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1.
 Phase A, Mg7Si2O8(OH)6, is a dense hydrous magnesium silicate whose importance as a host of H2O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between 3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (105 Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H2O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH o f =−7126±8 kJ mol-1, S o=351 J K-1 mol-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite. The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H2O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO2–H2O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressures above approximately 6 GPa. Received: 3 July 1995/Accepted: 14 December 1995  相似文献   

2.
Liquidus phase relations have been experimentally determined in the systems Qz-Ab-Or-(H2O), Qz-Ab-(H2O) and Qz-Or-(H2O) at H2O-undersaturated conditions (a H2O = 0.07) and P = 5 kbar. Starting materials were homogeneous synthetic glasses containing 1 wt% H2O. The liquidus temperatures were bracketed by crystallization and dissolution experiments. The results of kinetic studies showed that crushed glasses are the best starting materials to overcome undercooling and to minimize the temperature difference between the lowest temperature of complete dissolution (melting) and the highest temperature at which crystallization can be observed. At P = 5 kbar and a H2O = 0.07, the Qz-Ab eutectic composition is Qz32Ab68 at 1095 °C (±10 °C) and the Qz-Or eutectic is Qz38Or62 at 1030 °C (±10 °C). The minimum temperature of the ternary system Qz-Ab-Or is 990 °C (±10 °C) and the minimum composition is Qz32Ab35‐ Or33. The Qz content of the minimum composition in the system Qz-Ab-Or-H2O remains constant with changing a H2O. The normative Or content, however, increases by approximately 10 wt% with decreasing a H2O from 1 to 0.07. Such an increase has already been observed in the system Qz-Ab-Or-H2O-CO2 at high a H2O and it is concluded that the use of CO2 to reduce water activities does not influence the composition of the minima in quartz-feldspar systems. The determined liquidus temperature in melts with 1 wt% H2O is very similar to that obtained in previous nominally “dry” experiments. This discrepancy is interpreted to be due to problems in obtaining absolutely dry conditions. Thus, the hitherto published solidus and liquidus temperatures for “dry” conditions are probably underestimated. Received: 27 March 1997 / Accepted: 1 October 1997  相似文献   

3.
High temperature stability limit of phase egg, AlSiO3(OH)   总被引：1，自引：1，他引：0  
The stability relations of phase egg, AlSiO3(OH), have been investigated at pressures from 7 to 20 GPa, and temperatures from 900 to 1700 °C in a multi-anvil apparatus. At the lower pressures phase egg breaks down according to the univariant reaction, phase egg = stishovite + topaz-OH, which extends from 1100 °C at 11 GPa to 1400 °C at 13 GPa where it terminates at an invariant point involving corundum. At pressures above the invariant point, the stability of phase egg is limited by the breakdown reaction, phase egg = stishovite + corundum + fluid, which extends from the invariant point to 1700 °C at 20 GPa. Stishovite crystallized in the Al2O3-SiO2-H2O system contains Al2O3, and the amount of Al2O3 increases with increasing temperature. It is inferred that the Al2O3 content is controlled by the charge-balanced substitution of Si4+ by Al3+ and H+. Aluminum-bearing stishovite coexisting with an H2O-rich fluid may contain a certain amount of water. Therefore, phase egg and stishovite in a subducting slab could transport some H2O into the deep Earth. Received: 14 October 1998 / Accepted: 19 May 1999  相似文献   

4.
The low-pressure stability of clinohumite has been investigated in phase-equilibrium experiments on the reaction forsterite + brucite = clinohumite. The reaction was bracketed between 2.45 and 2.84 GPa at 650 °C, extending to between 1.37 and 1.57 GPa at 850 °C. At temperatures above the reaction brucite = periclase + vapour, the reaction clinohumite = forsterite + vapour was bracketed between 1.27 and 1.52 GPa at 900 °C, rising to between 1.90 and 2.00 GPa at 1000 °C. The position of the reaction forsterite + brucite = clinohumite is ∼0.5 GPa below the position determined in previous work, the difference arising either from pressure uncertainties in both studies, from enhanced reaction to clinohumite in this study due to the presence of excess brucite in the starting material, or from different concentrations of defects in the two samples. The brackets on the reaction were combined with other measured and estimated thermodynamic data for clinohumite to determine its enthalpy of formation and entropy, in a revised version of the thermodynamic dataset of Holland and Powell (1998). The values obtained were ΔH f =−9607.29±3.05 kJ mol−1, S=445 J mol−1 K−1. These data were used to calculate positions of other reactions involving clinohumite. The calculations suggest a larger stability field for clinohumite than implied by the results of previous experimental studies, indicating a need for more high-pressure phase-equilibrium studies to provide better thermodynamic data. Received: 30 April 1999 / Accepted: 8 November 1999  相似文献   

