Experimental petrology

Experimental petrology is the laboratory simulation of chemical and physical conditions within and at the surface of the earth. A wide variety of apparatuses are used to routinely obtain and control precisely the range of temperature and pressure conditions known to occur up to 150 kilometers in depth. Other apparatuses allow access to the much greater temperatures and pressures of the transition zone of the earth's mantle. Experimental data allow petrologists to interpret quantitatively the evolution of natural rocks and the earth as a planet.

Temperature and Pressure Conditions

A wide range of chemical and physical conditions exist within and at the surface of the earth. Experimental petrology is the simulation of these conditions in a carefully controlled laboratory environment. Temperature and pressure are the two primary physical parameters that change during geological processes. On or near the earth's surface, temperatures ranging between 0 and 1,300 degrees Celsius are observed. Such temperatures are easily attained at low pressure in the laboratory. Much higher temperatures occur deep within the earth and other planets. Pressure increases proportionally with increasing depth below the earth's surface as a direct result of the greater mass of material overlying the material below. The pressure at the earth's surface is that exerted solely by the overburden of the atmosphere, which defines the low pressure limit attained in geological processes.

At a depth of 3,000 kilometers below the earth's surface, near the boundary between the solid silicate mantle and the liquid outer core, the pressure approaches 1.2 million times that exerted by the atmosphere on the surface. As the pressure within the earth increases, so does the temperature, such that at the core-mantle boundary, the temperature approaches nearly 3,000 degrees Celsius. Experimental petrologists have apparatuses that will routinely obtain pressures of 50,000 times that of the atmosphere and temperatures up to 2,000 degrees Celsius, which corresponds to the physical conditions that occur at approximately 150 kilometers below the earth's surface. Other devices available to petrologists since 1980 allow access to temperatures and pressures as great as those found at 1,000 kilometers deep.

Natural- and Synthetic-System

The style of experiments done by petrologists to quantify the conditions of formation of a particular suite of rocks or some widely occurring rock type is nearly as varied as is the number of active investigators. These experiments, however, may be broadly grouped into two major categories: The first would include all experiments done on natural rock or mineral systems, while the second would include experiments done on simpler synthetic systems that are analogous to the much more complex natural systems in some way.

Most experimental petrologists tend to work within one of these categories almost exclusively. Experiments on natural systems are necessarily more complicated and difficult to interpret because even the simplest rocks contain three to four chemical components (the simplest chemical entity that may be used to describe the system under consideration), while most rocks contain ten major and several minor components. In contrast, experiments on synthetic systems can be designed to isolate and study an individual chemical component. In such experiments it is considerably less difficult to demonstrate attainment of equilibrium and interpret the finding in terms of classical thermodynamics.

Quench and Gas-Mixing Furnaces

Experimental petrologists have developed many different apparatuses that allow them to achieve conditions in the laboratory that mimic those found in the earth. The simplest apparatuses are quench and gas-mixing furnaces. Typical working conditions for these apparatuses are pressures that range from moderate vacuums to 1 atmosphere and temperatures from 0 to 1,600 degrees Celsius. Slight variations in pressure on the order of 1 percent normally occur during the operation of these furnaces. Temperatures may be controlled and measured with a precision of several to tens of degrees depending on the particular setup.

Standard 1-atmosphere quench furnaces are set up with a vertical ceramic tube around which some resistive heating elements are either wound or placed in close proximity. The elements are then heavily insulated to prevent large heat losses to the laboratory atmosphere. The vertical geometry is required to enable rapid quenching of material held in the hot zone of the furnace to water at 25 degrees Celsius (or some other suitable quench medium) at the lower end of the vertical tube. Samples are generally held at the end of a ceramic rod by thin platinum loops. At the end of an experiment, the platinum loops are melted by passing a small electrical current through them, which allows the samples to fall by gravity directly into the water, where they are cooled rapidly.

