Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p22c0418k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P22C-0418
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5410 Composition, 5455 Origin And Evolution, 6207 Comparative Planetology
Scientific paper
The extinct 182Hf-182W isotope system is particularly suitable as a chronometer for the accretion and subsequent chemical differentiation of planetary bodies. Three recent studies (Kleine et al., 2002, Schoenberg et al., 2002, Yin et al., 2002) independently report a deficit of ca. 2 ɛ units in the 182W/184W ratio of chondrites relative to the terrestrial value and a 182Hf/180Hf ratio at the start of the solar system that is by a factor of ca. 3 lower than previously accepted (Lee and Halliday, 2000). Our preferred value for ɛW of chondrites is -1.9 ɛ units relative to the terrestrial value and the initial 182Hf/180Hf of the solar system is (1.09+/-0.09) x 10-4 (Kleine et al., 2002). Using these newly defined parameters core formation ages of ~3 Myrs for Vesta, ~13 Myrs for Mars, and ~33 Myrs for Earth can be calculated (here, Myrs refers to the time elapsed since the start of the solar system). These core formation ages correlate with the planetś size suggesting a more protracted growth history for the larger planets. Accretion of Mars stopped earlier than that of Earth, which possibly is related to the role of Jupiter in planetary accretion. The less radiogenic ɛW values for the martian and terrestrial mantles compared to Vesta indicate later metal-silicate equilibration in Mars and Earth that most likely occurred in a magma ocean environment. Hf-W systematics suggest that core formation and mantle differentiation were two decoupled processes. Using the Hf-W whole-rock isochron for eucrites (Quitté et al., 2000) silicate differentiation and core formation on Vesta can be dated at ~4 Myrs and ~3 Myrs, respectively. Although the absolute ages for mantle differentiation and core formation on Vesta overlap within error, a relative time difference of ~1 Myr between these two processes is resolvable. This is because the errors on these two ages are correlated and therefore cancel when only the time difference between mantle differentiation and core formation is considered. A decoupling of core formation and mantle differentiation on Mars is indicated by elevated 182W/184W ratios (~2.2 ɛ units relative to chondrites) for samples that display chondritic 142Nd/144Nd ratios. Based on 142Nd systematics a timescale of 27 Myrs timescale can be estimated for silicate differentiation (Harper et al., 1995), which is clearly resolvable from the time of core formation on Mars dated at ~13 Myrs. Combined 146Sm-142Nd data and 92Nb-92Zr systematics on terrestrial samples, and Hf isotope compositions of Earthś oldest zircons suggest that there is no vestige of terrestrial silicate differentiation earlier than 20 to 70 Myrs after core formation. Although old silicate reservoirs on Earth might have been re-homogenized with primitive mantle reservoirs, these constraints tentatively suggest a decoupling of core formation and mantle differentiation on Earth. References: Kleine et al. (2002), Nature 418, 952-955. Schoenberg et al. (2002), Geochim. Cosmochim. Acta 66, 3151-3160. Yin et al. (2002), Nature 418, 949-952. Lee and Halliday (2000), Chem. Geol. 169, 35-43. Quitté et al. (2000), Earth Planet. Sci. Let. 184, 83-94. Harper et al. (1995), Science 267, 213-217.
Bischoff Addi
Kleine Thorsten
Mezger Klaus
Münker Carsten
Palme Herbert
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