ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.  相似文献   

4.
Zusammenfassung Teineit, CuTeO3·2H2O, kristallisiert in der RaumgruppeP212121-D 2 4 (a 0=6,634 Å,b 0=9,597 Å,c 0=7,428 Å;Z=4). Die an synthetischem Material durchgeführte Verfeinerung (R=0,047 für 541 beobachtete Reflexe) bestätigte die Kristallstruktur des natürlichen Teineits. Cu ist (4+1)-koordiniert; Te ist von drei Sauerstoffen umgeben und bildet mit diesen isolierte, weitgehend regelmäßige TeO3-Pyramiden. Die Struktur läßt sich unter Vernachlässigung der Wassermoleküle als ein dreidimensionales, dreifachver-knüpftes Netz beschreiben. Es wird ein Vergleich mit Chalkomenit, CuSeO3·2H2O, CuHPO3·2H2O sowie Cu(NH3)TeO3·H2O gegeben.
Structure refinement of synthetic teineite, CuTeO3·2H2O
Summary Teineite, CuTeO3·2H2O, crystallizes in the space groupP212121-D 2 4 (a 0=6.634 Å,b 0=9.597 Å,c 0=7.428 Å;Z=4). Data from synthetic teineite were refined (R=0.047 for 541 observed reflections) and the crystal structure of natural teineite was confirmed. Cu has a (4+1)-coordination; Te is surrounded by three oxygens and forms with them isolated, essentially regular TeO3-pyramids. Neglecting the hydrogen-bonds the structure may be described as a three-dimensional three-fold connected net. A comparison with chalcomenite, CuSeO3·2H2O, CuHPO3·2H2O, as well as Cu(NH3)TeO3·H2O is given.

Mit 2 Abbildungen  相似文献   

5.
Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules under anhydrous conditions at 22–40 kbar, 1057–1400 °C in a piston-cylinder apparatus. The concentration profiles that developed in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca. Because of their usually low concentrations and lack of sufficient compositional change across the interface of the diffusion couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe) and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D=D 0exp{−[Q(1 bar)+PΔV +]/RT}. The values of the activation energy (Q) and activation volume (ΔV +) depend on whether f O2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe:Q(1 bar)=65,532±10,111 cal/mol, D 0=3.50 (±2.30)×10−5 cm2/s, ΔV +=5.6(±2.9) cm3/mol, and for Mg:Q(1 bar)=60,760±8,257 cal/mol, D 0=4.66 (±2.48)×10−5 cm2/s, ΔV +=5.3(±3.0) cm3/mol. Here the ΔV + values have been taken from Chakraborty and Ganguly (1992). For the condition of constant f O2, the Q values are ∼9 kcal lower and ΔV + values are ∼4.9 cm3/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement with the Mg tracer diffusion data (D * Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750–900 °C, when all data are normalized to the same pressure and to f O2 defined by graphite in the system C-O. The D * Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature extrapolation of the present data, when all data are normalized to the same pressure and to f O2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional effects on D(I) along with the effects of thermodynamic nonideality, f O2, and pressure. Received: 8 May 1997 / Accepted: 2 October 1997  相似文献   

6.
The equilibrium intracrystalline distribution coefficient, k D *, of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k D * as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔS XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution. Received: 25 August 1998 / Accepted: 10 March 1999  相似文献   

