Brian Upton on the Unique Rift Zone of South Greenland
Note, transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is Geology Bites with Oliver Strimpel. Since the first geological surveys of southern Greenland in the 19th and early 20th centuries, it has been recognised that the region contains some remarkable igneous rocks. These include extraordinarily thick dykes up to 800 metres thick and elliptical intrusions, several kilometres in size, containing a profusion of exotic mineral. In the 1930s, this period of igneous activity was christened the Gardar period, after the Norse archbishopric of Gardar, established in the area in the 12th century. In the 1970s, the rocks of the Gardar period were dated using uranium lead radiometric dating and found to be between 1.1 and 1.3 billion years old. The dating also revealed that most of the rocks surrounding the thick dykes and elliptical intrusions of the Gardar period is a giant granite batholith that is 1.8 to 1.9 billion years old. The batholith was emplaced during a mountain building event accompanying the assembly of the Paleo Proterozoic supercontinent called Nuna, about 700 million years before the Gardar rocks were formed. For a refresher on Nuna and supercontinents in general, you can listen to David Evans in an earlier episode of Geology Bites. It was not until the 1950s that geological studies of the area revealed just how exceptional the Gardar rocks are. One of the leading researchers who brought this to light is Brian Upton, who has studied the region’s geology intensively and spent over 20 seasons in the field between 1957 and 2005. In what follows, Brian Upton discusses both types of Gardar intrusions, the giant dykes, and the elliptical intrusions that cross cut them. He refers to the latter variously as alkaline complexes and central complexes. Brian Upton is Emeritus Professor of Geology at the University of Edinburgh. Brian Upton, welcome to Geology Bites.
Brian Upton
Thank you very much. I'm very happy to be with you.
Oliver Strimpel
What led you to the Gardar region when you first arrived on the scene in 1955?
Brian Upton
Principal motivation for this was partly for looking for more uranium, but also looking for more cryolite for the aluminium industry. In the course of this, we were just mapping everything as we went along.
Oliver Strimpel
What structural features did you find?
Brian Upton
But I was put on to some fascinating ground that had recently been discovered by a helicopter reconnaissance work, which included some very large dykes and some new alkaline complexes. I was delighted by that, and it was only in the fullness of time that I realized it was part of the whole system, the whole evolutionary process that led to the extraordinary rocks.
Oliver Strimpel
So you found these enormously thick dykes. What do you think might have caused the dykes to be so very thick?
Brian Upton
Well, during the Precambrian, the heat flow to the upper mantle and into the crust was considerably greater than it was subsequently. We're talking about 1000 million years ago. And I think the energy was sufficient that when dykes were being intruded around the world, there were some rather big ones being produced. We see these in Canada. We see these in Sweden, extreme SW Australia, and the biggest of the lot is the Zimbabwe Great Dyke, which is about 13 kilometres across and hundreds of kilometres long. But I didn't know how these I was looking at in Greenland might relate to the others, and in fact it is chemically rather different. It's unusual in its high content of the so-called incompatible elements. It's an alkalic gabbroic dyke, whereas all of the others are by no means alkalic.
Oliver Strimpel
What can we infer about the tectonic context that led to the formation of these huge dykes in the Gardar Province?
Brian Upton
Really nothing major happened in that area for several 100 million years, but there was a large granite batholith that had been formed after the last ocean closure there, and the new activity, the Gardar activity, tended to be concentrated along the axis of this granite batholith. And it looks as if the centre of the batholith was mechanically weak and had given way to allow magmas to come up through fractures and a whole bunch of dykes, which in places amalgamated to form one which was as big as 800 metres.
Oliver Strimpel
You found dykes, but you also found some large mountain scale intrusions. Can we talk about those a bit.
Brian Upton
We started getting these elliptical things, which almost certainly were feeding volcanoes comparable to those of the East African Rift, but with the top two or three kilometres missing. Because this whole region of the Gardar Province and the rocks around it had been uplifted very, very gently during the course of the thousand million years or so. It had never been folded and never been badly altered in any way. It had been lifted up and in so doing it had been exposed to erosion, which had taken the skin off, it had taken the top 3 or 4 kilometres off. So we were looking modestly deep in the crust, but not very deep. But we're losing the surface products.
Oliver Strimpel
So these elliptical intrusive complexes are syenites. What are syenites and what minerals do they principally contain?
Brian Upton
Well, I have to say that the most widespread and the important mineral on the whole surface of our planet are those of the group called feldspars, which are aluminium silicates. But the aluminium silicates are held together by other elements, in particular by calcium, or by potassium, or by sodium, and those with potassium and sodium are called alkali feldspars. And if you've got a rock which is dominantly made of alkali feldspars, it's called a syenite.
