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1.
Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.  相似文献   

2.
Biaxiality in calcite (2 V10°, less commonly 15°) is widespread in metamorphic rocks of all grades and in carbonatites. It is a minor effect of late strain, in many cases not directly related to visible {01¯12} twinning, and partially independent of lattice bending. For values of 2V up to about 15° the optic axial plane coincides, within limits of error of measurement, with {10¯10}. For those rare crystals with 2V in the range 25–50°, there is a real departure—never greater than 10°—from this orientation; and biaxiality is certainly in some, probably in all such cases, confined to domains of overlap of single thin twin lamellae.Microscopic procedure is described for determining the orientation of the optical indicatrix relative to crystallographic coordinates.  相似文献   

3.
We report the result of H2O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded 40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only 5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded 50 vol% and 7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by 25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of minimum-temperature granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.  相似文献   

4.
A basanitoid flow of Miocene age, exposed near the West Kettle River, 25 km southeast of Kelowna, British Columbia, contains abundant ultramafic and mafic nodules. The subangular nodules are 1–20 cm across and typically show granular textures. A study of 250 nodules indicates that spinel lherzolite (60%) is the dominant type with subordinate olivine websterite (10%), websterite (7%), clinopyroxenite (4%), wehrlite (4%), pyroxene gabbro (4%), dunite (2%), harzburgite (1%) and granitic rocks (8%). Ultramafic nodules are of two types. Most of the wehrlites and clinopyroxenites belong to the black pyroxene (aluminous clinopyroxene) series, whereas the other clinopyroxene-bearing nodules belong to the green pyroxene (chromian diopside) series. Some spinel lherzolite nodules have distinctive pyroxene- and olivine-rich bands. Microprobe analyses of the constituent minerals of more than thirty nodules from the green pyroxene series indicate that grain to grain variations within individual nodules are small even when banding is present. Olivine, orthopyroxene, clinopyroxene and spinel in spinel lherzolite have average compositions of Fo90, En90, Wo47Fs5En48, Cr/(Cr+ Al+Fe3)=0.1 and Mg/(Mg+Fe2+)=0.8. Equilibration temperatures, which were calculated using the two pyroxene geothermometer of Wells (1977), range between 920–980° C. Based on published phase stability experiments, pressures of equilibration are between 10–18 kbar. In summary, the upper mantle beneath southern British Columbia is dominated by spinel lherzolite but contains some banding on a scale of cm to meters. The temperature in the upper mantle is 950° C at a depth of 30–60 km.On leave from the Geological Institute, University of Tokyo, Japan  相似文献   

5.
The solubility of water in melts in the NaAlSi3O8–H2O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at 0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to 15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to 12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above 0.45 GPa; it probably changes with T from retrograde below to prograde above 900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at 1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid.  相似文献   

6.
Summary Two assemblages have been identified in spinel-bearing metapelites from Rayagada, in the Eastern Ghats Belt, India: the first, an iron-rich assemblage, is characterized by iron-rich spinel and garnet; the second, iron-poor assemblage, by iron-poor spinel and garnet, together with cordierite and hematite. Garnet and sillimanite coronas around spinel show elongation parallel to the dominant fabric in the iron-rich assemblage but not in the iron-poor one. Both textures suggest the corona forming retrograde reaction spl + qtz = grt + sil. The P-T conditions for these corona-forming reactions are estimated at 950°C and 8.7-9.0 kbar for the iron-rich, and 800°C and 8 kbar for the iron-poor assemblage. Thermobarometric results and observed phase relations, using the model system FeO-MgO-Al2O3-SiO2 (FMAS) indicate near-isobaric cooling from 950° to 800°C and subsequent decompression from 8 to 6.5 kbar. The corona-forming event in the iron-rich assemblage has been correlated with the dominant second phase of deformation (D2), but that in the iron-poor assemblage apparently postdates D2.
Der retrograde P-T Pfad Spinell führender Metapelite in Rayagada, östliche Ghats, Indien
Zusammenfassung Zwei unterschiedliche Spinell-führende Mineralvergesellschaftungen in Metapeliten werden aus Rayagada in den östlichen Ghats Indiens beschrieben. Die erste, eine eisenreiche, ist durch Fe-reichen Spinell und Granat, die zweite eisenarme, durch Fe armen Spinell, Granat, Cordierit und Hämatit charakterisiert. Granat und Sillimanit Coronas um Spinell sind nur in der eisenreichen Vergesellschaftung parallel zum dominierenden metamorphen Gefüge elongiert, nicht aber in der eisenarmen. Beide Texturen belegen die retrograde Reaktion Spl + Qtz = Grt + Sil. Die P-T Bedingungen der Koronareaktionen in der Fe-reichen Assoziation werden mit ca. 950°C und 8.7-9.0 kbar, die in der Fe-armen mit ca. 800°C und 8 kbar abgeschätzt. Die Ergebnisse der Geothermobarometrie und die beobachteten Phasenbeziehungen im Modellsysem FeO-MgO-Al2O3-SiO2 (FMAS) belegen eine mehr oder weniger isobare Abkühlung von ca. 950 bis 800°C mit anschließender Dekompression von ca. 8 bis 6.5 kbar. Die koronabildende Raktion in der eisenreichen Vergesellschaftung wird mit der dominierenden zweiten (D2) Deformationsphase korreliert, überdauert diese aber in den Fe-armen Gesteinen.