7.
Diffusion couple experiments were carried out with San Carlos olivine (Fo90) and NiFe alloys (Ni100, Ni97Fe3, Ni90Fe10) or other olivine compositions (Fo100, Fo25) in order to determine the dependence on temperature, oxygen fugacity, composition and crystallographic orientation of Ni diffusion coefficient (DNi) in olivine. Experiments at 1 atmosphere total pressure cover a temperature range of 900-1445°C with run durations from 48 to 2155 h at different oxygen fugacities. In an Arrhenius plot the best fit for all data for Fo90 yields an activation energy (ED) of 220 ± 14 kJ/mol and an fO2 dependence of (1/4.25)·Δ log fO2 = Δ log DNi. The relationship between diffusion coefficients along different crystallographic axes at 1200°C is given by D[001] ≈ 6·D[100] ≈ 6·D[010]. DNi depends strongly on the major element (i.e. Fe/Mg) composition of olivine and decreases by about 1 order of magnitude as the olivine composition changes from Fo35 to Fo90. Thus, experimental investigations in Fe-free systems cannot be applied to natural samples. For calculation of residence times or cooling rates the present Ni data yield shorter timescales compared to those obtained using diffusion data published until now.In addition to Ni diffusion coefficients, Fe-Mg, Mn and Ca diffusion data were obtained from some of the same diffusion couples (Fo90-Fo100). It is found that the activation energies, ED[Ni] ≅ ED[Fe-Mg] ≅ ED[Mn] ≤ ED[Ca]. All diffusion coefficients are strongly dependent on the major element composition of olivine.  相似文献   

8.
The reaction chloritoid (ctd)=almandine (alm)+diaspore+H2O (CAD) has been reversed using Fe3+-free synthetic chloritoid and almandine, under fO2 conditions of the solid oxygen buffer Fe/FeO (CADWI), and using partially oxidized synthetic minerals under fO2 conditions of the solid oxygen buffer Ni/NiO (CADNNO). Experiments have been conducted between 550 and 700°C, 25 and 45 kbar. The equilibrium pressure and temperature conditions are strongly dependent on the fO2 conditions (CADNNO lies some-what 50°C higher than CADWI). This can be explained by a decrease in aH2O for experiments conducted on the Fe/FeO buffer, and a decrease in actd and aalm (through incorporation of ferric iron preferentially in chloritoid) for experiments conducted on the Ni/NiO buffer. The H2O activity has been calculated using the MRK equation of state, and the values obtained checked against the shift of the equilibrium diaspore=corundum+H2O bracketed on the Fe/FeO buffer and under unbuffered fO2 conditions. For fO2 buffered by the assemblage Fe/FeO, aH2O increases with pressure from about 0.85 at 600°C, 12 kbar to about 0.9 at 605°C, 25 kbar and 1 above 28 kbar. For fO2 buffered by the assemblage Ni/NiO, aH2O=1. The aH2O decrease from Ni/NiO to Fe/FeO is, however, too small to be entirely responsible for the temperature shift between CADNNO and CADWI. In consequence, the amount of ferric iron in almandine and chloritoid growing in the CADNNO experiments must be significant and change along the CADNNO, precluding calculation of the thermodynamic properties of chloritoid from this reaction. Our experimental data obtained on the Fe/FeO buffer are combined, using a thermodynamic analysis, with Ganguly's (1969) reversal of the reaction chloritoid=almandine+corundum +H2O (CAC) on the same oxygen buffer. Experimental brackets are mutually consistent and allow extraction of the thermodynamic parameters H o f,ctd and S octd. Our thermodynamic data are compared with others, generally calculated using Ganguly's bracketing of CACNNO. The agreement between the different data sets is relatively good at low pressure, but becomes rapidly very poor toward high pressure conditions. Using our thermodynamic data for chloritoid and KD=(Fe3+/Al)ctd/(Fe3+/Al)alm estimated from natural assemblages, we have calculated the composition of chloritoid and almandine growing from CADNNO and CACNNO. The Fe3+ content in chloritoid and almandine increases with pressure, from less than 0.038 per FeAl2SiO5(OH)2 formula unit at 10 kbar to at least 0.2 per formula unit above 30 kbar. This implies that chloritoid and almandine do contain Fe3+ in most natural assemblages. The reliability of our results compared to natural systems and thermodynamic data for Mg-chloritoid is tested by comparing the equilibrium conditions for the reaction chloritoid+quartz=garnet (gt)+kyanite+H2O (CQGK), calculated for intermediate Fe–Mg chloritoid and garnet compositions, from the system FASH and from the system MASH. For 0.65<(XFe)gt<0.8, CQKG calculated from FASH and MASH overlap for KD=(Mg/Fe)ctd/(Mg/Fe)gt=2. This is in good agreement with the KD values reported from chloritoid+garnet+quartz+kyanite natural assemblages.  相似文献   

