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Fourteen

Interview Professor Rod Brown

New answers for very old questions…

Interview Professor Rod Brown

Interview Rod Brown

New answers for very old questions

After helping to answer one of geology's most difficult problems – why southern Africa is so high – Professor Rod Brown and his team could now turn their attention to the landscape of Scotland, taking advantage of the latest technologies available right on their doorstep and new techniques they have developed to analyse the thermal history of rocks...

Twenty-three years ago at ICOG (The International Conference on Geochronology, Cosmochronology and Isotope Geology), held in 1990 in Canberra, Australia, (the “Geochemistry Olympics”), a small group of scientists met to discuss an exciting new idea. The topic of the session was how very rare isotopes, produced by cosmic rays from outer space which induce nuclear reactions within minerals on the Earth’s surface, could be used to date the surface. The idea may have seemed ‘out of this world’ but the science was more down to earth – and the dating technique utilising the products of these cosmogenic nuclear reactions is now being used by researchers worldwide to establish how the Earth’s surface formed as we see it today. The power of this cosmogenic isotope technique was that it provided earth scientists, for the first time, with a tool that could provide quantitative measurements of the age of land surfaces and how fast they were being lowered by erosion over hundreds of thousands to a few million years.

In parallel with the development of this new cosmogenic dating technique, another breakthrough in ‘conventional’ radiometric geological dating was also taking place. The basic science of radioactive decay had been known about for decades, since Ernest Rutherford had first suggested at the beginning of the 20th Century that radiogenic helium (He) could provide the key to dating the age of the Earth. It was not until the early 1990s, however, that scientists at Caltech (the California Institute of Technology) turned the concept of using measurements of He into a practical tool, using the latest mass spectrometer technology to do what is now known as uranium–thorium/helium (U–Th/He) analysis.

In parallel with the development of this new cosmogenic dating technique, another breakthrough in ‘conventional’ radiometric geological dating was also taking place. The basic science of radioactive decay had been known about for decades, since Ernest Rutherford had first suggested at the beginning of the 20th Century that radiogenic helium (He) could provide the key to dating the age of the Earth. It was not until the early 1990s, however, that scientists at Caltech (the California Institute of Technology) turned the concept of using measurements of He into a practical tool, using the latest mass spectrometer technology to do what is now known as uranium–thorium/helium (U–Th/He) analysis.

By analysing the helium isotopes in apatite crystals, which are found in virtually all rocks, geochemists can establish not only the age of the crystals but also their thermal history – or thermochronology.  Radiogenic helium is produced in apatite grains when uranium and thorium isotopes decay, and what makes it so useful in geology is that this helium is only retained in the apatite crystal once the rock is below about 80 degrees centigrade. Analysis of the helium isotopes thus provides a very accurate record of when the rocks cooled and how long it took for them to cool.

This information allows geologists to determine when and at what rate erosion has brought rocks to the surface from deeper down in the Earth’s crust. This is possible because the temperature in the Earth increases systematically with depth and so a rock cools steadily as it moves up to the surface. The power of the (U–Th)/He thermochronometry technique to measure large amounts of erosion (several km) over tens to hundreds of millions of years is that it complements beautifully the shorter time scale and smaller spatial scale tool provided by cosmogenic isotope analysis.

When used in concert, these two techniques can be used to explain processes which took place in the relatively recent past – i.e. hundreds of thousands of years ago – as well as tens of millions of years ago.

Rod Brown, now Professor of Earth Sciences at the University of Glasgow, was one of the scientists at that small gathering who realised the new technique could revolutionise our understanding of landscape evolution, his specialist subject.At that time, Brown was working at La Trobe University in Melbourne, Australia, using a thermochronology method called fission track analysis – looking at the “damage trails” in crystals for evidence of fission decay, in order to date rocks, very precisely, and measure erosion over very long time scales.

When Brown first came across the new cosmogenic technique in 1990, he immediately realised the impact it could have when combined with fission-track thermochronology, and even more so if it could be combined with the new (U–Th)/He technique, but it took several more years before he could put his ideas in action. 

In 2003, while still a lecturer in Melbourne, Brown learned that a new accelerator mass spectrometer (AMS) had been installed at a facility in Scotland for the primary use of geoscientists in the UK. (An AMS is essential for measuring the very small amounts of cosmogenic isotopes within minerals, but these very expensive machines are normally the preserve of nuclear physics research groups.) The following year, Brown successfully applied for a position in the School of Geographical and Earth Sciences in Glasgow, excited by the prospects of using the new AMS and the unique array of other dating techniques, including the new (U–Th)/He method, to pursue his research. 

“An accelerator mass spectrometer enables us to look at ludicrously rare isotopes,” Brown explains, “and the new AMS at SUERC (the Scottish Universities Environmental Research Centre) put Scotland on the map – it really opened people's eyes to what was possible.”

According to Brown, the facilities at SUERC provide “a unique array of technologies” and put them in the hands of geoscientists.  The centre also brings a range of disciplines together, to share ideas and different scientific perspectives in the quest for solutions, and also means that SUERC attrracts many leading names in geoscience.

