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Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System. Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation. Some hafnium-tungsten isotope data, however, require that Vesta formed later (approximately 16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time (62(-14)(+4504) Myr). Here we report measurements of tungsten isotope compositions and hafnium-tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results, the bulk of metal-silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation, and are also in agreement with dynamic accretion models that predict a relatively short time (approximately 10 Myr) for the main growth stage of terrestrial planet formation.

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Potential applications of the Lu-Hf isotope system have long been impeded by the analytical diculties of obtaining data on a wide variety of geological materials. Many of these limitations will now be eliminated because Hf isotopes can be readily measured with high precision and accuracy on small and/or Hf-poor samples using the newly developed magnetic sectormultiple collector ICP-MS, also known as MC-ICP-MS or the`Plasma 54'. We present here a new method to separate and determine isotopic compositions of both Hf and Lu from various types of geological materials using MC-ICP-MS. The chemical separation of Hf and Lu has been designed to take advantage of the characteristics of this unique instrument. The separation of Hf can be achieved with a straightforward two-step ionexchange column chemistry, which has a high eciency (better than 85% recovery) and low blanks (typical total blanks less than 150 pg for the largest samples of 1 g bulk rock). The isolation of Lu is achieved with a singlestage ion-exchange column procedure with near 100% yields and blanks below 20 pg. Hf isotopic compositions can be routinely measured on 50 ng Hf with an internal precision better than 20 ppm in less than 15 min and with an external precision better than 40 ppm. Our value for the 176 Hf/ 177 Hf ratio of the JMC 475 Hf standard currently is 0.282163 9 (2s). The Lu isotopic ratio is measured rapidly and precisely without isolating Lu from the bulk of Yb, and a mass fractionation correction increases the accuracy of the results compared with TIMS data. Our current reproducibility of the Lu/Hf ratio is »1%. Selected Lu-Hf isotope analyses of some modern and ancient geological samples validate the technique we have described here and illustrate the new opportunities for Lu-Hf isotope geochemistry that have opened up with the advent of magnetic-sector ICP mass spectrometry.

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