Accretion Earth core formation

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). 

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,... 

Just as the I-Pu-Xe system is useful for studying the rate of formation of the atmosphere and U-Pb and Hf-W are ideal for studying the rates of accretion and core formation, lithophile element isotopic systems are useful for smdying the history of melting of the silicate Earth. Two in particular, (Ti/2 = 36Myr) and Sm... 

Newsom H. E. (1990) Accretion and core formation in the Earth evidence from siderophile elements. In Origin of the Earth (eds. H. E. Newsom and J. H. Jones). Oxford University Press, Oxford, pp. 273-288. 

SNC meteorites (Harper et al., 1995 Borg et al., 1997). This isotopic anomaly requires early differentiation of mantle and crust. Because hafnium and mngsten fractionated into silicate and metal, respectively, the short-lived Hf- W system can be used to determine that martian core formation occurred within —13 milhon years of the planet s accretion (Kleine et al., 2002 Yin et al., 2002). Correlation between Nd and isotope anomalies, as well as the initial Os/ Os ratios for martian meteorites (Brandon et al., 2000), indicate synchronous differentiation of core, mantle, and cmst (Figure 14). On Earth, core formation took substantially longer, convection has stirred the mantle sufficiently to erase any evidence of early isotopic heterogeneity, and cmst formation continues throughout geologic history. 

O Neill H. St. C. and Palme H. (1998) Composition of the silicate Earth implications for accretion and core formation. In The Earth s Mantle Structure, Composition, and Evolution—the Ringwood Volume (ed. I. Jackson). Cambridge University Press, Cambridge, pp. 3-126. 

Geochemists and cosmochemists initially looked to models of planetary formation and comparison with other terrestrial planets to understand the earliest composition of Earth s atmosphere. During planetary accretion and core formation, volatile components were liberated... 

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... 

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

Tungsten and lead isotopic data can, however, be used to define the timescales for accretion, simply by assuming that core formation, the primary process that fractionates the parent/ daughter ratio, started very early and that the core grew in constant proportion to the Earth (Halliday et al., 1996, 2000 Harper and Jacobsen, 1996b Jacobsen and Harper, 1996 Halliday and Lee, 1999 Halliday, 2000). There is a sound basis for the validity of this assumption, as follows. 

Figure 4 Lead isotopic modeling of the composition of the silicate Earth using continuous core formation. The principles behind the modeling are as in Halliday (2000). See text for explanation. The Held for the BSE encompasses all of the estimates in Galer and Goldstein (1996). The values suggested by Kramers and Tolstikhin (1997) and Murphy et al. (2003) also are shown. The mean life (t) is the time required to achieve 63% of the growth of the Earth with exponentially decreasing rates of accretion. The p, values are the 2 U/2°4pb of the BSE. It is assumed that the p of the total Earth is 0.7 (Allegre et ah, 1995a). It can be seen that the lead isotopic composition of the BSE is consistent with protracted accretion over periods of 102-10 yr.

Earth, relative to average solar system (chondrites). However, the tungsten isotopic difference between early metals and the silicate Earth on its own does not provide constraints on timing. One needs to know the atomic abundance of Hf at the start of the solar system (or the ( Hf/ Hf)Bssn the bulk solar system initial ) and the composition of the chondritic reservoirs from which most metal and silicate reservoirs were segregated. In other words, it is essential to know to what extent the extra in the silicate Earth relative to iron meteorites accumulated in the accreted chondritic precursor materials or proto-Earth with an HfAV 1 prior to core formation, and to what extent it reflects an accelerated change in isotopic composition because of the high HfAV ( 15) in the silicate Earth. 

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