The timescale of accretion and core formation of the Earth inferred from Hf-W isotope systematics

Abstract

The Hf-W chromometer that was developed recently is a very powerful tool because of high time resolution due to short half life and significant partitioning of Hf into the mantle and W into the core. We have developed a mass conservation equation, which includes three important parameters, mass accretion rate, metal separation rate, and redox reaction rate, and searched for the conditions to satisfy the Hf/W and W isotopic compositions of the present Earth's mantle. Although previous work has suggested slow accretion (accretion time >50Myr), it could be shorter by an order of magnitude if the metal separation time or redox reaction rate is longer than -100Myr. The slow metal separation rate with rapid accretion is, however, inconsistent with constraints from thermal evolution and fluid dynamics, and slow redox reaction of oxidized materials is the plausible mechanism.