Experimental Petrology Machines
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. In all cases the experimentalist wishes to maintain the integrity of the sample and to control and accurately measure the temperature and pressure the sample is experiencing. In addition, the experimentalist wants to retrieve the sample and identify the final quenched phases and their compositions. What follows are short descriptions of the different machines used, starting from near-surface P-T conditions to mantle-core P-T conditions.
Quench and Gas-mixing Furnace
This is a simple box or tube shaped furnace that can maintain temperatures from 100 to 1500° C or more, or raise and lower temperatures in a controlled manner. Samples are placed in an open or sealed metal or ceramic capsule, depending on the experiment. Modifications include controlling oxygen or hydrogen fugacity by means of flowing gases. Applications: Igneous melting and crystallization studies, annealing, diffusion studies, crystal growth studies, preparation of starting materials.
The pressure vessel is a hollowed out rod of high strength metal. The sample and a small amount of water are added. The rod is placed in an external tube furnace and the whole assembly is heated. However, the open end of the rod where the thermocouple enters is outside the furnace, and sealed with a cone seal. Experimental conditions range from near surface conditions up to 800 ° C and 3, even to 5 kbar. Applications: simulations of upper crust conditions, including low-grade metamorphism, magma chamber studies, hydrothermal alteration, fluid inclusion studies and much more.
Internally Heated Gas Vessel
Samples sealed in precious metal capsules are placed inside a pressure vessel that is pressurized with argon gas. The whole vessel is heated and the argon is pressurized more to provide pressures up to 10 kbar (5 kbar is more typical). Temperatures can be up to 1400 ° C. Advantages include a T-P range that piston cylinders (see below) cannot reach, a larger sample size or ability to experiment on several samples at once, and more accurate and precise pressure measurements. Disadvantages include difficult (and dangerous) operation and upkeep.
A sample is placed in a precious metal capsule and packed into a pressure assembly, which is placed in a cylindrical opening in a pressure vessel (basically a large disc of high-strength metal). A piston advances into the cylinder, pressurizing the solid materials in the pressure assembly. Salt (NaCl) is one of the best pressure mediums to use as it deforms slowly, allowing experimental pressure to approach hydrostatic pressure. Pressures of 5 to 60 kbar (< 30 kbar more typical) can be reached. How pressure is measured and calibrated should be described in each experimental study as accurate and precise measurement can be a problem. The sample is heated by running current through a graphite sleeve. Temperatures of 1100 °C can be reached, or even higher if glass or other pressure media are used. Applications: simulations of middle and lower crust conditions, metamorphic reactions, high-pressure melting, diffusion studies and more.
A two stage process is used to reach pressures up to 250 kbar. Samples are placed in holes drilled into a ceramic octahedron. Six or eight anvils (designs vary) press upon on the octahedron. A second stage presses upon these anvils. As in the piston-cylinder, current is passed through a graphite or a lanthanum chromite furnace to heat the sample. Mantle pressures and temperatures can be reached. These machines are large and can be very expensive to run and maintain.
Diamond Anvil Apparatus
Two diamond anvils with a thin metal gasket and a layer of sample are pressed together in a small frame. Imagine two gem-quality diamonds mounted bottom point to bottom point. Because diamond is transparent, phase changes and X-ray diffraction measurements can be done in situ. Disadvantages are the small sample size and measuring experimental pressure and temperature accurately and precisely. Applications include lower mantle to core phase relations, equation of state measurements of high-pressure phases, and more.