5.
 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   

6.
We present H2O analyses of MgSiO3 pyroxene crystals quenched from hydrous conditions in the presence of olivine or wadsleyite at 8–13.4 GPa and 1,100–1,400°C. Raman spectroscopy shows that all pyroxenes have low clinoenstatite structure, which we infer to indicate that the crystals were high clinoenstatite (C2/c) during conditions of synthesis. H2O analyses were performed by secondary ion mass spectrometry and confirmed by unpolarized Fourier transform infrared spectroscopy on randomly oriented crystals. Measured H2O concentrations increase with pressure and range from 0.08 wt.% H2O at 8 GPa and 1,300°C up to 0.67 wt.% at 13.4 GPa and 1,300°C. At fixed pressure, H2O storage capacity diminishes with increasing temperature and the magnitude of this effect increases with pressure. This trend, which we attribute to diminishing activity of H2O in coexisting fluids as the proportion of dissolved silicate increases, is opposite to that observed previously at low pressure. We observe clinoenstatite 1.4 GPa below the pressure stability of clinoenstatite under nominally dry conditions. This stabilization of clinoenstatite relative to orthoenstatite under hydrous conditions is likely owing to preferential substitution of H2O into the high clinoenstatite polymorph. At 8–11 GPa and 1,200–1,400°C, observed H2O partitioning between olivine and clinoenstatite gives values of D ol/CEn between 0.65 and 0.87. At 13 GPa and 1,300°C, partitioning between wadsleyite and clinoenstatite, D wd/CEn, gives a value of 2.8 ± 0.4.  相似文献   

7.
 An experimental technique to make real-time observations at high pressure and temperature of the diamond-forming process in candidate material of mantle fluids as a catalyst has been established for the first time. In situ X-ray diffraction experiments using synchrotron radiation have been performed upon a mixture of brucite [Mg(OH)2] and graphite as starting material. Brucite decomposes into periclase (MgO) and H2O at 3.6 GPa and 1050 °C while no periclase is formed after the decomposition of brucite at 6.2 GPa and 1150 °C, indicating that the solubility of the MgO component in H2O greatly increases with increasing pressure. The conversion of graphite to diamond in aqueous fluid has been observed at 7.7 GPa and 1835 °C. Time-dependent X-ray diffraction profiles for this transformation have been successfully obtained. Received: 17 July 2001 / Accepted: 18 February 2002  相似文献   

8.
 Using the high-pressure differential thermal analysis (HP-DTA) system in a cubic multianvil high-pressure apparatus, we measured the melting points of portlandite, Ca(OH)2, up to 6 GPa and 1000 °C. We detected endothermic behavior at the temperature and pressure conditions of 800 °C and 2.5 GPa, 769 °C and 3.5 GPa, 752 °C and 4.0 GPa, 686 °C and 5.0 GPa, and 596 °C and 6.0 GPa, respectively, due to melting of portlandite. By in situ X-ray studies under pressure, the melting of portlandite was observed at 730 °C and 4.32 GPa and at 640 °C and 5.81 GPa, respectively. Results of both HP-DTA and X-ray studies were consistent within experimental error. The melting is congruent and has a negative Clapeyron slope, indicating that liquid Ca(OH)2 has higher densities than crystalline portlandite in this pressure range. Received: 19 June 1999 / Revised, accepted: 11 September 1999  相似文献   

