Understanding Our Planet Through Chemistry
One of the mysteries of the history of the Earth is the layer of clay that was deposited around the entire globe 65 million years ago. The layer marks the K-T boundary - the end of the Cretaceous and beginning of the Tertiary periods. It is best known as the time when not only the dinosaurs but nearly half of all life forms became extinct.
At the beginning of the last decade, Nobel Laureate Luis Alvarez and his team members discovered a 9 ppb abundance of the element iridium while using neutron activation analysis to study 1-cm-thick samples at the K-T boundary layer. The fact that the high level of iridium coincided exactly with the classic end of the Cretaceous mass extinction event led them to propose a theory linking these two observations. They theorized that an asteroid between 6 and 14 km in diameter struck the Earth, and the impact lofted enormous amounts of pulverized target material high into the Earth's atmosphere. They speculated that this dust-size, impact ejecta caused an environmental catastrophe.
Additional research by other scientists suggests that if the extraterrestrial object was an asteroid, it most likely impacted the Earth at a velocity of 50 times the speed of sound and measured 15 km in diameter. Because asteroids of this size are very few in number in our solar system, the object could also have been a comet, most likely moving even faster, possibly 170 times the speed of sound but measuring only 10 km in diameter.
To test the impact theories, we have applied a new analytical technique called laser ablation, inductively coupled plasma, quadrupole mass spectrometry (LA-ICP-QMS). To allow efficient, rapid, spatial sampling, a laser is used. The technique is highly sensitive for almost all elements.
As depicted below, the energy of the laser is focused onto a spot about 80 micrometers in diameter (slightly more than the diameter of a human hair) to vaporize and sputter material from small zones of the sample. The operating conditions of the laser range from 1 million to 1 trillion watts per square centimeter. This incredibly high energy density is created when the energy is packed into small bursts of 160 microseconds, which are then focused with a lens onto a very small spot.
The vapor from the sample is then carried by a stream of argon gas into a 7,000°C argon plasma, where the vapor is ionized. These ions are then drawn into a quadrupole mass spectrometer (QMS). The QMS consists of two sets of electrically charged, machined rods. A radio-frequency signal is applied to both sets of rods. Under specific operating conditions, one unique, mass-to-charge ratio of ions will be directed down the opening between the four rods and exit to the detector. All other ions will be lost.
The LA-ICP-MS is sensitive for all the platinum group elements (PGEs) that would appear from an asteroid impact. The laser, which has fine sampling resolution, was used to sample the 1-cm layer analyzed by Alvarez and coworkers but in bands only 0.25-mm thick. In this way, we were able to sample just the layer of PGE-enriched material and found the concentration in this zone to be nearly 1 ppm, a factor of 100 times higher than that previously reported. This greater concentration of the PGEs gives additional support to the theory that an extraterrestrial body collided with the Earth 65 million years ago.