Another way to cook rocks: in water!

Remember the blogpost where experiments were compared to cooking? Well, we can take that parallel even further and use more water to heat the rock with! As I mentioned in a previous post, rocks at the seafloor can get altered when fluids move through them. We call these hydrothermal systems, which can have spectacular venting sites on the seafloor. One way to find out more about the process behind these systems is studying natural examples that underwent hydrothermal fluid circulation; another way is by trying to reproduce these processes in the lab. But how can we do this when water is involved?

In general there are two ways to look at a hydrothermal system in the lab: as what we call a batch experiment (just put rock powder and fluid together in a container and heat them statically) or as a dynamic flow-through experiment, in which a continuous flow of fresh fluid goes in, reacts and exits at the other side of the rock mass. Both types of experiments can be done, and I would like to tell you more about the practical side of it. They both take longer than the average food preparation though; some scientist leave such experiments running for up to several years!

I will start with the batch experiment (left side of the figure), because this is easier to imagine and to perform in the lab. It is a simple matter of crushing rock, preparing a fluid of the desired composition by dissolving certain minerals or chemicals in it, closing the bag in which you combine the rock and fluid tightly, and start heating. Now here is our first problem. A hydrothermal system can often reach temperatures well above 100°C, at which water starts boiling. So if we use water-based fluids (water with dissolved minerals or chemicals), this will start to boil, expand and may rupture the bag that we put everything in. Fortunately we can resolve this problem by putting the whole system under pressure. Just like in real life at the seafloor, we apply pressure from the outside (similar to the water column) and the pressurised water boils at higher temperatures. For example at 200 bars (which corresponds to a depth of roughly 2 km beneath the sea surface), it only starts boiling around 370°C. We can apply the outer pressure by air and water, keeping the bag with the reacting rock-water mixture intact.

Figure 1. Sketch of the different setups in the laboratory. Left is a batch experiment and right is a flow-through experiment

A flow-through experiment is more tricky to perform. Here we need to set up a cell to isolate the rock powder, and a system of tubes that leads the fluid towards this cell. We then have to heat the fluid and the powder, and at the end we have to make sure the fluid can go out again, and that tubing does not get clogged. Of course here we also have the pressure problem, which can be solved by putting an outer pressure on the cell again. I made a sketch of an example of this setup that we have in Bremen (on the right side in the figure). The advantage of this type of experiment is that you get out reacted fluid at all times, which you can measure without problems. In a batch experiment you can take out fluid as well, but you would have to stop the experiment run, or take out the hot fluid through a sampler. But then you change the system: you take away the reacting fluid during the experiment and you cannot put it back, so you’re changing the experimental condition while it is ongoing. We like to take fluids from the reaction, because when we measure their chemical composition, we can ‘read’ the reaction inside from that fluid composition, and understand what’s going on is the whole point of the exercise. The flow-through experiment is also closer to what happens in nature when the fluids circulate in the porous rocks.

So, just like with Adriana’s post, this lab work looks a lot like cooking. Of course, both experiments are quite cool to do, and provide you good results if you are careful.

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