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1.
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–XCO2–fO2 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   

2.
A thermodynamic model for haplogranitic melts in the system Na2O–CaO–K2O–Al2O3–SiO2–H2O (NCKASH) is extended by the addition of FeO and MgO, with the data for the additional end‐members of the liquid incorporated in the Holland & Powell (1998) internally consistent thermodynamic dataset. The resulting dataset, with the software thermocalc , is then used to calculate melting relationships for metapelitic rock compositions. The main forms for this are P–T and T–X pseudosections calculated for particular rock compositions and composition ranges. The relationships in these full‐system pseudosections are controlled by the low‐variance equilibria in subsystems of NCKFMASH. In particular, the solidus relationships are controlled by the solidus relationships in NKASH, and the ferromagnesian mineral relationships are controlled by those in KFMASH. However, calculations in NCKFMASH allow the relationships between the common metapelitic minerals and silicate melt to be determined. In particular, the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks, particularly relating to open‐system behaviour of rocks under high‐grade conditions.  相似文献   

3.
New heat capacity measurements and cell volume data are presented for a very magnesian glaucophane from a Tauern Window eclogite. These data are combined with estimated entropy, thermal expansion, and compressibility data to generate an enthalpy of formation for glaucophane from experimentally determined phase equilibria. The data are supported by preliminary experiments of the author and provide consistent calculations on the pressure of formation of the Tauern eclogites and on the position of the blueschist-greenschist transformation reaction as studied experimentally by Maruyama et al. (1986). The resulting thermodynamic data for glaucophane may be combined with the dataset of Holland and Powell (1985) to calculate phase relations for blueschists and eclogites. The stability of magnesian glaucophane lies in the pressure range between 8 and 32 kbars at 400° C and between 13 and 33 kbars at 600° C, and the unusual eclogite assemblage of glaucophane+kyanite from the Tauern Window is restricted to pressures above 20 kbars at high water activity.  相似文献   

4.
Abstract This, the second of two papers, represents the application of a least squares approach, discussed in the previous paper, to the generation of an internally consistent thermodynamic dataset involving 60 reactions among 43 phases, in the system K2O–Na2O–CaO–MgO–Al2O3–SiO2–H2O–CO2. We make the assumption that all the thermodynamic data, with the exception of enthalpies of formation of the phases, are well known, and solve for an internally consistent set of enthalpies which reproduces the 60, experimentally determined, phase equilibrium reactions. An important difference between our dataset and that of previous alternatives in the literature is that we are able to determine the uncertainties on, and correlations between, the enthalpies of formation for all phases in the set, and hence are able to apply simple error propagation techniques to determine the uncertainties in any phase equilibrium calculations performed using this dataset. Selection of reactions, for geothermometry and geobarometry, may be more readily made by choosing equilibria with small uncertainties in their thermodynamics. Our data are in reasonably close agreement with the high temperature molten oxide calorimetry results on silicate minerals where available, a fact which lends a degree of confidence to the results.  相似文献   

5.
The thermodynamic properties of 254 end‐members, including 210 mineral end‐members, 18 silicate liquid end‐members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end‐members have been added, including several deep mantle phases and, for the first time, sulphur‐bearing minerals. Silicate liquid end‐members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine‐bearing equilibria, sulphur‐bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.  相似文献   

6.
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7.
A fully thermodynamic model for mafic melt in CaO–MgO–Al2O3–SiO2 (CMAS) has been calibrated, for calculation of melting equilibria in the pressure range 0–50 kbar. It is intended as a preliminary step towards a large‐system melt model, suitable for exploring melting, melt loss and crystallization processes in a wide range of natural rock compositions. Calibration was performed with attention to the model's behaviour in its compositional subsystems, as a rigorous test of model structure and parameterization. The model is consistent with the latest Holland & Powell thermodynamic data set, and can therefore be used to calculate phase relations in conjunction with the many solid‐phase activity–composition models written for the data set. Model calculations successfully reproduce experimental melting reactions in CMAS spinel lherzolite and garnet lherzolite assemblages, as well as sapphirine‐ and kyanite‐bearing assemblages, at moderate to high pressure. Thermodynamically sensitive features, such as thermal divides are also recovered. However, some changes to the model structure will be required before the model can describe the full range of mafic and ultramafic melt compositions known from experiment at low pressures.  相似文献   