9.
Summary ?Post-magmatic garnets occur in volcanic breccias at the base of the Neapolitan Yellow Tuff (NYT) formation in the north-western area of the Phlegraean Fields. We report the results of a comprehensive study of these grandites. Garnet is found on the surfaces of tuffaceous blocks or inside their micropores, and is associated with sodalite, sanidine, marialite and amorphous silica. Garnet samples were examined by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), powder and single-crystal X-ray diffraction (XRD) and infrared spectroscopy (IR). SEM observations on morphology showed typical dodecahedral and icositetrahedral habits. EPM analysis showed that they are close to grossular or andradite end members, with only moderate solid solution between them. X-ray study of single crystals showed cubic cell dimensions ao of 11.86 ? (grossular) and 12.04 ? (andradite). IR spectroscopy confirmed the presence of hydroxyls in coexisting garnet and sanidine, 0.06 wt% H2O (garnet) and 0.05–0.07 wt% H2O (sanidine), respectively. Well-crystallized sanidine of an earlier generation showed significantly higher water contents, in the range 0.13–0.23 wt% H2O. Type of occurrence and mineralogical features suggest a post-magmatic (pneumatolitic) genesis for these garnets. This is consistent with the physico-chemical processes linked to the eruptive dynamics of the breccias. Experimental studies of garnet synthesis at 550 °C and 2 kbar provide further support for this concept. Received January 16, 2002; accepted March 18, 2002  相似文献   

10.
 Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900 to 1300° C, and then quenched rapidly, at oxygen fugacities controlled between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers. The liquidus of both samples is suppressed ∼100° C at water-saturated conditions and 1500 bar. Both lavas contained between 3 and 4 wt% water at the stage of phenocryst precipitation. The partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f O2 units; when these determinations are combined with previous studies, Fe2O3/(Σ FeO total) of Mg-rich biotite can be calculated knowing log f O2, T, and X Fe. Thermodynamic modelling of biotite-liquid equilibria results in two expressions for calculating activity coefficients (γ) for annite and phlogopite in natural biotites. Based on the partitioning of BaO and TiO2 between biotite and liquid, we have formulated a thermometer and barometer. Over the range of 400° C, TiO2 partitioning between phlogopite and liquid is a function of temperature (±50° C), and is insensitive to pressure and H2O and O2 activities. BaO partitioning between phlogopite and liquid is a function of both temperature and pressure (±4 kbar), the latter being most important. Applying the TiO2 and BaO partitioning expressions to lamprophyre and lamproite suites shows that Mexican minettes equilibrated at low pressures (5 to 15 kbar;±4 kbar) and temperatures (1090 to 1160° C; ±50° C), while Australian lamproites equilibrated at higher P (up to 30 kbar; ±4 kbar) and T (1125 to 1400° C; ±50° C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO- and TiO2- based pressure calculations indicate that felsic minettes from the Mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures. Received: 1 August 1994/Accepted: 30 June 1995  相似文献   

11.
The H2O content of wadsleyite were measured in a wide pressure (13–20 GPa) and temperature range (1,200–1,900°C) using FTIR method. We confirmed significant decrease of the H2O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400–1,900°C. Wadsleyite contains 0.37–0.55 wt% H2O at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2–0.3 wt% H2O. The H2O contents of wadsleyite at 1,800–1,900°C are 0.22–0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H2O content of wadsleyite can be described by exponential equation C\textH2 \textO = 6 3 7.0 7 \texte - 0.00 4 8T , C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , where T is in °C. This equation is valid for temperature range 1,200–2,100°C with the coefficient of determination R 2 = 0.954. Temperature dependence of H2O partition coefficient between wadsleyite and forsterite (D wd/fo) is complex. According to our data apparent Dwd/fo decreases with increasing temperature from D wd/fo = 4–5 at 1,200°C, reaches a minimum of D wd/fo = 2.0 at 1,400–1,500°C, and then again increases to D wd/fo = 4–6 at 1,700–1,900°C.  相似文献   

12.
Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K2O+Na2O)/Al2O3 = 1.7–3.7], Si-poor (40–45 wt% SiO2), and Ti-rich (5.6–9.2 wt% TiO2) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca. 150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 °C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites. Received: 23 December 1996 / Accepted: 20 March 1997  相似文献   