With 5 Figures  相似文献   

7.
The equilibrium distribution of hydrous silicic melts in polycrystalline aggregates of quartz was characterized in a series of partial melting and melt distribution experiments in the systems quartz-albite-orthoclase-H2O and quartz-anorthite-H2O, at 650 to 1000 MPa and 800 to 900° C. Near-equilibrium textures in these experiments are characterized by very low quartz-quartz-melt wetting angles, and by a substantial number of thin melt films along grain boundaries. Wetting angles in the H2O-saturated experiments are as follows: 18° at 800° C-1000 MPa, and 12° at 900° C-1000 MPa in the granitic system; 18° at 850° C-650 MPa, 15° at 900° C-650 MPa, and 15° at 900° C-1000 MPa in the quartzanorthite system. In the granitic system at 900° C-1000 MPa, a decrease of H2O content in melt from 17 wt% (at saturation) to 6 wt%, results in a slight increase of wetting angle from 12° to 16°. These low wetting angles — and the observation that many grain boundaries are wetted by melt films-indicate that the ratio of quartz-quartz to quartz-melt interfacial energies (ss/s1) is high: 2. Secondary electron imaging of fracture surfaces of melt-poor samples provided a three-dimensional insight into the geometry of melt; at low melt fraction, melt forms an interconnected network of channels along grain edges, as predicted for isotropic systems with wetting angles below 60°. This high-permeability geometry suggests that the segregation of granitic melts is not as sluggish as previously anticipated; simple compaction calculations for a permeability range of 10-12 to 10-9 m2 indicate that segregation may operate at low to moderate melt fractions (below 30 vol. %), within relatively short time-scales, i.e., 105 to 106 years. Quartzmelt textures show significant deviations from the equilibrium geometries predicted for isotropic partially molten systems. The most consistent deviation is the pervasive development of crystallographically-controlled, planar faces of quartz; these faces provide definitive evidence for non-isotropic quartz-melt surface energy. For most silicates other than quartz, the grain-scale distribution of partial melts deviates even more significantly from equilibrium distributions in isotropic systems; accordingly, in order to describe adequately melt distributions in most natural source regions, the equilibrium model should be modified to account for anisotropy of solid-liquid interfacial energy.Contribution CNRS-INSU-DBT no 651  相似文献   

8.
The syenitic layered series in the Klokken intrusion is surrounded by a zone (500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro (200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series.Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at 1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at 900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.CRPG contribution 729  相似文献   

9.
A pink-colored (HUE 5R-7/4), predominantly calcium saturated, clay mineral from Tirebolu, NE-Turkey, is shown to be a dioctahedral, high-charge manganiferous smectite. It is probably an alteration product of volcanic tuffs of rhyodactitic composition. Ionic formula: (Si7.71Al0.29)IV(Al3.04Fe 0.12 3+ Fe 0.02 2+ Mg0.92Mn 0.07 2+ )VI O20 (OH)4 0.79 M+ Cation exchange capacity: 92 me/100 g (air dry sample). Unit cell parameters (ca. 20 ° C; ca. 40% RH): a09.02 Å b05.21 Å Mg++... Mn-smectite: c014.65 Å Ca++... Mn-smectite: c014.83 Å K+... Mn-smectite: c012.10 Å
Zusammenfassung Ein rosa (HUE 5R-7/4), weitgehend Calcium-gesättigtes Tonmineral von Tirebolu (NO-Türkei) wurde als ein dioktaedrischer, hochgeladener manganhaltiger Smektit identifiziert. Er ist wahrscheinlich ein Umwandlungsprodukt eines rhyodazitischen Tuffs. Strukturformel: (Si7.71Al0.29)IV(Al3.04Fe 0.12 3+ Fe 0.02 2+ Mg0.92Mn 0.07 2+ )VI O20 (OH)4 0.79 M+ Kationaustauschkapazität: 92 me/100 g (luftgetrocknet). Gitterkonstanten (ca. 20° C; ca. 40% RF): a09.02 Å b05.21 Å Mg++... Mn-Smektit: c014.65 Å Ca++... Mn-Smektit: c014.83 Å K+... Mn-Smektit: c012.10 Å
  相似文献   

