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.

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

The AI-in-hornblende barometer potentially offers a basis for estimating crystallization pressure for granitic batholiths. However, owing to the simplicity of its formulation, misuse of the barometer can occur. Many granitic intrusions are emplaced at conditions inconsistent with those of the existing experimental calibrations, including f02 2 kbar at typical crustal pressures. For the Mount Stuart batholith, consideration of temperature yields revised pressures that are in agreement with pressures obtained from wall rocks and eliminates much of the apparent domal structure. Low-f02 granites have amphibole Fe/(Fe + Mg) ratios that exceed the typical 0.40-0.65 range used in most experimental and empirical calibrations. Examples from anorogenic granitic batholiths of mid-Proterozoic age yield pressures that are too high by a factor of two to three in comparison with pressures obtained from adjacent metamorphic assemblages. Hornblende in these granites not only has high Fe/(Fe + Mg) but also low ratios of FeH to Fe2+. The anomalously high Al in Fe-rich, FeH -poor hornblende is inferred to be the result of increased [6JAIoccupancy of the M2 site not buffered by the Mg and FeH abundances typical of amphiboles in calc-alkaline and other high- f02 plutonic rocks.

/pdf/the-effects-of-temperature-and-f-o-2-on-the-al-in-hornblende-kxz4xeujbg.pdf

The Effects of Temperature and ƒ O 2 on the Al-in-Hornblende Barometer

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.

Experimental determination of the fluid-absent melting relations in the pelitic system

Although many recent reviews emphasize a uniformity in granulite pressure–temperature (P–T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P–T conditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900−1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P–T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P–T conditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P–T paths are important. Amphibolite–granulite transitions promoted by the passage of CO2-rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (1−2 mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P–T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels (< 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100−2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756800022330

The origins of granulites: a metamorphic perspective

An "internally consistent method" (Powell 1985) for performing thermobarometric calculations is described in which all possible equilibria implied by a given mineral assemblage are computed using an internally consistent set of thermodynamic data for end members and mixing propeilies. Results are presented graphically and as best estimates determined by weighted averaging of the graphical results, Several examples illustrate that convergence of all equilibria in a single P-T region correlates well with textural and chemical indications of equilibrium, suggesting that this technique makes it possible to reasonably assess the state of equilibration of samples for systems in which the thermodynamic properties appear to be well described, One implication is that portions of P-T paths may be reconstructed from rocks that display disequilibrium textures but that pass this test for local equilibrium. The general applicability of this technique is presently limited by the accuracy of thermodynamic data, particularly for solid solutions, but should expand as these properties are refined in the future. This method facilitates the refinement process by (1) highlighting which minerals are most incompatible in each calculation and thus may be in need of refinement, (2) illustrating the sensitivity of results to different solution-models and offering the chance to correlate these differences with petrographic observations, and (3) providing a means to select "well-equilibrated" samples that can be used to refine thermodynamic properties in the absence of appropriate experimental data.

Thermobarometry using multi-eouilibrium calculations: a new technioue, with petrological applications-

Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km2 bounded by younger fold-belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold-belt. Structural evolution of this central zone was determined by the geodynamics of the mega-blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold-belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post-Archaean sediments.



The Aldan shield is composed of Archaean high-grade granulites, while the Stanovoy fold-belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower-grade conditions. However, relics of the granulites are mapped within the fold-belt. They contain high-grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold-belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed.



Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north-south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.

Precambrian granulites of the Aldan shield, eastern Siberia, USSR

S U M M A R Y : Experimental data on equilibria involving rock-forming minerals in highgrade metapelites (garnet, orthopyroxene, cordierite and plagioclase) have been used to derive an internally consistent system of mineralogical thermometers and barometers based on component reactions and excess mixing properties of solid solutions. Application of these data to a granulite-facies metapelite sample reveals that compositional inhomogeneity of minerals on a thin-section scale is likely to be a result of several reactions opening simultaneously at the retrograde stage of metamorphic evolution. The widely accepted practice of estimating pressure and temperature from separate reactions may thus lead to errors and misinterpretation of P T paths.

Geothermobarometry of high-grade metapelites: simultaneously operating reactions

The paragenesis of cordierite-garnet-sillimanite-quartz is widespread in regionally metamorphosed high-grade pelitic rocks and in hornfelses of contact aureoles. Since Korzhinskii (1936) considered it as a potential indicator of metamorphic conditions, this assemblage has been of great interest to petrologists. Qualitative thermodynamic analysis by Marakushev (1965) showed that Cor + Gr + Sil + Qz paragenesis could be treated as divariant assemblage in the FeO-MgO-Al2O3-SiO2 system, the Fe-Mg minerals compositions having been defined by the P-T conditions. This approach allowed P-T estimates to be obtained from the intersection of isopleths of Mg/(Mg + Fe) of coexisting cordierite and garnet on a.P-T diagram, but unfortunately the experimental and theoretical studies provided conflicting calibrations of the diagram. In his review of the literature on the subject, Lonker (1981) has listed at least eight studies which disagree with each other concerning the effects of pressure, temperature, and water fugacity on the Cor-Gr-Sil-Qz equilibrium.

The Cordierite-Garnet-Sillimanite-Quartz Equilibrium: Experiments and Applications

The P–T partition function in statistical thermodynamics can be used to derive semi-empirical formulations of the Gibbs free energy G for minerals and fluids Parameterization of these equations includes simultaneous regression of experimental heat capacity and molar volume data, allowing fitting, appraisal and optimization of various data sources, as required in the construction of internally consistent petrological data bases This approach can also be extended to minerals with λ-transitions and to fluids by considering the Gibbs free energy as a function of pressure P, temperature T and an ordering parameter Xα, so that accurate modelled representation and extrapolation of the thermodynamic properties of large numbers of petrologically significant minerals and coexisting fluids can be attained The ordering parameter is chosen to denote the equilibrium mole fraction (thermodynamic probability) of ordered clusters (structural units) in a substance when G(T,P, Xα)=min The procedure is tested on existing experimental data for the system MgO–SiO2–H2O The proposed Gibbs free energy formulation permits thermodynamic properties of minerals, fluids and phase equilibria to be described and extrapolated over a wide range of pressure (0–800 kbar) and temperature (20–3000 K), thus allowing effective use in thermodynamic data bases of petrological interest

/pdf/semi-empirical-gibbs-free-energy-formulations-for-minerals-1wkbl9gu0b.pdf

Semi-empirical Gibbs free energy formulations for minerals and fluids for use in thermodynamic databases of petrological interest

Progress in mineral calorimetry and in the methods of calculation of the properties of rock-forming minerals by thermodynamic treatment of the experimental data on mineral equilibria is evident in a number of recent tabulations of standard thermodynamic data (Karpov et al., 1976, 1977; Robie and Waldbaum, 1968; Robie et al., 1978; Helgeson et al., 1978; etc.). A considerable body of data on the thermodynamic properties of solid solutions has also been obtained from experimental and theoretical studies (Perchuk and Andrianova, 1968; Perchuk, 1968, 1977; Wood, 1974, 1977; Cressey et al., 1978; Saxena, 1973).

K. K. Podlesskii

Papers

Precambrian granulites of the Aldan shield, eastern Siberia, USSR

Geothermobarometry of high-grade metapelites: simultaneously operating reactions

The Cordierite-Garnet-Sillimanite-Quartz Equilibrium: Experiments and Applications

Semi-empirical Gibbs free energy formulations for minerals and fluids for use in thermodynamic databases of petrological interest

Calculation of Thermodynamic Properties of End-Member Minerals from Natural Parageneses