13.
Determination of the phase boundary between ilmenite and perovskite structures in MgSiO3 has been made at pressures between 18 and 24 GPa and temperatures up to 2000 °C by in situ X-ray diffraction measurements using synchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present in situ X-ray observations, because the grain growth of ilmenite hindered the estimation of relative abundances of these phases. Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing pressure and temperature conditions during in situ X-ray diffraction measurements. Nevertheless, the phase boundary was well constrained by quench method with a pressure calibration based on the spinel-postspinel boundary of Mg2SiO4 determined by in situ X-ray experiments. This yielded the ilmenite-perovskite phase boundary of P (GPa) = 25.0 (±0.2) – 0.003 T (°C) for a temperature range of 1200–1800 °C, which is generally consistent with the results of the present in situ X-ray diffraction measurements within the uncertainty of ∼±0.5 GPa. The phase boundary thus determined between ilmenite and perovskite phases in MgSiO3 is slightly (∼0.5 GPa) lower than that of the spinel-postspinel transformation in Mg2SiO4. Received: 19 May 1999 / Accepted: 21 March 2000  相似文献   

14.
Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations. Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle – including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism. Resetting was very probably enhanced by the presence of fluids derived by H2O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright, metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression. (5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma, that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same continental crust. Received: 5 October 1998 / Accepted: 18 January 1999  相似文献   

15.
The fluid-absent reaction 12 zoisite = 3 lawsonite + 7 grossular + 8 kyanite + 1 coesite was experimentally reversed in the model system CaO-Al2O3-SiO2-H2O (CASH) using a multi-anvil apparatus. The upper pressure stability limit for zoisite was found to extend to 5.0 GPa at 700 °C and to 6.6 GPa at 950 °C. Additional experiments both in the H2O-SiO2-saturated and in the H2O-Al2O3-saturated portions of CASH provide further constraints on high pressure phase relationships of lawsonite, zoisite, grossular, kyanite, coesite, and an aqueous fluid. Consistency of the present experiments with the H2O-saturated breakdown of lawsonite is demonstrated by thermodynamic analysis using linear programming techniques. Two sets of data consistent with databases of Berman (1988) and Holland and Powell (1990) were retrieved combining experimental phase relationships, calorimetric constraints, and recently measured elastic properties of solid phases. The best fits result in G f ,1,298 ∘,zoisite=−6,499,400 J and S 1,298 ∘,zoisite=302 J/K, and G f ,1,298 ∘,lawsonite=−4,514,600 J and S 1,298 ∘,lawsonite=220 J/K for the dataset of Holland and Powell, and G f ,1,298 ∘,zoisite=−6,492,120 J and S 1,298 ∘,zoisite=304 J/K, and G f ,1,298 ∘,lawsonite=−4,513,000 J and S 1,298 ∘,lawsonite= 218 J/K for the dataset of Berman. Examples of the usage of zoisite as a geohygrometer and as a geobarometer in rocks metamorphosed at eclogite facies conditions are worked, profiting from the thermodynamic properties retrieved here. Received: 23 December 1996 / Accepted: 29 August 1997  相似文献   

16.
 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO2). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm−1, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/106 Si (at 5 GPa) to 1415 H/106 Si (at 10 GPa). Assuming a model of incorporation by (4H)Si defects, the data are fit well by the equation C OH=Af 2 H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/106 Si/GPa, and ΔV=20.6 × 10−6 m3 mol−1. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H2O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000  相似文献   

17.
Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H2O-poor melt and a silicate-poor H2O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H2O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an XH2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H2O, and the conjugate water-rich melt about 47 wt% H2O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H2O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. Received: 15 September 1999 / Accepted: 10 December 1999  相似文献   

18.
Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H2O; 5% CO2; 0.5% CH4; 0.2% N2; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO2, CH4 and N2). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO2, CH4 and N2), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO2)-rich aqueous inclusions, aqueous inclusions (H2O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999  相似文献   

19.
The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO2 < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO2 < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO2 > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996  相似文献   

20.
The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water has been experimentally investigated in the chemical system H2O−CaO− MgO−Al2O3−SiO2 over the range of 0.4–0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite + quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz, and water were obtained at 600, 650, and 700°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5±0.08 at the highest temperature. Thermodynamic analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S 0 (1 bar, 298 K) of 575.4±1.6 J/K · mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several experiments in the range of 1.0–1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770°C. Received: 24 January 1997 / Accepted: 2 October 1997  相似文献   

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