“It is not possible to overstate the uniqueness of being surrounded by equipment which is capable of dating more or less everything – and rubbing shoulders with so many world-class scientists,” says Brown. Geoscience tends to be hypothesis driven, and dating technologies are often viewed as “tools” to be simply applied when required, but the powerful new technologies at SUERC are “so advanced that they are pushing the boundaries of what’s possible analytically,” enabling geoscientists to invent new techniques and “new sample pre-processing recipes” to do things they couldn't have dreamed of before. Using these new techniques of thermochronometry, which had been written off before because they produced ages that were “too young,” to estimate the thickness of crust that is no longer there because it has been removed by erosion, could be compared to “dating rocks which don’t exist with methods  that don’t even work.”

Sometimes, geoscience takes dramatic twists and turns.  For example, says Brown, the study of cosmogenic nuclides (the rare isotopes created when cosmic rays hit the Earth and interact with atoms) has also revolutionised our view of how the surface of the planet has evolved, enabling geoscientists to “fill in the gaps” between what we can see in the landscape today and what happened millions of years ago, dating rocks and debris more accurately than ever before.

Out of Africa
Initially, Brown and his colleagues were puzzled by the data this produced.  Apatite crystals are typically elongated, prismatic shapes and are rarely extracted from their host rocks intact, and when you analyse the fragments, you get different readings for the helium content, depending on where the particular fragment has come from – e.g. from one of the ends of the crystal or from right in the centre. This means that the measurements determined on single crystals from the same rock produced a much wider range of ages than had been expected, but when they looked at the mean age of samples, the results made more sense. The problem, says Brown, is that “the mean of nonsense is nonsense.” In other words, there was no reason to think that the mean was correct. 

The breakthrough came after much racking of brains. “The penny dropped,” says Brown, “when we started to think about why the grains were dispersed in a particular pattern, and it was the pattern of dispersion that gave us the answer.”

Brown and Beucher decided to develop a mathematical model that specifically treated broken grains as fragments of larger crystals. This had not been done before, because until then the thermochronology community had used a simpler model that treated all grains as if they were spherical shapes. This was done because the calculations could then be performed much faster on computers. The new fragment model developed in collaboration with colleague Dr Steven Roper, from the School of Mathematics and Statistics at Glasgow, required the use of the high-performance computing services of ScotGrid, normally used by the Large Hadron Collider physicists at Glasgow, to perform the calculations. The results of these model experiments were very exciting, says Brown, because they showed for the first time that the very large amounts of dispersion seen in data sets such as those from South Africa were the result of the shape and size of crystals analysed and whether they were broken or not. With this new model, called Helfrag, Brown and his colleagues were able to make sense of what otherwise looked like “terrible” data.

Ironically, a single age from a whole grain or a single fragment may not be as useful in creating an accurate record of cooling as data from several fragments.  “The more terrible the data appears, the better it is,” Brown explains. “The record is encoded in extremely messy-looking data, but we think we have cracked it.”

According to Brown, the breakthrough was a “simple idea,” and treating broken crystals explicitly as fragments provided the key. Based on the new data, there is no evidence of recent uplift but, rather, strong evidence of an older upheaval.  This suggests that the first-order landscape was formed between 75 and 175 million years ago, probably because of a super plume pushing the land up, rather than as a result of more recent uplift and erosion. “We still can't date exactly when the surface rose,” Brown cautions, “because we are measuring erosion and not uplift directly, but we do know the timing and rate of this erosional process.” 

The cherry on the top is that new cosmogenic isotope measurements made on rock surfaces and on river terraces within South Africa have confirmed that there has been very little uplift of the land surface over the last four million years, thus supporting the conclusions drawn from the thermochronometry results. So, the tandem approach of cosmogenic dating and thermochronometry has finally provided some quantitative answers. Until this study, there were no constraints on timing, but Brown is confident the time scales can now be agreed. 

“The new (U–Th)/He data shows that the timing that you see at the margins (coastal erosion) is the same as that determined from samples from deep holes up on the plateau,” adds Brown. The fact that the timing on the coast is the same as the interior of the sub-continent suggests that the whole land mass rose because of the enormous energy pushing it up from below, rather than resulting from erosion migrating inland from the uplifted margins of the continent following continental break-up.

Following the success of this recent research, which was funded by the National Environmental Research Council (NERC), Brown would like to apply the new methods to other major problems in geology – including investigating the evolution of the topography of Scotland. 

Brown explains that the Highlands of Scotland, and indeed the similarly elevated topography along the eastern coast of Norway, have a long geological history; but as in Southern Africa, there are some who believe the present topography is quite young and others who think elements of the Highlands date back to when the North Atlantic was formed about 60 million years ago, or even earlier. The existing (U–Th)/He studies from these areas have been controversial because the ages have almost always been severely dispersed, and so the efficacy of this dating technique in these terrains has been questioned. With their new understanding and appreciation that “dispersion is good,” Brown and colleagues believe they can now tackle this frustrating problem head-on and turn “what looks like terrible data” into robust constraints on the timing of major uplift across Scotland and Fennoscandia.




 

 

 

 

 

"Interview Professor Rod Brown". Science Scotland (Issue Fourteen)
Printed from http://www.sciencescotland.org/feature.php?id=193 on 22/08/17 04:01:00 PM

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