Planetary differentiation core formation

Although not strictly related to mantle depletion and crust formation, Hf- W isotopic compositions do provide clear evidence for early planetary differentiation of the Earth, Moon, and Mars related to core formation. The results (Kleine et al., 2002 Yin et al., 2002 Schoenberg et al., 2002) from this short half-life t — 8 Myr) system provide convincing evidence that metal... 

Many meteorites have never been subjected to processes of planetary differentiation. These imdifferentiated meteorites come fix>m planetesimals that were never molten and resemble die composition of the solar nebula at the time and place of their formation. They exhibit approximately solar system composition, are called chondritic meteorites, and represent one big group of meteorites. The second group consists of the differentiated meteorites which represent pieces of partially or totally molten parent bodies. Examples of this group are meteoritic basalts (eucrites) or iron meteorites. The latter are pieces of the segregated core of the parent body. 

The decay of Hf to is well suited to date core formation in planetary objects mainly for three reasons. First, owing to the Hf half-life of 9 Myr, detectable W isotope variations can only be produced in the first -60 Myr of the solar system. This timescale is appropriate for the formation of the Earth and Moon in particular and to planetary accretion and differentiation in general. Second, both Hf and W are refractory elements such that there is only limited fractionation of Hf and W in the solar nebula or among different planetary bodies (see above). The HfrW ratio of the bulk Earth therefore can be assumed to be chondritic and hence can be measured today. Third, Hf is a lithophile and W is a siderophile element such that the chondritic HfrW ratio of the Earth is fractionated internally by core formation. If core formation took place during the effective lifetime of Hf, the metal core (HfrW-O) will develop a deficit in the abundance of whereas the silicate mantle, owing to its enhanced Hf/W, will develop an excess of (7-P). 

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-... 

There are four main types of non-chondritic meteorites (Table 10.1). Primitive achondrites, such as the acapulcoites and lodranites, are thought to be from asteroids that experienced only incipient or limited melting (Table 10.1). In contrast, achondrites, iron meteorites, and stony-irons are considered to represent parent bodies that featured widespread melting processes, which ultimately led to planetary differentiation and the formation of a metallic core and a silicate-rich mantle and crust [14, 15]. 

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],... 

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