# Practical Aspects of Mineral Thermobarometry

These pages provide information, tutorial material and worked examples for petrology students interested in extracting pressure-temperature information from rocks. It is assumed that you have completed a typical undergraduate petrology course. The sequence of topics, listed below, progresses from the acquisition of mineral analyses to computer-based P-T and phase diagram calculations.

The content of these pages gets updated periodically but irregularly. The current edition (2002 onwards) was prompted by the availability of version 3 of the THERMOCALC software for Mac and PC, and the old documentation is being brought into line with the capabilities of this release. Additionally, the documentation and support provided with the program is now much more comprehensive, and I shall gradually shift away from duplicating what Roger Powell and Tim Holland have already provided. Some material previously restricted to the Oxford domain has been moved to these public pages. The site redesign should make it easier to find things.

[ Main Index of Topics ] [ Other THERMOCALC Resources ]

#### Text-book

The theoretical content is not self-contained - you will need to
refer to other texts and papers. The most useful text is Frank
Spear's *Metamorphic Phase Equilibria and Pressure-Temperature-Time
Paths* (1993, Mineralogical Society of America), and many of the
sections below refer you directly to the appropriate part of this
book. If you want to buy this book, it's probably worth
ordering
direct from the MSA rather than going through a bookstore.

#### Thermodynamic calculation software

The P-T calculation method used involves Roger Powell and Tim Holland's program THERMOCALC with its associated thermodynamic data set, described in a number of papers, the most recent at the time of writing being Holland and Powell (1998), Journal of Metamorphic Geology 16, 309-343, and Powell et al.(1998), Journal of Metamorphic Geology 16, 577-588. See the bibliography for more detail.

### Main Index of Topics

- Microprobe analyses

Briefly, how they are performed, their precision and accuracy. How to spot good and bad ones. Tips for acquiring good ones. - Chemical mineralogy of common mineral
groups

Garnet, feldspar, mica, pyroxene, amphibole. Sites, assignments and preferences. Stoichiometry, formula recalculations, ferric iron estimation. - Solid solutions, mixing models and activity
calculations

Mixing-on-sites, simple and multi-site examples. Heats, volumes and entropies of mixing. Regular solution mixing parameters. Margules formulations for binary and ternary systems. Darken's quadratic formalism. Implementation of activity calculations. - Thermobarometer calibrations

Sources of the required thermochemical data. Self-consistent datasets. Calibrating individual thermobarometers. Simple linear expressions. - Error estimation in P-T
calculations

Relative importance of uncertainties associated with the calibration, mineral analyses, a-X relations, "geological" error. - Use of THERMOCALC for P-T
calculations

Some examples and advice: Himalayan metapelites; P-T-aH_{2}O of granulites from metapelites and metabasites; low-pressure assemblages. - Use of THERMOCALC in phase-diagram
mode

General aspects of phase diagram and pseudosection construction. - THERMOCALC tips and examples

e.g. bulk composition considerations: equilibration volumes, fractionation. Using mineral composition isopleths.

### Other THERMOCALC resources

THERMOCALC downloads and
documentation

Links to the official Cambridge and Melbourne sites for downloading
the programs and for access to online documentation. Other useful documentation,
kept on this site.

THERMOCALC v.3 coding for activity models

A library of data file fragments for various mineral species.

Example applications to petrological systems

Some systems we have investigated

Bibliography

A bibliography of papers related to derivation of the thermodynamic
data-set of Holland and Powell (1985, 1990, 1998) and its use in
thermobarometry and phase diagram calculation.

THERMOCALC bugs and quirks

A page for recording bugs, problems, unpredictable behaviour and other quirks of
the program.

Bulk compositions

Tips on how to estimate and refine a bulk composition for your modelled system.

Modal proportions

How THERMOCALC's "modes" (or oxide-molar proportions) work.

The Spreadsheet Tools Page

gives access to accessory programs, useful spreadsheets, and
other interactive material.

This page last modified 12 October 2004