Development of the method of measuring the age of rocks based on the lutetium-176/Hafnium-176 ratio

The pair hafnium-176/lutetium-176 call be used to date certain relatively widespread rare-earth ores. The authors undertook the development of methods of chemical analysis of Lu and Hf and of isotopic analysis of Hf which can be used for routine measurements of quantities of ore of the order of 1 g. The Lu is measured by flame photospectrometry using atomic absorption. In its present stage of development this method can be used to measure concentrations of the order of 10 ppm of Lu without serious interference from other rare earths. Mass spectrometry of Hf has been developed to measure the principal isotopic ratios using an amount of Hf of the order of 1 μg extract of the ore, with a relative precision of about ± 1%. This same method can be used to measure the Hf concentration on the scale from 0.1 to 1 ppm, using the method of isotopic dilution with a stable isotope. A fast activation analysis method has also been developed for Hf; it is based on the reaction 179Hf (n, γ) 180mHf (T 1/2 = 5.5 h), thus making the chemical separation of the zirconium unnecessary. These methods are at present being applied to a series of rare-earth ores dated by the Pb/U isotopic ratios. (author)