10.
Celadonite from the northwestern Mojave Desert area of California was examined by detailed Mössbauer spectroscopy at temperatures from 10 K to 400 K. In addition to the predominant Fe3+ doublet with isomer shift 0.4 mm s–1 and quadrupole splitting 0.4 mm s–1, another Fe3+ doublet with 0.4, 1.2 mm/s and two Fe2+ doublets with 1.1, 1.7, 2.7 mm s–1 at 300 K were distinguished. The minor Fe3+ component is ascribed to dehydroxylated surface sites. Most of the remaining Fe(90%) is M2 cis-OH octahedral in an ordered M+–M2+ array. However, about 10% is M1 trans-OH Fe2+. Isomer shift vs. T gives Debye temperatures of 570 K for Fe3+ in M2 and 380 K for both Fe2+ sites, indicating greater vibrational freedom for Fe2+. Quadrupole splitting vs. T for Fe2+ gives a valence electronic energy splitting of 760 cm–1 between the ground and first excited state for M2. The M1 sites have a more drastic variation in vs. T which indicates not only a lower first excited state but a rhombic distortion at these sites. A proposed explanation is a neighboring M2 site vacancy. The soil clay formed from this celadonite, which is mostly Fe-rich smectite, was also studied by Mössbauer spectroscopy. About half the Fe2+ has been oxidized in the clay, but the isomer shifts and quadrupole splittings are essentially the same as in the original celadonite. A texture orientation in the clay absorber was detected by measuring the absorber at 55° to the source radiation. This texture effect produces asymmetric doublets in the usual 90° measurement.  相似文献   

11.
Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of 650°–700° C at 12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.  相似文献   

12.
Relatively soon (0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions.Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite.Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.  相似文献   

13.
Sub-potassic nephelines in the system NaAlSiO4(Ne)-KAlSiO4(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO4) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion.  相似文献   

14.
Summary High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotite-dehydration melting at 820°C and 0.7–0.8GPa, melt segregation and near-isothermal decompression to 0.4–0.5GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between 400 and 230°C and fluid pressures between 0.3 and 0.1GPa. Metamorphic peak conditions are dated at 331–327Ma, while hydrothermal retrogression occurred significantly later at 315–310Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low fO2, very low XCO2 (<0.002), variable activities of Ca2+, K+, Na+ and H+ and relatively high activities of H2S and CH4. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite+calcite+quartz or (iii) K-feldspar+calcite+quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO2.  相似文献   

15.
Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with D –34 to –49. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with D –50 to –65. All fluids contain trace N2, CO2 and probably CH4. The carbonates have 13C –8 to –14. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich ( –35) low fO2, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.  相似文献   