8.
The recent publication of an updated thermodynamic dataset for petrological calculations provides an opportunity to illustrate the relationship between experimental data and the dataset, in the context of a new set of activity–composition models for several key minerals. These models represent orthopyroxene, clinopyroxene and garnet in the system CaO–MgO–Al2O3–SiO2 (CMAS), and are valid up to 50 kbar and at least 1800 °C; they are the first high‐temperature models for these phases to be developed for the Holland & Powell dataset. The models are calibrated with reference to phase‐relation data in the subsystems CaO–MgO–SiO2 (CMS) and MgO–Al2O3–SiO2 (MAS), and will themselves form the basis of models in larger systems, suitable for calculating phase equilibria in the crust and mantle. In the course of calibrating the models, it was necessary to consider the reaction orthopyroxene + clinopyroxene + spinel = garnet + forsterite in CMAS, representing a univariant transition between simple spinel and garnet lherzolite assemblages. The high‐temperature segment of this reaction has been much disputed. We offer a powerful thermodynamic argument relating this reaction to the equivalent reaction in MAS, that forces us to choose between good model fits to the data in MAS or to the more recent data in CMAS. We favour the fit to the MAS data, preserving conformity with a large body of experimental and thermodynamic data that are incorporated as constraints on the activity–composition modelling via the internally consistent thermodynamic dataset.  相似文献   

9.
Eclogites and garnet‐blueschists exposed at the deepest structural levels of the Oman Mountains in north‐eastern Saih Hatat, Oman, indicate that the Arabian continental margin was subducted and subsequently exhumed. The peak metamorphic pressure has been a matter of debate for over a decade, with initial thermobarometric estimates, based on garnet–clinopyroxene–phengite barometry and the presence of radial cracks around quartz inclusions in garnet, yielding values in excess of 20 kbar; these estimates have been questioned by some researchers. The high‐pressure minerals (glaucophane, omphacite and epidote) contain significant amounts of ferric iron, previously postulated to displace the stability fields of the eclogite and blueschist assemblages to less extreme conditions. In the present study, we have calculated phase diagrams and pseudosections in the model system NCFMASHO, using the program thermocalc and the thermodynamic database of Holland and Powell, which incorporates data for Fe3+‐bearing end‐members. It is found that the phase compositions and modal abundances for typical bulk compositions are matched successfully at 520 ± 15 °C and 20 ± 1.6 kbar for the eclogites and 510–530 °C and 17–20 kbar for the garnet blueschists. These results support the original high‐pressure estimates for the eclogites, and indicate that crossitic amphibole and aegirine‐rich pyroxene do not necessarily reflect lower pressure conditions. The data set and activity models are applicable to other oxidized high (and ultra‐high) pressure mineral assemblages.  相似文献   

10.
Internally consistent thermodynamic datasets available at present call for a further improvement of the data for nepheline (Holland and Powell 1988; Berman 1991). Because nepheline is a common rock-forming mineral, an attempt has been made to improve on the present state of knowledge of its thermodynamic properties. To achieve that goal, two heterogeneous reactions involving nepheline, albite, jadeite and a-quartz in the system NaAlSiO4-SiO2 have been reversed bylong duration runs in the range 460 ≤ T(°C) ≤ 960 and 10 ≤ P(kbar) ≤ 22. Given sufficiently long run times, thealbite run products approach internal equilibrium with respect to their Al,Si order-disorder states. Using appropriate thermochemical, thermophysical, and volumetric data, Landau expansion for albite, and the relevant reaction reversals, a refined thermodynamic dataset (ΔfHi0 and Si0) has been derived for nepheline, jadeite, a-quartz, albite, and monalbite. Our refined data agree very well with theircalorimetric counterparts, but have smaller uncertainties. The refined dataset for ΔfHi0 and Si0, including their uncertainties and correlation, help generate the NaAlSiO4-SiO2 phase diagram including 2a confidence interval for eachP-T curve (Fig. 5). Editorial responsibility: W. Schreyer  相似文献   

11.
《Lithos》2007,93(1-2):1-16
Metamorphic peak P–T conditions of five kyanite eclogites from the Tauern Window, Austria, are evaluated on the basis of recent calibrations of the assemblage garnet + omphacite + phengite + kyanite + quartz. Results are about 25 kbar, 630 °C according to the dataset of Holland and Powell [Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology 16, 309–343 (updated 2002)]. Mostly higher P–T values are calculated with the calibrations of Krogh Ravna and Terry [Krogh Ravna, E.J., Terry, M.P., 2004. Geothermobarometry of UHP and HP eclogites and schists—an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengitecoesite/quartz. Journal of Metamorphic Geology 22, 579–592] and Brandelik and Massonne [Brandelik, A., Massonne, H.-J., 2004. PTGIBBS—an EXCEL Visual Basic program for computing and visualizing thermodynamic functions and equilibria of rock forming minerals. Computers and Geosciences 30, 909–923], in part already in the stability field of coesite. However, no indications for this phase are evident from the Tauern samples. The presence of talc is consistent with these P–T values and high H2O activities. In contrast, the stability limits of paragonite and zoisite are situated at lower pressure and suggest a later formation during the decompression stage. THERMOCALC pseudosections in the NCFMASH system are constructed with the incorporation of fractional crystallization of garnet. Calculated garnet zonations are in better agreement with the observed compositions at peak pressures of about 25 kbar than results at lower pressures. This is also consistent with values from thermobarometry obtained with the same program.  相似文献   

