The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100 kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.

An internally consistent thermodynamic data set for phases of petrological interest

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

https://www.geol.umd.edu/~rudnick/PDF/Rudnick_1993_EPSL.pdf

Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical characteristics

Carbonatites are uncommon carbonate-rich rocks usually found in continental intra-plate regions and often associated with rifting. There has been much debate as to whether carbonatite magmas are primary melts derived from partial melting of mantle peridotite, or are formed by exsolution of an immiscible carbonate melt fraction from phonolitic or nephelinitic magmas. Our experiments on the phase relationships of carbonate and amphibole-bearing peridotite (containing 0.3% H2O and 0.5–2.5% CO2) show that sodic dolomitic carbonatite magma coexists with an amphibole lherzolite assemblage in a field ranging from 21 to 30 kbar and 930 to 1,080 °C, spanning a pressure and temperature interval between the solidus and the amphibole breakdown and melting curve. Thus primary carbonatite melts may occur under suitable geothermal conditions. The nature of the peridotite solidus and of the melting reactions differ considerably from published models1–3. The carbonatite melt composition, determined by a series of 'sandwich' experiments, was found to be rich in Na, Mg, Ca and Fe, with a small dissolved silicate content. This melt quenches to an assemblage of dolomite and Na–Mg carbonate minerals, producing textures similar to those preserved in samples from Oldoinyo Lengai4, Homa mountains, Tanzania5 and Kaiserstuhl, Germany6.

K.L. Harris

Papers

Pyroxene-carbonate reactions in the upper mantle