Earth core formation ages

For bodies the size of Earth, core formation did not occur as one single event but took place continuously during planetary growth. To determine realistic core formation ages for bodies like the Earth the W isotope evolution during continuous core formation must be considered (8, 35-38). During protracted accretion with concomitant core formation, the ratio of Earth s mantle... 

The calculation of core formation ages based on Hf-W observations is strongly model-dependent. The W isotope evolution of Earth s mantle does not only depend on the timescale of accretion and core formation, but also on the degree of re-equilibration between the metal cores of newly accreted planetesimals, the W isotope composition in the mantles of the newly accreted objects, and on the mechanisms of accretion. Hence, Hf-W chronometry alone cannot... 

Figure 5-8 A Pb-Pb isochron that determined the age of the Earth to be about 4.55 Ga. Stony and iron meteorites as well as a sediment of the Earth are plotted on a Pb-Pb isochron. The sediment, as a "bulk sample of the silicate Earth in terms of Pb isotopes, plots on the same line as the meteorites, suggesting that the Earth and meteorites formed at the same time and are the same age. Erom Patterson (1956). Later studies reveal a more detailed evolution history of the Earth, including core formation (about 4.53 Ga), atmospheric formation (about 4.45 Ga), and crustal evolution.

Chabot, N. L., Draper, D. S. and Agee, . B. (2005) Conditions of core formation in the Earth constraints from nickel and cobalt partitioning. Geochimica et Cosmochimica Acta, 69,2141-2151. 

The early history of Earth is greatly influenced by the probable impact of a Marssized body to form the Moon. Core-formation models suggest both Earth and the impactor were already differentiated by the time of the impact (Tonks Melosh 1992). The lack of a clear182W excess in uncontaminated lunar samples implies that the Moon-forming impact took place >50 Myr after the start of the Solar System (Touboul et al. 2007). The oldest known lunar samples are 150 Myr younger than CAIs, based on Sm-Nd dating (Touboul etal. 2007), which provides a lower limit on the Moon s age. 

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. 

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))...

Chabot N. L. and Agee C. B. (2003) Core formation in the Earth and Moon new experimental constraints from V, Cr, and Mn. Geochim. Cosmochim. Acta. 67, 2077-2091. 

Li J. and Agee C. B. (2001) The effect of pressure, temperature, oxygen fugacity and composition on partitioning of nickel and cobalt between liquidFe—Ni—S alloy and liquid silicate implications for the Earth s core formation. Geochim. Cosmochim. Acta 65, 1821—1832. 

Core formation model ages for Vesta, Mars, Earth and Moon... 

Small objects such as the parent bodies of differentiated meteorites underwent core formation early, in less than a few Myr after formation of the solar system. Over these timescales the decay of Al was a major heat source for differentiation. Accretion and differentiation of larger bodies such as Mars and Earth appear to have taken much longer, such that the energy required for differentiation was largely provided by collisions among planetary embryos. These Hf-W ages are consistent with numerical simulations that predict an early forma-... 

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