9.
In order to clarify the structural configurations observed in Diss in the Ca-rich region of the Di-En join (in which TEM observations show neither exsolution microstructures nor evidence of spinodal decomposition) single crystals large enough for X-ray diffraction analyses, with composition (Ca0.66Mg0.34)MgSi2O6, have been equilibrated close to the solvus atT=1350° C for 317 h, and quenched at room temperature. The refinement in C2/c space group shows that in the M2 site Ca and Mg are fully ordered in two split positions (M2occ: 0.66 Ca; M2occ: 0.34 Mg). Since the average structure shows a relevant elongation of anisotropic thermal ellipsoids of the O2 and O3 oxygen atoms, the refinement has been carried out according to a split model for O2 and O3 atoms: Ca appears 8-coordinated (as in diopside) and Mg shows a sixfold coordination similar to that of high-pigeonite. This coordination for Mg is significantly different from the fourfold coordination (Zn-like in Zn-cpx) proposed previously and it is a more probable coordination for Mg from a crystalchemical point of view. The same results were obtained refining a Di80En20 cpx, equilibrated atT=1230° C, according to the same O-split model. The data support the coexistence of a Di-like configuration for Ca and of a highPig-like configuration for Mg away from the solvus also. AtT very near toT solidus the different configurations, observed at room temperature in the quenched samples, should converge and Ca and Mg should retain a single disordered configuration in the M2 site.  相似文献   

10.
Thermal diffusivity (D) was measured using laser-flash analysis (LFA) from oriented single-crystal albite and glasses near LiAlSi3O8, NaAlSi3O8, CaAl2Si2O8, LiAlSi2O6 and CaMgSi2O6 compositions. Viscosity measurements of the supercooled liquids, over 2.6 × 108 to 8.9 × 1012 Pa s, confirm strongly non-Arrhenian behavior for CaAl2Si2O8, and CaMgSi2O6, and near-Arrhenian behavior for the others. As T increases, D glass decreases, approaching a constant near 1,000 K. Upon crossing the glass transition, D decreases rapidly. For feldspars, D for the melt is ~15% below D of the bulk crystal, whereas for pyroxenes, this difference is ~40%. Thermal conductivity (k lat = ρC P D) of crystals decreases with increasing T, but k lat of glasses increases with T because heat capacity (C P ) increases with T more strongly than density (ρ) and D decrease. For feldspars, k lat for the melt is ~10% below that of the bulk crystal or glass, whereas this decrease for pyroxene is ~50%. Therefore, melting substantially impedes heat transport, providing positive thermal feedback that could promote further melting.  相似文献   

