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1.
In the system CaO-MgO-A12O3-SiO2 the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatitess+sillimanite pyrope-grossularss+quartz (3), anorthite+enstatitess pyrope-grossularss+diopsidess+quartz (2), anorthite+pyrope-grossularss+ quartz diopsidess+kyanite (4) and anorthite+diopsidess grossular-pyropess +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En0.5Fs0.5). Enstatite and plagioclase (An0.5Ab0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.  相似文献   

2.
The distribution of sulfur between haplogranitic melt and aqueous fluid has been measured as a function of oxygen fugacity (Co-CoO-buffer to hematite-magnetite buffer), pressure (0.5-3 kbar), and temperature (750-850 °C). Sulfur always strongly partitions into the fluid. At a given oxygen fugacity, pressure and temperature, the distribution of sulfur between melt and fluid can be described by one constant partition coefficient over a wide range of sulfur concentrations. Oxygen fugacity is the most important parameter controlling sulfur partitioning. While the fluid/melt partition coefficient of sulfur is 468 ± 32 under Co-CoO buffer conditions at 2 kbar and 850 °C, it decreases to 47 ± 4 at an oxygen fugacity 0.5-1 log unit above Ni-NiO at the same pressure and temperature. A further increase in oxygen fugacity to the hematite-magnetite buffer has virtually no effect on the partition coefficient (Dfluid/melt = 49 ± 2). The dependence of Dfluid/melt on temperature and pressure was systematically explored at an oxygen fugacity 0.5-1 log units above Ni-NiO. At 850 °C, the effect of pressure on the partition coefficient is small (Dfluid/melt = 58 ± 3 at 0.5 kbar; 94 ± 9 at 1 kbar; 47 ± 4 at 2 kbar and 68 ± 5 at 3 kbar) and temperature also has only a minor effect on partitioning.The data show the “sulfur excess” observed in many explosive volcanic eruptions can easily be explained by the presence of a small fraction of hydrous fluid in the magma chamber before the eruption. The sulfur excess can be calculated as the product of the fluid/melt partition coefficient of sulfur and the mass ratio of fluid over melt in the erupted material. For a plausible fluid/melt partition coefficient of 47 under oxidizing conditions, a 10-fold sulfur excess corresponds to a 17.6 wt.% of fluid in the erupted material. Large sulfur excesses (10-fold or higher) are only to be expected if only a small fraction of the magma residing in the magma chamber is erupted.The behavior of sulfur, which seems to be largely independent of pressure and temperature under oxidizing conditions is very different from chlorine, where the fluid/melt partition coefficient strongly increases with pressure. Variations in the SO2/HCl ratio of volcanic gases, if they reflect primary processes in the magma chamber, therefore provide an indicator of pressure variations in a magma. In particular, major increases in the S/Cl ratio of an aqueous fluid coexisting with a felsic magma suggest a pressure reduction in the magma chamber and/or magma rising to the surface.  相似文献   

3.
Petrogenetic grids are a powerful tool for understanding metamorphic terrains and many theoretical grids have been suggested for the process of granulite formation in metapelitic rocks, via fluid-absent biotite melting reactions. However, application of these grids has been difficult due to the lack of suitable experimental constraints. We present here an experimentally determined and tightly constrained petrogenetic grid for KFMASH system metapelites which extends from 840–1000°C and 5.0–12.5 kbar. Sixty four experiments on three KFMASH, mineral-mix, bulk compositions (X Mg=0.62, 0.74, 0.86) provide phase composition and assemblage data from which a grid can be derived and constrained. Reversal experiments and consideration of the phase composition data show the experiments to be close to equilibrium. The KFMASH univariant fluid-absent biotite melting reactions occur between 850 and 870°C at 5 kbar and between 900 and 915°C at 10 kbar. These reactions are connected to equilibria beyond the stability of biotite to develop a fixed framework within which the phase assemblage evolution of metapelitic rocks can be interpreted. The effect of minor components on phase equilibria is evaluated using the experimentally determined grid as a simple-system reference. The temperature at which melting occurs in metapelites is strongly controlled by the concentrations of titanium and fluorine in biotite. Pressure-temperature pseudosections presented for each of the experimental compositions show both the univariant and divariant reactions available to a particular bulk composition, clearly illustrating the possible evolution of the phase assemblage. The pseudosections also constrain the stability limits of  相似文献   

