This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The geological history of central Europe is complicated. The region is composed of several continental blocks having quite distinct origins that came together over 300 million years ago in the Paleozoic Era. Then in the Mesozoic, many of the original rocks were overlaid, and continued plate movements caused mountain belts to form. In a previous Geology Bites podcast, Douwe van Hinsbergen explained how he used an analysis of the geological structure of mountain belts to reconstruct tectonic plate motions. In this episode, we hear about a totally different approach to reconstructing plate motions and paleogeography, and see how it was able to reveal the plate motions that assembled central Europe.

Ulf Linnemann is head of the geochronology department at the Senckenberg Museum of Mineralogy and Geology in Dresden. In his research, he uses detrital zircon crystals, which are crystals that have survived the erosion of the magmatic rocks in which they formed, and that have been recycled into sedimentary rocks. He and his team have determined the ages of large populations of detrital zircons as the basis for disentangling the events that led to the assembly of central Europe. Ulf Linnemann, welcome to Geology Bites.

Ulf Linnemann: Thank you very much for inviting me.

Oliver Strimpel: As I mentioned, detrital zircons, though they originated in igneous rocks, are found within sedimentary rocks. How do they get from the igneous rocks to the sedimentary rocks?

Ulf Linnemann: They come from magmatic rocks into sediments, first by the weathering of the magmatic rocks, then erosion of the magmatic rocks, transport by rivers, or by wind, or by ice, and then they become deposited in a basin, like on a marine shelf, or a riverbed, or a desert. They have an extremely high hardness. They can survive strong weathering processes, and that makes them a favorite for uranium-lead dating of minerals in sedimentary rocks.

Oliver Strimpel: OK. So the sedimentary rocks in which you find these detrital zircons are younger than the igneous rocks from which the zircons originated.

Ulf Linnemann: That is correct.

Oliver Strimpel: So when you get the date of a detrital zircon, what is the exact date you're getting?

Ulf Linnemann: That is the age when the zircon was formed, when a zircon crystallizes in the magma. When the magma becomes cool, it has a so-called closure temperature. The zircon closes its lattice at about 800 degrees, and so below 800 degrees normally there is no exchange with the surrounding materials. And that is the age that you finally measure in the lab.

Oliver Strimpel: Can you remind us briefly as to how you find the age of a zircon crystal?

Ulf Linnemann: The zircon is one of the minerals which collects uranium in very large volumes. It can store uranium up to 3.5 percent in the lattice, and it accepts normally no lead. Uranium is a radioactive element, and it decays over a decay chain finally to lead. If you are able to measure the content of the uranium parent isotopes and the content of the lead daughter isotope, then you can calculate the time by making use of the half-life. You can calculate the age quite easily.

Oliver Strimpel: So let's talk about how you went about using these detrital zircons to reconstruct the assembly of Europe. When you sample a particular sedimentary rock and you date the detrital zircons it contains, do all the zircons give you a single date, corresponding to the crystallization age of a single igneous rock that acted as their source?

Ulf Linnemann: In principle, you have different ages. I have to mention that zircons can be complex. Sometimes they have a core, an old core. That is a remnant from an older melt. If a melt remelts a rock, heats up a rock, then the zircon survives the melting process in part and swims in the magma. So an older zircon from an older rock swims in the magma of a new melt. And then, when this melt becomes cold, this old zircon grain becomes used as a crystallization core, and new zircon material grows around the core. So this is a complex zircon. The core has an old age, for instance 2 billion years, and the rim has a younger age, maybe 540 million years. So you can measure two ages. But normally each zircon has only one age, inherited from the magma in which the zircon has grown.

And if you have, in a sediment, in a sandstone, for instance, or in a quartzite, a mixture of zircons from different sources in the hinterland, from different rhyolites, gabbros, basalts, whatever, you have also different ages in the zircon populations. You have to measure at least 100 or 200 zircons per sandstone sample, and then you will get a representative age cluster. And the youngest zircon population gives you a maximum age of sedimentation. That means the youngest zircons you measure give you an age constraint, but it is not the age of origin of the sediment; it is the maximum age. That means if your youngest zircons in a sandstone are 330 million years old, that simply means the sandstone is younger than 330 million years. And then you have the older zircons, maybe a zircon population that is 2 billion, a zircon population which is 3 billion years old. So that tells you a lot about the hinterland composition from where this sandstone is derived.

