(September 6, 2001, Gazette)

Earth scientists use lasers to study the age of the earthGetting the date right

Photo by Alex Dalziel
Dr. Richard Cox

Dr. Richard Cox dates rock, and he thinks that recently he has hit on a real winner. But if you are thinking wine, flowers, and a night on the town might be the secrets to his dating success, you’re on the wrong track. When Dr. Cox dates a rock, he uses a laser.

Dr. Cox and some of his colleagues in the Department of Earth Sciences are working on a new radiometric technique: laser ablation. The radiometric dating of rocks is a staple of modern geochronological research, the science of the age of the Earth’s rocks. Radiometric dating exploits the decay of the unstable element uranium into the more stable elements including lead.

“Fortuitously in nature there are a number of minerals which have quite high concentrations of uranium but have little or no lead in the structure to begin with,” Dr. Cox said. Over a quantifiable period of time, uranium breaks down into lead — thus, the older the rock, the greater the lead content vis-a-vis uranium. “By measuring the uranium-lead isotope ratios we are effectively measuring the age at which the crystal grew.”

Laser ablation is but one of many ways to date rocks. “The technique that we have been developing is to use a laser attached to a different type of mass spectrometer called an ICPMS — Inductively Coupled Plasma Mass Spectrometer. What we do is use the laser to scour the surface of zircon or another mineral. The material then goes from the laser into the mass spectrometer and we measure the isotope ratios that way.”

The conventional dating method uses the Thermal Ionization Mass Spectrometer (TIMS). Geochronologists using this technique take “a small fraction of (zircons) and then treat them chemically and (then) separate the uranium and lead.” The next step is to “run the uranium and lead separates on the TIMS.” According to Dr. Cox, “that, undoubtedly, produces the most precise ages; typically we get down to something in the region of a million years or thereabouts on relatively old material.”

The role of the mineral zircon is crucial to the entire radiometric exercise. Zircon is what geologists call an “accessory mineral” — small grains sporadically embedded in other rocks. Uranium is generally only found (in statistically considerable amounts) in accessory minerals — it is rare in the surrounding “main phase” minerals such as feldspar and quartz. Geochronologists thus have to rely on tiny fractions of the rock to date the entire structure. This “makes it very difficult to relate the accessory mineral to the main phase minerals in the rock,” Dr. Cox explained.

TIMS, although very high tech, is rather heavy-handed — rocks must be smashed into grains before chemical treatment and then analysis in the spectrometer. The usefulness of the data can be limited, because zircon can survive multiple geological events, and grains from different sources, and therefore of different ages, can find themselves in the mix. The rock age data will then be skewed. Dr. Cox pointed out that “once you’ve crushed the rock, you’ve destroyed the textural information.”

Laser ablation is much more subtle, preserving the textural context of the grains. “What we can do is relate zircons texturally by seeing where they occur and then we can date them directly with the laser. With the laser system you can image every single grain individually and then see if there is a difference (between the origins of the zircons).”

According to Dr. Cox, the choice between the TIMS and ICPMS “comes down to accuracy versus precision. The TIMS method is by far the most precise method — you’ll get very much more precise ages. The problem is that it is not very accurate. So although laser ablation is much less precise, it is much more accurate. We can more readily relate what we see in zircon to what we see in the whole rock. We’re working all the time on improving the precision.”

Radiometric dating plays a central role in articulating the modern paradigm of earth sciences, plate tectonics. For instance, Dr. Cox is supplying crucial radiometric data with the laser to the department’s Dr. Joseph Hodych. “He’s a geophysicist who looks at paleomagnetic data,” Dr. Cox said. “If you measure the magnetic field direction in a rock and can relate that to the Earth’s magnetic field, that tells you what its latitude was when it was formed. The other piece of information you have to know is what time it was at that position. You need to know what the age of the rock is because you can (then) reconstruct continental movements. This is really a super test of plate tectonics.”

“Geochronology applies to everything in geology — it is very fundamental,” Dr. Cox said. A little dating, it seems, goes a long way.