11.
We employed the thin source technique to investigate tracer diffusion of Mg, Ca, Sr, and Ba in glasses and supercooled melts of albite (NaAlSi3O8) and jadeite (NaAlSi2O6) compositions. The experiments were conducted at 1 bar and at temperatures between 645 and 1025°C. Typical run durations ranged between 30 min and 35 days. The analysis of the diffusion profiles was performed with the electron microprobe. Diffusivities of Ca, Sr, and Ba were found to be independent of either duration t of the experiment or tracer concentration M, initially introduced into the sample. Mg exhibits a diffusivity depending on run time and concentration and tracer diffusivity is derived by extrapolation to M/√t = 0. Temperature dependence of the diffusivity D can be represented by an Arrhenius equation D = Do exp(−Ea/RT), yielding the following least-squares fit parameters (with D in m2/s and Ea in kJ/mol): DMg = 1.8 · 10−5 exp(−234 ± 20/RT), DCa = 3.5 · 10−6 exp(−159 ± 6/RT), DSr = 3.6 · 10−6 exp(−160 ± 6/RT), and DBa = 6.0 · 10−6 exp(−188 ± 12/RT) for albite; and DMg = 8.3 · 10−6 exp(−207 ± 18/RT), DCa = 3.8 · 10−6 exp(−153 ± 4/RT), DSr = 2.3 · 10−6 exp(−150 ± 4/RT), and DBa = 3.7 · 10−5 exp(−198 ± 4/RT) for jadeite composition. Ca and Sr diffusivities agree within error in both compositions and exhibit the fastest diffusivities, whereas Mg reveals the lowest diffusivity. The relationship between activation energy and radius shows a minimum at Ca and Sr for albite and jadeite compositions extending the relationship already observed elsewhere for alkalies. With increasing substitution of Si by (Na + Al), diffusivities increase, whereas activation energies decrease. Furthermore, a simple model modified from that of Anderson and Stuart (Anderson O. L. and Stuart D. A., “Calculation of activation energy of ionic conductivity in silica glasses by classical methods,” J. Am. Ceram. Soc.37, 573-580, 1954) is discussed for calculating the activation energies.  相似文献   

12.
 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H2O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)2Al4Si5O18+H2O ⇄ (Fe,Mg)2Al4Si5O18.H2O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ° r =–37141±3520 J and ΔS °  r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n H2O=1/[1+1/(K ⋅ f H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H2O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996  相似文献   

13.
The assemblage NiO+Ni-Pd alloy has been calibrated as a precise oxygen fugacity sensor in the temperature range 850–1250 K at 1 bar, using an electrochemical technique with oxygen-specific CSZ electrolytes, and Ni+NiO and Cu+Cu2O as the reference electrodes. Nine compositions were studied, ranging from 0.12 to 0.83 X Ni alloy . Steady EMFs, implying equilibrium, were rapidly achieved in all cells, and were found to be reversible on increasing and decreasing temperature with a precision approaching 0.1 mV. The estimated accuracy of the measurements on each cell is ±0.2 mV (1, corresponding to ±0.003 log-bar units in fo2 at 1273 K). Compositions of the Ni-Pd alloys were measured after each run by electron microprobe, and these compositions were then checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Nickel-rich alloys show positive deviations from ideality and endothermic enthalpies of mixing, but palladium-rich compositions have exothermic enthalpies of mixing and strong negative deviations from ideality. The excess entropies of mixing are positive for all compositions, and correlate approximately with the excess volumes of mixing. The highly asymmetrical deviations from ideality are well described by a polynomial expression of the Redlich-Kister form, with three terms for the enthalpies, and two for the excess entropies and volumes of mixing. The experimental data from this study have been used to re-formulate the Ni-Pd oxygen fugacity sensor to give an expression; O2 ss = O2 NNO – 2RT ln X Ni alloy – [2 · (1 – X Ni alloy )2 · [(–2165–7.958 · T) + (9409 – 0.888 · T) · (4 X Ni alloy – 1) + 2089 · (6 X Ni alloy – 1) · (2 X Ni alloy – 1)]](850<T<1300) where O2 ss is in J mol-1, T is in kelvins, and the expression for O2 NNO is that given by O'Neill and Pownceby (1993). Values in terms of log fo2 may be obtained from the above by dividing by RT ln 10. The estimated standard error in O2 ss is on the order of ±200 J mol-1, which is approximately ±0.01 log-bar units in fo2 at 1273 K.  相似文献   

