Models core formation

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. 

Jones J. H. (1998) Uncertainties in modeling core formation. Meteorit. Planet. Sci. 33, A79-A80. 

There are significant differences in the 142Nd and e182W values obtained for the different types of Martian meteorites (Fig. 9.15). These variations indicate that the meteorites were derived from distinct mantle reservoirs that were established early in the history of Mars. Modeling based on s l42Nd and e182W indicates that differentiation of the silicate mantle could have taken place contemporaneously with core formation or could have been... 

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. 

Figure 10 Schematic model of BFR core formation mechanism...

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). 

Radiogenic isotope geochemistry can help with the evaluation of the above models for accretion by determining the rates of growth of the silicate reservoirs that are residual from core formation. By far the most useful systems in this regard have... 

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.

While lead isotopes have been useful, the i 2jjf i82- chronometer (Ti/2 = 9 Myr) has been at least as effective for defining rates of accretion (Halliday, 2000 Halliday and Lee, 1999 Harper and Jacobsen, 1996b Jacobsen and Harper, 1996 Lee and Halliday, 1996, 1997 Yin et al, 2002). Like U-Pb, the Hf-W system has been used more for defining a model age of core formation (Kramers, 1998 Horan et al. 

Having made all of these cautionary statements, one still can state something useful about the overall accretion timescales. All recent combined accretion/continuous core formation models (Halliday, 2000 Halliday et al., 2000 Yin et al., 2002) are in agreement that the timescales are in the range 10 -10 yr, as predicted by Wetherill (1986). Therefore, we can specihcally evaluate the models of planetary accretion proposed earlier as follows. 

Eventually, models that involved successive changes in accretion and core formation replaced these. How volatiles played into this was not explained except that changes in oxidation state were incorporated. An advanced example of such a model is that presented by Newsom (1990). He envisaged the history of accretion as involving stages that included concomitant core formation stages (discussed under core formation). 

The major problem presented by the Earth s chemical composition and core formation models is providing mechanisms that predict correctly the siderophile element abundances in the Earth s upper mantle. It long has been recognized that siderophile elements are more abundant in the mantle than expected if the sihcate Earth and the core were segregated under low-pressure and moderate-temperature equilibrium conditions (Chou, 1978 Jagoutz et al, 1979). Several explanations for this siderophile excess have been proposed, including ... 

Jacobsen S. B. and Yin Q. Z. (2001) Core formation models and extinct nuclides. In Lunar Planet. Sci. XXXII, 1961. The Lunar and Planetary Institute, Houston (CD-ROM). 

Although HSE concentrations are low in the Earth s mantle, they are not as low as one would expect from equilibrium partitioning between core forming metal and residual mantle silicate, as emphasized by new data on metal/silicate partition coefficients for these elements (Borisov and Palme, 1997 Borisov et al., 1994). Murthy (1991) suggested that partition coefficients are dependent on temperature and pressure in such a way that at the high P-T conditions where core formation may have occurred, the observed mantle concentrations of HSEs would be obtained by metal/silicate equilibration. This hypothesis has been rejected on various grounds (O Neill, 1992), and high P-T experiments have not provided support for the drastic decrease of metal/silicate partition coefficients of HSE required by the Murthy model (Holzheid et al., 1998). 

Thus core-mantle equilibration can be excluded as the source of the HSEs in the Earth s mantle. It is more likely that a late accretionary component has delivered the HSEs to the Earth s mantle, either as single Moon-sized body which impacted the Earth after the end of core formation or several late arriving planetesimals. The impac-tors must have been free of metallic iron, or the metallic iron of the projectiles must have been oxidized after the collision(s) to prevent the formation of liquid metal or sulfide that would extract HSEs into the core of the Earth. The relative abundances of the HSEs in the Earth s mantle are thus the same as in the accretionary component, but may be different from those in the bulk Earth. The late addition of PGE with chondritic matter is often designated as the late veneer hypothesis (Kimura et al., 1974 Chou, 1978 Jagoutz et al., 1979 Morgan et al., 1981 O Neill, 1991). This model requires that the mantle was free of PGE before the late bombardment established the present level of HSEs in the Earth s mantle. 

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... 

If a small amount of core-forming metal was left behind in the mantle and then later oxidized, the mantle could potentially acquire a chon-dritic Ni/Co and even near chondritic HSE concentrations. This idea, called inefficient core formation, was advocated by Jones and Drake (1986) and provides an explanation by combining low-pressure and low-temperature metal/silicate partition coefficients and metal mobility constraints. This model awaits integration with more realistic mobility data acquired in a dynamic... 

The core accounts for one-third of the Earth s mass. The nature and abundance of the light elements in the core are of fundamental importance to the study of the Earth and the solar system. Once identified, the abundance and distribution of light elements in the core would place constraints on a variety of issues including core formation models, volatile element budget in the bulk Earth, thermal structure and evolution of the core, and convection in the liquid outer core. For instance, the temperature at the CMB might depend on the identity and concentration of light elements in the core. Whether or not compositional buoyancy is important in the outer core would affect the pattern of core convection, hence the structure and evolution of the geodynamo. 

Developing a model for the composition of the Earth and its major reservoirs can be established in a four-step process. The first involves estimating the composition of the silicate Earth (or primitive mantle, which includes the crust plus mantle after core formation). The second step involves defining a volatility curve for the planet, based on the abundances of the moderately volatile and highly volatile lithophile elements in the silicate Earth, assuming that none have been sequestered into the core (i.e., they are truly lithophile). The third step entails calculating a bulk Earth composition using the planetary volatility curve established in step two, chemical data for chondrites, and... 

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