12.
A petrogenetic grid is presented for the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O), including biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H2O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz-bearing and quartz-absent equilibria are shown. Quartz-absent equilibria are particularly relevant at high- T and low- P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg-1 and (Fe, Mg)SiAl-2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non-KFMASH components are taken into account.  相似文献   

13.
This Special Issue comprises a selection of the papers given at a two‐day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powell’s 60th birthday. At this milestone, it is fitting to review Roger’s career to date. He has published ～200 scientific papers on topics that range from low‐ to high‐grade metamorphism, from low‐ to ultrahigh‐pressure (UHP) metamorphism, and from thermodynamics to kinetics. Most of Roger’s papers are multi‐authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic properties of end members of phases necessary to undertake calculations on the conditions of formation and modification of metamorphic rocks. Additionally, Roger and Tim have developed activity–composition models for many of these phases, building on their important methodological developments in formulating such models. Roger is also responsible for the ongoing development of thermocalc , a thermodynamic calculation software package that may be used to undertake a wide range of phase diagram calculations, including P–T projections, P–T, P–X and T–X, compatibility diagrams and μ–μ diagrams. Together, Roger and Tim have changed the way we carry out quantitative phase equilibria studies. However, Roger’s contributions to metamorphic petrology go well beyond the development of phase equilibria methods and mineral thermodynamics. He has contributed significantly to our understanding of a range of metamorphic processes, and with an extensive array of co‐authors has shown how phase equilibria can be used to understand the evolution of metamorphic rocks in general terms as well as in specific terranes. The papers in this Special Issue cover the range from the stabilization of the continents to understanding the formation of orogenic gold deposits, from the stability of sapphirine–quartz‐bearing assemblages to the crystallization of melt in migmatites, from the effects of ferric iron and sulphur on the stability of metamorphic mineral assemblages in general to the effects of ferric iron and H2O on the stability of eclogite in particular, and to the quantification of UHP metamorphism. It is our hope that in reading these contributions, you will be stimulated to seek a better understanding of metamorphic processes and to improve our quantification of the variables in metamorphism.  相似文献   

14.
On thermobarometry   总被引：15，自引：2，他引：13  
Thermobarometry, the estimation of the pressure–temperature ( P – T ) conditions of metamorphism, can be undertaken by using pseudosection calculations as well as by conventional methods. Conventional thermobarometry uses only the equilibrium thermodynamics of balanced reactions between end-members of minerals, combined with the observed compositions of minerals. In contrast, pseudosections involve a forward calculation of mineral equilibria for a given rock composition. When related to observed rock data such as mineral assemblages, mineral proportions and mineral compositions, pseudosections have the power to provide valuable additional thermobarometric information. This is because the rock composition provides added constraints on P – T , unavailable in conventional thermobarometry, such as when minerals in the mineral assemblage are no longer stable, or when additional minerals join the mineral assemblage. Considering both conventional and pseudosection thermobarometry, a minimum requirement is that they use the same thermodynamic data and activity–composition models for the minerals involved. A new thermocalc facility is introduced that allows pseudosection datafile coding to be used for conventional thermobarometry. Guidelines are given and pitfalls discussed relating to pseudosection modelling and conventional thermobarometry. We argue that, commonly, pseudosection modelling provides the most powerful thermobarometric tools.  相似文献   

15.
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16.
The recent publication of new activity–composition models by Holland, Green and Powell (2018; Journal of Petrology 59 : 881–900), with a melt model calibrated for source compositions ranging from peridotitic to granitic, opens the door to the modelling of multiple petrogenetic processes at supersolidus conditions in which the composition of the melt phase changes considerably, without having to change the melt model. This melt model is also the first one using the internally consistent thermodynamic databases published by T. Holland and R. Powell that contains TiO2 and Fe2O3, further expanding the application of this model to more realistic geological scenarios. The accompanying mineral models are also the first in containing some minor elements, like TiO2 in garnet and K2O in clinopyroxene. Consequently, it is relevant to test the applicability of these new models to a large P–T–X range of conditions before they can be used in full. Thermodynamic calculations made with the software Perple_X using these models were compared to experimental results, namely the modal proportions and the composition of the melt and several mineral solution phases. The experiments chosen for the comparative study covered a wide range of source compositions (from mafic to felsic), pressure (from 0.3 to 2.1 GPa), temperature (from 700 to 1,150°C) and total and added water content (structural water: 0.15–1.48 wt%; added water: 0–8 wt%; total water: 0.15%–8.15%). The results indicate that the extended melt model reproduces well the composition of the experimental melts, with an inverse correlation between component amount and fit: the best match is found for SiO2 (−0.8% on average) and the worst match is found for those elements with the lowest amounts, TiO2 and MgO (+241% and +235% on average, respectively; values indicate calculated minus experimental, times 100 and divided by experimental). The TiO2 content in the melt model increases dramatically with increasing pressure, from +90% for P < 1.5 GPa to +593% for P > 1.5 GPa. No comparison was made on the Fe2O3 content, as the published iron contents of the experimental melts were always reported as FeOt. In some cases, there is a substantial mismatch in the modal proportions between experiments and calculations, with the reactant phases less abundant and product phases more abundant in the calculations, an effect that is attributed to kinetic effects in the experiments and to the selected clinoamphibole model. Finally, the extended melt model was compared to the tonalitic melt model of Green et al. (2016; Journal of Metamorphic Geology, 34 : 845–869). Both melt models produce very similar results for SiO2, Al2O3, Na2O and K2O, with slightly better results for the tonalitic melt model in FeOt and MgO and for the extended melt model in CaO. No comparison is made on TiO2 because the tonalitic melt model does not include this component. In summary, the new activity–composition models represent a significant contribution to thermodynamic calculations on the evolution of siliceous magmas where their composition, temperature and pressure changes substantially.  相似文献   

