experimental petrology

experimental petrology Experimental petrology is a branch of Earth science that attempts to reproduce, in the laboratory, the conditions under which igneous and metamorphic rocks are formed at great depth within the Earth where temperatures and pressures are extremely high. For example, diamonds form at depths of about 150 km and temperatures over 1300°C. Experimental petrologists grow samples of minerals, observe the melting behaviour of different compositions of rock, examine the movement of trace elements between minerals, and, in short, investigate many processes of geological interest.

Experimentalists use very small rock samples, often no more than a millimetre across. A tiny sample is used so that it will equilibrate quickly at the temperature and pressure of interest, and can quickly be cooled to ‘freeze in’ the experimental results. If the samples were large, long periods of time would be required for the sample to reach equilibrium, i.e., the point at which, if the samples remained indefinitely at the same temperature and pressure indefinitely no further change would take place. Processes that take thousands or even millions of years to occur in the Earth can be simulated, by using small enough samples, in only days or weeks in the laboratory.

Most of the work done before the 1950s was at atmospheric pressure. Notable contributions to the study of silicate systems were made by N. L. Bowen and his colleagues at the Geophysical Laboratory at Washington, D. C. Experiments at high pressure and temperature, corresponding to conditions at depth in the crust, became possible with the introduction in the late 1940s of the ‘Tuttle bomb’: essentially a thick metal test-tube closed by a screw cap. The ‘bomb’ was heated in a furnace and a fluid pumped in to raise the pressure. Higher pressures were achieved in the 1960s with the piston-cylinder apparatus, heated internally and using one or more pistons to compress the sample. The diamond-anvil apparatus carried the range of pressures still higher, with the advantage that the specimen could be observed during the experiment through the diamond anvil. (With the classical methods it was possible to examine the experimental charge only after it had been chilled and prepared for observation under a petrological microscope.) A laser beam is used to heat the sample. For the highest pressures, brute force is applied in the form of the shock-wave apparatus, in which a projectile is fired at the sample.

The equipment now available makes it possible to simulate conditions deep in the mantle. Experimental petrology is adding much to our understanding of the interior of the Earth and the way in which it melts to produce the material from which igneous rocks are derived.

Judith M. Bunbury