Oliver Strimpel: OK. So that's interesting. So a given sedimentary rock may have different zircons with radically different ages, each of which is indicative of a different origin, but those rocks still found a way into the same sandstone. When we talked earlier, you mentioned the idea of a zircon province. What is that?

Ulf Linnemann: A zircon province is a larger area which is characterized by certain zircon populations. We could figure out that the northwestern part of Africa has its own zircon population, or Arabia, or India, or Baltica. So these continents deliver the detrital material via the drainage system to the coasts. And at the coasts, the sea mixes the sands and the sediments in which the zircons occur, and finally you have sediment homogenization at the beaches. You mix the sand along the coast, maybe of northwestern Africa, by the currents of the sea, so the shelf sands of an entire region can be characterized by a certain zircon population. So today you can identify from where the sand is derived, if you have a huge database of potential source areas. For instance, you can say, OK, this sand must be from West Africa, or you can say this sand is from Atlantic Canada. So each continent has its own fingerprint of zircon populations in the recent sands along its coasts, and that was also the case in former times, in the Paleozoic or the Mesozoic or the Cenozoic.

Oliver Strimpel: That's fascinating. So in a given region, there'll be, if you like, a sort of catchment area where the detrital zircons come from many different rocks, which I suppose would correspond to different mountain ranges in the hinterland, that all gets washed down by different rivers into the same ocean and then mixed by tidal currents and waves and so on. How big a region are we talking about as this source? Is it tens or hundreds or thousands of kilometers across?

Ulf Linnemann: That is an interesting thing. If we talk about giant drainage systems, look at the River Nile or the Amazon or the Mississippi River. They have a huge hinterland. But there's also another limitation: how far can you transport a zircon in a river before it becomes totally destroyed? Even a zircon, which is characterized by its huge hardness, becomes eroded during its transport along the bottom of a river by mechanical interaction with the other sand grains. But there is also still a "but": zircons can also survive within a pebble. If you take a pebble that starts its journey at the source area of a river, and it is big in the beginning, the pebble becomes smaller and smaller and smaller, but when it comes to the river mouth, it is still large enough to have enough zircons in it. And then if it becomes weathered at the beach or in the river mouth area, it releases its zircons from the inside into the sediment. So it is difficult to say. There are cases where the zircons can survive thousands of kilometers. If you have a single grain, it cannot travel so far, maybe 100 kilometers or so.

But at the beach of a continent, you have sediment homogenization because of the mixing on the shelves and the interaction with the sea, with the breakers at the beach or on the shallow shelf. And then you have a mixture, but it's more or less identical over hundreds and thousands of kilometers along the coastline. So it is useful to think in terms of zircon populations which characterize a certain area or continent.

Oliver Strimpel: And so these form what you've been calling the zircon provinces.

Ulf Linnemann: Yes, these are the zircon provinces.

Oliver Strimpel: OK, so let's get back to Europe, and what are some of the specific zircon provinces that play a part in the disentangling of the history of Europe? I should mention that if you can look at the podcast web page while listening, the maps of the zircon provinces posted there will help you follow this part.

Ulf Linnemann: OK. If you look at today's Europe, you can identify a big northern continent: Scandinavia, large portions of northern Poland, Denmark, Sweden, Norway, Finland and parts of Russia. We call it Baltica. The southern margin of this continent has a zircon population we call the Sveco-Baltica Province, characterized by zircon populations with ages between 900 million years and 1.8 billion years. This is very significant for this northern continent, which forms the largest part of northern and eastern Europe.

Then you have Avalonia. That is very different. Avalonia is basically the southern part of Ireland, the southern part of the UK, England and the Midlands and Wales, and then northern Germany and Belgium, part of southern Denmark and parts of southern Poland, and you can follow it down into Eastern Europe via Bulgaria, finally into Turkey. On the other hand, you can follow it also in the Appalachians. So it is a huge ribbon continent.