Oliver Strimpel
So how did the rocks of these central complexes compare with those of the giant dykes?
Brian Upton
The one I was mostly interested in was one which cut right across both of the sets of giant dykes and was a very much more granitic type of rock. The big dykes were all near to the gabbros, very much richer in magnesium and calcium and iron. And these central complexes are much richer in potassium and sodium and aluminium and silicon. So I think they were probably products of crustal melting. It was probably remelting of the old batholithic granites, which were the source of these things, but we've never really been able to get a definitive answer.
Oliver Strimpel
You went back to study these rocks over 20 times over the course of many decades. What made these rocks so special?
Brian Upton
They were remarkable for minerals containing a whole variety of rare metals. I've mentioned uranium, zirconium and the elements that go with that like niobium and yttrium, but also elements which we know now are extremely important in 21st century technology. Whether we're talking about mobile phones or wind generators.
Oliver Strimpel
But quite a lot of syenites around the world do contain some exotic minerals containing rare elements, don't they?
Brian Upton
Nothing nearly like some of those from the Gardar Province. And the one were talking about specifically was the Ilimaussaq complex, which again cross cuts these giant dykes I'm talking about. It's on the same rift system, the same linear system, but much later from the dykes. And the Ilimaussaq itself is culminating all of that activity. It finished about 1104 million years ago, and since then there's been virtually no activity.
Oliver Strimpel
OK, so the intrusion that most attracted your attention was called the Ilimaussaq intrusion. How did it get all its exotic minerals and rare elements? Is it basically a result of extreme magma fractionation in a magma chamber?
Brian Upton
Yes, and fractionation was taking place with unusual ease and readiness, because these Gardar rocks have quite high fluorine contents to them, and fluorine is an element that makes a huge amount of difference to the melting of igneous rocks. First of all, if it's there, the melting points are lower than if it's not. Secondly floating, or rather its ion fluoride, is extremely efficacious in breaking down complicated silicate structures. And in so doing, it makes the magmas, the liquid, the molten material very, very much more fluid. It can flow very much more readily, not down to water or paraffin fluidity, but very much more than most natural molten magmas because it was easy for things to move in it. It means that again, not atoms, but ions can move quite long distances unimpeded to join up and form large crystals. And crystal fractionation depends on being able to remove crystals. And there are two ways of doing that. Either the crystals are so dense that when they form that they sink to the bottom of the magma chamber that they're in and leave the liquid above it impoverished in that type.
Oliver Strimpel
But the liquid above it would then be also enriched in those other materials that didn't make it into the crystals that sank.
Brian Upton
Or alternatively, and this applies particularly to Ilimaussaq, some other minerals are of low density can actually float, so they go to the top of the pot. I used to liken this to talking to students having a bucket of water and putting sand and fragments of cork into it. And of course all the sand settles off to the bottom and all the cork floats to the top, obviously depleted the water in between of both the lights and the heaviest. So a process very simply like that is involved in crystal fractionation and with the fluoride ions, as I say, the molten rocks, the magmas, are very, very much more fluid. So the crystals can float or sink much more readily than they can in most magmas.
Oliver Strimpel
So does that mean that because of this extreme mobility that's conferred by the presence of the fluorine and the chlorine, perhaps, that you essentially get much more extreme fractionation going on?
Brian Upton
Crystal fractionation occurs all over the world all the time, but it simply is able to proceed more effectively, more readily, faster and better in these fluorine-rich rocks than in those that are deficient in fluorine.
Oliver Strimpel
So part of the fractionation process you just described involves some elements forming crystals readily, while others do not and remain behind and become progressively more enriched. What determines whether an element remains behind or is a so-called incompatible element?
Brian Upton
Incompatible elements are those that just do not fit easily into crystal structures, so they tend to get left out. They get left as The Wallflowers around the dance hall. It's very small elements, small atoms. Lithium and beryllium are very small, and it's difficult for them to get taken into any structures. They tend to get left over and concentrated in late residuals of crystal fractionation. Or if you're a very big, fat ion, you can't get into any of these structures. You're robust. So things like potassium, barium, and particularly huge ones like uranium and sodium, are excluded and concentrated in late stuff. And then some of them just have very weird bonding habits to put it that way, and they get left out. Gold would be one of those. Nobody wants it, it just sits on its own, which is why you can go and find native gold in the rivers. You can't find most elements as native elements, so most of them combined, but some, like gold, platinum, mercury just get left out. That's why they’re all called incompatible. I tell the students it's a bit like human relationships. You're too fat, you're too small, or you're just awkward. You get left out.
Oliver Strimpel
And so the last rocks to form then are the ones that are most enriched in incompatible elements. Do we have any idea as to what percent of the original magma body that might have represented?
