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

Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites

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

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

https://science.sciencemag.org/content/sci/273/5277/924.full.pdf

Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001.

This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end-members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.

Julian R. Goldsmith

Papers

Oxygen isotopic fractionation in the system quartz-albite-anorthite-water

Oxygen isotope fractionations involving diopside, forsterite, magnetite, and calcite: Application to geothermometry

Mechanisms of hydrothermal crystallization of quartz at 250°C and 15 kbar

Oxygen and carbon isotope fractionations between CO2 and calcite

Lattice constants of the calcium magnesium carbonates