Torgeir Andersen on Ultrahigh-Pressure Metamorphism

Oliver Strimpel (0:04)

This is Geology Bites with Oliver Strimpel.

Along the coast of Western Norway, and for thousands of square kilometers inland, the rock is grey-banded gneiss, the sort of rock exposed on continents everywhere. Geologists have studied it for well over a century. Scattered through that gneiss are pods of a darker, denser rock, and some of the crystals in those pods have specks inside them, too small to see without a microscope. Those specks can only have formed under pressures found about a hundred kilometers down, far below the bottom of the crust, down inside the Earth's mantle. That demands an explanation because continental crust is buoyant, which is the reason continents stand high and ocean floors sit low. If pushed down into the mantle, it ought to bob back up or refuse to go at all. And yet here is a piece of it that went to mantle depths, stayed long enough for its minerals to recrystallize, and then came back up to the surface.

Let me define a couple of words that will come up in the podcast. The first is eclogite. When a rock is heated and compressed, its minerals rearrange themselves into whatever is stable under the new conditions. So the same starting material gives you one set of minerals at 10 kilometers down and quite a different set at 50 Take ordinary basalt deep enough and it reorganizes into an attractive looking rock containing red garnet and a green peroxene mineral called omphacite. That is eclogite. It is a dense rock even denser than the mantle. In Norway, it occurs as pods and lenses within the grey gneiss.

The second is coacite. Coacite is a high-density polymorph of quartz, meaning it has the same chemical composition as quartz, i.e. silicon dioxide, but packed into a more compact crystal structure. Coacite forms only at pressures above about 27,000 atmospheres, equivalent to a depth of 90 to 100 km. Metamorphism above the pressure at which quartz turns into coacite is called ultra-high-pressure metamorphism.

Torgeir Andersen is Professor of Geology at the University of Oslo, and for several decades, he has worked on the Western Gneiss Region of Western Norway, the world's largest and best exposed area of ultra-high-pressure rocks. Torgeir Andersen, welcome to Geology Bites.

Torgeir Andersen (2:57)

Thank you very much. It's a pleasure to be invited.

Oliver Strimpel (3:01)

I said that geologists have been studying the rocks of Western Norway for a long time, but coesite was only discovered in 1984 Why did it take so long and how was it eventually discovered?

Torgeir Andersen (3:15)

Well, it took a long time because all these ultra-high-pressure minerals, like the coesite and the diamond as well, they are tiny little inclusions sitting inside the other minerals that are the eclogite fascis minerals that we all know quite a lot about, like the garnet and the omfasite and so on.

Oliver Strimpel (3:37)

Just a word about the term fascis, which is a set of minerals that forms together at a particular pressure and temperature. So eclogite fascis means the conditions at which eclogite minerals become the stable ones.

Torgeir Andersen (3:52)

So it takes a very, very careful mineral study of a number of eclogites to actually find these small, magical ultra high pressure minerals that define this ultra high pressure metamorphism. So it took a long while, but it was discovered in Western Norway as it was in the Alps at the same time.

Oliver Strimpel (4:16)

And what was it that enabled them to distinguish it from quartz?

Torgeir Andersen (4:21)

In order to actually prove that you have cosite, you need Raman spectroscopy that is checking exactly the length of the bonds in these ultra high pressure minerals because they have a tendency to back react when the rocks are decompressed. So quite commonly, the cosite has back reacted to normal quartz and during that process, it expands a bit and exerts pressure on the rocks in which it sits. So it creates radial crack system around these inclusions, which is often how you first suspect that there has been cosite. You see these strange radial cracks, and then occasionally a little part of the cosite is preserved. It has a slightly higher refractive index, so you can actually see it with a microscope and suspect that it is a cosite. But then you need to do a Raman examination in order to actually confirm it. And actually, the same goes for the diamond.

You also should have a proper Raman examination to have them confirmed.

Oliver Strimpel (5:39)

So, apart from cosite and diamond, which you've already mentioned, are there any other minerals whose appearance in a rock would unambiguously point to ultra-high pressures that we're talking about?