And then to the south of Avalonia, you have a part we call Cadomia; other geologists call it Armorica. Cadomia and Avalonia both have a strong Neoproterozoic zircon population, with peaks at 600 and 570. And then you have in Cadomia a 2-billion-year-old event, the so-called Eburnean event, and then also a few Archean ages, let's say around 2.5 to 3.4 billion years. Avalonia, in contrast, has also the Neoproterozoic peak at 600 I mentioned, and then it has also a strong Mesoproterozoic portion of zircons (the Mesoproterozoic means between 1 billion and 1.6 billion years), and then a few 2-billion-year-old zircons.

And then you come finally to northwest Africa, characterized by a peak of most zircon ages between 1.8 and 2.2 billion years, which is the Eburnean event that formed most parts of northwest Africa. And then you have nothing younger than 1.8 billion years in the Precambrian. But you have a strong occurrence of Archean ages between 2.5 and 3.5 billion years. So you can distinguish all four regions. In the case of Baltica and the case of Africa, we talk about continents; in the case of Avalonia and Cadomia, we talk about terranes, or you can call them microcontinents.

Oliver Strimpel: So you have four zircon provinces relevant here, going from north to south here: Baltica, Avalonia, Cadomia and northwest Africa. Each of these provinces then has its own distinctive age distribution among its zircons.

Ulf Linnemann: That is right. And now between Africa and Cadomia is the Mediterranean Sea. But when we talk about the assembly of the supercontinent Pangea, there was no ocean between the European crust and Africa, and Baltica. All was really close together.

Oliver Strimpel: So what can the age distributions of these detrital zircon provinces tell us about the ancient positions of these four plates, and how they came together to form central Europe?

Ulf Linnemann: If you look at different time slices, for instance in the Cambrian or in the Ordovician, sometimes one plate collides with the other, and then detrital material is spilled from one plate to the other. For example, Baltica delivered its sand to Avalonia from 450 million years onward. So that means simply that Avalonia collided with Baltica at 450 million years. There's also strong evidence from paleomagnetic studies for that, but we see that also in the detrital zircon record: in the moment when Avalonia is docked by plate tectonic processes onto Baltica, then you find Baltica-derived sands on Avalonia.

Oliver Strimpel: So that's fascinating. What you're saying is that at the age of the sedimentary rock, when the sedimentary rock was put together, the population of zircons it had access to contains a signature not only of its own block, but the signature of a neighboring block that was adjacent to it. So in the population distribution of, say, an Avalonian rock, you find a signature that looks like it is that of a Baltica rock.

Ulf Linnemann: That is true. Baltica enlarged its drainage system into Avalonia and loaded Avalonia with the sands from Baltica.

Oliver Strimpel: Did it go the other way as well? Did Baltica get kind of polluted by Avalonian sand?

Ulf Linnemann: It depends where you are. There is a study from Japanese scientists in Estonia, and you find detrital material from Avalonia. If you go to northern Germany, to the Rhenish Massif, for instance, the basement is Avalonia, but in the Cambrian and in the Ordovician and in the Devonian you find only detrital material from Baltica. So there is a real exchange between Baltica and Avalonia. It depends where you are; it depends where the former drainage system was situated.

Oliver Strimpel: Do those detrital zircons that originated from Baltica, do they go even further, and you find them further south, in Cadomia as well, when Cadomia arrived later on?

Ulf Linnemann: Yes. If you close the ocean between Cadomia and Africa to the south, and you have the so-called Rheic Ocean in the north, and it collides later with Avalonia and Baltica in the north, then in the moment you have the closure done, you have an exchange of detrital materials. So in this moment we had for the first time all four landmasses together. You had at 540 Baltica in the north, then further south you have Avalonia, then you have a strip of Cadomia, and then you have West Africa. That we can say because in the lowermost Cambrian you have sediment homogenization on all the landmasses. And in this time slice you have a mixture of the Baltica provenance, the Avalonia provenance, the Cadomia provenance and the West African provenance. So we can use the detrital zircon composition of all lower Cambrian sediments on all four landmasses to say that Baltica collided at 540 million years with Avalonia and Cadomia and West Africa.

Oliver Strimpel: Wow, that's incredible. So it's a snapshot at the age at which a particular sedimentary rock was laid down, because that's the age at which all these detrital zircons were sourced. So if a rock is 540 million years old, a sedimentary rock, and it has zircons from West Africa and zircons from, say, Avalonia, then you know it must have been near both of those terranes at that time.

