Polar Podcasts

24: Allen Nutman – “Faraway places with unpronounceable names” – dating Greenland’s ancient rocks

December 08, 2020 Julie Hollis Season 1 Episode 24
Polar Podcasts
24: Allen Nutman – “Faraway places with unpronounceable names” – dating Greenland’s ancient rocks
Show Notes Transcript

In this episode, we hear more from Allen Nutman, Professor of Geology at the University of Wollongong in Australia, about his work on dating some of the oldest rocks in the world, in the Isua supracrustal belt, close to the inland ice in the Nuuk region.


24: Allen Nutman – “Faraway places with unpronounceable names” – dating Greenland’s ancient rocks

Based on interviews held on August 23–24, 2019 in Nuuk, Greenland

Note: Polar Podcasts are designed to be heard. If you are able, please listen to the audio, which includes emotion and emphasis that's not evident in the transcript.


Allen 0:01

There was an Australian postdoc in the laboratory called Lance Black and Stephen Moorbath went off on holiday and said, “Oh, can you analyze McGregor’s samples. For every sample which turns out to be very old, I’ll give you a bottle of wine.” So anyway, Stephen Moorbath came back from holiday and I can’t remember how many samples, maybe eight. And Moorbath says to Lance Black, “How many bottles of wine do I owe you?” And he said, “Eight.”

Julie 0:32

Welcome to Polar Podcasts, where you’ll hear stories from geologists who’ve spent their careers, their lives, exploring and studying the remarkable and remote geology of Greenland. Why did they become fascinated with Greenland? What were the problems and the discoveries that drove them? And what was it like working in these remote places, where few people venture, even now? I’m Julie Hollis.

In this episode, we hear more from Allen Nutman, Professor of Geology at the University of Wollongong in Australia, about his work on dating some of the oldest rocks in the world, in the Isua supracrustal belt, close to the inland ice in the Nuuk region.

Allen 1:15

One thing which is particularly important in geology is to actually know the age of a rock. And it’s exactly the same thing for a historian. They want to know when a particular event occurred relative to another event. So in exactly the same way, geologists want to try and know the actual absolute age of a rock, when it was formed. Also they want to be able to, if possible, work out the actual absolute ages of some of the things that have happened to that rock since it was first formed. So, in the 70s and 80s, this was an extremely specialized field, to be able to obtain ages of rocks. Also it needed large amounts of material. It needed very definite assumptions about the samples.

You were doing something called zircon dating. Zircon is a mineral which takes in uranium, which of course then decays to lead. So, these little crystals actually act as a little chronometer in the rock. So, in the old days that kind of dating, would be done by literally a handful of specialist people around the world. You would have to collect a very big sample. That very specialist analyst would collect maybe something like 50 milligrams of zircon out of that rock. They would have to, at any one time, analyze together thousands of those little crystals that have come out of the rock.

Nowadays, the technology is where you can actually get dates on individual growth rings of those crystals. So ones not only just simply dating a single crystal, you can actually see within the crystal growth zones which have developed at different geological events and on the scale of twenty microns. Twenty microns is something like a fiftieth of a millimeter.

Julie 3:29

Similar to or thinner than the width of human hair.

Allen 3:32

That kind of scale, you can obtain individual ages. So it’s like looking at the growth of a tree where you have different growth rings. Of course those growth rings have occurred in different years. The same way if you look at a zircon crystal, the different growth zones in it refer to events millions or tens of millions or even hundreds of millions of years apart.

So, there has been just unimaginable changes in the technologies involved, not only in the scale that you can analyze things but also because of a development of the technologies the vast amounts of data that are now produced annually around the world on the age of geological samples.

So for example, in the 1970s, from the Nuuk region you might be lucky if there would be, the age of one rock by the zircon method would be published a year. Nowadays people could produce that age literally something like three or four hours analytical session. So there is this explosion of data regarding the age and history of the rocks which, of course, have greatly improved our understanding of the events throughout not only the Nuuk region but anywhere in Greenland.

So with these technologies they are a lot more user-friendly. So the early technologies it was done by geochemists. They were specialist chemists who were interested in geological problems. They generally had no knowledge of field geology at all. So the program of dating was done by some collaboration between a geologist and a laboratory geochemist and there had to be a trust and understanding of the geochemist that the geologist actually knew what he was collecting. Nowadays, because a lot of the technology is so much easier to handle for the dating or other analysis of rocks, the actual geologist themselves can undertake the laboratory work as well.

