Scientists are a step nearer to understanding volcanic processes that occurred as far back as 8000 years ago.

Dating volcanic eruptions depends on detecting low levels of radioactive elements.

Akio Makishima and colleagues at Okayama University at Misasa, Japan, have developed a method for measuring attogram (10^-18 gram) amounts of radioactive elements in basalt rocks.

Basalt, one of the most common rock types on Earth, is formed when lava from volcanic eruptions cools. It contains small amounts of radium-226, a radionuclide that is formed over billions of years by the radioactive decay of uranium-238 via thorium-230. By measuring the quantities and ratios of these radionuclides in volcanic rocks, scientists can piece together the timescale of volcanic eruptions.

The team analysed basalts by multicollector inductively coupled plasma mass spectrometry. Makashima added a known amount of radium-228 to the samples to allow him to determine unknown amounts of radium-226. But radium-228 decays to form thorium-228 and because these radionuclides have the same mass, the mass spectrometer cannot distinguish between them. This causes errors in the dating process.

Thorium-228 can be removed from the radium fraction by column chromatography but Makishima found this inefficient. 'If a delay occurred in analysing the sample, we had to do column chemistry again because more thorium-228 formed. To overcome this problem, in-situ correction of thorium-228 was required,' he says. Makishima found that thorium-228, but not radium-228, forms a positively charged oxide ion in the mass spectrometer. By measuring the intensity of the thorium oxide peak, Makishima calculated the amount of thorium-228 in the radium samples and used it to correct the radium measurements.

Marcel Regelous, an expert in the analysis of volcanic rocks at the Royal Holloway University, London, UK, says, 'Analysis of such small quantities of radium has applications in dating geological processes, in particular for investigating timescales involved in the crystallisation of volcanic rocks. This requires analysis of individual mineral fractions separated from the crushed rock sample, which often contain very little radium.'

Ziva Whitelock