Hf and W both transition metals. Hf is lithophile and so will partition into silicate melts, whereas W is moderately siderophile and will partition into Fe-rich phases.
Hf and W are strongly fractionated by the early processes of melting and core formation.
The short-lived radioactive nuclide 182Hf was synthesised in a supernova event prior to formation of our solar system and is now extinct. Its decay products are preserved in some iron meteorites.
Early segregated materials in the solar system ave unradiogenic 182W/184W, indicating that these metals sampled early solar system before live 182Hf had decayed.
The chondritic Hf/W ratio is ~ 1. It was earlier thought that the 182W/184W ratio of the silicate Earth, with Hf/W ~15, was identical to carbonaceous chondrites with Hf/W ~1. It is now known that the 182W/184W of the silicate Earth is significantly more radiogenic (2 Epsilon-W units) than carbonaceous chondrites. This confirms that, as with iron meteorites, the Earth’s core has low Hf/W ratio. This implies that core formation, which increased the silicate Earth Hf/W to 15, occurred <35 Ma after start of the Solar System.
Earth’s accretion may have occurred in catastrophic steps due to major impacts.
The radiogenic 182W/184W of the moon is consistent with formation of the Moon ~50 Ma after solar system formation, when the Earth had reached 0.5 of its present mass before mars-sized impactor hit.