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1.
Recent activity–composition models for clinopyroxene and amphibole are revised to provide better consistency with observed phase relations in natural rocks. For clinopyroxene, the calibration in NCFMAS is retained, but the incorporation of acmite is revised to improve the partitioning of ferric iron between coexisting clinopyroxenes. For amphibole, the NCFMASH calibration is retained, but the addition of ferric iron is changed to provide consistency with the clinopyroxenes. The thermodynamics of orthoamphibole (gedrite) is also adjusted to resolve an unrelated inconsistency. The effects of these improvements are illustrated through comparison of calculated pseudosections produced with the existing and new models with natural data from lawsonite eclogites.  相似文献   
2.
Phase diagrams involving solid solutions are calculated by solving sets of non-linear equations. In calculating P–T  projections and compatibility diagrams, the equations used for each equilibrium are the equilibrium relationships for an independent set of reactions between the end-members of the phases in the equilibrium. Invariant points and univariant lines in P–T  projections can be calculated directly, as can coordinates in compatibility diagrams. In calculating P–T  and T–x / P–x pseudosections – diagrams drawn for particular bulk compositions – the equilibrium relationship equations are augmented by mass balance equations. Lines in pseudosections, where the mode of one phase in the lower variance equilibrium is zero, and points, where the modes of two phases are zero, can then be calculated directly. The software, THERMOCALC, allows the calculation of these and a range of other types of phase diagram. Examples of phase diagrams and phase diagram movies, with instructions for their production, along with the THERMOCALC input and output files, and the MathematicaTM functions for assembling them, are presented in this paper, partly in hard copy and partly on the JMG web sites (http://www.gly.bris.ac.uk/www/jmg/jmg.html, or equivalent Australian or USA sites).
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We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end-members made consistent with over 200 P–T–XCO2fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron-bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak-substituted end-members have been derived for iron and magnesium end-members of chlorite, talc, muscovite, biotite, pyroxene and amphibole. For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end-members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end-members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.  相似文献   
5.
The sea-floor morphology of two pronounced across-fjord bedrock thresholds located at the mouths of Ofotfjorden and Tysfjorden, northern Norway, has been analysed based on swath bathymetry and seismic data. The Younger Dryas ice front was located here during the recession of one of the large palaeo-ice streams of the Fennoscandian Ice Sheet. The thresholds are several kilometres long and wide, rising to several hundred metres above the adjacent sea floor, and the slopes are steep, up to 25°. The Ofotfjorden threshold is draped by acoustically discontinuous to chaotic sediments partly infilling the bedrock relief. A pattern of well-developed, subglacial bedforms (e.g. crag-and-tail formations, drumlins and glacial lineations) on top of both thresholds suggests fast-flowing ice. A series of smaller transverse ridges is identified on both thresholds and probably records ice-front oscillations during the final deglaciation. The distal parts of the sediments have been remobilized by slides that occurred after glacial retreat from the thresholds. Earthquake activity due to the isostatic rebound following ice retreat from this area was the most likely triggering mechanism for the slides. The location of the ice front on a prominent bedrock threshold indicates that the basin configuration was important in locating the maximum position of the climatically induced re-advance, i.e. a topographic control on the maximum Younger Dryas position in the Ofotfjorden and Tysfjorden area is suggested.  相似文献   
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A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H2O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe3+ content in clinopyroxene. Assuming no Fe3+ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe3+‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H2O‐undersaturated conditions (c. 1.3 mol.% H2O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H2O saturated, but if it is H2O‐undersaturated it contains assemblages with orthopyroxene. Increasing H2O undersaturation causes an earlier transition to SiO2 undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.  相似文献   
8.
An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.  相似文献   
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10.
The pressure dependence of the zirconium-in-rutile thermometer   总被引:19,自引:1,他引:19  
The solubility of ZrO2 in rutile is strongly temperature-dependent and has been identified as a potentially powerful thermometer when the rutile coexists with an appropriate buffer assemblage, e.g. zircon + quartz. In combination with experimental data at 10 kbar, previous consideration of data on natural rutile has not identified a pressure dependence for the thermometer. However, the expected volume change as a result of substitution of the larger Zr4+ cation for Ti4+ suggests that the Zr content of rutile should decrease with increasing pressure. To investigate the pressure dependence of the thermometer, piston cylinder (at 10, 20 & 30 kbar) and 1 atm furnace experiments were performed in the system ZrO2-TiO2-SiO2. The solubility of ZrO2 in rutile, in the presence of zircon and quartz was reversed at each pressure value. From these experiments, the thermodynamics of the end-member reaction ZrSiO4 = SiO2 + ZrO2 (in rutile) have been determined. There is a secondary pressure effect accompanying the primary temperature dependence of the Zr content of rutile. New thermometer equations are, in the α -quartz field: in the β -quartz field and in the coesite field in which φ is ppm Zr, P is in kbar and R is the gas constant, 0.0083144 kJ K−1. Thermometric results using these equations are shown for a range of geological settings.  相似文献   
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