Standard vertical tube furnaces may be modified to include the capability to have a gas mixture of known composition flow through the tube, replacing the static air environment. Such gas mixtures are commonly composed of species of carbon and hydrogen that, when mixed in known proportions, will fix the oxygen activity of the furnace atmosphere. The control of oxygen activity allows experimental petrologists to investigate chemical systems that contain transition metal cations, whose valence state would otherwise change in an uncontrolled and perhaps unwanted way. Samples range in size from 0.1 to 50 milliliters. One-atmosphere furnaces also may be mounted horizontally, if rapid quenching and gas mixing are not required. These apparatuses are found in virtually every experimental petrology laboratory around the world and have been used to investigate a wide variety of petrological problems.

Cold-Seal Vessels

Another common apparatus found in experimental petrology laboratories is the cold-seal vessel designed by O. F. Tuttle in the late 1940s. These apparatuses have been enormously important in the investigation of metamorphic and igneous processes that occur at middle to lower crustal levels. Cold-seal vessels typically are operated at pressures of several hundred to several thousand atmospheres and between 25 and 900 degrees Celsius. Modifications of the original design have allowed petrologists to obtain pressures up to 12,000 atmospheres and slightly higher temperatures. The pressure vessel is fabricated from a superalloy rod usually composed of nickel and chromium with smaller amounts of other metals. The rod has a small-diameter hole drilled into it to yield a container that is similar in shape to a test tube. The walls of the vessel are kept thick to support the high pressures and temperatures that occur during the course of an experiment. A pressure seal is formed by a cone-in-cone fitting at the open end of the vessel. High temperatures are obtained by placing the pressure vessel inside a simple muffle furnace (kiln), which is usually mounted vertically. The end of the vessel with the pressure seal remains outside the furnace and thus remains cold throughout the experiment. The pressure medium in most experiments is water, but to obtain pressures of more than 8,000 atmospheres, argon gas is used.

Piston Cylinders

The last common apparatus found in experimental petrology laboratories is called a piston cylinder. These apparatuses were originally designed and built in the late 1950s and early 1960s. Much of the current understanding of melting relations in basaltic and ultramafic systems has been gained by using the piston cylinder. The typical operating conditions range from pressures of 5,000 to 60,000 atmospheres and temperatures between 25 and 1,800 degrees Celsius. These conditions are similar to those of the earth's deep crust and shallow upper mantle. The apparatus consists of a small piston pressing into a cylinder, which compresses the solid materials of the furnace assembly. One end of the cylinder usually abuts a massive end load. The piston is pressed against the furnace assembly by the use of a hydraulic ram. The ratio of the areas of the piston and the ram allows one to calculate the pressure obtained inside the cylinder. Furnace assemblies consist of small graphite cylinders inside Pyrex or salt outer sleeves with inner sleeves of similar materials. Sample sizes typically range between 0.01 and 0.1 milliliter, which is an order of magnitude smaller than those used in 1-atmosphere experiments. Noble metal capsules that are welded shut are commonly used to contain the sample materials and to isolate them from the rest of the furnace assembly. Temperature is generated by passing a current through the graphite furnace as a result of its finite resistance. Although this apparatus is generally quite easy to operate, careful calibration of experimental temperatures and pressures is necessary prior to its use. Large pressure corrections as a result of frictional forces may arise depending on the materials and design of the furnace assembly.

Studying

The pioneer of igneous petrology, N. L. Bowen, used 1-atmosphere quench furnaces as described earlier to study a simple analog system for the evolution of basaltic rocks. Bowen investigated a synthetic iron-free diopside-albite-anorthite system as a function of composition and temperature. Because the system was iron-free, there was no need to control the oxygen activity during the course of the run by a gas-mixing apparatus. The system does contain sodium, however, which is notoriously volatile at high temperatures. To combat this problem, the sample charges were enclosed in platinum foil. At each bulk composition, a series of experiments was conducted to determine the onset and completion of melting. The starting materials were previously synthesized crystalline pyroxene and plagioclase.

