“I suppose you won’t be able to find one of your famous Clues on the thing?”
“Shouldn’t think so, sir. Not with all these fingerprints on it.”
Terry Pratchett, “Feet of Clay”
Captured in Pratchett’s satirical writing here is a key concept underpinning the advancement of science: In recognising the deficiencies of our understanding, we identify pathways to more fundamental, deeper insight.
If you might indulge me in illustrating this concept: The science of geochronology is only a little over a hundred years old. The genesis of this field – the direct measurement of the age of Earth materials in the millions and even billions of years, putting a timescale to the grinding wheels of geological process – is probably traceable most directly to the New Zealander (albeit that we should add the prefix ‘Colonial’ to that label, given he undertook his post-graduate education and scientific career at Cambridge University in Britain) Ernest Rutherford – one of the great figures of 19th and early 20th century physics. Scientists don’t often ascend to the pantheon of cultural heroes, so the fact that Rutherford’s distinguished portrait graces the $50 note of his country of birth is probably as effective a mark as any of the degree to which he bestrode the world stage, and the respect in which he is still held.
With his finger on the scientific pulse of the Edwardian age – and in particular the atomic theory at the heart of his own cutting-edge research – Rutherford was quick to appreciate the significance of the new phenomenon of radioactivity discovered by his contemporaries Marie and Pierre Curie. In the words of the great man himself:
“The helium observed in the radioactive minerals is almost certainly due to its production from the radium and other radioactive substances contained therein. If the rate of production of helium from known weights of the different radioelements were experimentally known, it should thus be possible to determine the interval required for the production of the amount of helium observed in radioactive minerals, or, in other words, to determine the age of the mineral.”
Ernest Rutherford – Silliman Lectures at Yale, 1905
With those words, a scientific revolution began.
Rutherford quickly set to work encouraging collaborators in the fields of chemistry, physics, and geology to put that principle into practice, but it didn’t take long for the community to recognise that his original elegant concept wasn’t going to be the simple path to greater knowledge that they had hoped for. The problem was that helium – the simple, easily extracted product of radioactive alpha decay – wasn’t fully retained in the mineral structures they were testing. In the words of John Strutt, one of the key figures in this early research:
“[helium ages provide only] minimum values, because helium leaks out from the mineral, to what extent it is impossible to say.”
R. J. Strutt (1910), Proceedings of the Royal Society of London
Some process – unknown at the time – was allowing the helium to escape from the crystals. Like a water clock with a leak in it then, there was no true fundamental way to calculate age from the system.
The key to our story here is that the contemporary paradigm to which these scientists were working was that the only age that mattered was the time at which a sample crystallised – nothing else entered their world view. When helium dating returned values that were clearly far too young and inconsistent to reflect such formation ages, the method was consequently abandoned, with the scientific community pursuing other isotopic systems – notably the pairing of uranium isotopes with their ultimate stable decay product of lead – as the pathway to temporal understanding of Earth evolution.
90 years later at the turn of the 21st century though, helium dating was back on the scene and a hot property (quite literally, as it turns out – but more of that later) in the field of geochronology – and it remains so right up to the present day. Why? Have we just forgotten the lessons of the past?
To understand the answer to that question, you need to appreciate that an isotopic ‘age’ is fundamentally just a ratio of chemical species – namely the abundance of a radioactive parent isotope – the ticking clock of the system – and the product of its decay within your sample. Ultimately, this is just a number – nothing more or less…unless you have a physical event you can relate that number to. If I toss you a rock and say “this rock is 7,000 years old” – what does that mean? Is it 7,000 years since the rock crystallised? 7,000 years since it was knocked from a large boulder upstream? That it has spent 7,000 years tumbling back and forth in the surf? 7,000 years lying on the beach? All these ‘ages’ might have meaning – telling us something interesting about the history of this particular sample – but unless you know which one I mean, the manifold possibilities obscure the potential insight.
To address this confusion from the perspective of helium, let’s drill down from the scale of rocks and hammers to the sub-microscopic world of a crystal lattice. The comforting solidity and discrete character of the everyday is replaced by a dynamic constellation of atomic structures held in place by overlapping and interfering clouds of electrons and opposing forces – a seething maelstrom of movement and change. As those particles spin and vibrate, the force balances governing their interactions rise and fall, bonds parting and re-forming in the blink of a conceptual eye as their stability waxes and wanes. Take a moment to watch this video clip from Dr Erik Laegsgaard at Aarhus University.
Scanning Tunneling Microscope imagery of atomic-scale diffusion in titanium dioxide, created by Dr Erik Laegsgaard, Aarhus University. Recorded at 300 degrees Kelvin, and at 8.6 seconds/frame.
Each of those glowing orange orbs is actually an atom of oxygen resolved by advanced scanning tunnelling microscopy of a sample of titanium dioxide. To my thinking, this movie is mind blowing – this is not a cartoon, or a fancy computer model – this is an actual resolved record of real individual atoms, in solid material, at room temperature. Reflect for a moment on just how we see those atoms behave as the movie advances through time. Rather than locked in place like mosaic tiles set in mortar, they skitter back and forth – momentarily held in the embrace of one bond, but then twisting away across the crystalline dance floor to some new partnership. The movement is random and unpredictable – particles as likely to jump one way as any other.
This atomic diffusion is what was responsible for Strutt’s anomalous ‘leakage’. Although the movements are individually random, if you’re building up an increased concentration of something (as with the helium produced by alpha decay in the example of our geochronometer), then you’re statistically more likely to have those random movements going out of the radioactive crystal structure than into it. It follows that this diffusion will prevent the build up of your daughter product (helium), keeping the isotopic age stuck stubbornly at zero.
So how then do we stop diffusion happening and allow our ticking clocks to record time? How do we set the geological stopwatch running? The simple answer is temperature – you cool things down. The rate at which diffusion occurs is proportional to temperature raised to an exponential power. In essence, this means that even a small change in temperature leads to a very large change in diffusivity, and the transition from rapid diffusion – so rapid that all the daughter product produced by radioactive decay is lost – to negligible diffusion where all that daughter product is retained – occurs across a very narrow temperature range.
Rather than the aberrant or spoiled data Strutt took them to be then, helium ages, once we understand this process and calibrate its thermal sensitivity, become sensitive records of the temperature change associated with dynamic geological history.
How does this help us?
When Gil Grissom finds a gun at the scene of a murder in CSI (yes, I know Grissom left the show after series 9, but I always thought he had excellent style as an on-screen scientist, and geologically speaking, his tenure is pretty much still within error of the present), his first thought isn’t “I must find out how old this gun is” – no – there are far more dynamic aspects of the weapon’s history he would like to see resolved. When was it bought? How long ago was it fired? Who pulled the trigger?
Similarly, if we focus purely on the crystallisation age of our samples, as Strutt, Rutherford, and their contemporaries were, there are many potential insights we will miss.
When were our samples last thrust beneath the crushing weight of an uplifting mountain range? When did they last feel the rush of superheated steam carrying rich mineral endowment through subterranean fluid conduits, or the frictional warmth induced by an active fault boundary radiating through the crust? When did erosion wear away its weighty overburden to exhume our rock from the hot interior of the Earth? With the thermal ages provided by helium dating and its correlatives, these dynamic episodes come within our grasp.
What was simply noise becomes, when we understand and can translate its origin, a sensitive new record of dynamic geological processes.
Unlike Pratchett’s protagonists, our FBI database is ready, and the fingerprints of geological systems are waiting to reveal themselves to our careful detective work.