4.
Experiments show that equilibration times across Shaw hydrogen diffusion membranes are variable. Using quartz-filled platinum membranes with fugacity differentials less than 20 bars, equilibration times at 750 ° C range from 1–3 days with new membranes to greater than 10 days for membranes with extensive use. New membranes at 650 ° C equilibrate in 7 days. At these temperatures the steady state hydrogen lag pressure between the bomb and membrane is less than one bar as long as relatively new membranes are in use.These slow hydrogen equilibrations necessitate that exacting procedures be followed when using the bombs for equilibrium determinations. The initial unequilibrated hydrogen fugacity in the bomb must be higher than the final equilibrated hydrogen fugacity for a reduced starting assemblage or must be lower than the final equilibrated hydrogen fugacity if the starting assemblage is oxidized. This criterion can be met by carefully controlling the initial hydrogen/argon ratio in the pressurizing fluid, and can be verified by monitoring changes in the membrane pressure.Determinations of the Ni-NiO equilibrium at 750 °C and 850 °C at 1 kbar have been performed in order to test the method. The corrected 1 bar values, log =–15.0±0.3 and –12.8±0.2 respectively, are in excellent agreement with electrochemical cell determinations.  相似文献   

5.
In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als.The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C.We demonstrate that, even under fluid-absent conditions, a large proportion (40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites.The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.  相似文献   

6.
The inability to synthesize pure tschermakite over the range of 6–15 kbar and 700–850° C, the optimal conditions reported by Oba (1978), prompted a reinvestigation of the join tremolite-tschermakite which was originally investigated by Jasmund and Schäfer (1972). The results of syntheses performed in 10 mol% increments along this join at 12 kbar and 850° C indicate that solid solution exists only in the composition range or TS0TR100 to TS50 TR50. This was confirmed by direct chemical analyses of the synthetic amphiboles and by monitoring the shift in the d310. Syntheses along this same join at 3 kbar and 850° C reveal an even more limited range of solid solution, namely from TS0TR100 to TS10TR90, which was also confirmed by direct analyses and variation in the d310 values. Comparison of the results at 3 kbar and 12 kbar indicates a pressure dependency of Al solubility in tremolite corresponding to 0.17 Al (total)/kbar on the basis of 23 oxygens. An attempt to re-equilibrate tremolite and an Al-rich amphibole at 6 kbar and 850° C to an intermediate composition resulted in the Al-rich amphibole displaying the best evidence of re-equilibration, which was to a lower Al content. This observation supports the results obtained from the synthesis experiments by showing that decreased pressure decreases the Al solubility. It is emphasized that the Al content of natural calcic amphiboles, whose compositions are approximately modeled by the join tremolite-pargasite, is probably a function of both pressure and temperature and must ultimately be calibrated in a fashion analogous to the pyroxenes.  相似文献   

7.
Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al2O3-SiO2 (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.  相似文献   

8.
Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.
Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre-date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope-rich) garnets ( X Mg= 0.71) yet found in granulite facies rocks. A high-temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet X Mg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower-temperature ( c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.
The major Late Proterozoic ( c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe-rich metapelites and associated paragneisses in which syn- to post-deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.  相似文献   

9.
Experiments with synthetic starting materials of muscovite, phlogopite, zoisite, kyanite and quartz were performed in the pressure temperature range 10–25 kbar, 640–780° C under water excess conditions. The reaction muscovite+zoisite+quartz+vapor=liquid+kyanite was bracketed at 10.5 kbar/689–700° C, 15.5 kbar/709–731° C and 20 kbar/734–745° C. The equivalent reaction in the Mg-bearing system muscovitess +zoisite+quartz+vapor=liquid+kyanite+phlogopitess lies at the same temperature around 10 kbar and approximately 10° C higher around 20 kbar, compared with the Mg-free reaction. At slightly higher temperatures formation of melt and tremolitess was reversibly observed from the assemblage phlogopitess+zoisite +kyanite+quartz around 10.5 kbar/690–710° C, 15.5 kbar/720–750° C and 20.5 kbar/745–760° C. In the subsolidus region, the reaction muscovitess+talcss+ tremolitess=phlogopitess+zoisite+quartz+vapor were located in the range 700° C/16.7–19.0 kbar and 740° C/19.7–20.8 kbar. From these data, a wedge shaped stability field of phlogopitess+zoisite+quartz appears with a high P, T termination around 21 kbar/755° C. Muscovite+tremolite+talc or kyanite comes in at higher pressures. These phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas. Formation and crystallization of melts in rocks of a wide compositional range involving zoisite/epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified KCMASH system.  相似文献   