Although this method could yield erroneous results because the melting point was only approached from the low-temperature side and not reversed from the high-temperature side, the analysis of the run products was extremely sensitive to small amounts of melting, recorded as glass. The run products were rapidly quenched and then ground to a fine powder. Gain mounts immersed in oil allowed Bowen to detect minute amounts of crystalline material and thus to determine precisely the location of the liquids in temperature-composition space. To determine the liquid composition, experiments were conducted to yield only quenched liquid (glass), which was then analyzed by conventional wet chemical techniques. Today, with highly developed electron microbeam capabilities, the liquid and two solid phase compositions could all be determined simultaneously. Bowen was able to apply his experimental results to the petrogenesis of basaltic rocks. The experiments clearly demonstrated that with decreasing temperature, plagioclase composition will become more sodic and less calcic when coexisting with a diopsidic pyroxene and liquid of approximately basaltic composition. Modern experiments on basalt and peridotite systems incorporating controlled amounts of volatiles began in earnest in the 1960s and continued through the 1980s. By the 1990s a reasonably self-consistent model for the evolution of terrestrial basalt magmas was available.

Studying Granitic Systems

Granitic systems were studied extensively by Tuttle and Bowen in the late 1950s. Until the time of their experiments, many geologists did not believe that granite batholiths (large intrusive igneous rocks) were the products of crystallization from silicate liquids at moderate pressure. The experiments of Tuttle and Bowen were conducted in cold-seal pressure vessels at temperatures below 700 degrees Celsius and pressures between 500 and 4,000 atmospheres. Their experiments were some of the first to use the apparatus designed by Tuttle. The solubility of water in synthetic granitic melts was determined as a function of temperature and pressure. Water was added directly to the experimental charges, which were welded shut inside platinum capsules. These experiments demonstrated conclusively that many granitic batholiths crystallized from water-bearing silicate melts.

Studying Magmas

A final example of experimental techniques for solving petrological problems is an investigation of the solubility of carbon dioxide in basaltic liquids at high pressure. In the late 1970s, D. Eggler, using a piston-cylinder apparatus similar to the one described earlier, demonstrated that carbon dioxide had significant solubility at pressures of 30,000 atmospheres. To study the effect of carbon dioxide in the melting relations of synthetic systems whose compositions approximated the earth's upper mantle, silver oxalate was added to the experimental charges, which were then sealed by welding in platinum capsules. At high temperatures, the silver oxalate decomposes to produce carbon dioxide or other carbon bearing species, depending on the composition of the liquid present and the oxygen activity during the course of the experiments. The addition of carbon dioxide decreases the solidus to lower temperatures at constant pressure and tends to favor the formation of orthopyroxene over olivine as a result of changes in the melt structure. These experiments, and others like them, are useful in helping to constrain the genesis and evolution of alkali-rich, silica-poor magmas.

Studying Volcanoes and

Experimental petrology has provided geologists with quantitative data that allow them to understand many complex geologic processes. The process of magma genesis deep in the earth's upper mantle, and its subsequent migration from depth to the surface is a process that would not be as well understood today if not for experimental petrologists and their work. The eruption of volcanoes at the surface of the earth is merely one example of the type of process upon which experimental petrology bears. One such eruption occurred in 1980 at Mount St. Helens, Washington. Studies of volcanic rocks, which generally integrate experimental data, geochemistry, and field geology, have revealed that the processes that governed the eruption of Mount St. Helens are still operating there and at other sites worldwide where oceanic crust is subducted below continental crust. In the Cascade province of the western United States, for example, Mount Shasta, Mount Bachelor, and Mount Rainier have many characteristics in common with Mount St. Helens, and these volcanoes might be expected to erupt in a similar manner in the near future. Such an eruption of Mount Rainier or Mount Bachelor could prove to be extremely dangerous to the large population centers of Seattle and Portland.

