Tungsten Isotopes in Planets

The short-lived Hf-W isotope system has a wide range of important applications in cosmochemistry and geochemistry. The siderophile behavior of W, combined with the lithophile nature of Hf, makes the system uniquely useful as a chronometer of planetary accretion and differentiation. Tungsten isotopic data for meteorites show that the parent bodies of some differentiated meteorites accreted within 1 million years after Solar System formation. Melting and differentiation on these bodies took ~1-3 million years and was fueled by decay of ²⁶Al. The timescale for accretion and core formation increases with planetary mass and is ~10 million years for Mars and >34 million years for Earth. The nearly identical ¹⁸²W compositions for the mantles of the Moon and Earth are difficult to explain in current models for the formation of the Moon. Terrestrial samples with ages spanning ~4 billion years reveal small ¹⁸²W variations within the silicate Earth, demonstrating that traces of Earth's earliest formative period have been preserved throughout Earth's history.