17.
The new, greatly expanded internally-consistent dataset of Holland and Powell includes thermodynamic data for a wide range of mineral end-members in common rock-forming minerals, in particular, including FeMg-1 substitutions in glaucophane, garnet, chloritoid and carpholite, and FeMg-1 and MgSiAl-1Al-1 substitutions in talc and chlorite. Moreover, we have the uncertainties and correlations for these data. With the data, we have calculated the full pressure-temperature phase diagram for the system Na2O–FeO–MgO–Al2O3–SiO2–H2O (NFMASH) for quartz (or coesite) and H2O in excess, in the range 300°–800°C and 5–50 kbars. By solving the set of non-linear equations formed by the equilibrium relationships for an independent set of equations between the end-members in an assemblage in NFMASH, the compositions of the minerals (and PT) can be calculated. Thus the changes in MgSiAl-1Al-1 along NMASH reactions, and FeSiAl-1Al-1 along NFASH reactions, are calculated, and the changes in FeMg-1 and MgSiAl-1Al-1 along NFMASH reactions are calculated. From this information it is straightforward to generate PT diagrams for specific rock compositions. Mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist, blueschist and eclogite facies assemblages in metapelitic rocks. It is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good. When semiquantitative extensions of the phase diagram to include Ca(MgFe)-1, NaSiCa-1Al-1, Fe3+Al-1 and KNa-1 substitutions are taken into account the agreement is essentially complete.  相似文献   

18.
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   

19.
High-precision relative thermobarometry: theory and a worked example   总被引：7，自引：1，他引：6  
A number of sources of uncertainty are involved in thermobarometric calculations, the most important of which are associated with analytical precision, activity–composition ( a – x ) relationships, and thermodynamic data. Statistical treatment of these uncertainties results in relatively large uncertainties on the calculated values of pressure and temperature. Little can be done, at least in the short term, about the magnitude of such uncertainties, and any thermobarometric calculations in which they are not taken into account should be treated with caution. Given that uncertainties associated with a–x models and thermodynamic data are systematic when applied to multiple samples with the same mineral assemblage, a solution to the problem of imprecise absolute thermobarometry can be obtained via a relative thermobarometric technique referred to as the Δ PT  approach. The Δ PT  approach offers a major improvement in the precision of thermobarometry if the calculations can be presented in a Δ PT  context.  相似文献   

20.
The kinetic problems associated with the experimental determination of reactions among complex solidsolution phases at low temperatures have hindered our understanding of the phase relations in greenschist facies rocks. In the absence of reliable experimental data, we have used the new, expanded internally-consistent thermodynamic dataset of Holland and Powell (1990), to present calculated phase equilibria for the system CaO–FeO–MgO–Al2O3–SiO2–H2O–CO2 (CaFMASCH) with quartz in eccess, in the range 400°–500°C at low to intermediate pressures, involving the minerals amphibole, chlorite, anorthite, clinozoisite, dolomite, chloritoid, garnet, margarite, andalusite, and calcite. By solving independent sets of non-linear equations formed from equilibrium relationships, we calculate not only the loci of reactions in pressuretemperature-x(CO2) space, but also the compositions of coexisting minerals in terms of the substitutions, FeMg-1 and (Fe,Mg)SiAl-1Al-1. Invariant, univariant and divariant equilibria are calculated and discussed in relation to naturally-occurring greenschist facies metabasic and siliceous dolomitic mineral assemblages. We thus avoid the use of activity-corrected curves so commonly presented in the literature as a substitute for genuine univariant phase diagram boundaries.  相似文献   

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