Laser Ablation Enhances Uranium Dating

Brent D. Johnson

To gain a better understanding of the history of the Earth, scientists have learned to peel back the layers of time, observing the growth rings of once-living organisms or the layers of sediment in a road cut. Another such dating technique uses mass spectrometry to investigate the uranium decay chain in coral reefs, and excimer laser technology is improving the sampling process.

Corals are filter feeders, straining microplankton and other food from the seawater around them. As they do so, the animals crystallize minute quantities of naturally occurring uranium, which they deposit in the elaborate calcium carbonate skeletons that form reefs. The uranium decays to a stable form of lead via a series of intermediary radioisotopes. By quantifying the presence of decay products such as thorium-230 in these reef structures using sensitive mass spectrometers, researchers can date the corals to up to approximately 400,000 years.

Peter van Calsteren, a senior research fellow in the department of earth sciences at the Open University in Milton Keynes, UK, is analyzing uranium-series isotopes in authigenic minerals -- those that formed in situ with the surrounding rock, such as in the skeletons of corals. Because carbonate precipitation rates are rather slow, high spatial resolution is required to make the most of the available information. Also, very high sensitivity is needed to detect variations of trace amounts of intermediary isotopes because even 2000-year-old samples have uranium concentrations of only 0.15 ppm.

As corals form reefs, they incorporate naturally occurring uranium into their calcium carbonate skeletons. Excimer lasers improve the analysis of uranium decay products in reefs, enabling researchers to date structures to hundreds of thousands of years in the past.

He performs his analyses using a UP193HE excimer laser ablation system from New Wave Research Inc. of Fremont, Calif., with a Nu Plasma mass spectrometer from Nu Instruments Ltd. of Wrexham, UK. The system incorporates an ExciStar M-100 ArF excimer laser from TuiLaser AG of Munich, Germany, which offers the required spatial resolution. A single pulse of the 193-nm laser at 10 mJ can ablate a 50-µm-diameter pit from the coral that weighs 1.2 ng and that contains approximately 140 atoms of ²³⁰Th.

The efficiency of the mass spectrometer is such that it detects one ion of ²³⁰Th per pulse. At 10 Hz, it takes roughly 16 minutes to collect 10,000 ions to give a 1 percent uncertainty in the measurement. The total amount of ablated material is approximately 12 µg.

The vaporized coral is sent through the plasma mass spectrometer, where the relative presence of the radioisotopes is detected. The ratios of the various isotopes can be used to calculate the age of the sample with an overall precision of 1 percent in favorable cases. By also studying the oxygen isotope ratios or the magnesium/calcium ratios of the carbonate, researchers can determine the water temperature in which the coral grew.

The system also can be used to investigate isotope abundance in volcanic rocks. The corals, however, have specific interest for van Calsteren because they may yield some clues to ocean processes as well as to events that lead to global warming.