16.
Zusammenfassung Die Phasenbeziehungen im Cu-W-S-System wurden zwischen 900 °C und Zimmertemperatur durch DTA- und Abschreckungsexperimente in Quarzglas-und in unter Druck kollabierenden Goldampullen, sowie durch eine Reihe von Verwitterungsversuchen und Umsetzungen in wässrigen Lösungen untersucht. Die im System auftretenden Verbindungen sind Cu2S (Kupferglanz), Cu1.97S (Djurleit), Cu1.75S (Anilit), Cu1+xS (blaubleibender Covellin), CuS (Covellin) und oberhalb 70°C Cu1.8S (Digenit), sowie WS2 (Tungstenit). Es gibt keine ternären Verbindungen. Von allen genannten Phasen ist nur der Tungstenit über den gesamten untersuchten Temperaturbereich stabil. Das System zeigt bei 900 °C neben Schwefelschmelze (L1) eine Sulfidschmelze (L2). Es handelt sich um das oberhalb 813 °C auftretende Monotektikum im Randsystem Cu-S, welches im ternären System 0.5 Gew.-% WS2löst. Die Phase WS2 koexistiert bei 900 °C mit L1, L2 und mit der bei dieser Temperatur lückenlosen Kupferglanz-Digenit-Mischkristallreihe sowie mit W. Außerdem besteht eine Konode zwischen W und Cu2S. Das gegenseitige Lösungsvermögen der Verbindungen ist selbst bei 900 °C gering. Während Digenit 0.5 Gew.-% WS2 in fester Lösung aufzunehmen vermag, beträgt umgekehrt die Löslichkeit von Kupfersulfid in WS20.2%. Die Phasenbeziehungen unter 900 °C sind charakterisiert durch das Stabilwerden des Covellins bei 507 °C. Kurz unterhalb dieser Temperatur werden WS2 und CuS nebeneinander stabil. Die Mischungsreihe zwischen Digenit und Kupferglanz ist unterhalb 430 °C nicht mehr lückenlos. Das System Cu-W-S zeigt daher bei 400 °C Konoden von WS2 zu Covellin, Digenit und Kupferglanz. Unterhalb 70 °C zerfällt der mit Tungstenit koexistierende Digenit zu Anilit und Djurleit. Bei künstlicher Verwitterung von Kupferglanz oder Digenit mit WS2-Einschlüssen konnten durch teilweise Oxidation mit verdünnter Fe-Sulfat- oder Cu-Sulfatlösung die Kupfersulfide in blaubleibenden Covellin überführt werden, während Tungstenit unter gleichen Bedingungen den Agenzien widerstand, wodurch sich die Koexistenz zwischen Cu1+xS und WS2 nachweisen ließ. Die bei niedrigen Temperaturen mit Tungstenit im Cu-W-S-System koexistierenden Phasen sind: Kupferglanz, Djurleit, Anilit, blaubleibender und normaler Covellin; bei Spuren von im Digenitgitter gelösten Fe tritt Anilit nicht auf, statt dessen ist Digenit mit Tungstenit stabil. Ein neues natürliches Tungstenitvorkommen (Kipushi/Katanga) wird beschrieben, das Mineral ist orientiert in massivem Kupferglanz eingewachsen (Abb. 4 und 5).
The phase relations in the Cu-W-S-system were investigated at various temperatures ranging from almost room temperature up to 900 °C. The experiments were performed in evacuated silica glass tubes with a minimum vapor space. At low temperatures alteration experiments were carried out in water solutions containing copper(II)-sulfate or iron(III)-sulfate. No ternary phase exists in the system. At 900 °C Cu2S and W are coexisting phases. Tie lines connect WS2 with the digenite-chalcocite solid solution and with a sulfuric liquid containing 0.5 wt.-% WS2. Below 813 °C the sulfuric liquid disappears in the Cu-S system (monotectic). On continuous cooling CuS will appear at 507 °C in the Cu-S system and shortly below this temperature covellite coexists with tungstenite. At temperatures below 70 °C tungstenite can coexist with covellite, blaubleibender covellite, anilite, djurleite, and with chalcocite in the pure system. If traces of iron are present anilite will not be formed and digenite remains stable with tungstenite. A new occurrence of tungstenite was observed from the Kipushi mine (Katanga), displaying excellent intergrowth with chalcocite (fig. 4 and 5).
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17.
We report the T-X(H2O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths. Under H2O-saturated conditions, a melting interval of 135° C separates the solidus at 610° C from the liquidus at 745° C. H2O-saturation at 15 kbar occurs at approximately 15.5 wt % H2O. The H2O-undersaturated liquidus extends along a curved path from 745° C at 15.5 wt % H2O to 1100° C at 2% H2O. Lower H2O contents were not investigated. At low H2O contents (<6%) sodic plagioclase (Pl, An32) is the liquidus phase followed at lower but still near-liquidus temperatures by quartz (Qz) and then garnet (Ga). At 6% H2O, Ga replaces Pl on the liquidus and is joined at slightly lower temperatures by Pl and hornblende (Hb). The field for liquidus Ga extends to only 7.5% H2O where it is replaced by Hb which is the liquidus phase up to 13% H2O. At all higher H2O contents, epidote (Ep) is the first phase to crystallize, followed by biotite (Bi) at slightly lower temperatures. Following the standard inverse approach, the near-liquidus phase assemblages are interpreted as potential residues from which trondhjemitic melts could be extracted. At high melt H2O contents (>7%), mafic residues consisting of some combination of Hb, Ga, Ep, and Bi are possible and could correspond to amphibolitic source rocks. At lower melt H2O contents (< 5%), possible residues consist of Na-Pl+Qz±Ga and could correspond to an earlier generation of tonalitic-trondhjemitic rocks. However, such residues would not impart the highly fractionated REE patterns characteristic of Archean trondhjemites. If a first generation of tonalitic-trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth, it would crystallize to an assemblage consisting almost entirely of Na-Pl+Qz with highly fractionated REE patterns. These rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated REE patterns because neigher Pl nor Qz is capable of significantly fractionating HREE from LREE.  相似文献   