10.
The hydration of peridotites modelled by the system H2O-CaO-MgO-Al2O3-SiO2 has been treated theoretically after the method of Schreinemakers, and has been investigated experimentally in the temperature range 700°–900° C and in the pressure range of 8–14 kbar. In the presence of excess forsterite and water, the garnet- to spinel-peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865±5° C and 15.2±0.3 kbar. At lower pressures, a model spinel lherzolite hydrates to both chlorite- and amphibole-bearing assemblages at an invariant point located at 825±10° C and 9.3±0.5 kbar. At even lower pressures the spinel-to plagioclase-peridotite transition boundary intersects the dehydration curve for amphibole+forsterite at an invariant point estimated to lie at 855±10° C and 6.5±0.5 kbar.Both chlorite and amphibole were characterized along their respective dehydration curves. Chlorite was found to shift continuously from clinochlore, with increasing temperature, to more aluminous compositions. Amphibole was found to be tremolitic with a maximmum of 6 wt.% Al2O3.The experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting, plagioclase, talc, anthophyllite, and antigorite to obtain a petrogenetic grid applicable to peridotites, modelled by the system H2O-CaO-MgO-Al2O3-SiO2, that covers a wide range of geological conditions. Direct applications of this grid, although quite limited, can be made for ultramafic assemblages that have been extensively re-equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism.  相似文献   

11.
The PT-phase relations of a Hawaiian tholeiite with 18.2% MgO has olivine–orthopyroxene multiple saturation at 20.5 kbar and 1,550°C. This pressure is less than the pressure at the lithosphere/asthenosphere transition, and it is suggested that tholeiites with this and lesser MgO contents are fractionated. Assuming a harzburgitic residuum it is shown that Hawaiian primary tholeiites contain about 23% MgO, and are generated at 36±5 kbar and 1,680±50°C. This pressure is equivalent to a depth of 112 km, which is consistent with the thickness of the lithosphere and thermal plume modeling. The minimal MgO content of primary Hawaiian tholeiites is suggested as 19% MgO.  相似文献   

12.
The join tremolite (Tr)-pargasite (Pa) has been studied in the temperature range 750 °–1,150 ° C under a water vapor pressure of 1 and 5 kbar. There is a continuous solid solution series between the compositions Tr85Pa15 and TroPa100 at 850 ° C and 5 kbar. Tremolite and pargasite are separated by a solvus at 1 kbar and the field of tremolitic amphibole +pargasitic amphibole+vapor is present in the region between Tr90Pa10 and Tr10Pa90 at 800 ° C. The phase assemblages at 850 ° C and 1 kbar change as follows with increasing pargasite component; clinopyroxene +orthopyroxene+quartz+vapor, tremolitic amphibole+vapor, tremolitic amphibole+clinopyroxene +forsterite+plagioclase+vapor, tremolitic amphibole+pargasitic amphibole+vapor, and pargasitic amphibole+vapor. The petrological significance of amphibole pairs in metamorphic rocks is discussed on the basis of the experimental results.  相似文献   

13.
Coexisting feldspars from across 2,000 km2 of the granulite facies Oaxacan Complex, southern Mexico exhibit variable amounts of solid solution from nearly binary (Ab-An and Ab-Or) to substantially ternary (Ab-An-Or). Reintegrated analyses of 21 coarsely exsolved perthite (AF)-plagioclase (PL) pairs yield AF=Or30–63 Ab30–56An2–15 and PL=Or1–2Ab70–84An11–28. These data have been used to test existing two feldspar geothermometers for this extended composition range.For all compositions, temperature estimates show relatively little spread in value (660° to 795° C, 7 kbar) using the Haselton et al. (1983) calibration (HHHR). These temperatures are in fair agreement with estimates of 750±40° C for feldspar pairs with nearly binary compositions using the Stormer (1975) thermometer (STO). However, STO temperatures increase significantly (to 990° C) with increasing ternary solid solution in AF, suggesting that thermometers derived for binary systems are inaccurate for ternary compositions. Isotherms drawn from HHHR which take into account variable anorthite solution in alkali feldspar show that estimated temperature decreased by 50–100° C for each 5 mole percent anorthite in alkali feldspar.Experimentally determined solvus relations (Seck 1971) require feldspars with significant ternary solid solution to have crystallized or to have equilibrated at higher temperature than feldspars with more binary compositions. However, petrographic and field relations of ternary and binary feldspars in the Oaxacan Complex suggest they were all equilibrated at similar metamorphic pressures and temperatures and do not support a model where ternary feldspars have preserved higher premetamorphic temperatures. The composition of coexisting feldspars from other Precambrian granulite-facies terranes are also inconsistent with Seck's (1971) results. Hence, thermometers which fit Seck's solvus relations may not yield accurate temperatures in high grade metamorphic terranes. Parallel tie-lines for ternary and binary feldspars in the Oaxacan Complex and the consistency of inferred temperatures (HHHR) for many granulite terranes suggest that estimation of temperature using tie-line slopes rather than solvus width may yield more accurate results for these samples.Peak metamorphic conditions in the Oaxacan Complex are inferred to have been 730±50° C, 7±1 kbar. Pressure estimates from four garnet-plagioclase barometers show good agreement. Results of feldspar thermometry are consistent with diopside-forsterite equilibria in marbles which restrict T=720–765° C at P=7 kbar.  相似文献   

