Hf-W


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.

Hf-W method

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.