So as a second part of my, shall we say, existence, besides having a very strong focus on the field geology and the mapping, my other speciality is dating of geological events by using these zircon crystals. I have had more than 30 years experience of this using a particular instrument which has got an acronym SHRIMP

Julie 6:14

Which stands for Sensitive High Resolution Ion MicroProbe.

Allen 6:19

When I have guest people coming to to work with me to do this zircon dating with SHRIMP, you know, quite literally, once I’ve got the machine set up and in a stable operational state, they can actually be doing an analyses with fifteen minutes tuition. As my er, ex-boss, who is actually one of the inventors of SHRIMP said, “Even a trained monkey can run SHRIMP,” So this actually led to a proliferation of all sorts of stuffed monkeys in the lab and even people say then sometimes they’ll say, Oh, I’m not going to do some SHRIMP work, they say they’re going to do some chimp work.

So, it has been an amazing change in the technologies. Part of it is been actually strongly driven by two fields. One is nuclear power and nuclear weapons. The other thing, big spin off, of course was the Apollo program, in that vast amounts of money were being spent on getting people to the Moon and bringing rocks back from the moon. And a spin off from that was the enhancement of all the analytical technologies in order to get much more valuable and worthwhile information from the rocks that had come back from the Moon. 

At that time, the Earth Sciences in Australian National University the Research School of Earth Sciences,

Julie 7:49

where Allen was based when he first moved to Australia.

Allen 7:53

did have a heavy engagement in the lunar program. At that stage, SHRIMP was still being developed and built. But the very early stages of ANU’s engagement in lunar work

Julie 8:06

ANU is an acronym for the Australian National University.

Allen 8:10

was actually on these funny spherules of glass, what are called impact spherules, which were dusted over the Moon’s surface, you see. So one technician in ANU actually had the job in, of course, special sterile conditions, to sort them out into sort of different categories of appearances before doing various analyses on them. And another technician told the guy who was doing it that there was still some concern about whether there was maybe some microbial life on the Moon, which might have actually been transferred back to Earth, and therefore people who were actually handling and working with lunar material had to be very carefully monitored. And he said that, the American Embassy in Canberra requires you to take a urine sample every day and then after 2 weeks, you should take your urine samples to the American Embassy. So this guy dutifully got all his urine samples and then went to the American Embassy and it was only then when he discovered he had, he had been fooled. He turned up at the American Embassy with these bottles of urine and said, “Here they are.” And they said, “What’s all this about?”

And I remember having a practical joke played on me just after I arrived in the dating laboratory. Because I had actually brought with me some rocks from Greenland to do some work on SHRIMP. There had already been a few samples dated by Professor Compston and also a guy called Peter Kinny but I wanted to expand the program. The samples did arrive. They’d been of course opened by customs and so on. And Ian Williams, who’s one of the, I guess, gurus of that analytical technology, wanted to play a little joke on me. So he said, “Oh, OK, yeah they’ve been opened and they’ve been actually been sprayed with a compound to really sterilize them. The only trouble is this compound in them has got some lead, you see.”

And of course, lead is something you’ve got to worry about when dating things by the uranium lead method. He could see the look of concern and anxiety come across my face and um, he said, “Oh, don’t worry, um, one can very easily deal with it. This stuff breaks down in ultra-violet light, you see,” and he said, “All you have to do is put your samples out in the garden and every day turn them and so on, and then after that, after a few days you can just you know, wash the samples off and then everything will be alright.”

And so of course I dutifully took these Greenland samples home, put them out in the bright Aussie sunlight you see, and was dutifully turning them for a few days. And then he relented and said, “I was playing a joke on you.”

Allen 11:14

So as we’re on the subject of dating rocks, there’s something very special for the Nuuk area. And that is the very old rocks, which do occur in the Nuuk area. These were discovered by a synergy between Vic McGregor and some people in the Oxford Isotope Laboratory. On the basis of field relationships, Vic McGregor came up with the idea that some particular sort of rocks were actually much older than the other rocks generally in the region. And he was very keen to have what we call absolute, in other words radiometric ages, presented on these rocks. However, the laboratory which the Greenland survey was collaborating with at that time, the Oxford Isotope Laboratory, were not particularly keen on taking these wild card samples to date, of course cause of the reason in those days, it was a lengthy and therefore expensive to date samples.