The techniques of experimental petrology also allow geologists to develop geothermometers and geobarometers, which may be applied to pieces of the earth that are entrained in magmatic eruptions worldwide. The compositions of the coexisting phases in these fragments of rock have permitted the petrologist to determine temperature-depth profiles for the upper 200 kilometers of the earth, gaining valuable information on the composition and state of regions of the earth that are not accessible to direct observation. This information is crucial to the understanding of how and why rocks deform during earthquakes. For rocks of fixed composition, ambient temperature is the primary variable that determines whether rocks will be able to deform in such a way as to produce an earthquake.

Principal Terms

component: a chemical entity used to describe the compositional variation of some phase

igneous rock: any rock that forms by the solidification of molten material, usually a silicate liquid

metamorphic rock: any rock whose mineralogy, mineral chemistry, or texture has been altered by heat, pressure, or changes in composition; metamorphic rocks may have igneous, sedimentary, or other, older metamorphic rocks as their precursors

mineral: a naturally occurring solid compound that has a specific chemical formula or range of composition; a mineral normally has regular crystal structures such that its internal arrangement of atoms is predictable

phase: a chemical entity that is generally homogeneous and distinct from other entities in the system under investigation; compositional variation within phases is described in terms of components

phase equilibria: the investigation and description of chemical systems in terms of classical thermodynamics; systems of specified composition are generally investigated as a function of temperature and pressure

thermodynamics: the area of science that deals with the transformation of energy and the laws that govern these changes; equilibrium thermodynamics is especially concerned with the reversible conversion of heat into other forms of energy

Bibliography

Blatt, Harvey, and Robert J. Tracy. Petrology: Igneous, Sedimentary, and Metamorphic. 3rd ed. New York: W. H. Freeman, 2005.

Campos, Cristina, et al. “Enhancement of Magma Mixing Efficiency by Chaotic Dynamics: An Experimental Study.” Contributions to Mineralogy & Petrology 161 (2011): 863-881.

Carmichael, Ian S. E., Francis J. Turner, and John Verhoogen. Igneous Petrology. New York: McGraw-Hill, 1974.

Edgar, Alan D. Experimental Petrology: Basic Principles and Techniques. Oxford, England: Clarendon Press, 1973.

Ernst, W. G. Petrologic Phase Equilibria. San Francisco: W. H. Freeman, 1976. This text outlines the elements of classical thermodynamics and experimental approaches to acquiring the thermodynamic data necessary for geothermometry and geobarometry.

Fyfe, W. S., and Mackenzie, W. S. “Some Aspects of Experimental Petrology.” Earth-Science Reviews 5 (1969): 185-215.

Gregory, Snyder A., Clive R. Neal, and W. Gary Ernst, eds. Planetary Petrology and Geochemistry. Columbia, Md.: Geological Society of North America, 1999.

Grotzinger, John, et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2006.

Hall, Anthony. Igneous Petrology. 2d ed. Harlow: Longman, 1996.

Holloway, J. R., and B. J. Wood. Simulating the Earth: Experimental Geochemistry. Winchester, Mass.: Unwin Hyman, 1988.

Mortimer, Charles E. Chemistry: A Conceptual Approach. 3rd ed. New York: D. Van Nostrand, 1975.

Newton, Robert C. “The Three Partners of Metamorphic Petrology.” American Mineralogist 96 (2011): 457-469.

Scaillet, Bruce and Michel Pichavant. "Himalayan Leucogranites: An Experimental Petrology Perspective." Elements, vol. 20, no. 6, 1 Dec. 2024, pp. 388-394, doi.org/10.2138/gselements.20.6.388. Accessed 9 Feb. 2025.

Ulmer, G. C. Research Techniques for High Pressure and High Temperature. New York: Springer-Verlag, 1971.

Uyeda, Seiya. The New View of the Earth: Moving Continents and Moving Oceans. San Francisco: W. H. Freeman, 1971.