Brian Upton
A very, very, very small percentage. You start off with a huge volume of slightly enriched material. You take away all of the other minerals which are not those of the enrichment, and the more you take out, the more you concentrate.
Oliver Strimpel
And so we must be talking about a really large volume of material. Would that originally have come from the mantle or from the deep crust near the mantle crust boundary?
Brian Upton
No, this comes from the mantle, and we're talking about processing of huge magma volumes, which must be very many cubic kilometres of magma down below. Which are busy undergoing crystal fractionation, and then the batches of the melts produced by this rise to the surface and form the things that we can see and get out and collect and analyze.
Oliver Strimpel
So do you think that this mantle source of unenriched magma that then wound up being fractionated to less than 1% to form the final rocks that we see in these intrusive complexes. Do you think that was already then enriched in fluoride ions and chlorine ions?
Brian Upton
It was relatively enriched in the mantle rocks at depth. And then it gets more and more concentrated as crystal fractionation processes proceed.
Oliver Strimpel
Are there any theories as to why some mantle rocks might at that time in the Neoproterozoic at 1.1 to 1.2 billion years ago, have already been enriched in these halogens?
Brian Upton
Why the Gardar Province in particular was enriched is unknown. It's not just fluorine, it's also chlorine and to a lesser extent, its fellow elements, bromine and iodine. But the ones of particular interest are fluorine, and then the next heaviest is chlorine. In the Ilimaussaq, as crystallisation of the syenites took place, chlorine or chloride ions became more and more concentrated, and they linked up with sodium and the sodium silicate to form a mineral which was of low density so it could float, and it was now acting as fragments of cork in the sand and cork experiment. And instead of going down, it floated up to the roof or whatever chamber there was there. And in the Ilimaussaq you've got one unit which is about 17 kilometres across. It's about 10 kilometres broad, so that's 170 square kilometres, and then the thing is about 5 or 6 kilometres thick. So finish up with an extraordinary concentration of fluorine or sodium fluoride, et cetera, which is totally unique across the globe.
Oliver Strimpel
You mentioned that the Gardar Province contains several of these elliptical intrusive complexes. In addition to all the exotic minerals that we're going to talk a bit more about is Ilimaussaq also the place with the greatest concentration of fluorine?
Brian Upton
Another extraordinary concentration not very far away was a place called Ivigtut, where the Inuits had known there was a remarkable white mineral there, and they used it, I think, for washing soda and various other things before it was discovered. It was extremely useful in extracting aluminium from aluminium ore, the rock called bauxite. And it's primarily because addition of fluoride to bauxite lowers the melting temperature greatly. So it's a hugely cheaper extraction process. The amount of energy required is much less, and in that particular case it also turns the melt into an electrolytic solution. You can pass an electric current through it and you can separate the aluminium at one of the electrodes, and that is still the worldwide method of separating bauxite. Ivigtut was known from certainly the 19th century. Inuits were using it. And European explorers found this stuff and also felt aluminium process important. It was mined for the best part of 100 years, but during World War II when aluminium was critically needed, particularly for aircraft production, something like 12 million tons of cryolite was mined from Ivigtut and sent to the United States for the war effort. So we've got this place called Ivigtut, which is extraordinarily rich in fluorine. The whole Gardar Province has got high fluorine, but Ivigtut was quite ridiculously highly concentrated, not just one fluoride mineral I've spoken of which is cryolite, but 16 others. So Gardar Province would have been famous if it was for Ivigtut alone, but then we've got this other one I'm talking about, Ilimaussaq, and it comes back to the question why you get these super-high concentrations of the so-called halogen elements, fluorine, chlorine, bromine and iodine. They were concentrated. Why? We still don't know an answer.
Oliver Strimpel
So in those two regions, Ilimaussaq and Ivigtut, are there lots of other exotic minerals? And are there minerals that are only known there, or for which it is the type location where they were first discovered?
Brian Upton
Ivigtut has quite a lot of the fluorides there would be just about globally unique. Most igneous rocks, syenites and some granites, you would be lucky to find perhaps a dozen different minerals in a rock. In Ilimaussaq you've got 220 known different minerals, so it's an order of magnitude bigger. It is a ridiculous high concentration of extremely exciting goodies. So which is why Ilimaussaq has become such a focus of international interest. You could imagine every country in the world with a technological basis is desperate to get at it. I would suspect very strongly that Mr. Trump's bid to buy Greenland from Denmark was first and foremost concentrated on Ilimaussaq and one or two others in the Gardar province.
Oliver Strimpel
That's extraordinary, though, so the variety of minerals, say in a piece of syenite, that you'll find in this complex, the Ilimaussaq complex, is literally over about 10 times the number that you would find in the next closest alkaline intrusion or similar kind of plutonic rock.