Julie 12:21

It was here that Emeritus Professor Kent Brooks, then working at Copenhagen University, played a key role in first introducing Vic McGregor to Stephen Moorbath, at the Oxford Lab where Kent had previously worked, and recommending that Stephen take the samples. Here is Kent Brooks.

Kent 12:39

GGU were not particularly interested in that. I mean, It wasn’t the sort of thing that GGU did. I mean GGU were there to do mapping, and er fancy things like radiometric dating were really not for them. So he couldn’t get any support for er getting an age on it. 

Julie 12:55

Here is Allen again.

Allen 12:57

There was geologist in the, in the Greenland survey, somebody called David Bridgwater, who thought that Vic McGregor had a good case and brokered a deal with the Oxford Isotope Laboratory with the chief person there being Stephen Moorbath, who died in 2018, that as a proviso of Stephen Moorbath having some particular samples he wanted to date, he had to date Vic McGregor’s samples as well. So that was the deal. So the samples went over to Oxford. But Stephen Moorbath was still not really keen on dating these things.

So at the time, there was an Australian postdoc in the laboratory called Lance Black and Stephen Moorbath went off on holiday and said, “Oh, while you’re away can you analyze McGregor’s samples, ok?” And he said, you know, “for every sample which, turns out to be, very old, I’ll give you a bottle of wine,” cause he was absolutely convinced that he would come back and not have to give any bottles of wine?

So anyway, Stephen Moorbath came back from holiday. And I can’t remember how many samples there were, maybe eight. And Moorbath says to Lance Black, “Oh, how many bottles of wine do I owe you?” and he said, “Eight.”

So that’s how it was actually realized there were these extremely old rocks in the Nuuk Isua area. And to give Stephen Moorbath his credit, he swallowed his pride after this and got very fully engaged and obviously championed a lot of further work in the area, throughout the 70s and 80s and also into the 90s.

Allen 14:54

I arrived in Australia carrying a rock bucket full of some rocks that Clark, Vic, and myself had collected in that 1988 field season, with the idea that eventually I’d be able to date some of them by the SHRIMP ion microprobe because I was joining the um, ion microprobe group in the Australian National University. However, I bided my time because it was made completely clear to me that my first brief was going to be working on the Narryer Gneiss Complex in Western Australia, which contain rocks very similar in character and in age to the Amitsoq Gneisses, as they were then known, in the Nuuk region. So therefore, I realized it was not a good thing to start pestering my new boss to start dating Greenland rocks immediately. It was about a year and a half until I broached the subject with Bill Compston as to you know, whether it’d be possible to actually, you know, do some Greenland rocks. And I think he could see that it was, I’d been a good boy and, you know, it wasn’t an unreasonable thing so he said yes.

So I started a small project of looking at some of the rocks that we’d brought, collected in 1988. And the first one I did was something which then became a very infamous sample, which was G88/66, which was a bit of banded gneiss right up against the supracrustal unit on Akilia Island.

Julie 16:37

Supracrustal rocks are rocks that were originally formed on the surface of the Earth, including sedimentary and volcanic rocks.

Allen 16:45

and at that stage the oldest rock in Greenland was about 3.82 giga years.

Julie 16:53

That’s three thousand eight hundred and twenty million years old, which is about 85% of the age of the Earth.

Allen 17:01

And then, just complete fluke, this first sample I started working on, it started coming out at 3.87. 

Julie 17:08

That is three thousand eight hundred and seventy million years old.

Allen 17:13

And on the basis of the field relations it could be argued very well that the Akilia Association amphibolites and banded iron formation

Julie 17:24

That is the supracrustal rocks that Allen referred to earlier, interpreted as sedimentary and volcanic rocks.

Allen 17:31

had to be older than that. This immediately started to cause great consternation amongst some people, particularly the Oxford group, who essentially had the idea that nearly all of the early Archean rocks in Greenland

Julie 17:49

That is rocks older than three thousand six hundred million years.

Allen 17:53

were very close to the similar age and there wasn’t this spread of ages now of over 200 million years. But that was a debate which reached its head a few years later.

