Planetary differentiation composition

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

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... 

There is a wide range of meteorite types, which are readily divided into three main groups the irons, the stony irons and the stones (see also Volume 1 of the Treatise). With this simple classification, we obtain our first insights into planetary dilferentiation. All stony irons and irons are differentiated meteorites. Most stony meteorites are chondrites, undifferentiated meteorites, although lesser amounts are achondrites, differentiated stony meteorites. The achondrites make up —4% of all meteorites, and <5% of the stony meteorites. A planetary bulk composition is analogous to that of a chondrite, and the differentiated portions of a planet—the core, mantle, and crust— have compositional analogues in the irons, stony irons (for core-mantle boundary regions), and achondrites (for mantle and crust). 

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. 

It is clear from table 21.1 and fig. 21.1 that the lanthanide distribution in the composite of North American shales (NASC) is different from that of the chondrites and the sun s atmosphere. Since the differences are a smooth function of atomic number (except for Eu), they are probably a result of processes of internal planetary differentiation. Differential condensation from a gas does not produce such smooth distributions (Boynton, 1975). What, then, does the NASC distribution represent, or tell us about the history of Earth ... 

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of... 

Some meteorites, and all planetary samples, have undergone melting and differentiation at some stage. Hence, the compositions of differentiated materials do not resemble solar system abundances. These samples can, however, tell us about various geochemical processes within asteroids and planets. 

Common igneous processes (partial melting and fractional crystallization) lead to element fractionations. Incompatible elements tend to be concentrated in melts and compatible elements in solids. Separation of partial melts from residual crystals as the melts ascend to higher levels, or accumulation of early-formed crystals from melts, ultimately produces rocks with compositions different from the starting materials. These processes account for the fractionations seen in differentiated meteorites and planetary samples. 

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

---