Isotopic determination of uranium in picomole and subpicomole quantities

A procedure for the separation of U and measurement of U isotopes in meteoritic, lunar, and terrestrial samples Is described. Accurate ²³⁸U/²³⁵U ratios and U concentration measurements can be done on a routine basis at the level of ~10⁻¹³ mol of U by isotope dilution using a double tracer composed of ²³³U and ²³⁶U. Precise determination of the number of U atoms in a sample can be done at the level of 10⁸-10⁹ atoms. The technique Involves careful control of the chemical procedures to eliminate laboratory contamination and precise calibration of the mass spectrometer in order to obtain high reproducibility. The ²³⁸U/²³⁵U ratios were measured on a wide variety of samples which include (1) bulk meterorites, (2) lunar samples, (3) coarse-grained Ca-Al-Tl-rich inclusions and (4) fine-grained AI-Mg-Na-rich Inclusions from the Allende meteorite, and (5) phosphates from three meteorites. The results show the same Isotopic values within limits of errors as that measured on normal terrestrial U to within 4‰. All of the analyzed meteorite samples are very ancient and several of them show large fractionations of U relative to Th and Nd. These data Imply that ²⁴⁷Cm was not abundant in the early solar system (²⁴⁷Cm/²³⁵U « 4 X 10⁻³) and that the last major "r" process terminated at ~10⁸ years prior to the formation of the solar system.