18.
Alkali feldspars in the Klokken layered syenite (South Greenland) are optically either coarse, turbid, deuteric patch perthites or clear, unaltered, fine braid microor cryptoperthites. Irregular, clear volumes can be found in most turbid grains. Electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion, from 40 nm to 300nm. They are composed of low albite and diagonally associated maximum microcline. A secondary coarsening is responsible for optically visible braid microperthites.The layered series consists of interleaved, granular syenites and coarser grained, laminated syenites with mineral layering. Microprobe analyses show a downward decrease in Ca and Ba in the granular syenite feldspars consistent with chilling from the roof inwards. Most laminated syenite feldspars contain 1% An increasing to 5% in mafic layers.The periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites. Periodicity is not influenced by Ab:Or or An content, but high-An feldspars have continuous zig-zag intergrowths, while others are more lozengeshaped. Lamellar periodicities largely reflect cooling rate in the pluton. Cooling times in the exsolution interval were probably about 103–104 years at the top of the series, whereas existing experimental data suggest that the periodicities would be reached in a few days or years. The periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition, suggesting that little or no coarsening occurred despite the long cooling times. Coarsening did occur lower in the pile. Framework ordering and twinning must slow or stop coarsening.The difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma. The exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites. Small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate.  相似文献   

19.
An analcime-rich mugearite near Spring Mount in north-eastern New South Wales contains dominant kaersutite, lesser anorthoclase and rare Ti-rich mica megacrysts, and also rare Group I peridotite xenoliths and granitoid inclusions. Mineralogical, experimental and Sr and Nd isotopic data are interpreted to indicate that the megacrysts are cognate with their host and that megacryst precipitation occurred at pressures in the vicinity of 1 GPa. Isotopic and other data indicate minimal crystal contamination of the melt. The generation of this evolved lava (100 Mg/(Mg+Fe2+), M55; differentiation index DI52–56) by high-pressure amphiboledominated fractionation of a mantle-derived hydrous alkali basaltic parent (M70) is inconsistent with observed megacryst abundances (<3%) and absence of Group II cumulates, and with the magnitude of the crystal extracts (45% crystals) indicated by crystal fractionation models. These data suggest the mugearite is not a high-pressure differentiate of a hypothetical primary parental magma (M70). Alternatively, its composition might approximate that of a primary magma derived from a relatively Fe-rich amphibole lherzolite mantle source.  相似文献   

20.
Carbon and oxygen isotopic covariations in hydrothermal calcites   总被引：5，自引：0，他引：5  
Isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes: (1) mixing between two different fluids which leads to the precipitation of calcite; (2) mixing between fluid and rock: (a) calcite precipitation due to fluid/rock interaction, (b) secondary alteration of primary calcite by interaction with a subsequent fluid. The models are derived from mass balance equations. A distinction among the three mixing processes can be made on a 13C vs 18O diagram, which places important constraints on the genesis of hydrothermal mineralization. The variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock, temperature, and dissolved carbon species. Owing to significant temperature-dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid, the mixing processes may be distinguished by telltale patterns of isotopic data in 13C vs 18O space. In particular, caution must be exercised in postulating the fluid mixing as the cause for mineral deposition. This is demonstrated for hydrothermal Pb-Zn deposits in the western Harz Mountains, Germany. A positive correlation between 13C and 18O values is observed for calcites from the Bad Grund deposit in the Upper Harz. Two sample profiles through calcite veins show similar correlations with the lowest -values at the center of the veins and the highest -values at the vein margins. Because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike, it is assumed that a fluid/rock interaction is responsible for the observed correlation and thus for the precipitation of calcite. A deep-seated fluid is inferred with a 13C value of — 7% and a 18O value of +10%., as well as H2CO3 as the dominant dissolved carbon species; precipitation temperatures of the calcites are estimated to be about 280 170°C. Quite different isotopic distributions are observed for calcites from the St. Andreasberg deposit in the Middle Harz. An alteration model is suggested based mainly on the isotopic distribution through a calcite vein. In addition to a primary fluid which has the same isotopic composition as that in the Bad Grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides, an evolved, HCO 3 - -dominant subsurface fluid with 13C about -20 — 15% and 18O 0% is deduced to alter the primary calcite at low temperatures of 70 40°C.  相似文献   

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