14.
Mercier's thermobarometer (Mercier 1980) and Saxena's thermometer (Dal Negro et al. 1982) were applied to single pyroxenes of both porphyroclastic (PF) and protogranular (PR) spinel peridotitic nodules enclosed in alkaline products related to necks of North-Eastern Brazil. Intercrystalline temperatures obtained using both orthopyroxene (opx) and clinopyroxene (cpx) compositions were in agreement, and were lower in protogranular than in porphyroclastic nodules (1051±57 and 1266±19°C respectively). In contrast, pressure estimates using cpx and opx were conflicting, in particular as regards PF nodules. In fact for PF nodules cpx compositions point to 27–32 kbar (mean 29±2) whereas opx compositions point to 17–19 kbar (mean 18±1). Conversely, PR nodule cpx and opx compositions point to similar values 17–24 and 15–18 kbar respectively (mean 19±4). The result obtained for PF nodules using cpx composition clearly contrasts with petrographic evidence and it is due to the peculiar composition of PF cpx (e.g. low Ca content, from 0.645 to 0.737 atoms per formula unit, a.f.u.) that strongly affects the barometric formulation. The PR and PF cpxs reveal similar mean intracrystalline temperature estimates (712±112 and 778±217°C, respectively). These, considering the difference of about 200°C in the intercrystalline temperature estimates, indicate that the exchange cation reaction between the M1 and M2 sites was a faster process in PF than in PR cpx, favoured by the low Ca content of PF cpx. Thus alternatively, the composition of PF cpx, characterized by a high rate of Ca » Mg substitution in the M2 site, may not affect the intercrystalline temperature. Therefore the difference of about 200°C found in intercrystalline temperatures between PR and PF cpxs, in spite of their same pressure values, may be interpreted as indicating an anomalous temperature gradient in the region.  相似文献   

15.
Clear evidence exists for a cordierite breakdown reaction to amphibole-kyanite-quartz in high-grade metamorphic rocks of the Arunta Complex, Australia. Using the natural minerals this reaction has been duplicated experimentally. It proceeds over a divariant band with a slope of 12±4 bars/°C, occurring between 8 and 10.4 kb at 750° C and between 9.5 and 11.3 kb at 850°C. The reaction is cut off at low temperature by the appearance of talc and at high temperature by the appearance of orthopyroxene. The maximum pressure stability of the amphibole-kyanite-quartz assemblage is about 20 kb. These data suggest that the natural rock was subjected to pressures of at least 8 kb at 750–850° C for high water fugacities. Other experimental data on the hydration of hypersthene and cordierite-hypersthene stability, point to a temperature below 820° C and an upper pressure limit of 9.5 kb at 750–820°C. Experiments at \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) total indicate that the breakdown of cordierite to amphibole-kyanite-quartz is a hydration reaction, and occurred in the natural rock as a result of an increase of water fugacity at constant total pressure (8–9.5 kb) and temperature (750–820°C).  相似文献   

