Models of plantary accretion and core formation based on the Hf-W clock

Details Models of plantary accretion and core formation based on the Hf-W clock Models of plantary accretion and core formation based on the Hf-W clock

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....13884j&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13884

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The Hf-W chronometer is in principle the best chronometer for determining the timescale for planetary accretion and core-formation. Hf is a refractory lithophile element, whereas W is refractory, moderately siderophile and depleted in the silicate Earth relative to Hf by a factor of ˜10-40 due to partitioning into the Earth's core. The Hf/W ratio is ˜0 for iron meteorites and chondrite metals, ˜1 for bulk chondrites and ˜ 10-40 for the Silicate Earth. Thus a radiogenic effect in the Silicate Earth and in chondrites relative to the initial value in iron meteorites and chondritic metals would be expected if 182Hf was live in the early solar system. This is exactly what is found (Yin et al., 2002) that the 182W/183W ratio of bulk chondrites are ˜ 1.5 ɛ-units higher than the solar initial value, the Earth's mantle is ˜ 3.5ɛ higher, the Moon appears to be ˜ 5ɛ higher, the mantle of Mars ˜ 6ɛ higher and the Vesta mantle ˜ 20ɛ higher. This confirms the utility of this system as a accretion and core formation chronometer. Here we present new model results for the Hf-W system using the constraints from the new chondrite parameters of Yin et al. (2002). The models are based on isotopic and chemical mass balance between a primitive nebular reservoir, the primitive mantle and the core. A variety of accretion histories were investigated. We conclude that the main stage of accretion of the Earth (˜65%) is completed in 10 Myr and the remaining growth must be effectively finished in another 20-40 Myr. The formation of the Moon, if by a giant impact, happened at ˜25-30 Myr subsequent to Solar System formation. A scenario in which the Earth's core is formed after 60 Myr and the Moon simultaneously by a giant impact is now implausible. The Martian core-mantle differentiation must have occurred in less than ˜ 5 Myr while for Vesta this timescale is less than ˜ 3 Myr.

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