Drilling allows us to collect samples from the inner Earth, which provide us an idea of its composition and internal hidden processes.The deepest scientific drilling on Earth reached 12.262 km into the crust (Kola Superdeep Borehole in Russia), but what does it represent? The Earth has a radius of 6,378 km which is significantly greater than the 12 km of crust drilled at depth in Russia (Image 1). However it is extremely costly, time consuming, and for the moment there is no existing technology that would allow us to drill deep in the upper mantle (from 5 to 100 km deep). Therefore in order to complete our knowledge of Earth’s interior, geologists have to use different strategies, for example geophysical imaging. Another, less known possibility is experimental petrology, or “how to cook your own rocks”.
Experimental petrology techniques aim to reproduce deep-Earth processes, such as magma formation, evolution or storage below a volcano, or rock-forming mineral transformations during mountain building. This discipline uses different devices that allow you to simulate the conditions that prevail at high depths, including pressure that can reach 25 Gigapascal (which corresponds to ~700km below Earth’s surface) and temperature which can reach 2500 °C.
What is pressure?
Pressure is the force force per unit area exerted against a surface by a solid or a fluid. It’s expressed in Pascal (Pa), where 1Pa = 1 kg/(m·s2) = 1N/m2. In experimental petrology we often use bar where 1 bar = 1.0 *105 Pa.Therefore, pressure is inversely proportional to the area where the force is applied: the smaller the surface, the larger the pressure. As an example, imagine a 80 kg man wearing hiking shoes and a 60 kg woman in stiletto heels. Which one will exert the higher pressure on the ground? Just do the math…the surface of stiletto heels is so small than even an elephant puts less pressure on the ground! With that in mind, how can high pressure be generated? You can likely imagine that it is easier to decrease the area of the device that will apply pressure than it is to increase the force applied by it. This is how we generate high pressures in geosciences. Experimental chambers and the samples they contain are very small, a few millimeters only.
There are different devices used to reproduce inner Earth conditions in laboratory. These devices include Externally Heated Pressure Vessel (EHPV), Internally Heated Pressure Vessel (IHPV), different kinds of piston-cylinder apparatuses, multi-anvil presses, and diamond anvil cells.
A piston-cylinder apparatus is able to reproduce upper mantle conditions (Figure 1) by generating pressures up to 6 GPa and temperatures up to 1600°C. In this kind of device, pressure is exerted by a piston (like in a syringe) to compress each of the successive sample containers in a confining cell called a “bomb”. Image 3 shows the entire process from the sample generation through to analysis. Temperature is produced by applying an electrical current on the graphite part of the assembly, which then acts as a furnace.
What do we put inside the “bomb”?
Inside the assembly there is a small cylindrical capsule of noble metal, gold, silver, or platinum, which maintains the sample in an isolated environment and prevents external interaction. This capsule is generally smaller than 1 cm in height and 2-4 mm in diameter. The small size is required in order to have better temperature constraint; the heat generated by the graphite furnace is not homogeneous and there is a hot spot located in the middle. The sample starting material depends on the process that is being investigated. Typically the starting material is a powder of either natural or synthetic rock or selected crystals. To this water, different chemical solutions and trace/minor elements, can be added.
Rocks are heterogeneous in terms of rock-forming minerals and they don’t have equal melting temperatures, therefore a process called partial melting can take place. In order to investigate partial melting of the mantle producing magmas, a petrologist needs to do the reverse: a mantle peridotite powder is put in the capsules and exposed to high pressure and temperature in order to cause melting and from that it is possible to observe the the formation of primary melts. When simulating the conditions of crystallization, melting of magmas, or of specific mineral transformations, a petrologist is able to investigate the role of some key controlling parameters of those processes. For example, samples are investigated to study the water content in rock-forming minerals, the impact of the experiment run time, cooling rate, and many others.
If a petrologist wants to study magma crystallization in a magma chamber, he uses a quenched lava that has been ground into a powder, puts it in the capsules, and “‘cooks” it for some hours or days, at pressure and temperature conditions expected for a magmatic chamber in the crust. Then he decreases the temperature to allow for crystallizing of this experimental melt.
After the experiment is finished, the capsule is extracted from the assembly, cut, embedded into epoxy to be handled, and polished in order to prepare it for microscopic observations and for analysis. The two images below are examples of what you can observe when you investigate the rocks within the capsules.
Thanks to experimental petrology, it is possible to recreate magma chamber processes and to study the evolution of magmatic systems. We can also constrain magma chamber size and depth for hazards, risks, or industrial applications as mining or geothermal use. Investigations on mineral transformations (metamorphism) enable us to recreate P,T,t (pressure, temperature, time) path of rocks and to understand tectonics.