14.
Gneisses in the Guri area of the Venezuelan Guayana Shield contain mineral assemblages with cordierite, garnet, sillimanite, hypersthene, biotite and Fe-Ti oxide intergrowths.Analysis of mineral assemblages and compositional relationships in the light of experimental data indicate metamorphic conditions of 725–800° C, 5–6 kb P T , <P T for the highest grade rocks and 650–700° C, 5–7 kb P T , approximating P T for the lowest grade rocks. Oxygen fugacities in different lithologies ranged between those of the MH and QFM buffers.The distribution coefficient K D (Mg-Fe) (gar-bio), decreases by 0.006 per atom percent increase in (Mn/Mn+Mg+Fe)gar, falls in the range of K D typical of the sillimanite-K feldspar zone and granulite facies, and is systematically lower in lower grade rocks-all in accord with observation in other localities. K D (Mg-Fe) (cord-bio) ranges from 3.0 in the highest grade rocks to 10.0 in the lowest grade rocks, appears independent of FeO/MgO of cordierite or biotite, and varies systematically with grade. In contrast with conclusions based on observation in other localities, data from the Guri area suggest -KD(cord-bio) may be a sensitive index of grade.A number of mineralogic and geologic observations are difficultly reconciled with existing experimental data.  相似文献   

15.
The phase relations of divariant and trivariant assemblages involving combinations of phengite, chlorite, biotite, K-feldspar, quartz and H2O in the KFASH, KMASH and KFMASH systems were calculated using a single thermodynamic data set (Holland and Powell 1998). The stability fields of the various equilibria are represented in P-T projections by contouring sets of compositional isopleths for the Tschermak (Al2(Fe,Mg)−1Si−1) and FeMg−1 exchanges controlled by the coexisting phases. Five multivariant continuous equilibria, which occur in different regions of P-T-X space, are calibrated as thermobarometers in metamorphic rocks of pelitic to quartzofeldspathic composition. More subtle P-T information, relating to the trajectories (dT/dz) along which reacting rocks have been buried or exhumed, can be extracted from the continuous reactions by investigating the recorded compositional trends in the Al2(Fe,Mg)−1Si−1 and FeMg−1 solutions. Singularities in P-T space are associated with some of these reactions and may result in unusual mineral textures and compositional trends. A fluid-absent singularity has particular petrological significance because it marks the transition between hydration and dehydration along a single reaction with increasing pressure and temperature. This behaviour causes the sequence of reactions among these minerals observed during metamorphism to be critically dependent on the P-T trajectory. Thermobarometric calculations show good agreement with respect to experimental and field-based data for phengite compositions less than about 50 mol% celadonite (<∼3.5 Si p.f.u. phengite). Received: 15 November 1999 / Accepted: 3 April 2000  相似文献   

16.
Experiments from 640 to 680?°C, 200 MPa H2O at?f O2?≈?NNO, employing a natural?F-rich?vitrophyric rhyolite from Spor Mountain, Utah, assessed the effect of variable Mg′ [100Mg/(Mg?+?Mn?+?Fe)] on the partitioning of fluorine and chlorine between biotite (Bt) and melt. Over this temperature interval, Bt (?±?fluorite, ?±?quartz) is the sole liquidus phase. Partition coefficients for fluorine between biotite and glass (DF Bt/melt) show a strong dependence on the Mg′ of Bt.?DF Bt/melt varies from???1.5 to 7.2 over the range of Mg′ from 21 to 76. A strong linear correlation between?DF Bt/melt?and Mg′ has a slope of 9.4 and extrapolates through the origin (i.e., DF Bt/melt?≈?0 at Mg′?=?0, an annite-siderophyllite solid solution in these experiments). DCl Bt/melt values (???1 to 6) in the same experiments vary inversely with Mg′. The Al-content of biotite does not vary with the aluminum saturation index (ASI?=?molar Al2O3/Σ alkali and alkaline earth oxides) of melt, but two exchange mechanisms involving Al appear to operate in these micas: (1) Alvi?+?Aliv?? Siiv?+?Mgiv, and Mgiv?+?2Aliv? 2Siiv?+?□iv. The effects of other components such as Li or other intensive parameters including f O2 have yet to be evaluated?systematically. At comparable Mg′ of Bt, however, the Spor Mountain rhyolite yields higher DF Bt/melt values than an Li-rich, strongly peraluminous melt previously investigated. The results indicate that the Mg′ of Bt exerts the principal control on halogen partitioning, with ASI and T as second-order variables. The experimental partition coefficients compare well with other experimental results but not with most volcanic rocks. Magmatic Bt from most rhyolites records higher DF Bt/melt due to reequilibration with degassed (H2O-depleted) magma and perhaps with F2O?1 exchange that may accompany oxidation ([Fe3+O] [Fe2+OH]?1). This behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near Morococala, Bolivia: Bt is sharply zoned with F-rich rims, but Bt(core)-melt inclusion pairs fall on our experimental curve for DF Bt/melt. These experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks. For plutonic rocks, the actual F-content of melt, not a relative activity ratio involving HF species, can be reasonably estimated if the mica has not undergone subsolidus reequilibration. This information is potentially useful for some shallow-level Ca-poor magmas that are thought to be rich in F (e.g., A- and S-type granites) but do not conserve F well as rocks.  相似文献   

