Core formation terrestrial

Lee DC, Halliday AN (1995) Hafnium-tungsten chronometry and the timing of terrestrial core formation. Nature 378 771-774... 

Kleine T., Munker C., Mezger K., and Palme H. (2002) Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry. Nature 418, 952-955. 

Harper, C. L. and Jacobsen, S. B. (1996) Evidence for 182Hf in the early solar system and constraints on the timescale of terrestrial core formation. Geochimica et Cosmochimica Acta, 60, 1131-1153. 

Harper, C. L., Volkening, J., Heumann, K. G., Shih, C.-Y., and Wiesmann, H. (1991a) l82Hf-182W new cosmochronometric constraints on terrestrial accretion, core formation, the astrophysical site of the r-process, and the origin of the solar system. Lunar Planet. Sci. XXD, 515-516. 

These models produced a zoned Earth with an early metallic core surrounded by silicate, without the need for a separate later stage of core formation. The application of condensation theory to the striking variations in the densities and compositions of the terrestrial planets, and how metal and silicate form in distinct reservoirs has been seen as problematic for some time. Heterogeneous accretion models require fast accretion and core formation if these processes reflect condensation in the nebula and such timescales can be tested with isotopic systems. The time-scales for planetary accretion now are known to be far too long for an origin by partial condensation from a hot nebular gas. Nevertheless, heterogeneous accretion models have become embedded in the textbooks in Earth sciences (e.g.. Brown and Mussett, 1981) and astronomy (e.g.. Seeds, 1996). 

Earth and its reservoirs, during the lifetime of i82Hf HfAV ratio of the silicate Earth is considered to be in the range of 10-40 as a result of an intensive study by Newsom et al (1996). This is an order of magnitude higher than in carbonaceous and ordinary chondrites and a consequence of terrestrial core formation. More recent estimates are provided in Walter et al (2000). 

Both, the uncertainty over ( Hf/ Hf)Bssi and the fact that the tungsten isotopic composition of the silicate Earth is now unequivocally resolvable from a now well-defined chondritic composition (Kleine et al, 2002 Lee and Halliday, 2000a Schoenberg et al., 2002 Yin et al, 2002), affect the calculated timescales for terrestrial accretion. It had been argued that accretion and core formation were fairly protracted and characterized by equilibration between accreting materials and the silicate Earth (Halliday, 2000 Halliday et al,... 

Nb decays by electron capture to Zr with a half-life of 36 3 Myr. At one time it was thought to offer the potential to obtain an age for the Moon by dating early lunar Umenites and the formation of Umenite-rich layers in the lunar mantle. Others proposed that it provided constraints on the time-scales for the earhest formation of continents on Earth (Miinker et al, 2000). In addition, it was argued that it would date terrestrial core formation (Jacobsen and Yin, 2001). There have been many attempts to utilize this isotopic system over the past few years. To do so, it is necessary to first determine the initial Nb abundance in early solar system objects accurately and various authors have made claims that differ by two orders of magnitude. 

Kramers J. D. and Tolstikhin I. N. (1997) Two terrestrial lead isotope paradoxes, forward transport modeling, core formation and the history of the continental crust. Chem. Geol. 139, 75-110. 

Righter K. and Drake M. J. (1999) Effect of water on metal-silicate partitioning of siderophile elements a higji pressure and temperature terrestrial magma ocean and core formation. Earth Planet. Set Lett. 171, 383-399. 

Schonberg R., Kamber B. S., Collerson K. D., and Eugster O. (2002) New W isotope evidence for rapid terrestrial accretion and very early core formation. Geochim. Cosmo-chim. Acta 66, 3151-3160. 

Yin Q. Z. and Jacobsen S. B. (1998) The Tc- Mo chronometer and its imphcations for timing of terrestrial accretion and core formation. In Lunar Planet Sci. XXIX, 1802. The Lunar and Planetary Institute, Houston (CD-ROM). 

Models for core formation in the Earth and other terrestrial planets are based on the distribution of siderophile elements between core and mantle. Interpretations of these data have focused on several characteristics of siderophile elements in... 

Figure 18 Metallic melt interconnectivity occurs when the dihedral angle between grains becomes <60 . At low pressures, this only occurs at high total anion content (sulfur, carbon, and oxygen). As a result, most metallic liquids relevant to core formation in the Earth and terrestrial planets have dihedral angles > 60°, and thus are unable to connect. In a solid mantle, metallic liquids will be trapped and unable to percolate (after Rushmer et al., 2000 homestead matrix data (2-23 GPa Shannon and Agee (1996) 25 GPa Shannon and Agee (1998))...

Holzheid A. and Grove T. L. (2002) Sulfur saturation hmits in sihcate melts and their implications for core formation scenarios for terrestrial planets. Am. Mineral. 87, Til-Til. 

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