So anyway, that was very encouraging and interesting and then I actually got Australian National University funding to come back in Greenland in 1991 in order to do some more fieldwork, more collecting specifically for a larger dating program. And of course that was done with close collaboration with Vic and with Clark Friend. Also along on that trip was Peter Kinny. Peter Kinny, he’s an Australian geologist. He had done a little bit of SHRIMP work as part of his PhD thesis, very soon after the first SHRIMP instrument was er, built. At that stage, the only sample dated from Isua by the SHRIMP zircon method was a unit generally interpreted as volcanics and that had come out at 3,800 million years. And that was then regarded as a good approximation of the age of the whole of the Isua supracrustal belt. But on that trip, we actually collected from another big unit of felsic volcanic rocks up by the inland ice. And when we started to do the dating, things didn’t turn out as expected. I had expected that what we were going to do at Isua with more SHRIMP work was to refine a chronology around about 3,800, that one would start to take, a provisional stratigraphy

Julie 19:41

That is, an understanding of how the rocks were arranged as layers, originally deposited one on top of the other.

Allen 19:48

perhaps differing by a few million years, or ten million years apart, in that stratigraphy in the same way that one can see within the Pilbara greenstone belt successions

Julie 19:59

Also an very old area of rocks in Western Australia.

Allen 20:04

or some of the um Barberton ones, for example.

Julie 20:07

In South Africa.

Allen 20:09

Instead, the first sample which I did from Isua again came out as a shock because it was a felsic volcanic rock which was 3,700 – a hundred million years different from the first one. And one suddenly start to realise, oh Isua is a lot more complicated than people think. And on the basis of the tectonostratigraphic terrane model that we’d been working on, one started to think, maybe there’s some fundamental tectonic division within Isua. That was one idea. Another idea is that you might have a 3.8 sequence

Julie 20:47

That is a group of rocks three thousand eight hundred million years old

Allen 20:51

and then rocks a hundred million years younger on top of that. So that, that was the nature of this first round of SHRIMP work. Often every single one was a surprise. So those were the really early exciting days when you’re getting lots and lots of surprises, but overall this new model of terranes was actually holding.

So with new visions of the complexity of early Archean crust and Archean crust in general in Greenland, my then boss, Bill Compston, decided it was worth throwing more money this way, even though he had this very special terminology for working out of Australia, and that was working on “faraway places with unpronounceable names.” So that’s where we went. We came to as far away as possible place with as unpronounceable as possible names.

And in 1993 came up again supported by the Australian National University. I was joined by another very long-term collaborator, Vicky Bennett. So Vicky Bennett is now head of geochemistry in Australian National University and she’s also president of the Geochemical Society. So we spent summer of 1993 starting off actually at Isua. So that was the first time I actually returned and camped in Isua since the early 80s when I’d been doing contract mapping with the Greenland survey. And it was actually quite interesting because at that time for a very short fleeting time in Greenland history there was a float plane in Nuuk. So we actually went up on a float plane to Isua and one landed on one of the big lakes there. That was fantastic. 

We actually started going over in a slower, more methodical fashion in eyes which were many years older than when I first went there. One had under ones belt a lot more years experience as a, as a geologist. So we were able to reinforce, the idea that there were definitely two sequences present in Isua, about a hundred million years difference in age. So one lot is 3,700 approximately. One lot’s about 3,800, whereas if you do regional whole rock isochrones on Isua

Julie 23:34

The isochron method is a method of dating rocks that effectively averages the result from a range of different rocks to calculate a single age. The method assumes that all the samples have the same age.

Allen 23:47

you get something about 3,750 plus or minus a big error. So in other words, they were averaging um, the two completely unrelated groups of rocks. So to give you an idea of the game-changer in the precision of the rocks those isochrons usually had errors of about three hundred million years, which is from now back to the end of the Carboniferous,

Julie 24:12

About fifty million years before dinosaurs first appeared on Earth.

Allen 24:17

to put it into a more modern context. On the other hand, with the SHRIMP, one was regularly churning out ages which were plus or minus five million years. So it was a complete game-changer in that one could be really sure yes, here we have a bunch of rocks which are 3,700. Here they’re 3,800. And also we were getting more and more convinced that the break between the sequences wasn’t a unconformity and stratigraphic break, it was a tectonic break.

Julie 24:46

By which Allen means, it wasn’t just a period of time where no sediments or volcanic rocks were deposited on top of the older rocks. These were completely unrelated rocks that had been pushed together by plate tectonic processes well after they formed.

Allen 25:01

but at that stage we were not brave enough to put that in print. 

Julie 25:07

I’m Julie Hollis and you’ve been listening to Polar Podcasts.

Julie 25:18

In the next episode, we hear more from Emeritus Professor Brian Upton about his expeditions to Northeast and North Greenland.