What is Experimental Petrology?
Experimental work involves simulating igneous or metamorphic conditions with specialized machines, described below. Samples may include synthetic minerals or mixes of minerals, fluids such as H2O and/or CO2 or actual rock compositions.
Other than visiting on-going lava flows in Hawaii and Iceland, the formation of igneous and metamorphic rocks cannot be directly observed. However, in the lab samples of a known composition can be held at known pressure and temperatures,and results can be analyzed. Certain processes (melting, crystallization, diffusion) can also be simulated.
Igneous conditions can be reproduced in high temperature furnaces. The furnaces can be modified to allow control of oxygen activity (fugacity). Low-grade metamorphic conditions can be reproduced with machines called a cold-seal apparatus or gas vessels. Conditions in the deep crust and upper mantle can be simulated with a piston-cylinder apparatus. Conditions in the deep mantle and even the core can be simulated in a multi-anvil device or diamond anvil device.
Strict limitation of variables allows us to understand what happens with changes in one variable, for example, composition or temperature. Phase diagrams for a mineral or mineral assemblage can be determined. Thermodynamic data such as enthalpy of formation can be extracted. Diffusion rates can be determined. In some cases Activity-composition data are obtained for mineral activity models.
Strengths and Limitations
Experimental conditions can be varied at the discretion of the experimenter. Pressure and temperature are accurately and precisely known in most cases. Products can be analyzed by XRD and electron microprobe. Past experience has led to improved techniques of quenching and preserving the high temperature-pressure assemblages. Past experience has also improved techniques demonstrating approach to equilibrium by the experimental charges.
The major limitation is time. Metamorphic processes may take millions of years, but experimentalists are not that patient. Experiments may last minutes or up to two weeks. Therefore for kinetic reasons many experiments are run at higher temperatures than what is normally hypothesized for crustal conditions.
Another limitation is sample size. In general machines simulating higher pressures and temperatures have very small sample sizes, from 10 mg (piston cylinder) to 0.1 mg (multi-anvil device). Such small samples may be difficult to analyze.
Some conditions or situations may be impossible to simulate. Disequilibrium assemblages may persist, even at high temperatures or over long experimental times. The machines may be expensive to run and maintain.
Geoscientists interested in experimental petrology are encouraged to visit this site. LEPR is a database of results of published experimental studies involving liquid-solid phase equilibria relevant to natural magmatic systems. The LEPR Database can be searched by author, laboratory, or you can perform an advanced search by sorting on experimental conditions, phase assemblages, and/or compositional data on phases. You have to register to use this site, but access is free and there is a wealth of great information to be discovered! The LEPR site is closely allied with the MELTS program (see the tutorial on MELTS later in this module on Teaching Heterogeneous Phase Equilibria).
See also: Hirschmann, M. M., M. S. Ghiorso, F. A. Davis, S. M. Gordon, S. Mukherjee, T. L. Grove, M. Krawczynski, E. Medard, and C. B. Till (2008), Library of Experimental Phase Relations (LEPR): A database and Web portal for experimental magmatic phase equilibria data, Geochem. Geophys. Geosyst., 9, Q03011, http://dx.doi.org/10.1029/2007GC001894 14 March 2008
- Holloway, J.R. and Wood, B.J. (1988) Simulating the Earth: Experimental Geochemistry. Winchester Ma: Unwin Hyman, 181 p.
- Working with Electron Microprobe Data from a High Pressure Experiment - Calculating Mineral Formulas, Unit Cell Content, and Geothermometry - Brandon Schwab, Humboldt State University