16.
K–Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.  相似文献   

17.
We have simulated the dehydration-melting of a natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) in piston-cylinder experiments at 10 kbar and 750–1000°C, for 1–9 days. The solidus temperature is lower than 750°C; garnet appears at 850°C. The overall reaction is: Hb+PL+Cpx+Al-Hb+Ca-Hb+Ga+Opx. Three stages of reaction are: (1) melting dominated by the growth of clinopyroxene and garnet, with little change in composition of liquid or garnet, (2) a reversal of this reaction between 875°C and 900°C, with decreases in the amounts of liquid and garnet, and (3) a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase. Garnet is enriched in pyrope and zoned from Fe-cores to Mg-edges (range 3 mol % pyrope); liquid composition is enriched first in An (to 950°C) and then in Ab. The liquids are more calcic and aluminous than natural tonalites, which is attributed to the plagioclase composition (An90). The formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite — not H2O-saturated conditions. The results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet-absent field (900–1000°C), but with a solidus backbend at 7–9 kbar, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation in a closed system must make H2O available for H2O-undersaturated melting right down to the H2O-saturated solidus, below 700°C, which defines a large low-temperature PT area where hydrous granitoid melts can be generated with residual garnet and hornblende.  相似文献   

18.
The polymorphic transformation between synthetic pyroxmangite and rhodonite of MnSiO3 composition has been reversibly bracketed in the presence of water at 3 kbar (between 425 ° and 450 ° C), 6 kbar (between 475 ° and 525 ° C), 20 kbar (between 500 ° and 900 ° C), 25 kbar (between 800 ° and 900 ° C) and 30 kbar (between 900 ° and 1,000 ° C), using standard cold-seal pressure vessels and piston cylinder apparatus. Oxygen fugacities buffered by the bomb walls and piston-cylinder cell assemblies sufficed to keep manganese in the divalent state. Pyroxmangite of MnSiO3 composition is shown to be the high-pressure, low-temperature polymorph with respect to rhodonite of the same composition. It is a stable phase at atmospheric pressure below 350–405 ° C.X-ray data for synthetic pyroxmangite are presented. The unit-cell parameters (a0=6.717(2) Å, b0=7.603(1)Å, c0=17.448(5) Å, =113 °50(1), = 82 °21(2), =94 °43(1); space group P-1) give a unit-cell volume (807.5(0.3) Å3) which, in accordance with other recent least squares lattice refinements of hydrothermally synthesized material, is slightly smaller than that obtained by single-crystal work on anhydrously synthesized material.Application of the present results to natural rocks is severely restricted due to the great variety and extent of cationic substitutions observed in natural pyroxenoids. The univariant polymorphic transformation determined for the MnSiO3 composition is thus replaced in natural systems by a divariant field in which pyroxmangite and rhodonite of differing composition will stably coexist.  相似文献   

19.
Activity-composition relationships for pyrope-grossular garnet   总被引：1，自引：0，他引：1  
Activity coefficients () for grossular in pyrope-grossular garnet have been determined experimentally using the divariant assemblage garnet-anorthite-sillimanite (kyanite)-quartz. Values of for garnets with 10–12 mole % grossular have been obtained at 1000 °, 1100 °, 1200 ° and 1300 ° C at pressures between 15 and 21 Kb. The data are consistent with a symmetrical regular solid model for grossular-pyrope solid solutions. The interaction parameter (W) increases linearly with decreasing temperature and is given by W = 7460-4.3 T cals (T in °K). A solvus in the pyrope-grossular solid solution is predicted with a temperature of critical mixing of 629°C±90 ° C.  相似文献   

20.
Fluid-solid-solid dihedral angles in the NaCl-H2O-CO2-calcite-dolomite-magnesite system have been determined at pressures ranging from 0.5 to 7 kbar and temperatures from 450°C to 750°C. At 1 kbar and 650°C, both dolomite and magnesite exhibit a dihedral angle minimum for intermediate H2O-CO2 fluids similar to that previously determined by the present authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60° for both dolomite and magnesite for all fluid compositions. Calcite-calcite-brine dihedral angles at 650°C have been determined in the pressure range 1–5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of about 65° for strong brines at pressures above 2 kbar. There is a general increase of angle with increasing pressure which is most marked for strong brines. A positive correlation of angle with pressure is also observed in calcite-H2O-CO2 fluids, the position of the minimum moving towards higher angles and towards H2O-rich fluids with increasing pressure. The permeability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1.5 kbar. The results demonstrate that the permeability of carbonates to grain edge fluid flow is only possible at low pressures and for fluids of restricted H2O-CO2-NaCl compositions. However, geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted. From examination of published studies of infiltrated carbonates we conclude that deformation plays a critical role in enhancing carbonate permeability. Possible mechanisms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, and dynamically maintained transient grain boundary fluid films.  相似文献   

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