An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program

TLDR

This approach maximizes the benefit of an internally consistent thermodynamic dataset compared to specific geothermometers and geobarometers. The paper presents methods and a Pascal program, thermocalc, for thermodynamic calculations using the dataset, and recommends uncertainties on activities for rock calculations, stressing avoidance of underestimates at low mole fractions. Thermocalc incorporates dataset uncertainties to compute uncertainties in geothermometry and geobarometry, and applies a least‑squares combination of independent reactions between mineral end‑members and the dataset to determine rock conditions. Illustrative phase‑diagram and rock‑calculation examples show the program’s utility, and thermocalc is available for IBM PC and Mac at A$25 or £10.

Abstract

Abstract 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.