Behind the scenes: starting a petrological experiment!

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1. A: Gold capsules (not bullets) containing rock and fluid, 2 cm in length. B: Balance used to weigh capsules and their content

I had never thought experimental petrology could resemble cooking to such an extent. With some imagination you can replace the basic ingredients of your favourite pie recipe and use a rather special type of oven to make the magic happen. Actually, every experiment has its own experimental “cooking” setup, with the desired quantities of components raised to specific pressures and temperatures that will (hopefully) lead your experiment to success.

So what is all this for? I am performing experiments aiming to simulate environmental conditions found in nature, to reproduce geological processes in the laboratory. The experiments I am doing, specifically, aim to simulate the reaction between a rock and a fluid found deep in the lower oceanic crust.

But this is no ordinary experiment. It’s actually not an endeavour you can get away with at home; the temperatures and pressures needed for experimental petrology are far above baking a pie in your own kitchen (see Justine’s post). To do this you need an oven (furnace, Image 2A) that will heat the capsule to the same temperatures consistent with the geological environment you’re trying to reproduce. In addition, you will need high pressures, as we find at the depths of the Earth’s lower crust, below kilometers of rocks. For this we will be using a system that pumps water into the system at high pressure. This is called a water cold seal pressure vessel.

As a start, you will need some tens of milligrams of rock or single minerals. In the case shown here the rock used is olivine gabbro and it has been drilled from the lower oceanic crust (for more information, check out Barbara’s post on the structure of the oceanic crust). Now you add a few microlitres of the desired fluid, in this case salt dissolved in pure water. Finally you will need a couple of centimetres of gold tube to keep the ingredients in (as a substitute of a baking dish), and you will assemble your capsules (Image 1A) with the help of various tools you can find in an experimental lab and a welding device. Now that you’ve gathered your “ingredients” I will proceed to take you through the experiment.

The first step is to prepare the golden capsules that will contain our ingredients. It’s one of the most important and delicate steps since the tiniest mistake could jeopardize the entire experiment. To check if the capsules are airtight, a condition that will avoid any leakage of material, I proceed to a double weight measurement (Image 1B). The first one is made once the capsules are welded shut and the second one is done after they have been placed in an oven at above 100°C overnight. If the two measurements are equal, this means that we did not lose any water upon heating. We can then proceed to the next step which takes place in the workshop.

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2. A: Cold seal pressure vessel furnaces. B: Vessel attached to pressure line and spanners that want to get it out.

Once at the workshop we switch on the furnace so that it’s hot enough and ready to start the experiment as soon as possible. First we need to remove the vessel from the experiment I finished the previous evening. Sometimes it’s pretty tough to unscrew from the pressure line, so we will use some spanners to get it out (Image 2B). Now let’s open the vessel (a bit of elbow grease is needed here, as well as the help a really oversized spanner, Image 3B). Once we’ve unscrewed the top of the vessel we are able to remove the capsule of the previous experiment and place the one we just filled inside, close it and fit it back onto the pressure line.

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3. A: Vessel with added wire and cable for temperature detection. B: Myself opening a vessel.

After this we have to add a temperature-sensitive wire to the vessel (Image 3A), which is attached to a computer with a cable to enable us to monitor the temperature in the vessel as the experiment proceeds.

Now our furnace is hot and ready. It’s very important to wear gloves for the next step, remember we’re dealing with very high temperatures (Image 4A). With very little pressure inside (Image 4B) we will put the vessel in the furnace and wait for it to heat up. While the temperature progressively increases, the pressure in the system will also go up thanks to water heating.

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4. A: Wear protective gloves before putting the vessel into the furnace, it’s really hot! B: Pressure is still 50 bar. C: Pressure has increased!

After a while the vessel with our capsule inside has reached the desired temperature (750°C in this case) and the pressure has also increased (Image 4C). However, we need to give the pressure one last push to reach the 2000 bar required by our experiment, that is almost 2000 times standard atmospheric pressure (1 atm = 1.013 bar). So we pump the remaining water into the system and then we can close all the valves and go back to the office. But always remember to come back later to check how the experiment is progressing! We need to make sure that our experiment is running smoothly and kept at the right temperature and pressure.

What do you think? Was it not a bit like the procedure of assembling ingredients and baking a cake? The delicious bite in our case will be the analysis that we will perform on our piece of rock, after the experiment, in the electron microprobe, to see what changes have happened and what reactions have taken place while the sample was at high pressure and temperature.

There are many kinds of apparatuses and experimental possibilities for simulations of Earth Science processes. Check out what is available at the facilities of the Mineralogy Institute of Leibniz University Hannover. And don’t forget to check out Justine’s post on How to reproduce inner Earth pressure and temperature in a laboratory.

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