Ulf Linnemann: That is correct. And that's why detrital zircon analysis is so important for plate tectonic reconstructions and reconstructions of paleogeography.

Oliver Strimpel: I want to ask how far it's possible to push this detrital zircon technique. For example, if you have in a particular terrane, say Avalonia, populations that seemed to come from Baltica, that have the Baltica signature, and also some zircons have the age distribution corresponding to West Africa, are you able to look at the relative abundances of those two and say, well, if there's more of the Baltica zircons, then maybe Avalonia was closer to Baltica at the time than it was to West Africa, or if it's the other way around, you know that it was closer to West Africa rather than Baltica? Is that pushing the technique too far? Or is that something you can infer?

Ulf Linnemann: One of our studies has shown that in some cases this is possible. If you take this collage at 540 million years, composed of Baltica in the north, then Avalonia to the south, and further south Cadomia, and further south West Africa, you have a mixing pattern. You can see it in the ratios between the certain zircon populations. So the Baltica zircon provenance you can follow from the shorelines of Baltica into Avalonia and then into Cadomia, but it becomes less and less and less. And if you come from the south, the West African zircon provenance just pinches out to the north. So in Cadomia it's very strong, but in Avalonia you have less and less and less. In some cases you can find also, in that time slice, West African zircon populations in Baltica, and vice versa, Baltica zircons in West Africa, but less than 5 percent, less than 1 percent.

Oliver Strimpel: Obviously, it's going to go from high ground. Like if you have mountains, it's going to travel down to the lowlands. So I guess it reflects the paleotopography, if you like. I presume, to be able to do these kinds of studies, you must have dated an enormous number of zircons. Can you give a ballpark estimate of how many zircons you've actually dated, and sort of how long it takes to do each one, or each group of them?

Ulf Linnemann: Good question. We've been doing it for 10 years. In the beginning, when it had just become established, you could date 60 or 100 per normal working day. Let's start with the SHRIMP, the sensitive high-resolution ion microprobe, first established in Australia. They could date 60 grains in 24 hours.

Oliver Strimpel: Is that one of the laser-based techniques?

Ulf Linnemann: No, that is an ion microprobe. You analyze the zircons with the help of an ion beam. The SHRIMP was the principal instrument, established in the early '80s, for in situ measurements: you drill holes into a zircon. If you analyze with the laser, on a normal working day you can do between 200 and 250.

Oliver Strimpel: So you must have literally dated thousands or tens of thousands of zircons in the last 10 years.

Ulf Linnemann: Yeah, it's hard to say. I guess a hundred thousand, no, not one but two hundred thousand grains, maybe more. I don't know.

Oliver Strimpel: Can you see any technological innovation coming along? Or do you fantasize about some new techniques or machines that would make your work quicker, less labor-intensive?

Ulf Linnemann: It would make it quicker if you could reduce the analytical time. But I don't like that idea, because you have also to process the samples; you have to calculate the ages. So you need time for that too. I think we will see an improvement in the speed of analyses and in the speed of calculating the ages from the raw data, but there's also a lot of potential to make the processes before measurement more effective. You have to bring out the zircons from the rock. You have to crush it, you have to make a heavy mineral separation with heavy liquids and complicated processes, you have to use a magnetic separator, and you have to pick the zircons from a concentrate under the binoculars. So maybe these processes can be made much more effective. On the other hand, maybe other dating methods will come on the market.

What we want is more precision. That is maybe more important: not to speed up so much, but to get more precise ages. The manufacturers who do the construction of the lasers and of the mass spectrometers develop better and better mass spectrometers, and in my point of view, we should look for a development of much better precision of measurement.

Oliver Strimpel: So I can see how the dream is the accuracy of one part in a million that you get from thermal ionization mass spectrometry techniques, but with the ease, convenience and speed of the laser ablation or the SHRIMP methods. That would be the Holy Grail.

Ulf Linnemann: Yes, indeed. Hopefully.

Oliver Strimpel: Ulf Linnemann, thank you very much.

Ulf Linnemann: It was a pleasure. Thanks a lot.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.