Brian Upton
And many of them are absolutely totally unique to Ilimaussaq. The mineral that they were most interested in for uranium is an extraordinary mineral because it's basically a silicate. It's got silicon and oxygen, but it also doubles up by being a phosphate mineral, and so it's a silico-phosphate mineral. Now, I've never, ever heard of any comparable mineral anywhere else to steenstrupine, and it's the steenstrupine that contains the relatively high uranium.
Oliver Strimpel
I want to come back to the tectonics of the region and the history that we think must have happened. So because you've got dykes and you've got these intrusions, are we talking about an inferred region of continental rifting?
Brian Upton
It was an area of continental rifting about 1100 million years ago; it's quite narrow. It's only about 15 kilometres across.
Oliver Strimpel
So we're talking about rifting that was going on about 1.1 to 1.2 billion years ago. We're talking about the quarter of the age of the Earth. Can we use this extraordinary glimpse into an ancient rift zone to infer anything about how the planet might have been different then, or whether plate tectonics was different?
Brian Upton
But the planet started about four and a half thousand million years ago, and it must have been extremely hot, and it's been losing heat ever since. It's losing heat at the present time. But very, very, very slowly for the last few 100 million years. But 1000 million years ago it was still really quite warm, and as I used to tell students in the Precambrian they just did things bigger and better because they had more energy to drive things around so. We're back when the earth was not hot, but a lot warmer. This thermal loss through the crust was a lot more than it is at the present time.
Oliver Strimpel
And did that contribute to the extraordinary thickness of these giant dykes, and perhaps to the extreme mobility, then, of this magma body that led to this extreme fractionation in Ilimaussaq?
Brian Upton
Definitely! If you start looking at modern geology, let's say from about 700 or 800 million years ago. It doesn't sound very modern, but it is. You're getting dykes formed all over the place. But a dyke 20 metres across would be quite unusual. Getting dikes which are approaching a kilometre or more is really very, very rare. Indeed, there was just more energy available to pull things apart and to cause damage than there has been subsequently.
Oliver Strimpel
One of the things you mentioned when we spoke earlier was that these igneous rocks of the Gardar period appear to be in quite pristine condition. So before reliable radiometric dating results became available, did you conclude therefore that the rocks were relatively young, geologically speaking?
Brian Upton
They are in such beautiful, pristine condition that our first thought, these must be perhaps about as old as the volcanic rocks in the West of Scotland and the Hebridean region, which are about 20 million years or so old, but certainly not Precambrian. To say it's a bit of continental crust that the gods forgot about. They just left it alone after the Gardar period. Some things did happen because Greenland split apart from Canada, from Labrador. The continental region containing the Gardar rocks was elevated just very, very, very gently and slowly. It was lifted up by magical hands from a few kilometres down towards the surface. So of course everything on the surface is continually being rubbed away. Continually being sand-papered by wind and rain and glacial period. It's been rubbed away very effectively by ice. But even so, we're talking about an area all over of about 20,000 square kilometres where these Gardar rocks outcrop, and it's all been recently scoured over the last million years or so in the Ice Ages. But it's been extremely gently treated. It hasn't been folded. Watery fluids have passed through it so there’s alteration in your minerals, so it is a very, very remarkable example of a fossil continent. And I find that quite wonderful.
Oliver Strimpel
I want to end by asking you what it was like to do field work in Greenland, starting in the1950s, well before any of today's tourist infrastructure and mobile phone service was in place.
Brian Upton
Well, first of all, Greenland is a very big place. It's about 2000 kilometres from top to bottom. So if you cut it out on your globe and pasted it, say in the North End, over Scotland, the tail end would probably be somewhere over Iraq or Persia. It's a lot of it. It has an enormous coastline and thousands and thousands of kilometres of theodred coasts. It's not gonna get overcrowded. What was it like? Well, there must be something wrong with me, but I love empty places, unspoiled places. And those of us that are working there, you form very, very close friendships, very close, bonding with all sorts of mates which last for decades. I've worked all over the world. The times I've spent in the Arctic, everywhere from Siberia to Canadian Arctic. I love the people I meet. Extremely interesting people, very resilient, or they wouldn't be living there. People that don't know about hardships and can live very simply. We had no radio at the beginning. My mate and I would be dumped by boat on the coast with a load of supplies and the tent, and we’d be told we'll be back in so many weeks’ time. You look after him and he looks after you. So the Buddy system is critical. You need your mate and your mate needs you. So things don't fall in and don't fall off. You look after yourselves. The best years of my life.
Oliver Strimpel
Brian Upton, thank you very much.
Brian Upton
Enjoyed talking to you, Oliver. Many thanks.
Oliver Strimpel
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