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.
