Kelvin Revisited: Cooling and Core Formation after the Giant Impact

Details Kelvin Revisited: Cooling and Core Formation after the Giant Impact Kelvin Revisited: Cooling and Core Formation after the Giant Impact

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v31f..05w&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #V31F-05

Other

1015 Composition Of The Core, 1040 Radiogenic Isotope Geochemistry, 3672 Planetary Mineralogy And Petrology (5410)

Scientific paper

The 182Hf-182W chronometer is now accepted as indicating relatively rapid accretion and core formation on Earth. Assuming that all accreting material mixed isotopically with the silicate earth the 182W/184W ratio of the mantle yields a mean-life of accretion of 15 Myr after the origin of the solar system. In contrast, if we assume that Pb is siderophile, the U-Pb system indicates much slower rates of core segregation, with substantial accretion of the Earth after the moon-forming impact at 45+/- 5 Myr. This means either that the late loss of Pb from the silicate Earth is due to some mechanism other than core formation or the U-Pb age reflects a later stage of core formation than does Hf-W. Recent metal-silicate partitioning data, when applied to the current composition of the mantle, support the `deep magma ocean' model of core formation, combined with a substantial increase of oxygen fugacity during accretion. Here we argue that the increase of oxygen fugacity during accretion was a consequence of the crystallization of silicate perovskite in the lower mantle. Due to the affinity of this phase for ferric iron, a planet larger than Mars should undergo progressive self-oxidation during accretion and core segregation. The oxidation process leads to destabilization of metal so that sulphide is the only `metallic' phase which can coexist with the silicate. Our explanation of the two timescales of core-formation is, therefore, as follows. The Hf-W timescale refers to the principal phase of core-formation before the giant impact. Crystallisation of silicate perovskite in the lower mantle during this phase produced Fe3+ which was released during the giant impact. This oxidation resulted in later segregation of sulphur-rich metal into which Pb dissolved readily, re-setting the U-Pb age of the Earth. Separation of the latter metal occurred 30+/-10 Myrs after the Moon-forming impact. Over this timespan the Earth cooled by about 4000K in returning from a fully-molten to a partially crystalline state. The result is in surprisingly good agreement with Lord Kelvin's estimate of the cooling age of the Earth.

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