Hf-W, ACCRETION OF THE EARTH, CORE FORMATION AND THE ORIGIN OF THE MOON. Stein

Introduction: The time scale and functional shape of Earth’s accretion and core formation may be investigated using the Hf (half-life: 9 Myr)-W extinct nuclide chronometer [1,2] because the Hf/W ratio is fractionated both strongly and uniquely by core formation. W is partitioned into the metal phase and Hf into the silicate mantle during the formation of the Earth. The core will develop a deficit in W, while the silicate Earth will develop a W excess during the early stages of accretion. If the accretion process continues for more than 50 Myr, this difference may be erased. The two most important parameters used for such calculations in the past have now been drastically changed by the new results of Yin et al. [3,4]. First, according to the new results, the silicate Earth has a radiogenic W isotope signature of εW(CHUR) = +2 (in contrast to the previous value of +0.2 ±0.3 of Lee and Halliday [4]), and second, the initial solar Hf/Hf has been lowered from 2.75x10 [5] to 1.0x10 [3,4]. Results and discussion: Here we present new model results for the Hf-W system using the constraints from the revised CHUR parameters. The models are based on isotopic and chemical mass balance between a primitive nebular reservoir, the primitive mantle and the core. An example of accretion histories investigated is shown in Fig. 1.