17.
The space group and hydrogen positions of -(Al0.84Mg0.07Si0.09)OOH are investigated using a single crystal synthesized using a multi-anvil apparatus under conditions of 1000 °C and 21 GPa. The space group determined by single-crystal X-ray diffraction is to Pnn2, with unit-cell parameters of a=4.6975(8) Å, b= 4.2060(6) Å, c=2.8327(4) Å, and V=55.97(1) Å3. Partial occupancy of the Al site by Mg and Si suggests the possibility of a limited solid solution between -AlOOH, stishovite, and a hypothetical CaCl2-type Mg(OH)2 that is 16% denser than brucite. Difference-Fourier maps reveal two small but significant Fourier peaks attributable to hydrogen atoms. Atomic distances and angles around the first peak indicate a hydrogen bond with O···O distances of 2.511 Å, while those around the second peak are suggestive of a bifurcated hydrogen bond with O···O distances of 2.743 and 2.743 Å.  相似文献   

18.
The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (KD = (XNiS/XFeS)liquid/(XNiSi12O2/XFeSi12O2)olivine; Xi = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO2 = 10−8.1 to 10−10 and fS2 = 10−0.9 to 10−1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO2 of −8.7 ± 0.1, and log fS2 of −0.9 to −1.7, KD is relatively insensitive to sulfur fugacity, but comparison with previous results shows that KD increases at very low sulfur fugacities. KD values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO2 of −8.1, lessens with decreasing fO2, and KD becomes independent of melt Ni content at log fO2 ≤ −9.5. The origin of this variation in KD with fO2 and fS2 is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts.Application of these experimental results to natural samples shows that the relatively large dispersion that exists in KD values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO2, fS2, and the nickel content of the sulfide liquid. Estimates of fO2 based on KD and sulfide melt composition in natural samples yields a range from fayalite-magnetite-quartz (FMQ)-1 to FMQ-2 or lower, which is in good agreement with previous values determined for oceanic basalts that use glass ferric/ferrous ratios. Anomalously high KD values recorded in some suites, such as Disko Island, probably reflect low fS2 during sulfide saturation, which is consistent with indications of low fO2 for those samples. It is concluded that the variation in KD values from natural samples reflects olivine-sulfide melt equilibrium at conditions within the T-fO2-fS2 range of terrestrial mafic magmas.  相似文献   

19.
Prograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M1–2 and M3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M1) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M2 assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M1, whereas the inclusion‐poor outer part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (<2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.  相似文献   

20.
This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 108 – 2 × 108 Pa, and initial H2O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In all experiments a NaCl melt was present as well. In the absence of H2O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 105 Pa: lgDca=−3. 88−5140/T, lgDk =−3. 79−4040/T, and lgDNa, =−4.99−3350/T, where D is in cm2 /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H2O content in the melt. The presence of about 2 wt% H2O m the melt will lead to a dramatical increase in diffusivity, but higher H2O content has only a minor effect. This change is probably the result of a change in the melt structure when H2O is present. The diffusion coefficients measured in this study are significantly different from those in previous works. This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient, of the two contacting melts.  相似文献   

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